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Mortars, plasters and pigments—research questions and sampling criteria

Mortars, plasters and pigments—research questions and sampling criteria Within the Topical Collection, this paper represents an introductory contribution aimed at describing and discussing the research questions and the sampling criteria in the field of mortars, plasters and pigments studies. The paper is divided into three parts. In the first part, some terminological issues are clarified and the building archaeology is introduced as an indis- pensable method for sampling and interpreting archaeometric results. In the second part, the most common research questions are presented and discussed. Some case studies are also reported to clarify what the expected results may be. The sampling problem is faced in the third part, where the criteria for a representative, functional and suitable selection are provided. Keywords Mortars and plasters · Pigment analysis · Building archaeology · Archaeometry and archaeology · Research questions · Sampling criteria Premise organisation is further provided by DeLaine (2021). Fur- thermore, several issues concerning the degradation and This Topical Collection (TC) covers several topics in the conservation of mortars and plasters are addressed from field of study, in which ancient architecture, art history, practical and technical standpoints (La Russa and Ruffolo archaeology and material analyses intersect. The chosen 2021; Caroselli et al. 2021). perspective is that of a multidisciplinary scenario, capable The second group of contributions is focused on pig- of combining, integrating and solving the research issues ments, starting from a philological essay on terminology raised by the study of mortars, plasters and pigments. (Becker 2021). Three archaeological reviews on prehistoric The first group of contributions explains how mortars (Domingo Sanz and Chieli 2021), Roman (Salvadori and have been made and used through the ages (Arizzi and Sbrolli 2021) and Medieval (Murat 2021) wall paintings Cultrone 2021, Ergenç et al. 2021; Lancaster 2021; Vitti clarify the archaeological and historical/cultural frame- 2021). An insight into their production, transport and on-site work. A series of archaeometric reviews illustrate the state of the art of the studies carried out on Fe-based red, yel- low and brown ochres (Mastrotheodoros et al. forthcom- This article is part of the Topical Collection on Mortars, plasters ing);  Cu-based greens and blues (Švarcová  et al. 2021); and pigments: Research questions and answers As-based yellows and reds (Gliozzo and Burgio 2021); Pb-based whites, reds, yellows and oranges (Gliozzo and * Elisabetta Gliozzo elisabetta.gliozzo@uniba.it; elisabetta.gliozzo@gmail.com Ionescu 2021); Hg-based red and white (Gliozzo 2021); and organic pigments (Aceto 2021). An overview of the use Antonio Pizzo antonio.pizzo@eehar.csic.es of inks, pigments and dyes in manuscripts, their scientific examination and analysis protocol (Burgio 2021) as well as Mauro Francesco La Russa mauro.larussa@unical.it an overview of  glass-based pigments (Cavallo and Riccardi forthcoming) is also presented. Furthermore, two papers on Department of Humanities, University of Bari, Bari, Italy cosmetic (Pérez-Arantegui 2021) and bioactive (antibacte- Escuela Española de Historia y Arqueología en Roma, CSIC, rial) pigments (Knapp et al. 2021) provide insights into the Roma, Italy variety and different uses of these materials. Department of Biology, Ecology and Earth Sciences, University of Calabria, Rende, Italy Vol.:(0123456789) 1 3 193 Page 2 of 30 Archaeol Anthropol Sci (2021) 13:193 Table 1 Definitions and uses of pigment, ink, dye and de/colouring agents to be used in archaeometry Pigment (Ink) Dye* De/colouring agent Definition Substance which, finely dispersed in Fine substance which penetrates the Substance which chemically bonds the water or other solvent, colours the substrate to which it is being applied material to which is added substrate by overlapping Characteristics - Chiefly inorganic - Chiefly organic - Only inorganic - Fine/coarse grained - Fine grained - Different types of compounds and grain - Insoluble - Soluble sizes - Dispersed suspension that covers the - Solution that is absorbed by the -Insoluble substrate substrate -Incorporated by the object Few examples Ochres (Mastrotheodoros et al. forthc.) Carmine; Gamboge Metallic Cu 2+ Cu-based (Švarcová et al. 2021) Indigo; Lac**; Oxydes and hydroxides containing Cu 2+ 3+ 4+ 2+ Hg-based (Gliozzo 2021) Madder; Purpurissimum Fe , Fe Mn, Co As-based (Gliozzo and Burgio 2021) Sepia Bronze Pb-based (Gliozzo and Ionescu 2021) Bindheimite Smalt (Cavallo & Riccardi forthc.) Use Painting, writing Textiles and leathers (painting, writ- Glass ing***) For a review on organic colouring materials used in wall painting, see Aceto (2021) in this TC ** The term lac may refer to two very different types of inorganic compounds: a) the gommalacca or shellac, i.e. “a pink-red–purple organic col- ourant derived from an insect and used as a lake pigment or a dye” (Berbers et al. 2019); b) a mixture of nitrocellulose and resin/wax *** Limited to specific chronologies and artefacts are provided here, based on the International Standards Introduction Organization (ISO 6707–1:2020 ): This paper serves as an anchor for the numerous contribu- (a) Mortar is described as a mixture of binder, aggregate tions of this TC. It intends to provide the criteria for both the and water; formulation of sound archaeometric questions and the execu- (b) Concrete is a mixture of aggregate, cement and water; tion of a suitable sampling in the field of mortars, plasters (c) Cement is an inorganic binder mixed with water to and pigments studies. form a paste “that sets by means of hydration reactions To frame these issues in a methodologically clear and and  processes, and that, after hardening, retains its exhaustive panorama, two brief explanatory sections are pro- strength and stability”. vided at the beginning: (1) what is meant by mortar, plaster and pigment and (2) what is meant by building archaeology. Both these introductory texts function to explain which Furthermore, in the context of Roman architecture, con- archaeometric questions are valuable for historical recon- crete is defined as the mixture of lime, aggregates and water struction and to guide researchers in the direction of repre- to form the so-called opus-caementicium (see Vitti 2021 in sentative sampling. These last two issues represent the bulk this TC). of the present contribution and are both presented in light of In this context, mortar is a recipe where the components the most recent advances made in this field. The interpreta- are the variables and its end-use is the archaeometric study’s tive problems that frequently arise from the archaeometric object. This basic distinction is fundamental in organising study of mortars are discussed in the last section of this the research questions and provides a key tool for their inter- paper, exploring the value and potentiality of archaeometric pretation. Consequently, correct identification and charac- research for historically meaningful reconstructions. terisation of the variables represent the starting point for any technological and provenance issue. Moreover, while the recipe’s reconstruction mostly regards the technological Mortars and pigments: basic definitions issue, the end-use of a mortar provides the necessary link to establish its suitability and functionality. Basically, there The term mortar basically defines a mixture of different components used to bond bricks or stones (Table 1). Ter- minological issues may arise when trying to distinguish a mortar from concrete and cement; therefore, the definitions https:// www. iso. org/ obp/ ui/# iso: std: iso: 6707:-1: ed-6: v1: en: term:3. 4.4. 27 1 3 Archaeol Anthropol Sci (2021) 13:193 Page 3 of 30 193 Fig. 1 Main types of plasters and mortars, depending on the starting raw material are two types of mortars: the aerial (Ergenç et al. 2021 in Typical examples in ancient buildings include: this TC) and the hydraulic ones (Arizzi and Cultrone 2021 in this TC). – Clay plasters, mixing clay, sand and plant fibres; Aerial mortars are characterised by higher flexibility, – Aerial lime mortars and plasters, mixingslaked lime and plasticity and permeability than the hydraulic ones; how- aggregate (e.g. sand or a mixture of sand and other inert); ever, the former shows a greater shrinkage during hardening, – (Feebly hydraulic ) magnesian mortars and plasters, mix- lower mechanical strength and resistance to moisture, salt ing slaked magnesia-lime and aggregate; attack and frost compared with the latter. Based on these – Gypsum mortars and plasters, consisting of calcium sul- main properties, aerial mortars are mainly used in sheltered fate hemihydrates and sand; areas, while hydraulic ones are favoured in exposed build- – Hydraulic pozzolanic mortars and plasters, mixing ings, such as port infrastructures (Vicat 1837; Ashurst and hydrated lime with natural or artificial pozzolanas (e.g. Ashurst 1988; Gibbons et al. 1995; Holmes and Wingate fly ashes and/or powdered ceramics); 1997; Cowper 1998). – Hydraulic mortars and plasters, mixing a natural hydrau- The term plaster includes several types of mixtures, lic lime and aggregate. depending on the type of binder and aggregate used. Techni- cal terminology (ISO 6707–1:2020 ) distinguishes between A schematic distinction between the various types of mixtures of one or more binders, defined as plasters, and plasters and mortars is provided in Fig. 1, while the main mixtures of one or more binders with aggregate (and other reactions addressed henceforth are listed in Fig. 2. possible admixtures), defined as renders. Moreover, the The use of the term pigment may be tricky, as several terms plaster and render should apply to mixtures used for other terms, such as colouring agent, ink and dye may appear the internal and external finish, respectively. to be synonyms; however, several differences exist among In archaeometry, plaster is used as a more general term, them regarding grain size, composition, application and use. including the meaning and properties of both modern plas- The definitions provided by different authorities such as the 4 5 ters and renders. This should perhaps remain so to main- ETAD, the CPMA and the DIN Standards Committee tain terminological coherence with the history of studies Pigments and Extenders are based on current commercial and highlight the distinction between ancient materials and use and classification of colour pigments. They sometimes technical materials used, for example, for restoration and conservation. On the other hand, the modern classification The hydraulicity of magnesia-limes is debated. Its characterisation of mortar and cement (e.g. UNI EN 197/1) foresee subdivi- as feebly hydraulic relies on Chever et al. (2010). sions that do not apply to the ancient world’s materials and Ecological and Toxicological Association of Dyes and Organic Pig- could not be extensively used tout court. ments Manufacturers (https:// etad. com/). Color Pigments Manufacturers Association, Inc. (https:// www. pigme nts. org/). 2 6 https:// www. iso. org/ obp/ ui/# iso: std: iso: 6707:-1: ed-6: v1: en: term:3. Responsible for the European standardization (https:// www. din. de/ 4.4. 27en/ getti ng- invol ved/ stand ards- commi ttees/ npf). 1 3 193 Page 4 of 30 Archaeol Anthropol Sci (2021) 13:193 Fig. 2 Chemical reactions occurring during mortar production and weathering (from Davidson et al. 1965; Böke et al. 2006; Uğurlu and Böke 2009; Jakić et al. 2016; Ponce-Antón et al. 2018; Li et al. 2020). The magnesian lime cycle is oversimplified because the formation of the different phases (e.g. artinite, brucite, calcite, dolomite, dypingite, huntite, hydromag- nesite, lansfordite, magnesite, nesquehonite, periclase and portlandite) depends on the temperature, the C O concentra- tion and the pH and RH (i.e. relative humidity). Therefore, it is not straightforward to indicate which phases are formed for each slaking, setting and hard- ening phase (see also Lanas and Alvarez 2004 on this topic) include both organic (i.e. containing carbon) and inorganic – Extenders (“material in granular or powder form, prac- substances under the general definitions of “pigment” and tically insoluble to somewhat soluble in the application “dye”. In the current regulation (ISO 18,451–1:2019 ), a medium and used to modify or influence certain physical pigment is defined as a “ colourant consisting of particles, properties”); insoluble in the application medium (e.g. coating material – Fillers (“coating material with a high proportion of or plastic)” where colour is a “generic term for all colour- extender, intended primarily to even out irregularities in ing substances” and subcategories are allowed based on substrates to be painted and to improve surface appear- their chemical composition and properties (e.g. inorganic, ance”). organic, coloured, white, effect, corrosion-inhibiting, mag- netic). Main distinctions are traced as follows: Undoubtedly, the increase and the variation in terminol- ogy that we have witnessed in recent years follow the devel- – Dyes (“colourant, soluble in the application medium”); opment of technological products that did not exist in ancient – Pigments used for ceramics and glass (called stains); times and the creation of products used for conservation. https:// www. iso. org/ obp/ ui/# iso: std: iso: 18451:-1: ed-2: v1: en: term:3. 1 3 Archaeol Anthropol Sci (2021) 13:193 Page 5 of 30 193 It is not uncommon, in modern literature, to find the term Born at the end of the 1980s by Italian medieval archae- inorganic dyes used in place of pigment or the term colour- ologists (Siena, Genova and Padova universities), the disci- ing agent used in the sense of chromophore and applied to pline picked up the need to renew the archaeological meth- both pigments and dyes. Therefore, while it is necessary to ods applied to historical architecture. consider and adopt it when necessary, it is also worth provid- BA applies the stratigraphic method to any building type, ing adequate terminological explanations and maintaining whether or not preserved in its entirety. The theoretical and a certain consistency with our study field, hence avoiding methodological system used in BA has been, by now, thor- unnecessary anachronisms. oughly discussed and formalised. The main objective is to To make an example, each definition produced by the reconstruct the historical diachrony (phases of construction, sector authorities agrees on the basic distinction: pigment- use, transformation, abandonment and destruction) of indi- insoluble, dye-soluble; however, no further specification is vidual structures or entire buildings. given on the state (solid or liquid) or nature (organic/inor- Moreover, in addition to identifying the historical ganic and natural/synthetic), meaning that all combinations sequences of architectural artefacts, the BA offers a fun- appear possible. Conversely, in archaeometry, these distinc- damental tool for conservation, eventual restoration and, tions have important implications regarding the analytical overall, the protection and safeguarding of historic buildings. techniques to be used for their characterisation and the prob- To achieve a diachronic reading of the elevation, the basic lems related to conservation. method of the BA includes: In the current archaeometric literature and practice (Table  1), pigments are considered inorganic materials The stratigraphy, for the definition of relative chronolo- (chiefly minerals and earths). They may be either natural gies; or synthetic (e.g. Egyptian blue) and colour the surfaces The study of building techniques, which, in turn, to which they are applied by simply covering it. Their use includes: for different types of paintings (e.g. paintings, frescoes and icons) is the most common one; however, when used for The characterisation of the building materials (stone, dark writing in manuscripts, the name switches from pig- bricks, mortar, plaster, etc.); ment to ink. Conversely, dyes mainly include organic mate- The technical and technological study of both materi- rials, frequently soluble (also insoluble, e.g. indigo), that als and structures and the way materials are assem- can chemically disperse into and bind to the materials. They bled. are frequently obtained from plants (e.g. leaves, berries and roots) or animals (e.g. insects and molluscs), and their mul- In practice, the BA consists of (a) distinguishing the tiple uses range from textiles and food colouring to painting. constructively coherent parts on a wall/structure, (b) Finally, the terms de/colouring agents are used only in the identifying their contours and (c) organising the individ- field of glass studies and are sometimes synonymous with ual stratigraphic units in a diagram according to a rela- chromophore, to identify elements and phases—exclusively tive chronology. This procedure represents the basis of inorganic so far—that chemically bond the material to which archaeological stratigraphy, as it allows us to understand they are added. the before and after of each action. To conclude, we must add that (a) mixed inor- To better understand the role of mortars and pigments, ganic–organic compounds are also known, as evidenced by it is necessary to clarify the role of building techniques the mixture of palygorskite clay and indigo used for Maya under a theoretical (Mannoni 1997) and methodologi- blue; (b) the term colourant should be avoided in scientific cal (Parenti 1988a) point of view, as to be aware of the literature, as it is a general term that includes both pigments limitations in using building techniques to date buildings and dyes. (Parenti 1988b). Moreover, this is the study’s field, where a fruitful collab- Building archaeology oration between different expertise is essential, as it includes the investigation of mortars, plasters and pigments. The building archaeology (BA) is a research methodology The analysis of building techniques has to be multi-level, and a discipline that provides a reading of the macro trans- from the scale of the entire building to the materials con- formations of the investigated building, such as changes in stituting the wall, the mortars, the plasters, the binders and the original project, added structures, interventions for the the pigments. redistribution of spaces, use and functions of buildings and In this sense, it is an indispensable premise to any archae- analysis of the environmental or built context related to the ometric research. To interpret the results at the large scale building. of the entire building, it is necessary to start from sampling at the small scale of the single wall. 1 3 193 Page 6 of 30 Archaeol Anthropol Sci (2021) 13:193 At the small scale, BA provides information on the con- between research phases, such as sampling, analytical study struction morphology, details of the workforce and interpre- and results interpretation, with the history and intended use tations of the structures. of the investigated “structure” implies a close collaboration All these features, combined together, provide the sam- in planning the best research strategy. pling criteria for archaeometric research. The sections below clarify the main research question After the completion of archaeometric analyses, the raised during the study of the masonry works, including results complete and improve the description of the tech- both those that are possible to solve through archaeometric nique itself. In substance, BA and archaeometry initially methods and those in which archeometry plays a marginal work separated and then converge towards an agreed, accu- role. The aim is to provide as complete a picture as possible, rate and comprehensive result. albeit within the single article’s limits. At the large scale, the construction techniques and the stratigraphy provide the instruments for the interpretation of results. The con- textualisation of the results is the passage that allows us to reconstruct The provenance of raw materials used for building the history of the building. By integrating all the results obtained materials from the joint research, it is possible to reconstruct all stages of pro- cessing: from the quarry chosen to supply the most suitable material, While building material characterisation provides precious to the production of lime and mortar, to assembling of materials in information on the type of raw material used, the identi- the masonry, to the decoration, to the uses and, eventually, to the fication and localisation of the supply area inform us on collapse of the building. the aspects that determined its selection. This is the main At both the small and large scale, the building technique reason why we should consider this question as intended represents the main instrument to establish the absolute chro- to investigate a territory rather than limited archaeologi- nology of a built context. Archaeological dating uses direct cal samples and geological outcrops. Moreover, this is a and indirect sources. The former includes historical, carto- multi-fold question since it regards all different compo- graphic or iconographic sources, while the latter are deduced nents: mortars, plasters, pigments, bonded stones or clayey from BA or from structural elements (e.g. stamped bricks of raw materials used for brick making. While it is possible to known chronology or coins; see also Mannoni 1984). Funda- limit the study to mortars, plasters or pigments, an in-depth mental information is also obtained by studying local chrono- understanding of the whole context certainly grants higher logical clusters, i.e. construction techniques closely linked to quality research. As a matter of fact, this approach makes the a territory and the consistency of the material used. Archaeo- difference between mere material analysis and meaningful logical dating is, thus, the main tool to decide whether the archaeometric research. archaeometric analysis of dating is a path to be attempted or In practice, the localisation of the supply areas or quar- not and, if it is, to select the most significant samples. ries allows a deepening of our knowledge on various issues: It goes without saying that, in practice, the chronological sequence of our studies generally corresponds to the small/ (1) To define what was the builders’ awareness of the large scale distinction proposed above. resources available in their territory; (2) To determine when the functionality criterion prevailed over that of availability or vice versa; (3) To find rational explanations to technologically contro- The archaeometric questions versial choices; (4) To answer all those “whys” that follow the provenance Taking up the concept already expressed in Gliozzo (2020a), issue’s resolution and require a thorough knowledge the archaeometric questions guide both sampling and experi- of both the territory and the ancient production tech- mental choices, besides providing the key criterion of eval- niques. uation for the entire research project. Research questions should be wide and necessarily contextualised in a historical In addition, the reconstruction of georesources supply strat- perspective (the “big picture” in Gliozzo 2020a). The tradi- egy and trade patterns clarifies which was the commercial net- tional—now anachronistic—distinction between archaeolog- work of reference for the site in a given chronological period ical and scientific questions finds a practical compendium and, therefore, allows the researcher to evaluate the choices in the archaeometric question. Consequently, archaeometric made by the builders in a broader framework that includes the research implies and requires a multidisciplinary team to social and market economy, the geology of the territory and the provide the necessary skill-sets on a permanent basis. complex production activities carried out on the site. In the field of mortar studies, this unity of purposes and In the practice of archaeometric research, this question practices is particularly evident and essential. The close link involves a preliminary in-depth knowledge of the geological 1 3 Archaeol Anthropol Sci (2021) 13:193 Page 7 of 30 193 setting of a large territory, including the archaeological site, out after the stratigraphic and the typological study have the archaeometric analysis of archaeological materials and that been completed; otherwise, the risk is to select a collection of numerous types of natural georesources that are believed to of non-representative samples. Moreover, (a) once the first have been used. All this information provides the indispensable round of materials analyses is completed, the need for tar- reference database. geted field campaigns arises, both to study the territory and To make a shortlist, materials analyses and investigation to sample the geological materials to be analysed for com- may regard: parison, and (b) after the characterisation of archaeological and geological materials, a third experimental phase may A. Mortars, plasters and pigments from which the study follow, aimed at verifying some working hypotheses. This is starts (henceforth, “the study’s object”), further divided the procedure generally adopted because the archaeometric into: approach is sometimes intended as a “tool” to find some answers and not as a consolidated procedure, intimately A(1). Geological sands, which can be compared to linked to the archaeological research methodology. those used in plasters and mortars; On the contrary, if the archaeometrist is involved from the A(2). Geological raw materials suitable for the produc- beginning of the excavations, he/she can carry out the geo- tion of the binder; logical “reading” of the territory and a part of the samplings A(3). Any material that may have been added to both in advance of the usual timetable, the objectives can be con- mortars and plasters (e.g. plants, ground ceramic certed and programmed more systematically and effectively fragments, or rock fragments) to confer particular and well-integrated results may become expendable in less properties; time. A(4). The water; A(1) The aggregate: sands Based on the International Stand- B. In stonework, building stones and relative geological ards Organization (ISO 6707–1:2020 ), the aggregate is an samples. In brickwork, bricks and geological clayey “inert granular material” that can be divided into fine and materials, which can be compared to those used in heavy, the latter having oven dry-particle ≥ 3000 kg/m . bricks; In historical mortars, most of the aggregate is usually C. Minerals and pigments naturally present in the area constituted by sands; therefore, investigating their nature— under investigation. “which kind of sand is it?”—typically represents the first step. To better explain how the research deals with the “prov- These are usually classified as fluvial, lacustrine, enance of raw materials”, we propose some examples from marine, fossil and quarry sands, depending on whether published case studies, but, for details, we refer to Ergenç they are taken from rivers, lakes, sea beaches or other ter- et al. (2021) and Arizzi and Cultrone (2021 in this TC). restrial deposits. The canonical particle size classifica- tion defined by Wentworth (1922) distinguishes sands as A. The study’s objects very fine (> 63 < 125 μm), fine (> 125 < 250 μm), medium (> 250 < 500  μm), coarse (> 500  μm < 1  mm) and ver y Undoubtedly, the study’s objects, whether they are mortars, coarse (> 1 < 2 mm) and the presence of the coarser gravel plasters or pigments, are typically the first to be investigated; fraction distinguishes concretes from mortars. It is possi- otherwise, we would have no information available to guide ble to use other classifications, but the reader must bear in the search for the raw materials that have allowed their reali- mind that, while the Wentworth classification was specifi- zation. Indeed, the research begins with the basic question: cally formulated in the geological field, others such as the “What is it made of?”. ISO 14,688–1:2002 have been elaborated for engineering The specific methodologies are addressed in the various purposes. The shape and rounding of the grains, the sorting contributions of this TC; therefore, we do not dwell on the and the composition represent other important characteris- subject here. However, we would like to highlight two pro- tics that, combined with previous ones, provide the neces- cedural aspects that we believe are important but not always sary information to determine the nature of sands and guide clear to newbies: (1) it is important to perform an accurate the search for natural outcrops to be sampled for comparison. visual inspection of mortars and plaster before any sampling is planned, and (2) analyses are not a quick procedure. In https:// www. iso. org/ obp/ ui/# iso: std: iso: 6707:-1: ed-6: v1: en: term:3. most cases, it is not enough to insert the sample into an 4.4. 27 instrument to get the answers we want. Still from a proce- In this regard, it is also worth underlining that “whereas concrete dural point of view, the researcher must bear in mind that is itself a building material, mortar is a bonding agent that typically holds bricks, tiles and other masonry units together” (Allen and Iano sampling and subsequent analyses should only be carried 2013). 1 3 193 Page 8 of 30 Archaeol Anthropol Sci (2021) 13:193 In this way, it is possible to answer the second question absent since they easily burn completely. Conversely, when aimed at locating the quarry, site, or area of supply of the sand: carbonatic rocks are used, the presence of these relics is more “where does it outcrop and from where was it taken?”. frequent and may favour raw materials’ search; however, the The following questions depend on the results obtained by researcher is frequently obliged to deal with very small quanti- characterising the mortar/plaster. For example, if the sand that ties of little dimensions even in this case. Furthermore, differ - was used has a significant clay component, we may wonder why ent types of binders may have been used in combination (e.g. the type of sand that is least suited to making these products lime and gypsum). In these cases, determining the nature and was used. If, on the other hand, the sand is rich in an earthy origin of the binder may be a complicated task, which requires component, we may ask ourselves why the ancient artisans did the use of numerous analytical techniques, capable of provid- not feel the need to wash them before use. If, finally, the particle ing complementary information. size is poorly/well sorted, we may ask ourselves how to explain The finding of the lime kiln or a dump of waste materials this evidence: comparable with the natural supply deposit or the would represent an ideal situation. A micro-stratigraphic sam- result of a technological procedure (sieving)? pling of contextual materials may guarantee the identification of These are just a few examples of questions that may arise the raw material, provide information about its processing and, from the initial two. Therefore, it is good to remain flex- when it is possible to establish a direct connection between the ible when planning the overall research and, especially, the place of production and the structure of use, indicate the degree initial sampling because it is not possible to predict them in of mortars elaboration. In such an advantageous situation, the advance. In several cases, in-depth knowledge of the terri- field campaign may be precisely directed towards specific out- tory likely provides the key to answer the various research crops, and the ensuing laboratory analyses may provide results questions. Still, the researcher should be aware that this ranging from a high likelihood to certainty. knowledge often makes it necessary for a supplement of In the absence of this archaeological evidence, the level the investigation—aimed at the characterisation of natural of accuracy of the answers is strongly conditioned by the materials—because literature data may be insufficient for characteristics of the binder itself (e.g. nature, composition, archaeometric research. Apart from this, an equal in-depth abundance and size) and is, therefore, not predictable. knowledge on the different yield of fine sands (e.g. requiring high amounts of binder and water and thus resulting in less A(3)  The additives (~ modern additives, admixtures and workable, too porous and less resistant mortars) and coarse aggregates) In modern terminology, the additive is a material sands (e.g. requiring less binder and water, thus resulting “added in small quantities to a liquid or granular material in better workable, adequately porous and highly resistant to produce some desired modification to its characteristics” mortars) and, in general, the importance of sorting, or pref- (ISO 6707–1:2020), while the admixture is a material “added erences tuned to certain types of sands (e.g. river and lake) in small quantities before or during a mixing process in order over others (e.g. quarry sands due to impurities; sea sands to modify the characteristics of a mixture”. The two terms for aerial mortars due to salts) is preliminarily required. describe apparently similar materials that, added in small quantities, can modify the properties of the starting mixture. A(2) The binder The characterisation of the binder always However, the difference between the two terms lies mainly represents a key question: “which kind of binder was in that the additive is added during the manufacturing of the used?”. The analyses combined with the knowledge of the cement, while the admixture is added to the concrete during geological settings provide the necessary clues to the identi- mixing. Moreover, the UNI EN 934–2 standard states that the fication and localisation of the raw materials used for binder admixture for concrete should be in an amount not greater making, i.e. to solve the related question: “where do the raw than 5% by mass, compared with the cement. Therefore, under materials to make the binder come from?”. the current rules, the addition of materials, such as ground Since binders may be a variety of materials, several ceramic fragments, in considerable proportions would fall into types of sources should be targeted, such as earths (with the category of aggregates and not that of additives. earth mortars), clays (with clay mortars), limestone and/ However, one of the main aims of the archaeometric or sea-shells (with lime-based mortars/plasters), dolomite study on ancient mortars and plasters is to reconstruct the and dolostones (with magnesian mortars/plasters), gypsum production’s phases and the supply modalities of each com- deposits (with gypsum mortars/plasters) and marls (with ponent. In this case, the term additive, used in its etymologi- lime-based mortars with feeble hydraulic properties). cal sense (from addere, to add) has the advantage of clearly However, even if, in most cases, the material is of a geo- recalling the difference between a primary component (fre- genic nature, its study implies an additional difficulty since quently sand) and an added component (i.e. the additive) we no longer see the “original” material but only its prod- in the aggregate. This is the meaning we have given to the uct, further reacted after use. For example, when seashells term additive in this article. Still, the explanation provided are used (e.g. Gleize et al. 2009), lumps of un-burnt lime are above accounts for the fact that modern terminology partly 1 3 Archaeol Anthropol Sci (2021) 13:193 Page 9 of 30 193 overlaps and partly articulates itself differently, based on The discovery of recycled amphorae containing north- the contemporary world’s different needs. It follows that, as Latial pozzolans in the ship B of Pisa (Augustan Age) is a long as terminological uncertainties persist, it is always good clear example of how the archaeometric analyses can effec- to specify the choice made in the description of the results. tively trace the movements of these raw materials, apparently Hence, after having accurately characterised the main also used as ballast despite its low weight. As further evidence components (aggregate and binder), the following question that the investigation of the provenance is only the first step concerns the possible presence of additives: “are there any to seek historically much more significant reconstructions, deliberately added materials?”. If yes: “which types of the studies conducted on volcanic scoria in some buildings of materials were added?” and “where did they come from ancient Rome (Lancaster et al. 2011) have not only updated our or how were they made?” (other questions concern suit- knowledge on the chronologically diversified supply of these ability and functionality and are reported in the dedicated materials from Rome, the Vesuvius and the Campi Flegrei, but paragraphs). also imposed a revision of the land and sea transport system The addition of inclusions to improve the final product’s and hypothesised an imperial involvement in the trade of these performance was a common practice that frequently left rec- raw materials. Similarly, the studies conducted on Tunisian ognisable traces in literary sources and archaeological evi- (from Sardinia or Pantelleria) and Turkish (from short-range dence. Consequently, citable examples are numerous and pro- resources) analogous materials have shed new light on the vide us with an extremely varied picture in which inorganic methods of importation (primary or secondary loads associ- and organic materials are enlisted. Therefore, it is possible to ated with other goods, such as grain mills), as well as demon- formulate the characterisation question in terms of: “are the strated their long-range transportation (Lancaster et al. 2010). additives of an inorganic or organic nature?” (we will see In a nutshell, from the provenance investigation of a particular later that there are additional difficulties in this second case). additive in the mortar, it is possible to find ourselves investigat- To properly solve this question, it is necessary, perhaps ing the favourability of winds along one sea route compared to even more than in previous cases, to know which types of another, the distribution of primary and secondary ports, the type additives were used to facilitate—or even allow—their iden- and capacity of ancient boats, the circulation of associated goods tification during the analysis of the “study’s object”. For this (e.g. foodstuffs, ceramic, millstones, building materials) and the reason, we present a shortlist of inorganic and organic addi- ancient navigation methods (e.g. coastal navigation): it is precisely tives (and admixtures); however, we refer to the other contri- here that the study becomes interesting and fruitful. butions for the necessary insights on their characteristics and Proceeding further, it is worth adding that the term poz- properties (Arizzi and Cultrone 2021; Ergenç et al. 2021). zolan may also apply to non-volcanic, natural and artificial Beginning with inorganic additives, the most common were materials with similar characteristics (chiefly silica and alu- natural and artificial pozzolans that are defined as “siliceous or mina, along with ferrites). Therefore, the question may turn siliceous and aluminous materials which in themselves possess into: “what type of pozzolan was added?”. little or no cementitious value but will, in finely divided form Main examples are represented by: and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperatures to form compounds pos- – Ceramic powder (especially tiles). Based on literary sessing cementitious properties” (Dodson 1990). sources, this was one of the most typical inclusion (“coc- Natural pozzolans are volcanic origin materials, such as ciopesto” and opus signinum in the Roman world; Kho- the volcanic ashes from the region of Pozzuoli from which rasan in Turkey; Surkhi in India; semen merah in Indo- they take their name (pozzolan s.s.). These are often found nesia; Homra in Arabic countries; Spence 1979). During in the mortars/plasters of sites located within or close to the Roman age, both Vitruvius and Plinius reported on volcanic regions (Barba et al. 2009; Özkaya and Böke 2009; the effectiveness of such inclusions in constructions built Villaseñor and Graham 2010; Kurugöl and Güleç 2012), but along a river or the sea exposed to humidity and marine when they are found in sites far from supply areas, archaeo- sprays. During the Middle Ages, Villard de Honne- metric research is called upon to localise their origin and court (French master mason of the thirteenth century reconstruct the trade routes that allowed their import and usage. The provenance question may thus remain confined to the scale of the site and its territory or expand to long- Vitruvius, De Architectura II, 5, 1 “etiam in fluviatica aut marina si qui testam tunsam et succretam ex tertia parte adiecerit, efficiet scale range imports and cross other issues such as those materiae temperaturam ad usum meliorem”. (If to river or sea sand, concerning the methods and extent of the marketing of raw potsherds ground and passed through a sieve, in the proportion of one materials in a given period (e.g. “what was the diffusion of third part, be added, the mortar will be better for use). Pliny, Natura- these materials?”; “what were the routes and commercial lis Historia, XXXVI, 54, 175 “si et testae tusae tertia pars addatur, melior materia erit” (If, too, one third of the mortar is composed of methods involved in the transport of these important raw bruised earthenware, it will be all the better). materials?”). 1 3 193 Page 10 of 30 Archaeol Anthropol Sci (2021) 13:193 AD) reported a hydraulic paste recipe based on lime, – Processed clays such as metakaolins (see, e.g. Baronio pounded fragments of bricks and linseed oil. In the and Binda 1997; particularly used for restoration, e.g. Renaissance period, the famous architect Leon Battista Aggelakopoulou et al. 2011; Vejmelková et al. 2012a; Alberti (1404–1472) reported the common opinion that, Loureiro et al. 2020); if you add a third of crushed brick, the resulting mixture – Diatomaceous earths (see, e.g. Franzini et al. 1999, 2000) becomes much more tenacious. A century later, Pietro and opal-A (Sarp Tunçoku and Caner-Saltık 2006); di Giacomo Cataneo (Sienese architect, 1510–1574) – Specific soils in earthen mortars (see, e.g. Cantù et al. handed down the recipe for mortar, recommending two 2016). lime parts and two other parts of tile powder, with half a part of iron flakes. The list could go on for long but Finally, marble (or travertine) dust was used in mix- we believe it is already clear how the masters configured tures with sand and lime for plasters to be decorated or the use of this material as “typical” over the centuries. for the bed mortar layer of mosaics or for relief decora- The archaeometric analyses support this reconstruc- tions (stuccos) that had the splendour and luster of marble tion. Ceramic powders, sometimes mixed with other (opus albarium or caementum marmoreum; see Vitruvius, pozzolans and volcanic materials, have been frequently De Architectura, VII 2, 1–2 and Cataneo 1567, II 11; for found in mortars and plaster dated to the: a review on some historical treatises see Salavessa et al. 2013; for some archaeometric evidence see Toniolo et al. • Late Bronze Age (Theodoridou et al. 2013); 1998; Riccardi et  al. 2007; Kriznar et  al. 2008; Weber • Hellenistic (see, e.g. in Italy, Montana et al. 2016); et  al. 2009; Duran et al. 2010; Miriello et al. 2011; De • Roman and early Byzantine periods (see, e.g. in Luca et al. 2012; Robador and Arroyo 2013; Lezzerini Italy, Bugini et al. 1993; Damiani et al. 2003; Berto- et al. 2014, 2019). lini et al. 2013; Izzo et al. 2016; Columbu and Garau As for the identification of organic additives, additional 2017; Graziano et al. 2018; Miriello et al. 2018, difficulties need to be known in advance. The substantial dif- Montana et  al. 2018; Sitzia et  al. 2020; in Spain ference between inorganic and organic materials is that the Alonso-Olazabal et al. 2020; in Tunisia, Farci et al. former are generally visible to the naked eye or at medium 2005; and in Turkey, Bakolas et al. 1998; Miriello magnification. In contrast, many of the organic ones (e.g. et al. 2011); milk and egg whites) are not visible. Therefore, the research • Medieval periods (see, e.g. in Greece Moropoulou question “are there any organic additives?” stems more et al. 2000; in Italy, Lezzerini et al. 2014; in Por- from the preliminary knowledge of ancient methods and tugal, Adriano et al. 2009; in Turkey, Kurugöl and techniques than from macroscopic and microscopic obser- Güleç 2012) and beyond (e.g. in the Czech Republic, vation but is archaeometrically resolvable through an ad hoc Přikryl et al. 2011; in Italy, Cantù et al. 2016; in analytical strategy. Turkey, Böke et al. 2006; Uğurlu and Böke 2009; The next step concerns the characterisation: “what kind Binici et al. 2010); of organic substance was used?”. The ancient authors documented the extensive use of both – Processed slags from metalworking (see, e.g. Diekamp plant and animal origin substances to improve the perfor- et  al. 2006; Cacciotti et  al. 2015; and Kropáč and mance of mortars and plasters. For example, Vitruvius sug- Dolníček 2013 for correlated evidence); gested the use of lime tempered with oil (and dregs of oil) for waterproofing and preventing frosting (De Architectura, VII 1, 6–7; VIII 6, 8), thus demonstrating that the water- repellent properties of natural oils and fats were well known in Antiquity. Pliny reported that the temple of Minerva of Elis had been plastered by Panænus (brother of Phidias) Album de Villard de Honnecourt, Plate XLII “On prend chaux et tuile de paiens pilee, et vous ferez autant de l'une que de l'autre, with the addition of milk and saffron. mettant un peu plus de tuile de paiens, jusqu'a, ce que sa couleur domine l'autre. Detrempez ce ciment d'huile de lin, et vous en pour- rez faire un vaisseau a contenir l'eau.” (Take lime and pounded pagan (Roman) tiles in equal quantities, adding a little more of the latter until its colour predominates. Moisten this cement with linseed oil, and with it you can make a vessel that will hold water)”. Waste generated during mining and processing of marble is nowa- De re aedificatoria, III, 4 “tertiam si tunsae testae partem adiec- days re-evaluated for the same use (see e.g. Kore et al. 2020). eris, affirmant omnes futuram multo tenaciorem”. Naturalis Historia XXXVI, 55, 177 “Elide aedis est Minervae, in I Quattro Primi Libri di Architettura, II, 12  “piglisi per ogni due qua frater Phidiae Panaenus tectorium induxit lacte et croco subac- staia di calcina due altre staia di polvere di tegole, con mezzo staio di tum, ut ferunt; ideo, si teratur hodie in eo saliva pollice, odorem croci scaglia di ferro”. saporemque reddit”. 1 3 Archaeol Anthropol Sci (2021) 13:193 Page 11 of 30 193 The archaeometric research allowed for the individuation monumental buildings and offices, temples, pagodas, houses of several others organic additives in archaeological finds, and tombs) dated from the Taosi phase to the Qing Dynasty for example: (2300 BC–1911 AD). Different analytical approaches allowed these authors to determine the presence of starch – Wood, straw and charcoal in Roman mortars and plasters in 112 samples, oils in 87, proteins in 59, sugar in 14 and from Petra in Jordan (Al-Bashaireh and Hodgins 2011); blood in 5; furthermore, 48 and 5 samples showed two and – A saccharide material-based additive of plant origin and three organic additives, respectively. Apart from the com- a natural gum in the mortars of the medieval shipyard of prehensive and diachronic reconstruction proposed by these Amalfi Arsenale (Rampazzi et al. 2016); authors, it is also interesting to learn how the choices made – Egg white and urea in the mortars from the Yoros Castle by the ancient artisans were directly correlated with (a) the th th (13 –14 AD) at Anadolukavağı (Kurugöl and Güleç variable climatic environments, (b) the different seismicity 2012); of the various areas, (c) the development of agriculture and th th – Fibers in 13 –18 AD mortars from Erzurum in Turkey (d) socio-cultural factors linked to the relationship between (Binici et al. 2010); man and nature and Confucianism. – Proteins and/or animal blood in post-medieval earthen Also in this case, therefore, the characterization of addi- mortars from Cremona in Italy (Cantù et al. 2016); tives may lead to studies regarding ancient agriculture, – Egg yolk, oil and some resin in mortars repairs from the beliefs, philosophies and religions, the seismicity of specific St. Engracia Basilica at Zaragoza in Spain (Luxán et al. territories, the societies, understood as cultural ensembles, 1995); and the dynamics of trade in perishable products. – Sticky rice, egg white and tung oil, brown sugar, pig Finally, it can be concluded that the provenance of an blood and tung oil in several Chinese mortars, variously additive is more likely a starting point rather than an arrival dated from 563 to 1381 AD (Yang et al. 2009; Yang et al. point: “beyond the technical aspect, what further infor- 2010; Zhang et al. 2014; Zhao et al. 2014a, b, 2015). mation can I deduce from the presence of these additives in mortars/plasters, starting from their provenance?”. These few examples should have clarified that the use of organic additives is ancient and seamlessly has come A(4) The water Water supply is often taken for granted, down to us, in practically every part of the world. The sec- especially if the archaeological site is located along a water- ond aspect to consider is that not all archaeometric methods course or near the sea. However, water procurement deserves allow for accurate identification of the organic substances; a closer study—“which kind of water did they use and therefore, the analytical procedure must be strategically tar- where did they get it from?”—as it was necessary to sup- geted. This appears even more evident if we try to draw up ply considerable quantities, and different types of water (i.e. a list of possible organic substances used for mortars and from wells, springs, rivers and sea) have different yield over plasters. Using only the reference quoted here, along with time. For example, salt content can damage an aerial mortar those discussed by Sickels (1981, 1982), it is possible to list while enhancing the binding properties of a hydraulic mortar (in alphabetical order) animal glue, barley, beer, beeswax, (Davidson et al. 1965; Karim et al. 2017; Li et al. 2020; see blood (also of hippopotamus), butter, charcoal, cheese, cot- also Fig. 2). ton, curd, dung, egg white and yolk, elm bark, fibres, fruit The presence of chlorine and sodium in the analysed juices (e.g. fig), gluten, gum Arabic or tragacanth, hair, hogs’ mortars may give a clue on this aspect and clarify specific lard, keratin, malt, milk (casein), molasses, oil (e.g. linseed technological choices. For example, these two components oil and tung oil), resin, rice, rye flour, saffron, shellac, starch, in the binding matrices and pozzolanic lumps of some Nora straw, suet, sugar, tallow, tannin, urea, wine and wort. plasters (used for waterproofing this Punic-Roman site’s cis- It goes without saying that the variety of materials is wide terns in Sardinia, Italy) made the authors suggest the use of and that, most likely, we do not know it fully yet. What is seawater for their preparation (Secco et al. 2020). certain, however, is that the provenance question may not Looking at the same topic from the point of view of conserva- directly regard a geographic area but a certain animal or tion, the decay induced by salt crystallisation cycles (salt weather- vegetal species, e.g. “from which animal does the blood ing) is a factor that puts monuments at risk; moreover, it is foresee- come?”; “which plant does the wood come from?”; able that it will be more and more significant due to the climate “where was the species from which the oil was extracted breakdown we are witnessing (see below. On salt weathering also grown or fished/bred?”. see Ergenç et al. 2020; Randazzo et al. 2020). Based on these exam- As a final example, we propose a comprehensive study on ples, the question “where was the water for the mortar/plaster Chinese organic–inorganic composite mortars performed by mix from?” certainly acquires a much more meaningful motivation Li and Zhang (2019). Their research focused on 358 mor- and it would also be useful to investigate “what quantities were tar samples taken from 159 buildings (city walls and forts, needed?” for the masonry under examination. 1 3 193 Page 12 of 30 Archaeol Anthropol Sci (2021) 13:193 – The binder and then the lime processing (e.g. “are there B. Stones and bricks unburnt portions?”; “is it possible to trace the kiln and evaluate its function and effectiveness?”) In this case, the materials under investigation may either be worked stones and/or ceramic bricks and tiles. The ques- – The additives (e.g. “have they been chopped or pulver- ized?”) tions arising from their study are only partially different: (a) dealing with stoneworks, the main questions regard the It is useless to deny that the answers that we can realisti- characterisation (“what kind of stone was used?”) and the localisation of the geological outcrops (“where do the cally obtain may be very uncertain or partial, often dictated by common sense than by real archaeometric evidence. In stones come from?”); (b) when ceramic bricks were used, the questions regard the characterisation of the raw materi- the most fortunate cases, the evidence of production struc- tures such as lime kilns are found (e.g. Vaschalde et  al. als (“what kind of clayey raw materials were used?”), the localisation of the clay outcrop that supplied them (“from 2016; Toffolo et al. 2017a; Casas et al. 2020; Goguitchaich- vili et al. 2020), and the researcher can thus reconstruct the which outcrop were they taken?”) and, possibly, the locali- sation of the production plant (“where were the raw mate- lime production process. There are also particular cases in which the slags, sintered during the burning process inside rials prepared and fired?”). In this second case, the study of mortars is closely linked with both that of stone materials the inner lining of the lime kiln, have been reused inside the mortars. Their discovery may indirectly provide informa- and that of ceramic productions and, therefore, it becomes obvious how a global approach to an archaeological site may tion about the production and preparation of both the binder and the additives (Kropáč and Dolníček 2013). A similar prove economically much more sensible, as well as histori- cally more interesting, than a single material approach. It fortunate case—although very rare—is represented by lay- ers of abandoned raw materials from which we can obtain should also be added that the additives found in mortars may result from waste from stone processing or from the important information about sand processing. Despite the difficulty in tracing this information, its use shredding of local ceramics; therefore, the investigation of stone materials may provide information that can be used in for both conservation and formulation of compatible mortars is undeniable. It is well known, for example, that the use more than one field of investigation (see Fort et al. 2021). In this case, the example we propose is regarding the of unwashed marine sands introduces harmful Cl-salts into aerial mortars; moreover, the evaporation of saline solutions archaeological site of Thamusida-Sidi Ali ben Ahmed in Morocco (Fig. 3), where both stones and bricks were used can lead to discontinuities. Indeed, saline solutions represent a critical factor that may enter the mortar also a posteriori, for the masonries and a global archaeometric approach was applied to its Cultural Heritage remains. In this case, the for example, during flooding events or the circulation of thermal waters. presentation goes through the series of questions that typi- cally arise during the research and the provided answers Finally, this question and the following one imply a whole series of questions related to the organisation of the produc- (Q&A), as summarised in Table 2. tion, through which we can move from the analysis of the material culture to the study of workers, observing what the The processing of raw materials used for mortars and plasters production relationships were, how it was their work, what surfaces they were able to achieve at a certain time and so In the study of masonry works, the preparation concerns on (DeLaine 2021). every single component found in the mortar but also, as seen in the previous example, the processing of stones and The reconstruction of mortars and plasters recipes bricks (i.e. “is it possible to recognize traces left by stone- working?”; “how were raw clays prepared for brick This technological issue includes a simple question to which making?”). Focusing on mortars and plasters, the questions, there- a necessarily simplistic answer is usually provided. The question “in what proportions were the various compo- fore, concern: nents inserted and mixed?” may be misleading if we con- sider some non-computable variables, such as: – The sands (e.g. “were the sands prepared or used as they were?”; “were they washed to eliminate sodium a. The compositional variability that is certainly present in chlorides?”; “were they ground, sieved or mixed to obtain the desired granulometry and sorting?”) the different days of work; 1 3 Archaeol Anthropol Sci (2021) 13:193 Page 13 of 30 193 Fig. 3 (a) The Moroccan area including the archaeological site of Thamusida in Morocco. Thamusida corresponds to the present area of Sidi Ali ben Ahmed and is located along the left bank of the Sebou River (15 km north of the city of Kénitra and about 20 km north of the estuary. The widest supply area for calcarenite is that of Sidi Bouknadel (about 30 km SW as the crow flies). The two black dots above and below Bouknadel correspond to two quarries whose calcarenite was characterised for com- parison. (b) An open quarry of calcarenite at Bouk Nadel. (c) A wall of the archaeological site of Thamusida where calcarenite was used 1 3 193 Page 14 of 30 Archaeol Anthropol Sci (2021) 13:193 1 3 Table 2 Q&A on building materials provenance raised and provided during the development of the Thamusida research project (years 2003–2008) Questions Actions required Answers What kind of stone was used? Representative sampling of stones in Thamusida’s masonries Rocks macroscopically the same in the site. Samples distin- guished into quarry cuts (squared), roughly hewn stones (with a flat face) and stones used as they are (shapeless) Characterization through OM, SEM–EDS, XRD, XRD The samples consist of polygenic clasts, rounded in shape and constant granulometry (150 and 200 μm). The clasts are mainly of quartz, calcite, fragments of calcareous shells and lithic fragments (esp. sandstone with a clayey matrix). The rock used for stone masonry is a calcarenite (for details Gior- getti and Gliozzo 2009) Does calcarenite outcrop at the site? Short-range distance field campaign No So, where did they get it from? Geological study of the area and longer-range field campaign Calcarenite outcrops are common along a large part of the coastal strip (approximately corresponding to the yellow strip in Fig. 3). Numerous quarries are still active along National Route 1, especially in the area comprised between Salè and Sidi Bouknadel. Compared to the site, the most consistent and close outcrops are those of Sidi Bouknadel Is there any way to know exactly where the artisans took it Sampling campaign of un-worked rocks Samples were taken along the coast, in several sites north and from? south of the site Characterization through OM, SEM–EDS, XRD, XRD Set up of an ad hoc database of unworked calcarenites to be used for compositional comparison. The analyses showed both a great overall similarity between worked and un-worked samples and an intra-site compositional heterogeneity that prevented a more precise localisation (for details Giorgetti and Gliozzo 2009) Can we try to figure out how they transported it to the site, Knowledge on the modes of transport adopted in antiquity and The most likely path includes a first section of marine naviga- although we don't know the exact spot where it came GIS-based network analysis tion along the coast, moving from north or south to the Sebou from? River’s mouth. The second section of river navigation runs against the current, moving from the mouth to the site. In this second section, they can have used the (alaggio) hauling method, i.e. boats like the naves caudicariae pulled with ropes by animals or by men who proceeded on roads parallel to the course of the river, specially made for this purpose. GIS-based network analysis also informs that all possible land paths require much more time than the navigated ones So, why did they go to get that kind of stone off the site? Detailed knowledge of the geological setting of the territory Although this lithotype is not readily available in the surround- ings of Thamusida, it still represents the lithotype available at the shortest distance from the site. Therefore, calcarenite choice was still the cheapest one, in terms of time and effort to transport it to the site (compared for instance with limestones and basalts). Moreover, this is the most common type of stone in the area and, in fact, the same stone is also used in today's city of Rabat (e.g. for the Tour Hassan or the Kasbah des Oudayas), although its carbonatic composition and porous structure cause significant conservation issues (for details Zaouia et al. 2005) Archaeol Anthropol Sci (2021) 13:193 Page 15 of 30 193 1 3 Table 2 (continued) Questions Actions required Answers On the other hand, the bricks with what were they produced Representative sampling of bricks and tiles in Thamusida’s Collection of 67 bricks and tiles: 27 from the area of the Islamic th and where they come from? masonries and brick wastes from the ceramic kiln ceramic workshop (active from the 8 cen. AD onwards); 40 st rd from bricks and tile in the Roman settlement (1 -3 AD), mostly in situ Characterization of the ceramics through OM, SEM–EDS, Both Roman and Islamic bricks used local grey tirs (illitic clays) XRD, XRD as received, i.e. without manipulation or temper addition and fired them at 600–950 °C (for details Gliozzo et al. 2011) Short-range distance field campaign for the individuation and Two were the raw materials closely available for ceramic pro- sampling of clay outcrops duction: the grey and black tirs that are clay soils more (black) or less (grey) rich in humus (for details Arnoldus Huyzenveld 2008) Characterization of the clayey samples through OM, SEM– Set up of an ad hoc database of clayey raw materials to be used EDS, XRD, XRD for compositional comparison. Local ceramists selected the grey tirs and were particularly active in the earlier phase of Roman occupation (first–third centuries AD). Conversely, the later phase witnesses a contraction of production favouring imports which end up supplanting local production (for details Gliozzo et al. 2011) Epigraphic study of stamps on bricks Imports from Banasa (bricks stamped QAP and APP) and, likely, from the bay of Tangiers (bricks stamped CN and HADRIAVG) (for details Gliozzo et al. 2011) In conclusion, what was the operational criterion underlying Integration of all results achieved The best criterion, combining economy and functionality at the the construction of the masonry works of Thamusida? same time Abbreviations: OM optical microscopy, SEM–EDS scanning electron microscopy coupled with energy dispersive X-ray spectroscopy, XRD X-ray diffraction, XRF X-ray fluorescence 193 Page 16 of 30 Archaeol Anthropol Sci (2021) 13:193 b. A dismissal that led to the replacement of the person in The suitability and functionality of mortars charge with another worker who introduced a partially and plasters different recipe; c. A momentary shortage of one of the raw materials that These issues include several questions that may widely vary led to the mortar’s completion by not respecting the depending on the types of examined structure. original recipe, and other cases of this type, that are For example, assessing the hydraulic properties of mor- possible but not traceable. tars/plasters is a typical research question, especially when exposed to high moisture levels, such as thermal baths Furthermore, mortars and plasters are materials that tolerate and cisterns, or wet environments, such as harbours. The a wide compositional variability. By changing the ratio of com- questions that can be made in these cases are various, for ponents, various properties may change, such as setting time, example: “is the plaster suitable for the environment in durability or mechanical properties. Still, it is not certain that which the workers applied it?”; “is the composition of the the ancient workers understood a decrease in these properties mortar suitable for setting underwater?”; “is the plaster- as we are used to interpreting them today. The error of anach- ing of the cistern functional to water containment?”; and ronism—that is to evaluate the quality of a product based on “is the mortar/plaster suitable for the intended use?”. modern parameters—must always be kept in mind. In several Questions about the functionality of a mortar/plaster are cases, this distinction does not make much sense or is not drawn all the more complicated, the greater the number of vari- based on realistic or sensible parameters. ables to consider. Mortars and plasters must be regarded Another, often discouraging, phenomenon concerns the as systems in which each component plays a specific (but approach with ancient texts. For a long time, researchers possibly different) role, depending on the components with have debated whether and how the indications of Vitruvius— which it is mixed. Aerial lime-based mortars/plasters appear or other ancient architects—swere respected or neglected, as the most straightforward system, as they are reduced to giving rise to a series of interpretations of little use. The two components, i.e. the aggregate and the binder; how- text of Vitruvius has often been read with all the rigidity ever, even assuming an invariable binder’s composition, the of a black and white text, in order to deny the validity of result is not obvious. The aggregate may vary in terms of his teaching or to identify any detachments from the norm. composition, grain size, roundness and sorting and each of How ancient texts should be read is explained by Lancaster these properties corresponds to a different yield of the final (2021) in this TC. product (see, e.g. Grassl et al. 2010; Idiart et al. 2012). Nev- Operationally, archeometry can answer—with different ertheless, the binder’s composition may vary when the lime levels of approximation—the questions: “how much aggre- is obtained from impure limestones. In this case, the type gate, how much binder and how many additives (if any) and amount of impurities present in the raw material will were mixed to make this mortar/plaster?”, or “what is the determine the characteristics of the binder. ratio between the various components?” and the results If additives are added to a lime-based mortar/plaster, the relate to the individual sample analysed and not to the entire number of variables and, consequently, the complexity of the masonry work. To obtain a realistic estimate of the propor- system increase. Each type of additive will trigger specific tions adopted for the entire work, it is necessary to select reactions depending on the system and different character - and collect a stratigraphically appropriate and numerically istics will be achieved by the final mortar/plaster. representative collection of samples and to evaluate homoge- This reasoning serves to convey that the characterisation neity/heterogeneity induced by random and non-calculable of a mortar/plaster, as aerial or hydraulic, is only a first step factors, such as those described above. to defining its suitability and functionality. The latter proper - The characterisation of the components and the ties require the evaluation of numerous features such as, for evaluation of the relationship between them is a tool to example, shrinkage, workability, plasticity, stiffness, brit- evaluate changes made during the work or diversified tleness, hardening with high relative humidity, pozzolanic choices, based on the type or location of the structure activity, thermal conductivity, porosity, water permeability, under eximination. For example, if the ratio varies sig- adhesion, mechanical strength, tensile strength, compressive nificantly within the plasters of the same room, it may strength, moisture resistance and frost resistance. be possible to imagine a change of the workers or a Each of these parameters varies according to the number and series of renovations. If the ratio changes significantly type of variables that constitute the system. To give just a few between buildings with different use (e.g. dry/humid, examples related to the strength of a mortar, it is possible to list: internal/external environment), a deliberate diversifica- tion (i.e. to confer different properties to the materials) • Using natural hydraulic lime (NHL), high strength is becomes possible. obtained when a suitable grain size distribution of the aggregate is achieved; however, the strength decreases 1 3 Archaeol Anthropol Sci (2021) 13:193 Page 17 of 30 193 if the aggregates are siliceous, while it increases if the the introduction of fibres made these materials more suitable aggregates consist of limestone and/or the percentage of for use in seismic areas and therefore, in these cases, the binder also increases (Lanas et al. 2004); question may become: “are mortars and plasters suitable The compressive strength varies, for example, with the for that given structure, considering the seismicity of the variation of the binder's metakaolin content, while it area?”. remains almost unchanged if crushed brick particles are There are also cultural factors to consider when the struc- added (Nežerka et al. 2014). The addition of metakao- ture in question is large enough to include both environments lin can increase the compressive strength of lime-based open to external visitors and work environments open to plasters up to five times (Vejmelková et al. 2012b); owners and professionals only. In this case, a question may Using Mg-limes, the strength is comparable to that of concern the value given to suitability/functionality with natural feebly hydraulic lime, according to the param- respect to the owners’ self-representation (e.g. “is there a eters established by EN459-1. With respect to pure variation in the suitability/functionality of mortars/plas- calcium-lime, the flexural strength and the compressive ters based on the destination of the rooms?”). strength can be 1.75 and 2.4 times higher (Pavia et al. Undoubtedly, this case is closely linked to the use of deco- 2005; Chever et al. 2010); rations and other questions may arise on the suitability/func- The strength is low in pure lime matrix pastes, due to tionality of the plasterwork for paintings (e.g. “is the plaster the formation of cracks around the aggregate; therefore, suitable / functional to the realisation of wall painting?”). the characteristics of the aggregate becomes crucial Furthermore, from a diachronic perspective, the recon- (Nežerka et al. 2014); struction of the recipe may also be tackled in chronological The mechanical strength can also be increased through terms and, thus, the questions become “how have the reci- the introduction of casein or animal glue (Ventolà et al. pes evolved?”; “from the beginning of pyrotechnology in 2011) or even sticky rice soup (Yang et al. 2009, 2016). the distant Epi-Paleolithic Geometric Kebaran (Gourdin and Kingery 1975; Kingery et al. 1988) is it possible to This is an example related to a single property, the observe radical changes, progressive adaptations or con- strength, but highlights how suitability and functionality servative choices?”. depend not only on multiple variables but also on the char- Hence, the various questions that can be raised concern a acteristics of each component. whole series of how, when and why, the solution of which To make just one example of the suitability/functionality helps us to understand better a wide range of aspects related of historic mortars, Singh and Kumar (2020) investigated a to the cost-effectiveness of the work, the technical skills of series of plasters from the late fifteenth century Mughal’s the workers, the priorities and the variable tastes of clients summer palace of Farah Bagh (India). Although the most bound to the fashions of the time and the relationship between striking feature of this monument is perhaps that of being the intended use of a structure and its functional performance. equipped with a complex system of air cooling, the equally The archaeometric answers that can be obtained for these complex preparation and application of different types and questions are often decisive, thanks to the numerous experi- layers of plasters are indicative of an advanced technology, mental works that have addressed the technical characteris- aimed at their suitability and durability during processing tics of ancient and modern mortars/plasters. and use in an environment that is repeatedly subjected to Essential for this topic are the works carried out by Baro- moisture. The workers obtained flexibility, strength and nio and Binda (1997) and Böke et al. (2006) on the hydrau- permeability by adding several types of inclusions, ranging licity of different types of bricks and clays for cocciopesto. from jute fibres and dry paddy stems to grinded ceramics These studies led to clarification of the conditions necessary and basaltic rocks. for the reaction to occur and the ceramic material to be con- Of a very different nature are cases in which the hydrau- figured as pozzolanic: licity of a mortar/plaster is involuntary or not functional for any purpose (e.g. in some residential rooms in predomi- a. Ceramic firing must not exceed 900 °C; otherwise min- nantly dry climates). The question thus becomes “why was a erals destabilization can lead to the formation of new hydraulic mortar used when it was not necessary?” and/ thermodynamically stable phases (Gliozzo 2020b); or “did they really want to hydraulicise the mortar, or b. The temperatures typically range between 450 and recycle materials already available, resulting, for exam- 800 °C according to the type of clay minerals (He et al. ple, from demolition of old structures?”. 1995); This is a more common situation than imagined and clari- c. Ceramic fragments must be finely ground; fies how a mortar/plaster’s hydraulicity is a characteristic to d. Not all clays fired at low temperatures develop pozzo- be interpreted with caution and above all else, not individu- lanic properties (e.g. when they have a low percentage ally. For example, we know that some precautions such as of the clayey fraction). 1 3 193 Page 18 of 30 Archaeol Anthropol Sci (2021) 13:193 Numerous outstanding insights on colour variations, water- from industrial research (therefore, not related to archeom- proofing and mechanical properties that can be conferred to a etry or restoration). For example, some experimental pro- mortar/plaster through the addition of specific additives have also grams aimed at the development of commercial materials been gained through experimental reproductions. For example: may provide useful information for a better understanding of the properties of ancient mortars and plasters (e.g. Grist – Centauro et al. (2017) carried out a series of ageing tests et al. 2013; Ergenç et al. 2018). adding linseed oil, brown sugar and cow’s milk; – Nunes et al. (2018) investigated the microstructure and Particular cases composition of lime and metakaolin pastes with linseed oil added and aged for 68 months; The study of mortars and plasters may also present particular – Zendri et al. (2004) reproduced “cocciopesto” mortars, in suitability/functionality cases depending on the archaeologi- order to investigate the reactions responsible for confer- cal context or find in which they are found. ring hydraulic properties to these products; For example, the archaeometric investigations made on – Nežerka et al. (2014) compared the pozzolanic activity the wall plasters of a round installation (Feature 6), found of metakaolin and crushed bricks; at the Late Natufian village of Nahal Ein Gev II (NEG II, – Işikdaǧ and Topçu (2013) compared the mechanical ca. 12,000 cal BP) helped to shed light on the likely use of strength of mortars, including tile powder, crushed tile, that structure as one of the very first storage installations lime and granulate blast furnace slag; (Grosman et al. 2020). Firstly, the authors reconstructed – According to ancient recipes, Salavessa et  al. (2013) the morphology of the installation as a plastered domed compared the compressive and flexural strength of plas- pit. Then, they characterised the walls as an alternation of ters prepared with either waste marble or limestone dust; mud and lime plastering (the latter was absent in other areas – Vejmelková et al. (2012b) investigated the mechanical of the site). The heating experiments verified that NEG II and fracture-mechanical properties, hydric parameters, inhabitants did not use the installation for pyrotechnological durability characteristics and thermal properties of sev- processes and the filling was characterised as waste dumped eral lime–pozzolana composites, using a particular clay inside after its original use. By integrating all the various shale that is available in the Czech Republic that may information, the authors were finally able to identify the use have been the natural substitute of volcanic pozzolanas of “Feature 6” installation for cereals and legumes storage. (absent in Central Europe); The discovery is of outstanding importance, as it testified – Yang et al. (2009) and Yang et al. (2016) reproduced the the practice of (short-term) storage just before the Neolithic mechanism of solidification of sticky rice mortar; and subsequent expansion of agriculture. In this case, how- – Ventolà et al. (2011) tested different types of non-hydrau- ever, the research question appears somehow reverted: “can lic lime mortars, adding animal glue, casein, nopal and the plaster provide information about the function of the olive oil to develop new compatible products for repairs; structure it was associated with?”. – Lima et al. (2020) investigated the effects of clay min- Another rather frequent case concerns the possibly con- eralogy on drying behaviour, pore size distribution, textual use of lime for other purposes. The lime produced for mechanical strength, vapour adsorption, desorption masonry works could have been used in tobacco processing capacity, water capillary absorption, linear dry shrinkage, (see, e.g. Villaseñor and Graham 2010), in maize processing cracking and thermal conductivity. They also verified the (see, e.g. Katz et al. 1974), in waterproofing amphorae (see, greater suitability of illitic clayish earths compared with e.g. Dorrego et al. 2004) and boats, in glass production and montmorillonitic or kaolinitic ones. tanning (Foy et al. 2000; Heth 2015), not to mention that calcium oxide was also a thrown weapon (a smokescreen From these studies, it becomes clear that the sampling irritating to the eyes similar to modern smoke bombs or pep- of archaeological finds and comparative geological mate- per stinging sprays) in battles against opponents. Taking a rials may not be sufficient and research must be open to look at other materials found at the same time, for example, experimentation to validate/discard the working hypotheses. in an excavation, may therefore raise new questions (e.g. In practice, this procedure leads to an increase in samples, “what was the organization of lime production in light analyses and time, but it certainly offers the necessary tools of the many possible uses of this material?”) and provide for correct and meaningful interpretations. interesting information on the versatility of a single material. Indeed, not all studies require an additional experimental procedure. Knowledge of the properties conferred by the dif- Mortars and plasters dating ferent constituents is already largely known (see also Moro- poulou et al. 2005) and must be deeply known in advance. Mortars dating, including radiocarbon and optically stimu- Furthermore, it is also possible to gather useful information lated luminescence (OSL) dating, has been comprehensively 1 3 Archaeol Anthropol Sci (2021) 13:193 Page 19 of 30 193 addressed in the paper by Ergenç et al. (2021) in this TC. We workable” involved the mechanical separation of the binder will, therefore, limit ourselves to discussing some problem- from the aggregate (Folk and Valastro 1976). atic aspects related to the radiocarbon technique on which The difficulty in obtaining reliable dating of well-dated numerous publications are recently flourishing. samples have held back the application of this technique In principle, the method is straightforward. It is based (Malone et al. 1980; Zouridakis et al. 1987; Heinemeier on the extraction and measurement of pyrogenic C aCO , et al. 2010; see also Van Strydonck et al. 2011, 2015 and formed after the incorporation (fixation) of atmospheric De Mulder et al. 2014 on lime burials) but did not stop its CO (which contains C) by slaked lime (Ca(OH) ) experimentation and consequent development (Van Stry- 2 2 (Labeyrie and Delibrias 1964). donck et al. 1982, 1983, 1986, 1989, 1992; Pachiaudi et al. The binder’s age corresponds to that of the mortar’s 1986; Hajdas et al. 2017; Hayen et al. 2017). hardening, and, consequently, its C date corresponds to Sampling and analytical problems are well known to date that of the mortar from which the sample was extracted. (Boaretto and Poduska 2013; Ringbom et al. 2014; Toffolo However, this “simple” concept brings with it several et  al. 2020; Urbanová et al. 2020) and partially obviated practical complications, essentially due to the very indi- through the preliminary analysis of the samples. On the viduation and extraction of appropriate samples from the other hand, the development of accelerator mass spectrom- carbonaceous binder. etry (AMS) has made it possible to step forward further, Calcite may have different origins, not exclusively related thanks to reducing the sample required for the analysis (less to the hardening process; therefore, the possible “contami- than 1 mg). nants” are many: Several treatments and analyses are required both for the selection and measurement of the samples and the valida- a. Geogenic calcite, i.e. calcite naturally present in the tion of the proposed date. In the first category, it is possible aggregate, introduced during mixing the aggregate with to list: the binder; b. Calcite contained in the ashes; – Dry/wet sieving and purification treatments, to separate c. Unreacted calcite, i.e. limestone not completely trans- specific fractions and eliminate part of the contaminants formed during the lime production cycle; (Addis et al. 2019; Michalska 2019; Ricci et al. 2019); d. Lime lumps, i.e. lime that did not react to form calcite – Density separation, to separate components, based on after hardening of the mortar; their specific gravity (see, e.g. Moropoulou et al. 1995; e. Secondary calcite, i.e. calcite formed by recrystallisation Toffolo et al. 2017b); beginning from the mortar’s use up to the present day. – Optical and scanning electron microscopy, to identify the different components of the mortar (authors are too The whole range of contaminants can move the date far numerous in this case because this represents a quasi- back, as it happens with geogenic calcite (i.e. dating refers to mandatory research step); the age of formation of the carbonates present in the sand), – Cathodoluminescence, to identify the different genera- or far forward, as it happens with secondary calcite. tions of carbonates (see, e.g. Heinemeier et al. 1997; This problem became immediately evident. Stuiver Lindroos et al. 2007; Murakami et al. 2013; Toffolo et al. and Smith (1965) highlighted chronological discrepan- 2019b): cies related to C dilution due to residues of limestones – Fourier transform infrared spectroscopy (FTIR), to indi- or calcareous sands used to prepare CaO (Baxter and Wal- viduate geogenic, biogenic and pyrogenic calcites and ton 1970). Moreover, the method was initially presented as thus, avoid samples showing extensive recrystallisa- unsuitable for hydraulic mortars whose solidification, in tion (see, e.g. Anastasiou et al. 2006; Regev et al. 2010; the presence of water, led to the formation of calcium sili- Poduska et al. 2011, 2012; Toffolo et al. 2019a); cates and aluminates (Delibrias and Labeyrie 1965). Subse- – X-ray diffraction (XRD), to obtain a qualitative and quently, other researchers have confirmed that this difficulty quantitative measurement of both crystalline phases and is mainly due to the poor permeability to atmospheric CO , amorphous fraction. This technique is particularly advan- the enduring reactivity of this type of mortar and the pres- tageous to investigate the possible presence of secondary ence of lime lumps or phases formed from hydraulic reac- phases, especially those occurring in hydraulic mortars tions, such as layered double hydroxides (Artioli et al. 2017; such as the double-layer hydroxide (LDH) minerals (i.e. Toffolo et al. 2020). typical products of the pozzolanic reaction; see, e.g. Hence, it soon became clear that these carbonatic resi- Artioli et al. 2017; Ponce-Antón et al. 2018); however, dues had to be eliminated before proceeding with the analy- this task can also be aided by the following techniques; sis and ensuing attempts “to refine the technique to make it – Solid state nuclear magnetic resonance (SS-MAS-NMR), to identify traces of LDH phases (Richardson et  al. 1 3 193 Page 20 of 30 Archaeol Anthropol Sci (2021) 13:193 2010), whose contents are below the detection limits of In conclusion, despite all the advances made, the method the XRD; is very laborious and frequently inconclusive. It is not yet – Differential thermal analysis and thermogravimetric able to provide a date that unquestionably relates to the analysis (DTA-TGA), to study the thermal decomposition mortar under investigation; consequently, a series of age of the various types of carbonates, based on the experi- control investigations are made for comparison with other mental evidence that lower temperatures are required to techniques, which may, however present other types of prob- decompose pyrogenic CaCO than geogenic CaCO (see, lems. In the second category, it is possible to list: 3 3 e.g. Moropoulou et al. 1995; Anastasiou et al. 2006; Tof- folo et al. 2017b); – TL and OSL, to compare mortar dating with that obtained – Sequential dissolution, to provide representative C O on silica-based fragments such as those included in coc- samples of the binder, excluding contaminants (Lindroos ciopesto or, specifically, to date their last exposure to et al. 2007; Ringbom et al. 2014). This is a fundamental light before being put “in the dark” inside a mortar (OSL preparation stage of the AMS dating technique (Tubbs applications in Moropoulou et al. 2018 and Ergenç et al. and Kinder 1990; Hale et al. 2003; Regev et al. 2017), 2021); chiefly because the sequentiality of carbonates dissolu- – Dendrochronology and palaeoenvironmental studies, to tion (secondary carbonates → pyrogenic CaCO → geo- compare mortar dating with that obtained on other con- genic aggregates) allows for C O isolation during textual materials; hydrolysis. The times with which the sequential dissolu- – Archaeological stratigraphy for a relative frame. tion takes place are not known with certainty. Therefore, further investigations are needed to verify the reliability Despite all these limitations, researchers are certainly of the selected sample and, consequently, of the proposed not discouraged. Future research will surely solve the raised dating. In an attempt to solve this problem, Toffolo et al. issues; however, it was necessary to describe the criticalities (2017a) and Toffolo and Boaretto ( 2014) started from of the method to not misguide researchers on chronological the observations made on aragonite and developed a issues. new procedure involving the thermal decomposition (at 500 °C) of pyrogenic calcite. Mortars and plasters alteration and degradation Among all these methods, there is no one that, taken The alteration and degradation of stone materials, mortars individually, guarantees a correct selection of the sample. and plasters represent a critical issue in the field of restora- The combined use of two or more techniques is always tion and conservation of built heritage (Price and Doehne necessary, even if not necessarily decisive. As clearly 2011). Alteration and degradation are often used as syno- demonstrated by experimental and cross-laboratory test- nyms, but they are not. The term alteration refers to a phys- ing, different or analogous treatments can yield very dif- ico-chemical change that generally occurs on the surfaces, ferent dates (Hayen et al. 2017; Michalska 2019). In the for example, a colour change. It can be seen, but hardly can former study (Hayen et al. 2017), a detailed compositional it be linked to something that threatens the integrity and the analysis of the samples was preliminarily performed and function of the stone. On the contrary, the degradation of the same four mortars were investigated using different a mortar/plaster represents something that must be taken techniques and laboratories. The dates obtained on dif- into account to preserve the material itself. In other words, ferent parts of the sample and/or with different analytical degradation implies a physico-chemical modification of the techniques varied by many centuries between them and materials, which leads to a worsening of its properties and, if there is no possibility of establishing which is the “right” it has a great entity, a restoration/conservation intervention is one, except with a combined investigation of a numeri- needed. It is important to establish a common language and cally high and compositionally heterogeneous collection let the researchers and stakeholders understand each other. of samples (e.g. lime lumps, charcoal fragments, specific For this reason, national and international organizations grain fractions, etc.). In the latter study, Michalska (2019) have set up common glossaries to give the same terminol- compared the C results obtained for 37 mortars with ogy to alteration and degradation forms. One of the most different compositions and enucleated three groups: (1) used is the glossary issued by ICOMOS (Vergès-Belmin air-hardening and slightly hydraulic mortars that can be 2008). Chemical, physical and biological agents can cause successfully dated, (2) highly hydraulic mortars affected the alteration of mortars and plasters; however, it is common by rejuvenation effects and (3) mortars with very high con- that an alteration/degradation pattern is due to a combination tent of ageing components, for which “obtaining the true of the abovementioned agents. age of mortar production is not possible with the current The characterisation of the alteration and degrada- state of knowledge”. tion forms, both on a macroscopic and microscopic level, 1 3 Archaeol Anthropol Sci (2021) 13:193 Page 21 of 30 193 requires a multidisciplinary approach since chemical, min- from each other. On the other hand, the great compositional eralogical, and biological skills are needed (La Russa and homogeneity of certain other pigments, such as cinnabar, Ruffolo 2021). The interventions are planned based on the makes it difficult to discriminate between geographically materials’ conservation state (Caroselli et al. 2021). That is close resources. In practice, it all depends on the composi- why alteration/degradation analyses should be carried out tion and/or the information we have available for a compo- before restoration. sitional comparison (see the specific contributions, pigment When we deal with archaeometric analysis of mortar and by pigment, of this TC). plaster samples, a particular attention has to be paid to their degradation since it can induce wrong interpretations. For The pictorial technique example, when the dating of mortars is performed on degraded samples, it is easy to observe a rejuvenation of the C age Understanding “how was the pigment applied?” and/or because of the generation of secondary carbonates (Michalska “how was the surface prepared?” means investigating the 2019). Generally speaking, the degradation of mortars induces link between the pigment and its support. Each technique a change in terms of elemental and mineralogical composi- corresponds to different archaeometric evidence that must be tion, as well as microstructural (i.e. porosity, binder-aggregate investigated and unravelled (see Salvadori and Sbrolli 2021 ratio), which must be taken into account when approaching an and Murat 2021 in this TC). archaeometric study based on such parameters. For example, some pigments, such as ochers, lend them- selves to being used with the a fresco technique, while for C. The characterisation of pigments others, such as those based on lead, a secco technique is preferred. Therefore, investigating the painting technique The first questions concern pigments identification (“what means investigating the palette and technique of the painter pigment is it?”) and nature (“is it a natural or synthetic and verifying the appropriateness and effectiveness of some pigment?”). techniques compared to others. Most of the time, answering the first question also pro- Moreover, we know that some pigments, such as those vides the answer to the second question. For example, if based on arsenic, are particularly sensitive to light and the answer is “Egyptian blue” or “lime white”, we already sometimes obliged to adopt particular techniques for their know from previous studies that these compounds were arti- application. Therefore, the information we obtain from the ficially produced (see Gliozzo and Burgio 2021; Gliozzo painting technique analysis provides useful information for and Ionescu 2021; Cavallo and Riccardi forthcoming, and a correct display of the artefact and its conservation. Švarcová et al. 2021 in this TC for futher examples). Finally, the information that we can obtain from the study Conversely, if the answer is “yellow ochre” or “green of a single artefact or artwork represents a piece of the wider earth”, the natural origin will be straightforwardly deter- mosaic that describes the evolution of painting techniques mined (see Mastrotheodoros et al. forthcoming in this TC). and is, therefore, of value in the broad field of art history. On the other hand, however, even pigments of natural ori- gin, such as ochers, could be processed to vary their colour Linking pigments with the social status or consistency. For example, it is well known that goethite- of the clients based yellow ochres could turn to hematite-red upon heating. Still another case is presented by arsenic-based pigments Up to this point, the questions posed to pigments are all for which, after a certain period, both natural and artificial technical. As we have seen, the answer we can get depends compounds of realgar and orpiment were in use (Gliozzo largely on the composition of the pigment itself and the qual- and Burgio 2021 in this TC). ity of the database available for comparison. Nevertheless, Therefore, if the answer to the second question is “natu- there are also other questions related to the use of specific ral”, we should proceed further with the other research ques- pigments that go beyond technological choices. For example, tions. On the contrary, if the answer is “processed” or “arti- “is it possible to establish whether the type of pigment ficial”, the next question will be aimed at investigating “how used corresponds to a desire for representation?” or, in was the pigment processed and produced”. other words, “can the low/high cost of a pigment faithfully reflect the social status of the clients?”. The provenance of minerals and pigments The use of cinnabar instead of ocher, for example, has often been referred to as a desire for social self-represen- The provenance study is often very complicated and not tation on the part of the clients rather than to a different— always conclusive. On the one hand, the great heteroge- more vivid and brilliant—rendering of cinnabar compared neity of widespread raw materials, such as ochers, makes to ocher. Similar assumptions have been made regarding the it difficult to distinguish supply areas that are very distant 1 3 193 Page 22 of 30 Archaeol Anthropol Sci (2021) 13:193 use of Egyptian blue and lapis lazuli, although there were no A sampling that complies with the representativeness other choices in the ancient world with which to obtain blue. criterion is first and foremost guided by the definition of the To avoid possible anachronism, the study of the relation- construction techniques and the stratigraphic sequence. The ship between pigment and client may help clarify if pig- parameters to be evaluated are many and the resulting sam- ments were used as a demagogic and self-representative tool pling must thus be representative of (1) the types and charac- and cast an eye on the tastes of the time. teristics of all construction materials (e.g. lithotypes/bricks and mortars), (2) the way they were assembled, i.e. the con- struction techniques and (3) the construction phases in which they were employed as delineated by the stratigraphy. Sampling criteria and methods In the first case, a preliminary evaluation is essential to identify macroscopic similarities/differences between In the cases that are addressed below, the sampling aims building materials. A range of simple tests that can be per- to answer the questions that we have analysed in the previ- formed in the field (e.g. the effervescence in dilute hydro- ous chapters; however, the most important aspect to keep in chloric acid) and the use of a magnifying lens may help this mind concerns contextualisation. first phase. A preliminary catalogue of the mortars is also Mortars, plasters and pigments constitute a complex essential. It should take into account the broadest range of system of structural and aesthetic importance that visible characteristics, such as colour, texture, clasts types, must be contextualised within the various phases of a dimensions and distribution, state of aggregation, and the historic building. Attributing a specific context to the possible presence of cracks and traces of weathering. These various components is essential for correct sampling characteristics, combined with the parameters used for the for archaeometric purposes. To this end, it is necessary definition of construction techniques (e.g. structure, equip- to apply the method of “reading” historical buildings ment, size of elements, percentage of pieces placed at the established precisely by the BA to correct extrapolation tip or end, size distribution of segments, the possible pres- of the components according to their space–time ence of plasters), provide the framework on which to set the dimension. selection of representative samples based on the first two As anticipated above, we must also consider that the parameters. research in progress may raise new questions not stated from In the third case, the stratigraphic sequence may suggest the beginning. For these reasons, the sampling and, above duplicating the selection of some types of materials or con- all, the quantity of material to be taken should always be struction techniques, in order to evaluate any changes or per- slightly greater than what we establish at the table, as long sistence on a diachronic basis. For example, the acquisition as the conservation requirements are respected. of two samples of the same lithotype from structures dated to chronologically different periods could demonstrate slight How to obtain a representative sample set variations in raw materials supply. of mortars and plasters Finally, the criterion of representativeness must guide a selection that accounts for all types of materials used, with The first assumption is that there is no arbitrary number of all construction techniques, for each period considered. samples that can be considered representative tout court. A sampling that conforms to the functionality criterion Although the most obvious and frequent question is “how is guided by the research questions. The representativeness many samples are needed?”, not only is there no straight- criterion has previously provided useful samples to answer forward answer but it is worth bypassing this question with some questions such as “what materials were used for con- another type of question: “what samples are needed and structing the walls?” but it cannot be considered sufficient. from where should they be taken?”. If, for example, we want to know where the workers took The main characteristics of sampling must be. the raw materials from, it is frequently necessary to expand the sampling outside the building under investigation. The a. Representativeness, which is given by stratigraphic, investigation moves to the surrounding territory to an extent typological and chronological criteria; that the researcher can establish, based on geological maps b. Functionality, given by sample collection’s intrinsic and historical information. Since a detailed characterisation ability to answer established research questions (e.g. of the lithotypes is required to focus the search for supply production technology, provenance, alteration and deg- basins, it is also foreseeable that sampling will be a multi- radation); step activity conducted on several occasions. c. Suitability, which concerns the suitability of the material For other types of questions, such as those aimed at recon- sample to be investigated by the analytical techniques structing the production technology of mortars, the “repre- (Gliozzo 2020a, Fig. 4). sentative sampling” already considers numerous features 1 3 Archaeol Anthropol Sci (2021) 13:193 Page 23 of 30 193 derived from the production processes and their eventual quick, cheap and non-destructive data that cannot pro- diachronic evolution. Still, only laboratory analyses can vide unquestionable answers and, on the other hand, data decide if the sampled collection is sufficient, or an expan- inaccuracy can lead to a waste of time and money. sion is necessary. Furthermore, evaluating the production Therefore, the choice of analytical techniques should processes and the mortars’ physical–mechanical character- first find a balance between the accuracy requirements istics may require a series of experimental reproductions for of quality research and the conservation needs and then which further sampling of raw materials may be necessary. guide sampling to the selection of suitable samples. In Similarly, it may be possible to answer questions relat- this regard and based on what has been said previously, ing to alteration and degradation through the set of samples it is good to underscore that sampling does not necessar- obtained with the first criterion. In these cases, however, it is ily involve the physical taking of material. In the case of more frequent to carry out targeted sampling. In fact, while non-destructive techniques, sampling may simply address unaltered samples are preferred for the characterisation of the selection of the analysis spots. building materials, altered specimens are also required to To conclude, while before proceeding with sampling, evaluate their conservation state. it is necessary to collect all available data from any field As far as the dating question is concerned, it was previ- of study; during sampling, it is mandatory to properly ously explained why the sampling issue is crucial for radiocar- record and document all phases and, after sampling, it bon dating and why it may be necessary to carry out multiple is required to remain open and f lexible to answer new samplings before obtaining a suitable sample for analysis. questions that may arise. Before deciding and after tak- In summary, while the “representative sampling” may ing the samples, the overall research should record the already provide the necessary material to answer some various actions and use a critical survey. This survey research questions, the functionality criterion aims precisely should be equipped with specific signs that provide a to test the completeness of the overall sampling from the cartographic mapping functional to sampling. Regard- point of view of research questions. less of the use of new technologies for the graphic rep- The research methodology guides sampling compliant resentation of historical buildings, we should create with the third criterion, i.e. the suitability. In addition to standardised protocols to record sampling through, for the example recalled above on selecting suitable samples example, a precision topographic positioning. for radiocarbon dating, another clarifying example con- cerns visibly altered mortars. By adding all the three cri- How to obtain a representative sample set teria together, the sample must be (a) representative of of pigments both the original composition and the altered composition; (b) functional to solve research questions, such as those A sampling of pigments is carried out primarily based on relating to the recipe and those relating to the evaluation colours. In this way, by counting the number of colours vis- of degradation; and, finally, (c) suitable for the analytical ible in the painting, the number of samples to be investi- techniques that the researcher will use to investigate it. gated is also obtained. In large-scale paintings such as wall In this case, it is well known that (1) the thickness of paintings, the sampling of a single sample per colour may the surface alteration layer is variable, (2) the preparation prove insufficient. Therefore, it will be necessary to carry out of a thin section requires a small volume but a suitable multiple samplings for each colour to evaluate any changes. surface and (3) an accurate chemical analysis may require This reasoning is valid both in the case of natural and arti- a variable volume depending on the chosen technique. ficial pigments, as it provides the possibility of identifying Therefore, the sampling methods will be crucial to obtain a different technologies and/or provenances. suitable sample while observing the fundamental principle In complex structures with several decoration phases, the of conservation, or in other terms, of minimum sampling. stratigraphic sequence of superimposed layers provides the Archaeometric analyses can move along the path of tool for guiding sampling. non-destructiveness or destructiveness. Between these When the analysis involves non-destructive instrumenta- two end-members, different levels of destructivity tion, it is undoubtedly desirable to consider a large increase and accuracy can be achieved (e.g. highly destructive, in the spots to be analysed. When, on the other hand, the minimally-destructive). Provided the same quality of research question requires the use of destructive investiga- the results obtained, the most obvious approach would tions, the number of samples will adapt to the minimum nec- be that of non-destructiveness; however, it should be essary and will possibly be taken in the least exposed portion. emphasised that, often, non-destructive techniques can- For the study of the painting technique, there is an addi- not provide sufficiently accurate or comparable results tional difficulty, as the analysis needs to go beyond the pic- to those obtained with destructive methods. Therefore, torial film, in order to reach the preparatory layer. In this on the one hand, the researcher risks collecting a lot of case, the researcher may choose either to analyze the gaps or 1 3 193 Page 24 of 30 Archaeol Anthropol Sci (2021) 13:193 take a sample that shows the cross-section. The first method on the workers and the construction site's organisation does not necessarily damage the artefact but is not always (“cantiere”). resolutive, while the second has the advantage of ensuring The integration with other historical information on eco- an exhaustive response but is destructive. The researcher nomic trades sheds light on the commercial dynamics and on must, therefore, choose the first or second method based on the routes of trade, as well as on the status of the clients and conservation needs. A third option is to be considered when the functional destination of some buildings. the analysis concerns mobile artwork and consists of instru- Finally, the integration with technological knowledge can add ments with a higher penetration capacity than conventional useful pieces of information to reconstruct the history of technolo- techniques, such as Raman or portable X-ray fluorescence. gies, in a diachronically and geographically wide perspective. In any case, while it is not acceptable that the availabil- The main assumption that derives from these examples must ity/absence of specific instrumentation or funds decides the be that of a plurality of questions that follow one another, rather analytical protocol, the diversity offered by the artworks and than a rigid and pre-set list of questions. This last method is their needs (wall painting/mobile artefact, alteration issues functional to the start of the research but must then open up and conservation needs, the impossibility of transferring the to the unexpected how, when and why that arise during the find to the laboratory, etc.) always guide the selection of the advances/progress of the research. most suitable sampling and analytical techniques. Consequently, sampling must remain flexible and, subject to conservation requirements, must answer both pre-set and unexpected questions. Mortar sampling should be guided by Concluding summary of key concepts three main parameters: representativeness, functionality and suitability. Pigment's sampling should be based on colours and, The archaeometric study of mortars, plasters and pigments when applicable, guided by stratigraphy. provides information about: Non-destructive methodologies are certainly an advantage in the study of pigments. Still, they cannot always provide – The origin of the raw materials used as aggregate, addi- sufficiently accurate measurements and, being superficial tives or pigments or as raw material for the production analyses, they can mislead due to surface alterations. On of lime; the other hand, destructive methodologies are almost always – The production technology of lime, mortars, plasters and preferable (if not indispensable) in the study of mortars and pigments as well as the painting technique; plasters. A reasoned sampling can minimise the damage. – The state of alteration and/or degradation of both build- Finally, three important aspects should be born in mind. First, ings and paintings, as well as the yield of products for the results obtained on a single sample or a poorly selected sam- restoration; ple collection cannot be extended to entire rooms or buildings. – The chronology of some components of the mortar with Secondly, mortars/plasters that are compositionally different levels of likelihood depending on whether they very different from each other may show comparable are siliceous materials, datable through TL-OSL (relatively performance, at least adopting ancient standards. We accurate results), or carbonaceous, datable through radio- should avoid anachronisms putting ourselves in the shoes carbon (results not trustable as received but to be verified). of people who do not necessarily think with our param- eters. Leaving aside qualitative evaluations (that we can- From the first series of questions addressing the “study’s not estimate except with current criteria) is likely the best object” directly, the integration of laboratory results with the approach in most cases. geological knowledge of the territory provides information about: Thirdly, there is an unquantifiable level of randomness that we cannot in any way reconstruct; therefore, we must – The exploitation of the territory itself over the centuries be careful in not falling into attractive overinterpretations. or in a specific period; Some choices or variations of the “products” we analyse – The relationship between knowledge of the raw materials should not necessarily be interpreted as a decline in quality available and their exploitation for production purposes; or an evolution of the technique. They may (simply) repre- – The tools to understand better certain technical choices sent the best choice at that given moment. relating to the use of certain materials compared to oth- ers. Authors’ contributions Building Archaeology by AP; Mortars and plas- ters alteration and degradation by MFLR; Concluding summary of key The integration with the stratigraphy allows the observa- concepts AP-MFLR-EG; other contents by EG. English professional tion of changes during the same phase and/or specify the revision provided by MFLR. chronological seriation and/or provide indirect information 1 3 Archaeol Anthropol Sci (2021) 13:193 Page 25 of 30 193 Code availability Not applicable Cementitious materials: Composition, properties, application. De Gruyter, Berlin, pp 147–158 Ashurst J, Ashurst N (1988) Practical Building Conservation 3: Mor- Declarations tars, Plasters and Renders. (English Heritage technical hand- book). 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Mortars, plasters and pigments—research questions and sampling criteria

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Abstract

Within the Topical Collection, this paper represents an introductory contribution aimed at describing and discussing the research questions and the sampling criteria in the field of mortars, plasters and pigments studies. The paper is divided into three parts. In the first part, some terminological issues are clarified and the building archaeology is introduced as an indis- pensable method for sampling and interpreting archaeometric results. In the second part, the most common research questions are presented and discussed. Some case studies are also reported to clarify what the expected results may be. The sampling problem is faced in the third part, where the criteria for a representative, functional and suitable selection are provided. Keywords Mortars and plasters · Pigment analysis · Building archaeology · Archaeometry and archaeology · Research questions · Sampling criteria Premise organisation is further provided by DeLaine (2021). Fur- thermore, several issues concerning the degradation and This Topical Collection (TC) covers several topics in the conservation of mortars and plasters are addressed from field of study, in which ancient architecture, art history, practical and technical standpoints (La Russa and Ruffolo archaeology and material analyses intersect. The chosen 2021; Caroselli et al. 2021). perspective is that of a multidisciplinary scenario, capable The second group of contributions is focused on pig- of combining, integrating and solving the research issues ments, starting from a philological essay on terminology raised by the study of mortars, plasters and pigments. (Becker 2021). Three archaeological reviews on prehistoric The first group of contributions explains how mortars (Domingo Sanz and Chieli 2021), Roman (Salvadori and have been made and used through the ages (Arizzi and Sbrolli 2021) and Medieval (Murat 2021) wall paintings Cultrone 2021, Ergenç et al. 2021; Lancaster 2021; Vitti clarify the archaeological and historical/cultural frame- 2021). An insight into their production, transport and on-site work. A series of archaeometric reviews illustrate the state of the art of the studies carried out on Fe-based red, yel- low and brown ochres (Mastrotheodoros et al. forthcom- This article is part of the Topical Collection on Mortars, plasters ing);  Cu-based greens and blues (Švarcová  et al. 2021); and pigments: Research questions and answers As-based yellows and reds (Gliozzo and Burgio 2021); Pb-based whites, reds, yellows and oranges (Gliozzo and * Elisabetta Gliozzo elisabetta.gliozzo@uniba.it; elisabetta.gliozzo@gmail.com Ionescu 2021); Hg-based red and white (Gliozzo 2021); and organic pigments (Aceto 2021). An overview of the use Antonio Pizzo antonio.pizzo@eehar.csic.es of inks, pigments and dyes in manuscripts, their scientific examination and analysis protocol (Burgio 2021) as well as Mauro Francesco La Russa mauro.larussa@unical.it an overview of  glass-based pigments (Cavallo and Riccardi forthcoming) is also presented. Furthermore, two papers on Department of Humanities, University of Bari, Bari, Italy cosmetic (Pérez-Arantegui 2021) and bioactive (antibacte- Escuela Española de Historia y Arqueología en Roma, CSIC, rial) pigments (Knapp et al. 2021) provide insights into the Roma, Italy variety and different uses of these materials. Department of Biology, Ecology and Earth Sciences, University of Calabria, Rende, Italy Vol.:(0123456789) 1 3 193 Page 2 of 30 Archaeol Anthropol Sci (2021) 13:193 Table 1 Definitions and uses of pigment, ink, dye and de/colouring agents to be used in archaeometry Pigment (Ink) Dye* De/colouring agent Definition Substance which, finely dispersed in Fine substance which penetrates the Substance which chemically bonds the water or other solvent, colours the substrate to which it is being applied material to which is added substrate by overlapping Characteristics - Chiefly inorganic - Chiefly organic - Only inorganic - Fine/coarse grained - Fine grained - Different types of compounds and grain - Insoluble - Soluble sizes - Dispersed suspension that covers the - Solution that is absorbed by the -Insoluble substrate substrate -Incorporated by the object Few examples Ochres (Mastrotheodoros et al. forthc.) Carmine; Gamboge Metallic Cu 2+ Cu-based (Švarcová et al. 2021) Indigo; Lac**; Oxydes and hydroxides containing Cu 2+ 3+ 4+ 2+ Hg-based (Gliozzo 2021) Madder; Purpurissimum Fe , Fe Mn, Co As-based (Gliozzo and Burgio 2021) Sepia Bronze Pb-based (Gliozzo and Ionescu 2021) Bindheimite Smalt (Cavallo & Riccardi forthc.) Use Painting, writing Textiles and leathers (painting, writ- Glass ing***) For a review on organic colouring materials used in wall painting, see Aceto (2021) in this TC ** The term lac may refer to two very different types of inorganic compounds: a) the gommalacca or shellac, i.e. “a pink-red–purple organic col- ourant derived from an insect and used as a lake pigment or a dye” (Berbers et al. 2019); b) a mixture of nitrocellulose and resin/wax *** Limited to specific chronologies and artefacts are provided here, based on the International Standards Introduction Organization (ISO 6707–1:2020 ): This paper serves as an anchor for the numerous contribu- (a) Mortar is described as a mixture of binder, aggregate tions of this TC. It intends to provide the criteria for both the and water; formulation of sound archaeometric questions and the execu- (b) Concrete is a mixture of aggregate, cement and water; tion of a suitable sampling in the field of mortars, plasters (c) Cement is an inorganic binder mixed with water to and pigments studies. form a paste “that sets by means of hydration reactions To frame these issues in a methodologically clear and and  processes, and that, after hardening, retains its exhaustive panorama, two brief explanatory sections are pro- strength and stability”. vided at the beginning: (1) what is meant by mortar, plaster and pigment and (2) what is meant by building archaeology. Both these introductory texts function to explain which Furthermore, in the context of Roman architecture, con- archaeometric questions are valuable for historical recon- crete is defined as the mixture of lime, aggregates and water struction and to guide researchers in the direction of repre- to form the so-called opus-caementicium (see Vitti 2021 in sentative sampling. These last two issues represent the bulk this TC). of the present contribution and are both presented in light of In this context, mortar is a recipe where the components the most recent advances made in this field. The interpreta- are the variables and its end-use is the archaeometric study’s tive problems that frequently arise from the archaeometric object. This basic distinction is fundamental in organising study of mortars are discussed in the last section of this the research questions and provides a key tool for their inter- paper, exploring the value and potentiality of archaeometric pretation. Consequently, correct identification and charac- research for historically meaningful reconstructions. terisation of the variables represent the starting point for any technological and provenance issue. Moreover, while the recipe’s reconstruction mostly regards the technological Mortars and pigments: basic definitions issue, the end-use of a mortar provides the necessary link to establish its suitability and functionality. Basically, there The term mortar basically defines a mixture of different components used to bond bricks or stones (Table 1). Ter- minological issues may arise when trying to distinguish a mortar from concrete and cement; therefore, the definitions https:// www. iso. org/ obp/ ui/# iso: std: iso: 6707:-1: ed-6: v1: en: term:3. 4.4. 27 1 3 Archaeol Anthropol Sci (2021) 13:193 Page 3 of 30 193 Fig. 1 Main types of plasters and mortars, depending on the starting raw material are two types of mortars: the aerial (Ergenç et al. 2021 in Typical examples in ancient buildings include: this TC) and the hydraulic ones (Arizzi and Cultrone 2021 in this TC). – Clay plasters, mixing clay, sand and plant fibres; Aerial mortars are characterised by higher flexibility, – Aerial lime mortars and plasters, mixingslaked lime and plasticity and permeability than the hydraulic ones; how- aggregate (e.g. sand or a mixture of sand and other inert); ever, the former shows a greater shrinkage during hardening, – (Feebly hydraulic ) magnesian mortars and plasters, mix- lower mechanical strength and resistance to moisture, salt ing slaked magnesia-lime and aggregate; attack and frost compared with the latter. Based on these – Gypsum mortars and plasters, consisting of calcium sul- main properties, aerial mortars are mainly used in sheltered fate hemihydrates and sand; areas, while hydraulic ones are favoured in exposed build- – Hydraulic pozzolanic mortars and plasters, mixing ings, such as port infrastructures (Vicat 1837; Ashurst and hydrated lime with natural or artificial pozzolanas (e.g. Ashurst 1988; Gibbons et al. 1995; Holmes and Wingate fly ashes and/or powdered ceramics); 1997; Cowper 1998). – Hydraulic mortars and plasters, mixing a natural hydrau- The term plaster includes several types of mixtures, lic lime and aggregate. depending on the type of binder and aggregate used. Techni- cal terminology (ISO 6707–1:2020 ) distinguishes between A schematic distinction between the various types of mixtures of one or more binders, defined as plasters, and plasters and mortars is provided in Fig. 1, while the main mixtures of one or more binders with aggregate (and other reactions addressed henceforth are listed in Fig. 2. possible admixtures), defined as renders. Moreover, the The use of the term pigment may be tricky, as several terms plaster and render should apply to mixtures used for other terms, such as colouring agent, ink and dye may appear the internal and external finish, respectively. to be synonyms; however, several differences exist among In archaeometry, plaster is used as a more general term, them regarding grain size, composition, application and use. including the meaning and properties of both modern plas- The definitions provided by different authorities such as the 4 5 ters and renders. This should perhaps remain so to main- ETAD, the CPMA and the DIN Standards Committee tain terminological coherence with the history of studies Pigments and Extenders are based on current commercial and highlight the distinction between ancient materials and use and classification of colour pigments. They sometimes technical materials used, for example, for restoration and conservation. On the other hand, the modern classification The hydraulicity of magnesia-limes is debated. Its characterisation of mortar and cement (e.g. UNI EN 197/1) foresee subdivi- as feebly hydraulic relies on Chever et al. (2010). sions that do not apply to the ancient world’s materials and Ecological and Toxicological Association of Dyes and Organic Pig- could not be extensively used tout court. ments Manufacturers (https:// etad. com/). Color Pigments Manufacturers Association, Inc. (https:// www. pigme nts. org/). 2 6 https:// www. iso. org/ obp/ ui/# iso: std: iso: 6707:-1: ed-6: v1: en: term:3. Responsible for the European standardization (https:// www. din. de/ 4.4. 27en/ getti ng- invol ved/ stand ards- commi ttees/ npf). 1 3 193 Page 4 of 30 Archaeol Anthropol Sci (2021) 13:193 Fig. 2 Chemical reactions occurring during mortar production and weathering (from Davidson et al. 1965; Böke et al. 2006; Uğurlu and Böke 2009; Jakić et al. 2016; Ponce-Antón et al. 2018; Li et al. 2020). The magnesian lime cycle is oversimplified because the formation of the different phases (e.g. artinite, brucite, calcite, dolomite, dypingite, huntite, hydromag- nesite, lansfordite, magnesite, nesquehonite, periclase and portlandite) depends on the temperature, the C O concentra- tion and the pH and RH (i.e. relative humidity). Therefore, it is not straightforward to indicate which phases are formed for each slaking, setting and hard- ening phase (see also Lanas and Alvarez 2004 on this topic) include both organic (i.e. containing carbon) and inorganic – Extenders (“material in granular or powder form, prac- substances under the general definitions of “pigment” and tically insoluble to somewhat soluble in the application “dye”. In the current regulation (ISO 18,451–1:2019 ), a medium and used to modify or influence certain physical pigment is defined as a “ colourant consisting of particles, properties”); insoluble in the application medium (e.g. coating material – Fillers (“coating material with a high proportion of or plastic)” where colour is a “generic term for all colour- extender, intended primarily to even out irregularities in ing substances” and subcategories are allowed based on substrates to be painted and to improve surface appear- their chemical composition and properties (e.g. inorganic, ance”). organic, coloured, white, effect, corrosion-inhibiting, mag- netic). Main distinctions are traced as follows: Undoubtedly, the increase and the variation in terminol- ogy that we have witnessed in recent years follow the devel- – Dyes (“colourant, soluble in the application medium”); opment of technological products that did not exist in ancient – Pigments used for ceramics and glass (called stains); times and the creation of products used for conservation. https:// www. iso. org/ obp/ ui/# iso: std: iso: 18451:-1: ed-2: v1: en: term:3. 1 3 Archaeol Anthropol Sci (2021) 13:193 Page 5 of 30 193 It is not uncommon, in modern literature, to find the term Born at the end of the 1980s by Italian medieval archae- inorganic dyes used in place of pigment or the term colour- ologists (Siena, Genova and Padova universities), the disci- ing agent used in the sense of chromophore and applied to pline picked up the need to renew the archaeological meth- both pigments and dyes. Therefore, while it is necessary to ods applied to historical architecture. consider and adopt it when necessary, it is also worth provid- BA applies the stratigraphic method to any building type, ing adequate terminological explanations and maintaining whether or not preserved in its entirety. The theoretical and a certain consistency with our study field, hence avoiding methodological system used in BA has been, by now, thor- unnecessary anachronisms. oughly discussed and formalised. The main objective is to To make an example, each definition produced by the reconstruct the historical diachrony (phases of construction, sector authorities agrees on the basic distinction: pigment- use, transformation, abandonment and destruction) of indi- insoluble, dye-soluble; however, no further specification is vidual structures or entire buildings. given on the state (solid or liquid) or nature (organic/inor- Moreover, in addition to identifying the historical ganic and natural/synthetic), meaning that all combinations sequences of architectural artefacts, the BA offers a fun- appear possible. Conversely, in archaeometry, these distinc- damental tool for conservation, eventual restoration and, tions have important implications regarding the analytical overall, the protection and safeguarding of historic buildings. techniques to be used for their characterisation and the prob- To achieve a diachronic reading of the elevation, the basic lems related to conservation. method of the BA includes: In the current archaeometric literature and practice (Table  1), pigments are considered inorganic materials The stratigraphy, for the definition of relative chronolo- (chiefly minerals and earths). They may be either natural gies; or synthetic (e.g. Egyptian blue) and colour the surfaces The study of building techniques, which, in turn, to which they are applied by simply covering it. Their use includes: for different types of paintings (e.g. paintings, frescoes and icons) is the most common one; however, when used for The characterisation of the building materials (stone, dark writing in manuscripts, the name switches from pig- bricks, mortar, plaster, etc.); ment to ink. Conversely, dyes mainly include organic mate- The technical and technological study of both materi- rials, frequently soluble (also insoluble, e.g. indigo), that als and structures and the way materials are assem- can chemically disperse into and bind to the materials. They bled. are frequently obtained from plants (e.g. leaves, berries and roots) or animals (e.g. insects and molluscs), and their mul- In practice, the BA consists of (a) distinguishing the tiple uses range from textiles and food colouring to painting. constructively coherent parts on a wall/structure, (b) Finally, the terms de/colouring agents are used only in the identifying their contours and (c) organising the individ- field of glass studies and are sometimes synonymous with ual stratigraphic units in a diagram according to a rela- chromophore, to identify elements and phases—exclusively tive chronology. This procedure represents the basis of inorganic so far—that chemically bond the material to which archaeological stratigraphy, as it allows us to understand they are added. the before and after of each action. To conclude, we must add that (a) mixed inor- To better understand the role of mortars and pigments, ganic–organic compounds are also known, as evidenced by it is necessary to clarify the role of building techniques the mixture of palygorskite clay and indigo used for Maya under a theoretical (Mannoni 1997) and methodologi- blue; (b) the term colourant should be avoided in scientific cal (Parenti 1988a) point of view, as to be aware of the literature, as it is a general term that includes both pigments limitations in using building techniques to date buildings and dyes. (Parenti 1988b). Moreover, this is the study’s field, where a fruitful collab- Building archaeology oration between different expertise is essential, as it includes the investigation of mortars, plasters and pigments. The building archaeology (BA) is a research methodology The analysis of building techniques has to be multi-level, and a discipline that provides a reading of the macro trans- from the scale of the entire building to the materials con- formations of the investigated building, such as changes in stituting the wall, the mortars, the plasters, the binders and the original project, added structures, interventions for the the pigments. redistribution of spaces, use and functions of buildings and In this sense, it is an indispensable premise to any archae- analysis of the environmental or built context related to the ometric research. To interpret the results at the large scale building. of the entire building, it is necessary to start from sampling at the small scale of the single wall. 1 3 193 Page 6 of 30 Archaeol Anthropol Sci (2021) 13:193 At the small scale, BA provides information on the con- between research phases, such as sampling, analytical study struction morphology, details of the workforce and interpre- and results interpretation, with the history and intended use tations of the structures. of the investigated “structure” implies a close collaboration All these features, combined together, provide the sam- in planning the best research strategy. pling criteria for archaeometric research. The sections below clarify the main research question After the completion of archaeometric analyses, the raised during the study of the masonry works, including results complete and improve the description of the tech- both those that are possible to solve through archaeometric nique itself. In substance, BA and archaeometry initially methods and those in which archeometry plays a marginal work separated and then converge towards an agreed, accu- role. The aim is to provide as complete a picture as possible, rate and comprehensive result. albeit within the single article’s limits. At the large scale, the construction techniques and the stratigraphy provide the instruments for the interpretation of results. The con- textualisation of the results is the passage that allows us to reconstruct The provenance of raw materials used for building the history of the building. By integrating all the results obtained materials from the joint research, it is possible to reconstruct all stages of pro- cessing: from the quarry chosen to supply the most suitable material, While building material characterisation provides precious to the production of lime and mortar, to assembling of materials in information on the type of raw material used, the identi- the masonry, to the decoration, to the uses and, eventually, to the fication and localisation of the supply area inform us on collapse of the building. the aspects that determined its selection. This is the main At both the small and large scale, the building technique reason why we should consider this question as intended represents the main instrument to establish the absolute chro- to investigate a territory rather than limited archaeologi- nology of a built context. Archaeological dating uses direct cal samples and geological outcrops. Moreover, this is a and indirect sources. The former includes historical, carto- multi-fold question since it regards all different compo- graphic or iconographic sources, while the latter are deduced nents: mortars, plasters, pigments, bonded stones or clayey from BA or from structural elements (e.g. stamped bricks of raw materials used for brick making. While it is possible to known chronology or coins; see also Mannoni 1984). Funda- limit the study to mortars, plasters or pigments, an in-depth mental information is also obtained by studying local chrono- understanding of the whole context certainly grants higher logical clusters, i.e. construction techniques closely linked to quality research. As a matter of fact, this approach makes the a territory and the consistency of the material used. Archaeo- difference between mere material analysis and meaningful logical dating is, thus, the main tool to decide whether the archaeometric research. archaeometric analysis of dating is a path to be attempted or In practice, the localisation of the supply areas or quar- not and, if it is, to select the most significant samples. ries allows a deepening of our knowledge on various issues: It goes without saying that, in practice, the chronological sequence of our studies generally corresponds to the small/ (1) To define what was the builders’ awareness of the large scale distinction proposed above. resources available in their territory; (2) To determine when the functionality criterion prevailed over that of availability or vice versa; (3) To find rational explanations to technologically contro- The archaeometric questions versial choices; (4) To answer all those “whys” that follow the provenance Taking up the concept already expressed in Gliozzo (2020a), issue’s resolution and require a thorough knowledge the archaeometric questions guide both sampling and experi- of both the territory and the ancient production tech- mental choices, besides providing the key criterion of eval- niques. uation for the entire research project. Research questions should be wide and necessarily contextualised in a historical In addition, the reconstruction of georesources supply strat- perspective (the “big picture” in Gliozzo 2020a). The tradi- egy and trade patterns clarifies which was the commercial net- tional—now anachronistic—distinction between archaeolog- work of reference for the site in a given chronological period ical and scientific questions finds a practical compendium and, therefore, allows the researcher to evaluate the choices in the archaeometric question. Consequently, archaeometric made by the builders in a broader framework that includes the research implies and requires a multidisciplinary team to social and market economy, the geology of the territory and the provide the necessary skill-sets on a permanent basis. complex production activities carried out on the site. In the field of mortar studies, this unity of purposes and In the practice of archaeometric research, this question practices is particularly evident and essential. The close link involves a preliminary in-depth knowledge of the geological 1 3 Archaeol Anthropol Sci (2021) 13:193 Page 7 of 30 193 setting of a large territory, including the archaeological site, out after the stratigraphic and the typological study have the archaeometric analysis of archaeological materials and that been completed; otherwise, the risk is to select a collection of numerous types of natural georesources that are believed to of non-representative samples. Moreover, (a) once the first have been used. All this information provides the indispensable round of materials analyses is completed, the need for tar- reference database. geted field campaigns arises, both to study the territory and To make a shortlist, materials analyses and investigation to sample the geological materials to be analysed for com- may regard: parison, and (b) after the characterisation of archaeological and geological materials, a third experimental phase may A. Mortars, plasters and pigments from which the study follow, aimed at verifying some working hypotheses. This is starts (henceforth, “the study’s object”), further divided the procedure generally adopted because the archaeometric into: approach is sometimes intended as a “tool” to find some answers and not as a consolidated procedure, intimately A(1). Geological sands, which can be compared to linked to the archaeological research methodology. those used in plasters and mortars; On the contrary, if the archaeometrist is involved from the A(2). Geological raw materials suitable for the produc- beginning of the excavations, he/she can carry out the geo- tion of the binder; logical “reading” of the territory and a part of the samplings A(3). Any material that may have been added to both in advance of the usual timetable, the objectives can be con- mortars and plasters (e.g. plants, ground ceramic certed and programmed more systematically and effectively fragments, or rock fragments) to confer particular and well-integrated results may become expendable in less properties; time. A(4). The water; A(1) The aggregate: sands Based on the International Stand- B. In stonework, building stones and relative geological ards Organization (ISO 6707–1:2020 ), the aggregate is an samples. In brickwork, bricks and geological clayey “inert granular material” that can be divided into fine and materials, which can be compared to those used in heavy, the latter having oven dry-particle ≥ 3000 kg/m . bricks; In historical mortars, most of the aggregate is usually C. Minerals and pigments naturally present in the area constituted by sands; therefore, investigating their nature— under investigation. “which kind of sand is it?”—typically represents the first step. To better explain how the research deals with the “prov- These are usually classified as fluvial, lacustrine, enance of raw materials”, we propose some examples from marine, fossil and quarry sands, depending on whether published case studies, but, for details, we refer to Ergenç they are taken from rivers, lakes, sea beaches or other ter- et al. (2021) and Arizzi and Cultrone (2021 in this TC). restrial deposits. The canonical particle size classifica- tion defined by Wentworth (1922) distinguishes sands as A. The study’s objects very fine (> 63 < 125 μm), fine (> 125 < 250 μm), medium (> 250 < 500  μm), coarse (> 500  μm < 1  mm) and ver y Undoubtedly, the study’s objects, whether they are mortars, coarse (> 1 < 2 mm) and the presence of the coarser gravel plasters or pigments, are typically the first to be investigated; fraction distinguishes concretes from mortars. It is possi- otherwise, we would have no information available to guide ble to use other classifications, but the reader must bear in the search for the raw materials that have allowed their reali- mind that, while the Wentworth classification was specifi- zation. Indeed, the research begins with the basic question: cally formulated in the geological field, others such as the “What is it made of?”. ISO 14,688–1:2002 have been elaborated for engineering The specific methodologies are addressed in the various purposes. The shape and rounding of the grains, the sorting contributions of this TC; therefore, we do not dwell on the and the composition represent other important characteris- subject here. However, we would like to highlight two pro- tics that, combined with previous ones, provide the neces- cedural aspects that we believe are important but not always sary information to determine the nature of sands and guide clear to newbies: (1) it is important to perform an accurate the search for natural outcrops to be sampled for comparison. visual inspection of mortars and plaster before any sampling is planned, and (2) analyses are not a quick procedure. In https:// www. iso. org/ obp/ ui/# iso: std: iso: 6707:-1: ed-6: v1: en: term:3. most cases, it is not enough to insert the sample into an 4.4. 27 instrument to get the answers we want. Still from a proce- In this regard, it is also worth underlining that “whereas concrete dural point of view, the researcher must bear in mind that is itself a building material, mortar is a bonding agent that typically holds bricks, tiles and other masonry units together” (Allen and Iano sampling and subsequent analyses should only be carried 2013). 1 3 193 Page 8 of 30 Archaeol Anthropol Sci (2021) 13:193 In this way, it is possible to answer the second question absent since they easily burn completely. Conversely, when aimed at locating the quarry, site, or area of supply of the sand: carbonatic rocks are used, the presence of these relics is more “where does it outcrop and from where was it taken?”. frequent and may favour raw materials’ search; however, the The following questions depend on the results obtained by researcher is frequently obliged to deal with very small quanti- characterising the mortar/plaster. For example, if the sand that ties of little dimensions even in this case. Furthermore, differ - was used has a significant clay component, we may wonder why ent types of binders may have been used in combination (e.g. the type of sand that is least suited to making these products lime and gypsum). In these cases, determining the nature and was used. If, on the other hand, the sand is rich in an earthy origin of the binder may be a complicated task, which requires component, we may ask ourselves why the ancient artisans did the use of numerous analytical techniques, capable of provid- not feel the need to wash them before use. If, finally, the particle ing complementary information. size is poorly/well sorted, we may ask ourselves how to explain The finding of the lime kiln or a dump of waste materials this evidence: comparable with the natural supply deposit or the would represent an ideal situation. A micro-stratigraphic sam- result of a technological procedure (sieving)? pling of contextual materials may guarantee the identification of These are just a few examples of questions that may arise the raw material, provide information about its processing and, from the initial two. Therefore, it is good to remain flex- when it is possible to establish a direct connection between the ible when planning the overall research and, especially, the place of production and the structure of use, indicate the degree initial sampling because it is not possible to predict them in of mortars elaboration. In such an advantageous situation, the advance. In several cases, in-depth knowledge of the terri- field campaign may be precisely directed towards specific out- tory likely provides the key to answer the various research crops, and the ensuing laboratory analyses may provide results questions. Still, the researcher should be aware that this ranging from a high likelihood to certainty. knowledge often makes it necessary for a supplement of In the absence of this archaeological evidence, the level the investigation—aimed at the characterisation of natural of accuracy of the answers is strongly conditioned by the materials—because literature data may be insufficient for characteristics of the binder itself (e.g. nature, composition, archaeometric research. Apart from this, an equal in-depth abundance and size) and is, therefore, not predictable. knowledge on the different yield of fine sands (e.g. requiring high amounts of binder and water and thus resulting in less A(3)  The additives (~ modern additives, admixtures and workable, too porous and less resistant mortars) and coarse aggregates) In modern terminology, the additive is a material sands (e.g. requiring less binder and water, thus resulting “added in small quantities to a liquid or granular material in better workable, adequately porous and highly resistant to produce some desired modification to its characteristics” mortars) and, in general, the importance of sorting, or pref- (ISO 6707–1:2020), while the admixture is a material “added erences tuned to certain types of sands (e.g. river and lake) in small quantities before or during a mixing process in order over others (e.g. quarry sands due to impurities; sea sands to modify the characteristics of a mixture”. The two terms for aerial mortars due to salts) is preliminarily required. describe apparently similar materials that, added in small quantities, can modify the properties of the starting mixture. A(2) The binder The characterisation of the binder always However, the difference between the two terms lies mainly represents a key question: “which kind of binder was in that the additive is added during the manufacturing of the used?”. The analyses combined with the knowledge of the cement, while the admixture is added to the concrete during geological settings provide the necessary clues to the identi- mixing. Moreover, the UNI EN 934–2 standard states that the fication and localisation of the raw materials used for binder admixture for concrete should be in an amount not greater making, i.e. to solve the related question: “where do the raw than 5% by mass, compared with the cement. Therefore, under materials to make the binder come from?”. the current rules, the addition of materials, such as ground Since binders may be a variety of materials, several ceramic fragments, in considerable proportions would fall into types of sources should be targeted, such as earths (with the category of aggregates and not that of additives. earth mortars), clays (with clay mortars), limestone and/ However, one of the main aims of the archaeometric or sea-shells (with lime-based mortars/plasters), dolomite study on ancient mortars and plasters is to reconstruct the and dolostones (with magnesian mortars/plasters), gypsum production’s phases and the supply modalities of each com- deposits (with gypsum mortars/plasters) and marls (with ponent. In this case, the term additive, used in its etymologi- lime-based mortars with feeble hydraulic properties). cal sense (from addere, to add) has the advantage of clearly However, even if, in most cases, the material is of a geo- recalling the difference between a primary component (fre- genic nature, its study implies an additional difficulty since quently sand) and an added component (i.e. the additive) we no longer see the “original” material but only its prod- in the aggregate. This is the meaning we have given to the uct, further reacted after use. For example, when seashells term additive in this article. Still, the explanation provided are used (e.g. Gleize et al. 2009), lumps of un-burnt lime are above accounts for the fact that modern terminology partly 1 3 Archaeol Anthropol Sci (2021) 13:193 Page 9 of 30 193 overlaps and partly articulates itself differently, based on The discovery of recycled amphorae containing north- the contemporary world’s different needs. It follows that, as Latial pozzolans in the ship B of Pisa (Augustan Age) is a long as terminological uncertainties persist, it is always good clear example of how the archaeometric analyses can effec- to specify the choice made in the description of the results. tively trace the movements of these raw materials, apparently Hence, after having accurately characterised the main also used as ballast despite its low weight. As further evidence components (aggregate and binder), the following question that the investigation of the provenance is only the first step concerns the possible presence of additives: “are there any to seek historically much more significant reconstructions, deliberately added materials?”. If yes: “which types of the studies conducted on volcanic scoria in some buildings of materials were added?” and “where did they come from ancient Rome (Lancaster et al. 2011) have not only updated our or how were they made?” (other questions concern suit- knowledge on the chronologically diversified supply of these ability and functionality and are reported in the dedicated materials from Rome, the Vesuvius and the Campi Flegrei, but paragraphs). also imposed a revision of the land and sea transport system The addition of inclusions to improve the final product’s and hypothesised an imperial involvement in the trade of these performance was a common practice that frequently left rec- raw materials. Similarly, the studies conducted on Tunisian ognisable traces in literary sources and archaeological evi- (from Sardinia or Pantelleria) and Turkish (from short-range dence. Consequently, citable examples are numerous and pro- resources) analogous materials have shed new light on the vide us with an extremely varied picture in which inorganic methods of importation (primary or secondary loads associ- and organic materials are enlisted. Therefore, it is possible to ated with other goods, such as grain mills), as well as demon- formulate the characterisation question in terms of: “are the strated their long-range transportation (Lancaster et al. 2010). additives of an inorganic or organic nature?” (we will see In a nutshell, from the provenance investigation of a particular later that there are additional difficulties in this second case). additive in the mortar, it is possible to find ourselves investigat- To properly solve this question, it is necessary, perhaps ing the favourability of winds along one sea route compared to even more than in previous cases, to know which types of another, the distribution of primary and secondary ports, the type additives were used to facilitate—or even allow—their iden- and capacity of ancient boats, the circulation of associated goods tification during the analysis of the “study’s object”. For this (e.g. foodstuffs, ceramic, millstones, building materials) and the reason, we present a shortlist of inorganic and organic addi- ancient navigation methods (e.g. coastal navigation): it is precisely tives (and admixtures); however, we refer to the other contri- here that the study becomes interesting and fruitful. butions for the necessary insights on their characteristics and Proceeding further, it is worth adding that the term poz- properties (Arizzi and Cultrone 2021; Ergenç et al. 2021). zolan may also apply to non-volcanic, natural and artificial Beginning with inorganic additives, the most common were materials with similar characteristics (chiefly silica and alu- natural and artificial pozzolans that are defined as “siliceous or mina, along with ferrites). Therefore, the question may turn siliceous and aluminous materials which in themselves possess into: “what type of pozzolan was added?”. little or no cementitious value but will, in finely divided form Main examples are represented by: and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperatures to form compounds pos- – Ceramic powder (especially tiles). Based on literary sessing cementitious properties” (Dodson 1990). sources, this was one of the most typical inclusion (“coc- Natural pozzolans are volcanic origin materials, such as ciopesto” and opus signinum in the Roman world; Kho- the volcanic ashes from the region of Pozzuoli from which rasan in Turkey; Surkhi in India; semen merah in Indo- they take their name (pozzolan s.s.). These are often found nesia; Homra in Arabic countries; Spence 1979). During in the mortars/plasters of sites located within or close to the Roman age, both Vitruvius and Plinius reported on volcanic regions (Barba et al. 2009; Özkaya and Böke 2009; the effectiveness of such inclusions in constructions built Villaseñor and Graham 2010; Kurugöl and Güleç 2012), but along a river or the sea exposed to humidity and marine when they are found in sites far from supply areas, archaeo- sprays. During the Middle Ages, Villard de Honne- metric research is called upon to localise their origin and court (French master mason of the thirteenth century reconstruct the trade routes that allowed their import and usage. The provenance question may thus remain confined to the scale of the site and its territory or expand to long- Vitruvius, De Architectura II, 5, 1 “etiam in fluviatica aut marina si qui testam tunsam et succretam ex tertia parte adiecerit, efficiet scale range imports and cross other issues such as those materiae temperaturam ad usum meliorem”. (If to river or sea sand, concerning the methods and extent of the marketing of raw potsherds ground and passed through a sieve, in the proportion of one materials in a given period (e.g. “what was the diffusion of third part, be added, the mortar will be better for use). Pliny, Natura- these materials?”; “what were the routes and commercial lis Historia, XXXVI, 54, 175 “si et testae tusae tertia pars addatur, melior materia erit” (If, too, one third of the mortar is composed of methods involved in the transport of these important raw bruised earthenware, it will be all the better). materials?”). 1 3 193 Page 10 of 30 Archaeol Anthropol Sci (2021) 13:193 AD) reported a hydraulic paste recipe based on lime, – Processed clays such as metakaolins (see, e.g. Baronio pounded fragments of bricks and linseed oil. In the and Binda 1997; particularly used for restoration, e.g. Renaissance period, the famous architect Leon Battista Aggelakopoulou et al. 2011; Vejmelková et al. 2012a; Alberti (1404–1472) reported the common opinion that, Loureiro et al. 2020); if you add a third of crushed brick, the resulting mixture – Diatomaceous earths (see, e.g. Franzini et al. 1999, 2000) becomes much more tenacious. A century later, Pietro and opal-A (Sarp Tunçoku and Caner-Saltık 2006); di Giacomo Cataneo (Sienese architect, 1510–1574) – Specific soils in earthen mortars (see, e.g. Cantù et al. handed down the recipe for mortar, recommending two 2016). lime parts and two other parts of tile powder, with half a part of iron flakes. The list could go on for long but Finally, marble (or travertine) dust was used in mix- we believe it is already clear how the masters configured tures with sand and lime for plasters to be decorated or the use of this material as “typical” over the centuries. for the bed mortar layer of mosaics or for relief decora- The archaeometric analyses support this reconstruc- tions (stuccos) that had the splendour and luster of marble tion. Ceramic powders, sometimes mixed with other (opus albarium or caementum marmoreum; see Vitruvius, pozzolans and volcanic materials, have been frequently De Architectura, VII 2, 1–2 and Cataneo 1567, II 11; for found in mortars and plaster dated to the: a review on some historical treatises see Salavessa et al. 2013; for some archaeometric evidence see Toniolo et al. • Late Bronze Age (Theodoridou et al. 2013); 1998; Riccardi et  al. 2007; Kriznar et  al. 2008; Weber • Hellenistic (see, e.g. in Italy, Montana et al. 2016); et  al. 2009; Duran et al. 2010; Miriello et al. 2011; De • Roman and early Byzantine periods (see, e.g. in Luca et al. 2012; Robador and Arroyo 2013; Lezzerini Italy, Bugini et al. 1993; Damiani et al. 2003; Berto- et al. 2014, 2019). lini et al. 2013; Izzo et al. 2016; Columbu and Garau As for the identification of organic additives, additional 2017; Graziano et al. 2018; Miriello et al. 2018, difficulties need to be known in advance. The substantial dif- Montana et  al. 2018; Sitzia et  al. 2020; in Spain ference between inorganic and organic materials is that the Alonso-Olazabal et al. 2020; in Tunisia, Farci et al. former are generally visible to the naked eye or at medium 2005; and in Turkey, Bakolas et al. 1998; Miriello magnification. In contrast, many of the organic ones (e.g. et al. 2011); milk and egg whites) are not visible. Therefore, the research • Medieval periods (see, e.g. in Greece Moropoulou question “are there any organic additives?” stems more et al. 2000; in Italy, Lezzerini et al. 2014; in Por- from the preliminary knowledge of ancient methods and tugal, Adriano et al. 2009; in Turkey, Kurugöl and techniques than from macroscopic and microscopic obser- Güleç 2012) and beyond (e.g. in the Czech Republic, vation but is archaeometrically resolvable through an ad hoc Přikryl et al. 2011; in Italy, Cantù et al. 2016; in analytical strategy. Turkey, Böke et al. 2006; Uğurlu and Böke 2009; The next step concerns the characterisation: “what kind Binici et al. 2010); of organic substance was used?”. The ancient authors documented the extensive use of both – Processed slags from metalworking (see, e.g. Diekamp plant and animal origin substances to improve the perfor- et  al. 2006; Cacciotti et  al. 2015; and Kropáč and mance of mortars and plasters. For example, Vitruvius sug- Dolníček 2013 for correlated evidence); gested the use of lime tempered with oil (and dregs of oil) for waterproofing and preventing frosting (De Architectura, VII 1, 6–7; VIII 6, 8), thus demonstrating that the water- repellent properties of natural oils and fats were well known in Antiquity. Pliny reported that the temple of Minerva of Elis had been plastered by Panænus (brother of Phidias) Album de Villard de Honnecourt, Plate XLII “On prend chaux et tuile de paiens pilee, et vous ferez autant de l'une que de l'autre, with the addition of milk and saffron. mettant un peu plus de tuile de paiens, jusqu'a, ce que sa couleur domine l'autre. Detrempez ce ciment d'huile de lin, et vous en pour- rez faire un vaisseau a contenir l'eau.” (Take lime and pounded pagan (Roman) tiles in equal quantities, adding a little more of the latter until its colour predominates. Moisten this cement with linseed oil, and with it you can make a vessel that will hold water)”. Waste generated during mining and processing of marble is nowa- De re aedificatoria, III, 4 “tertiam si tunsae testae partem adiec- days re-evaluated for the same use (see e.g. Kore et al. 2020). eris, affirmant omnes futuram multo tenaciorem”. Naturalis Historia XXXVI, 55, 177 “Elide aedis est Minervae, in I Quattro Primi Libri di Architettura, II, 12  “piglisi per ogni due qua frater Phidiae Panaenus tectorium induxit lacte et croco subac- staia di calcina due altre staia di polvere di tegole, con mezzo staio di tum, ut ferunt; ideo, si teratur hodie in eo saliva pollice, odorem croci scaglia di ferro”. saporemque reddit”. 1 3 Archaeol Anthropol Sci (2021) 13:193 Page 11 of 30 193 The archaeometric research allowed for the individuation monumental buildings and offices, temples, pagodas, houses of several others organic additives in archaeological finds, and tombs) dated from the Taosi phase to the Qing Dynasty for example: (2300 BC–1911 AD). Different analytical approaches allowed these authors to determine the presence of starch – Wood, straw and charcoal in Roman mortars and plasters in 112 samples, oils in 87, proteins in 59, sugar in 14 and from Petra in Jordan (Al-Bashaireh and Hodgins 2011); blood in 5; furthermore, 48 and 5 samples showed two and – A saccharide material-based additive of plant origin and three organic additives, respectively. Apart from the com- a natural gum in the mortars of the medieval shipyard of prehensive and diachronic reconstruction proposed by these Amalfi Arsenale (Rampazzi et al. 2016); authors, it is also interesting to learn how the choices made – Egg white and urea in the mortars from the Yoros Castle by the ancient artisans were directly correlated with (a) the th th (13 –14 AD) at Anadolukavağı (Kurugöl and Güleç variable climatic environments, (b) the different seismicity 2012); of the various areas, (c) the development of agriculture and th th – Fibers in 13 –18 AD mortars from Erzurum in Turkey (d) socio-cultural factors linked to the relationship between (Binici et al. 2010); man and nature and Confucianism. – Proteins and/or animal blood in post-medieval earthen Also in this case, therefore, the characterization of addi- mortars from Cremona in Italy (Cantù et al. 2016); tives may lead to studies regarding ancient agriculture, – Egg yolk, oil and some resin in mortars repairs from the beliefs, philosophies and religions, the seismicity of specific St. Engracia Basilica at Zaragoza in Spain (Luxán et al. territories, the societies, understood as cultural ensembles, 1995); and the dynamics of trade in perishable products. – Sticky rice, egg white and tung oil, brown sugar, pig Finally, it can be concluded that the provenance of an blood and tung oil in several Chinese mortars, variously additive is more likely a starting point rather than an arrival dated from 563 to 1381 AD (Yang et al. 2009; Yang et al. point: “beyond the technical aspect, what further infor- 2010; Zhang et al. 2014; Zhao et al. 2014a, b, 2015). mation can I deduce from the presence of these additives in mortars/plasters, starting from their provenance?”. These few examples should have clarified that the use of organic additives is ancient and seamlessly has come A(4) The water Water supply is often taken for granted, down to us, in practically every part of the world. The sec- especially if the archaeological site is located along a water- ond aspect to consider is that not all archaeometric methods course or near the sea. However, water procurement deserves allow for accurate identification of the organic substances; a closer study—“which kind of water did they use and therefore, the analytical procedure must be strategically tar- where did they get it from?”—as it was necessary to sup- geted. This appears even more evident if we try to draw up ply considerable quantities, and different types of water (i.e. a list of possible organic substances used for mortars and from wells, springs, rivers and sea) have different yield over plasters. Using only the reference quoted here, along with time. For example, salt content can damage an aerial mortar those discussed by Sickels (1981, 1982), it is possible to list while enhancing the binding properties of a hydraulic mortar (in alphabetical order) animal glue, barley, beer, beeswax, (Davidson et al. 1965; Karim et al. 2017; Li et al. 2020; see blood (also of hippopotamus), butter, charcoal, cheese, cot- also Fig. 2). ton, curd, dung, egg white and yolk, elm bark, fibres, fruit The presence of chlorine and sodium in the analysed juices (e.g. fig), gluten, gum Arabic or tragacanth, hair, hogs’ mortars may give a clue on this aspect and clarify specific lard, keratin, malt, milk (casein), molasses, oil (e.g. linseed technological choices. For example, these two components oil and tung oil), resin, rice, rye flour, saffron, shellac, starch, in the binding matrices and pozzolanic lumps of some Nora straw, suet, sugar, tallow, tannin, urea, wine and wort. plasters (used for waterproofing this Punic-Roman site’s cis- It goes without saying that the variety of materials is wide terns in Sardinia, Italy) made the authors suggest the use of and that, most likely, we do not know it fully yet. What is seawater for their preparation (Secco et al. 2020). certain, however, is that the provenance question may not Looking at the same topic from the point of view of conserva- directly regard a geographic area but a certain animal or tion, the decay induced by salt crystallisation cycles (salt weather- vegetal species, e.g. “from which animal does the blood ing) is a factor that puts monuments at risk; moreover, it is foresee- come?”; “which plant does the wood come from?”; able that it will be more and more significant due to the climate “where was the species from which the oil was extracted breakdown we are witnessing (see below. On salt weathering also grown or fished/bred?”. see Ergenç et al. 2020; Randazzo et al. 2020). Based on these exam- As a final example, we propose a comprehensive study on ples, the question “where was the water for the mortar/plaster Chinese organic–inorganic composite mortars performed by mix from?” certainly acquires a much more meaningful motivation Li and Zhang (2019). Their research focused on 358 mor- and it would also be useful to investigate “what quantities were tar samples taken from 159 buildings (city walls and forts, needed?” for the masonry under examination. 1 3 193 Page 12 of 30 Archaeol Anthropol Sci (2021) 13:193 – The binder and then the lime processing (e.g. “are there B. Stones and bricks unburnt portions?”; “is it possible to trace the kiln and evaluate its function and effectiveness?”) In this case, the materials under investigation may either be worked stones and/or ceramic bricks and tiles. The ques- – The additives (e.g. “have they been chopped or pulver- ized?”) tions arising from their study are only partially different: (a) dealing with stoneworks, the main questions regard the It is useless to deny that the answers that we can realisti- characterisation (“what kind of stone was used?”) and the localisation of the geological outcrops (“where do the cally obtain may be very uncertain or partial, often dictated by common sense than by real archaeometric evidence. In stones come from?”); (b) when ceramic bricks were used, the questions regard the characterisation of the raw materi- the most fortunate cases, the evidence of production struc- tures such as lime kilns are found (e.g. Vaschalde et  al. als (“what kind of clayey raw materials were used?”), the localisation of the clay outcrop that supplied them (“from 2016; Toffolo et al. 2017a; Casas et al. 2020; Goguitchaich- vili et al. 2020), and the researcher can thus reconstruct the which outcrop were they taken?”) and, possibly, the locali- sation of the production plant (“where were the raw mate- lime production process. There are also particular cases in which the slags, sintered during the burning process inside rials prepared and fired?”). In this second case, the study of mortars is closely linked with both that of stone materials the inner lining of the lime kiln, have been reused inside the mortars. Their discovery may indirectly provide informa- and that of ceramic productions and, therefore, it becomes obvious how a global approach to an archaeological site may tion about the production and preparation of both the binder and the additives (Kropáč and Dolníček 2013). A similar prove economically much more sensible, as well as histori- cally more interesting, than a single material approach. It fortunate case—although very rare—is represented by lay- ers of abandoned raw materials from which we can obtain should also be added that the additives found in mortars may result from waste from stone processing or from the important information about sand processing. Despite the difficulty in tracing this information, its use shredding of local ceramics; therefore, the investigation of stone materials may provide information that can be used in for both conservation and formulation of compatible mortars is undeniable. It is well known, for example, that the use more than one field of investigation (see Fort et al. 2021). In this case, the example we propose is regarding the of unwashed marine sands introduces harmful Cl-salts into aerial mortars; moreover, the evaporation of saline solutions archaeological site of Thamusida-Sidi Ali ben Ahmed in Morocco (Fig. 3), where both stones and bricks were used can lead to discontinuities. Indeed, saline solutions represent a critical factor that may enter the mortar also a posteriori, for the masonries and a global archaeometric approach was applied to its Cultural Heritage remains. In this case, the for example, during flooding events or the circulation of thermal waters. presentation goes through the series of questions that typi- cally arise during the research and the provided answers Finally, this question and the following one imply a whole series of questions related to the organisation of the produc- (Q&A), as summarised in Table 2. tion, through which we can move from the analysis of the material culture to the study of workers, observing what the The processing of raw materials used for mortars and plasters production relationships were, how it was their work, what surfaces they were able to achieve at a certain time and so In the study of masonry works, the preparation concerns on (DeLaine 2021). every single component found in the mortar but also, as seen in the previous example, the processing of stones and The reconstruction of mortars and plasters recipes bricks (i.e. “is it possible to recognize traces left by stone- working?”; “how were raw clays prepared for brick This technological issue includes a simple question to which making?”). Focusing on mortars and plasters, the questions, there- a necessarily simplistic answer is usually provided. The question “in what proportions were the various compo- fore, concern: nents inserted and mixed?” may be misleading if we con- sider some non-computable variables, such as: – The sands (e.g. “were the sands prepared or used as they were?”; “were they washed to eliminate sodium a. The compositional variability that is certainly present in chlorides?”; “were they ground, sieved or mixed to obtain the desired granulometry and sorting?”) the different days of work; 1 3 Archaeol Anthropol Sci (2021) 13:193 Page 13 of 30 193 Fig. 3 (a) The Moroccan area including the archaeological site of Thamusida in Morocco. Thamusida corresponds to the present area of Sidi Ali ben Ahmed and is located along the left bank of the Sebou River (15 km north of the city of Kénitra and about 20 km north of the estuary. The widest supply area for calcarenite is that of Sidi Bouknadel (about 30 km SW as the crow flies). The two black dots above and below Bouknadel correspond to two quarries whose calcarenite was characterised for com- parison. (b) An open quarry of calcarenite at Bouk Nadel. (c) A wall of the archaeological site of Thamusida where calcarenite was used 1 3 193 Page 14 of 30 Archaeol Anthropol Sci (2021) 13:193 1 3 Table 2 Q&A on building materials provenance raised and provided during the development of the Thamusida research project (years 2003–2008) Questions Actions required Answers What kind of stone was used? Representative sampling of stones in Thamusida’s masonries Rocks macroscopically the same in the site. Samples distin- guished into quarry cuts (squared), roughly hewn stones (with a flat face) and stones used as they are (shapeless) Characterization through OM, SEM–EDS, XRD, XRD The samples consist of polygenic clasts, rounded in shape and constant granulometry (150 and 200 μm). The clasts are mainly of quartz, calcite, fragments of calcareous shells and lithic fragments (esp. sandstone with a clayey matrix). The rock used for stone masonry is a calcarenite (for details Gior- getti and Gliozzo 2009) Does calcarenite outcrop at the site? Short-range distance field campaign No So, where did they get it from? Geological study of the area and longer-range field campaign Calcarenite outcrops are common along a large part of the coastal strip (approximately corresponding to the yellow strip in Fig. 3). Numerous quarries are still active along National Route 1, especially in the area comprised between Salè and Sidi Bouknadel. Compared to the site, the most consistent and close outcrops are those of Sidi Bouknadel Is there any way to know exactly where the artisans took it Sampling campaign of un-worked rocks Samples were taken along the coast, in several sites north and from? south of the site Characterization through OM, SEM–EDS, XRD, XRD Set up of an ad hoc database of unworked calcarenites to be used for compositional comparison. The analyses showed both a great overall similarity between worked and un-worked samples and an intra-site compositional heterogeneity that prevented a more precise localisation (for details Giorgetti and Gliozzo 2009) Can we try to figure out how they transported it to the site, Knowledge on the modes of transport adopted in antiquity and The most likely path includes a first section of marine naviga- although we don't know the exact spot where it came GIS-based network analysis tion along the coast, moving from north or south to the Sebou from? River’s mouth. The second section of river navigation runs against the current, moving from the mouth to the site. In this second section, they can have used the (alaggio) hauling method, i.e. boats like the naves caudicariae pulled with ropes by animals or by men who proceeded on roads parallel to the course of the river, specially made for this purpose. GIS-based network analysis also informs that all possible land paths require much more time than the navigated ones So, why did they go to get that kind of stone off the site? Detailed knowledge of the geological setting of the territory Although this lithotype is not readily available in the surround- ings of Thamusida, it still represents the lithotype available at the shortest distance from the site. Therefore, calcarenite choice was still the cheapest one, in terms of time and effort to transport it to the site (compared for instance with limestones and basalts). Moreover, this is the most common type of stone in the area and, in fact, the same stone is also used in today's city of Rabat (e.g. for the Tour Hassan or the Kasbah des Oudayas), although its carbonatic composition and porous structure cause significant conservation issues (for details Zaouia et al. 2005) Archaeol Anthropol Sci (2021) 13:193 Page 15 of 30 193 1 3 Table 2 (continued) Questions Actions required Answers On the other hand, the bricks with what were they produced Representative sampling of bricks and tiles in Thamusida’s Collection of 67 bricks and tiles: 27 from the area of the Islamic th and where they come from? masonries and brick wastes from the ceramic kiln ceramic workshop (active from the 8 cen. AD onwards); 40 st rd from bricks and tile in the Roman settlement (1 -3 AD), mostly in situ Characterization of the ceramics through OM, SEM–EDS, Both Roman and Islamic bricks used local grey tirs (illitic clays) XRD, XRD as received, i.e. without manipulation or temper addition and fired them at 600–950 °C (for details Gliozzo et al. 2011) Short-range distance field campaign for the individuation and Two were the raw materials closely available for ceramic pro- sampling of clay outcrops duction: the grey and black tirs that are clay soils more (black) or less (grey) rich in humus (for details Arnoldus Huyzenveld 2008) Characterization of the clayey samples through OM, SEM– Set up of an ad hoc database of clayey raw materials to be used EDS, XRD, XRD for compositional comparison. Local ceramists selected the grey tirs and were particularly active in the earlier phase of Roman occupation (first–third centuries AD). Conversely, the later phase witnesses a contraction of production favouring imports which end up supplanting local production (for details Gliozzo et al. 2011) Epigraphic study of stamps on bricks Imports from Banasa (bricks stamped QAP and APP) and, likely, from the bay of Tangiers (bricks stamped CN and HADRIAVG) (for details Gliozzo et al. 2011) In conclusion, what was the operational criterion underlying Integration of all results achieved The best criterion, combining economy and functionality at the the construction of the masonry works of Thamusida? same time Abbreviations: OM optical microscopy, SEM–EDS scanning electron microscopy coupled with energy dispersive X-ray spectroscopy, XRD X-ray diffraction, XRF X-ray fluorescence 193 Page 16 of 30 Archaeol Anthropol Sci (2021) 13:193 b. A dismissal that led to the replacement of the person in The suitability and functionality of mortars charge with another worker who introduced a partially and plasters different recipe; c. A momentary shortage of one of the raw materials that These issues include several questions that may widely vary led to the mortar’s completion by not respecting the depending on the types of examined structure. original recipe, and other cases of this type, that are For example, assessing the hydraulic properties of mor- possible but not traceable. tars/plasters is a typical research question, especially when exposed to high moisture levels, such as thermal baths Furthermore, mortars and plasters are materials that tolerate and cisterns, or wet environments, such as harbours. The a wide compositional variability. By changing the ratio of com- questions that can be made in these cases are various, for ponents, various properties may change, such as setting time, example: “is the plaster suitable for the environment in durability or mechanical properties. Still, it is not certain that which the workers applied it?”; “is the composition of the the ancient workers understood a decrease in these properties mortar suitable for setting underwater?”; “is the plaster- as we are used to interpreting them today. The error of anach- ing of the cistern functional to water containment?”; and ronism—that is to evaluate the quality of a product based on “is the mortar/plaster suitable for the intended use?”. modern parameters—must always be kept in mind. In several Questions about the functionality of a mortar/plaster are cases, this distinction does not make much sense or is not drawn all the more complicated, the greater the number of vari- based on realistic or sensible parameters. ables to consider. Mortars and plasters must be regarded Another, often discouraging, phenomenon concerns the as systems in which each component plays a specific (but approach with ancient texts. For a long time, researchers possibly different) role, depending on the components with have debated whether and how the indications of Vitruvius— which it is mixed. Aerial lime-based mortars/plasters appear or other ancient architects—swere respected or neglected, as the most straightforward system, as they are reduced to giving rise to a series of interpretations of little use. The two components, i.e. the aggregate and the binder; how- text of Vitruvius has often been read with all the rigidity ever, even assuming an invariable binder’s composition, the of a black and white text, in order to deny the validity of result is not obvious. The aggregate may vary in terms of his teaching or to identify any detachments from the norm. composition, grain size, roundness and sorting and each of How ancient texts should be read is explained by Lancaster these properties corresponds to a different yield of the final (2021) in this TC. product (see, e.g. Grassl et al. 2010; Idiart et al. 2012). Nev- Operationally, archeometry can answer—with different ertheless, the binder’s composition may vary when the lime levels of approximation—the questions: “how much aggre- is obtained from impure limestones. In this case, the type gate, how much binder and how many additives (if any) and amount of impurities present in the raw material will were mixed to make this mortar/plaster?”, or “what is the determine the characteristics of the binder. ratio between the various components?” and the results If additives are added to a lime-based mortar/plaster, the relate to the individual sample analysed and not to the entire number of variables and, consequently, the complexity of the masonry work. To obtain a realistic estimate of the propor- system increase. Each type of additive will trigger specific tions adopted for the entire work, it is necessary to select reactions depending on the system and different character - and collect a stratigraphically appropriate and numerically istics will be achieved by the final mortar/plaster. representative collection of samples and to evaluate homoge- This reasoning serves to convey that the characterisation neity/heterogeneity induced by random and non-calculable of a mortar/plaster, as aerial or hydraulic, is only a first step factors, such as those described above. to defining its suitability and functionality. The latter proper - The characterisation of the components and the ties require the evaluation of numerous features such as, for evaluation of the relationship between them is a tool to example, shrinkage, workability, plasticity, stiffness, brit- evaluate changes made during the work or diversified tleness, hardening with high relative humidity, pozzolanic choices, based on the type or location of the structure activity, thermal conductivity, porosity, water permeability, under eximination. For example, if the ratio varies sig- adhesion, mechanical strength, tensile strength, compressive nificantly within the plasters of the same room, it may strength, moisture resistance and frost resistance. be possible to imagine a change of the workers or a Each of these parameters varies according to the number and series of renovations. If the ratio changes significantly type of variables that constitute the system. To give just a few between buildings with different use (e.g. dry/humid, examples related to the strength of a mortar, it is possible to list: internal/external environment), a deliberate diversifica- tion (i.e. to confer different properties to the materials) • Using natural hydraulic lime (NHL), high strength is becomes possible. obtained when a suitable grain size distribution of the aggregate is achieved; however, the strength decreases 1 3 Archaeol Anthropol Sci (2021) 13:193 Page 17 of 30 193 if the aggregates are siliceous, while it increases if the the introduction of fibres made these materials more suitable aggregates consist of limestone and/or the percentage of for use in seismic areas and therefore, in these cases, the binder also increases (Lanas et al. 2004); question may become: “are mortars and plasters suitable The compressive strength varies, for example, with the for that given structure, considering the seismicity of the variation of the binder's metakaolin content, while it area?”. remains almost unchanged if crushed brick particles are There are also cultural factors to consider when the struc- added (Nežerka et al. 2014). The addition of metakao- ture in question is large enough to include both environments lin can increase the compressive strength of lime-based open to external visitors and work environments open to plasters up to five times (Vejmelková et al. 2012b); owners and professionals only. In this case, a question may Using Mg-limes, the strength is comparable to that of concern the value given to suitability/functionality with natural feebly hydraulic lime, according to the param- respect to the owners’ self-representation (e.g. “is there a eters established by EN459-1. With respect to pure variation in the suitability/functionality of mortars/plas- calcium-lime, the flexural strength and the compressive ters based on the destination of the rooms?”). strength can be 1.75 and 2.4 times higher (Pavia et al. Undoubtedly, this case is closely linked to the use of deco- 2005; Chever et al. 2010); rations and other questions may arise on the suitability/func- The strength is low in pure lime matrix pastes, due to tionality of the plasterwork for paintings (e.g. “is the plaster the formation of cracks around the aggregate; therefore, suitable / functional to the realisation of wall painting?”). the characteristics of the aggregate becomes crucial Furthermore, from a diachronic perspective, the recon- (Nežerka et al. 2014); struction of the recipe may also be tackled in chronological The mechanical strength can also be increased through terms and, thus, the questions become “how have the reci- the introduction of casein or animal glue (Ventolà et al. pes evolved?”; “from the beginning of pyrotechnology in 2011) or even sticky rice soup (Yang et al. 2009, 2016). the distant Epi-Paleolithic Geometric Kebaran (Gourdin and Kingery 1975; Kingery et al. 1988) is it possible to This is an example related to a single property, the observe radical changes, progressive adaptations or con- strength, but highlights how suitability and functionality servative choices?”. depend not only on multiple variables but also on the char- Hence, the various questions that can be raised concern a acteristics of each component. whole series of how, when and why, the solution of which To make just one example of the suitability/functionality helps us to understand better a wide range of aspects related of historic mortars, Singh and Kumar (2020) investigated a to the cost-effectiveness of the work, the technical skills of series of plasters from the late fifteenth century Mughal’s the workers, the priorities and the variable tastes of clients summer palace of Farah Bagh (India). Although the most bound to the fashions of the time and the relationship between striking feature of this monument is perhaps that of being the intended use of a structure and its functional performance. equipped with a complex system of air cooling, the equally The archaeometric answers that can be obtained for these complex preparation and application of different types and questions are often decisive, thanks to the numerous experi- layers of plasters are indicative of an advanced technology, mental works that have addressed the technical characteris- aimed at their suitability and durability during processing tics of ancient and modern mortars/plasters. and use in an environment that is repeatedly subjected to Essential for this topic are the works carried out by Baro- moisture. The workers obtained flexibility, strength and nio and Binda (1997) and Böke et al. (2006) on the hydrau- permeability by adding several types of inclusions, ranging licity of different types of bricks and clays for cocciopesto. from jute fibres and dry paddy stems to grinded ceramics These studies led to clarification of the conditions necessary and basaltic rocks. for the reaction to occur and the ceramic material to be con- Of a very different nature are cases in which the hydrau- figured as pozzolanic: licity of a mortar/plaster is involuntary or not functional for any purpose (e.g. in some residential rooms in predomi- a. Ceramic firing must not exceed 900 °C; otherwise min- nantly dry climates). The question thus becomes “why was a erals destabilization can lead to the formation of new hydraulic mortar used when it was not necessary?” and/ thermodynamically stable phases (Gliozzo 2020b); or “did they really want to hydraulicise the mortar, or b. The temperatures typically range between 450 and recycle materials already available, resulting, for exam- 800 °C according to the type of clay minerals (He et al. ple, from demolition of old structures?”. 1995); This is a more common situation than imagined and clari- c. Ceramic fragments must be finely ground; fies how a mortar/plaster’s hydraulicity is a characteristic to d. Not all clays fired at low temperatures develop pozzo- be interpreted with caution and above all else, not individu- lanic properties (e.g. when they have a low percentage ally. For example, we know that some precautions such as of the clayey fraction). 1 3 193 Page 18 of 30 Archaeol Anthropol Sci (2021) 13:193 Numerous outstanding insights on colour variations, water- from industrial research (therefore, not related to archeom- proofing and mechanical properties that can be conferred to a etry or restoration). For example, some experimental pro- mortar/plaster through the addition of specific additives have also grams aimed at the development of commercial materials been gained through experimental reproductions. For example: may provide useful information for a better understanding of the properties of ancient mortars and plasters (e.g. Grist – Centauro et al. (2017) carried out a series of ageing tests et al. 2013; Ergenç et al. 2018). adding linseed oil, brown sugar and cow’s milk; – Nunes et al. (2018) investigated the microstructure and Particular cases composition of lime and metakaolin pastes with linseed oil added and aged for 68 months; The study of mortars and plasters may also present particular – Zendri et al. (2004) reproduced “cocciopesto” mortars, in suitability/functionality cases depending on the archaeologi- order to investigate the reactions responsible for confer- cal context or find in which they are found. ring hydraulic properties to these products; For example, the archaeometric investigations made on – Nežerka et al. (2014) compared the pozzolanic activity the wall plasters of a round installation (Feature 6), found of metakaolin and crushed bricks; at the Late Natufian village of Nahal Ein Gev II (NEG II, – Işikdaǧ and Topçu (2013) compared the mechanical ca. 12,000 cal BP) helped to shed light on the likely use of strength of mortars, including tile powder, crushed tile, that structure as one of the very first storage installations lime and granulate blast furnace slag; (Grosman et al. 2020). Firstly, the authors reconstructed – According to ancient recipes, Salavessa et  al. (2013) the morphology of the installation as a plastered domed compared the compressive and flexural strength of plas- pit. Then, they characterised the walls as an alternation of ters prepared with either waste marble or limestone dust; mud and lime plastering (the latter was absent in other areas – Vejmelková et al. (2012b) investigated the mechanical of the site). The heating experiments verified that NEG II and fracture-mechanical properties, hydric parameters, inhabitants did not use the installation for pyrotechnological durability characteristics and thermal properties of sev- processes and the filling was characterised as waste dumped eral lime–pozzolana composites, using a particular clay inside after its original use. By integrating all the various shale that is available in the Czech Republic that may information, the authors were finally able to identify the use have been the natural substitute of volcanic pozzolanas of “Feature 6” installation for cereals and legumes storage. (absent in Central Europe); The discovery is of outstanding importance, as it testified – Yang et al. (2009) and Yang et al. (2016) reproduced the the practice of (short-term) storage just before the Neolithic mechanism of solidification of sticky rice mortar; and subsequent expansion of agriculture. In this case, how- – Ventolà et al. (2011) tested different types of non-hydrau- ever, the research question appears somehow reverted: “can lic lime mortars, adding animal glue, casein, nopal and the plaster provide information about the function of the olive oil to develop new compatible products for repairs; structure it was associated with?”. – Lima et al. (2020) investigated the effects of clay min- Another rather frequent case concerns the possibly con- eralogy on drying behaviour, pore size distribution, textual use of lime for other purposes. The lime produced for mechanical strength, vapour adsorption, desorption masonry works could have been used in tobacco processing capacity, water capillary absorption, linear dry shrinkage, (see, e.g. Villaseñor and Graham 2010), in maize processing cracking and thermal conductivity. They also verified the (see, e.g. Katz et al. 1974), in waterproofing amphorae (see, greater suitability of illitic clayish earths compared with e.g. Dorrego et al. 2004) and boats, in glass production and montmorillonitic or kaolinitic ones. tanning (Foy et al. 2000; Heth 2015), not to mention that calcium oxide was also a thrown weapon (a smokescreen From these studies, it becomes clear that the sampling irritating to the eyes similar to modern smoke bombs or pep- of archaeological finds and comparative geological mate- per stinging sprays) in battles against opponents. Taking a rials may not be sufficient and research must be open to look at other materials found at the same time, for example, experimentation to validate/discard the working hypotheses. in an excavation, may therefore raise new questions (e.g. In practice, this procedure leads to an increase in samples, “what was the organization of lime production in light analyses and time, but it certainly offers the necessary tools of the many possible uses of this material?”) and provide for correct and meaningful interpretations. interesting information on the versatility of a single material. Indeed, not all studies require an additional experimental procedure. Knowledge of the properties conferred by the dif- Mortars and plasters dating ferent constituents is already largely known (see also Moro- poulou et al. 2005) and must be deeply known in advance. Mortars dating, including radiocarbon and optically stimu- Furthermore, it is also possible to gather useful information lated luminescence (OSL) dating, has been comprehensively 1 3 Archaeol Anthropol Sci (2021) 13:193 Page 19 of 30 193 addressed in the paper by Ergenç et al. (2021) in this TC. We workable” involved the mechanical separation of the binder will, therefore, limit ourselves to discussing some problem- from the aggregate (Folk and Valastro 1976). atic aspects related to the radiocarbon technique on which The difficulty in obtaining reliable dating of well-dated numerous publications are recently flourishing. samples have held back the application of this technique In principle, the method is straightforward. It is based (Malone et al. 1980; Zouridakis et al. 1987; Heinemeier on the extraction and measurement of pyrogenic C aCO , et al. 2010; see also Van Strydonck et al. 2011, 2015 and formed after the incorporation (fixation) of atmospheric De Mulder et al. 2014 on lime burials) but did not stop its CO (which contains C) by slaked lime (Ca(OH) ) experimentation and consequent development (Van Stry- 2 2 (Labeyrie and Delibrias 1964). donck et al. 1982, 1983, 1986, 1989, 1992; Pachiaudi et al. The binder’s age corresponds to that of the mortar’s 1986; Hajdas et al. 2017; Hayen et al. 2017). hardening, and, consequently, its C date corresponds to Sampling and analytical problems are well known to date that of the mortar from which the sample was extracted. (Boaretto and Poduska 2013; Ringbom et al. 2014; Toffolo However, this “simple” concept brings with it several et  al. 2020; Urbanová et al. 2020) and partially obviated practical complications, essentially due to the very indi- through the preliminary analysis of the samples. On the viduation and extraction of appropriate samples from the other hand, the development of accelerator mass spectrom- carbonaceous binder. etry (AMS) has made it possible to step forward further, Calcite may have different origins, not exclusively related thanks to reducing the sample required for the analysis (less to the hardening process; therefore, the possible “contami- than 1 mg). nants” are many: Several treatments and analyses are required both for the selection and measurement of the samples and the valida- a. Geogenic calcite, i.e. calcite naturally present in the tion of the proposed date. In the first category, it is possible aggregate, introduced during mixing the aggregate with to list: the binder; b. Calcite contained in the ashes; – Dry/wet sieving and purification treatments, to separate c. Unreacted calcite, i.e. limestone not completely trans- specific fractions and eliminate part of the contaminants formed during the lime production cycle; (Addis et al. 2019; Michalska 2019; Ricci et al. 2019); d. Lime lumps, i.e. lime that did not react to form calcite – Density separation, to separate components, based on after hardening of the mortar; their specific gravity (see, e.g. Moropoulou et al. 1995; e. Secondary calcite, i.e. calcite formed by recrystallisation Toffolo et al. 2017b); beginning from the mortar’s use up to the present day. – Optical and scanning electron microscopy, to identify the different components of the mortar (authors are too The whole range of contaminants can move the date far numerous in this case because this represents a quasi- back, as it happens with geogenic calcite (i.e. dating refers to mandatory research step); the age of formation of the carbonates present in the sand), – Cathodoluminescence, to identify the different genera- or far forward, as it happens with secondary calcite. tions of carbonates (see, e.g. Heinemeier et al. 1997; This problem became immediately evident. Stuiver Lindroos et al. 2007; Murakami et al. 2013; Toffolo et al. and Smith (1965) highlighted chronological discrepan- 2019b): cies related to C dilution due to residues of limestones – Fourier transform infrared spectroscopy (FTIR), to indi- or calcareous sands used to prepare CaO (Baxter and Wal- viduate geogenic, biogenic and pyrogenic calcites and ton 1970). Moreover, the method was initially presented as thus, avoid samples showing extensive recrystallisa- unsuitable for hydraulic mortars whose solidification, in tion (see, e.g. Anastasiou et al. 2006; Regev et al. 2010; the presence of water, led to the formation of calcium sili- Poduska et al. 2011, 2012; Toffolo et al. 2019a); cates and aluminates (Delibrias and Labeyrie 1965). Subse- – X-ray diffraction (XRD), to obtain a qualitative and quently, other researchers have confirmed that this difficulty quantitative measurement of both crystalline phases and is mainly due to the poor permeability to atmospheric CO , amorphous fraction. This technique is particularly advan- the enduring reactivity of this type of mortar and the pres- tageous to investigate the possible presence of secondary ence of lime lumps or phases formed from hydraulic reac- phases, especially those occurring in hydraulic mortars tions, such as layered double hydroxides (Artioli et al. 2017; such as the double-layer hydroxide (LDH) minerals (i.e. Toffolo et al. 2020). typical products of the pozzolanic reaction; see, e.g. Hence, it soon became clear that these carbonatic resi- Artioli et al. 2017; Ponce-Antón et al. 2018); however, dues had to be eliminated before proceeding with the analy- this task can also be aided by the following techniques; sis and ensuing attempts “to refine the technique to make it – Solid state nuclear magnetic resonance (SS-MAS-NMR), to identify traces of LDH phases (Richardson et  al. 1 3 193 Page 20 of 30 Archaeol Anthropol Sci (2021) 13:193 2010), whose contents are below the detection limits of In conclusion, despite all the advances made, the method the XRD; is very laborious and frequently inconclusive. It is not yet – Differential thermal analysis and thermogravimetric able to provide a date that unquestionably relates to the analysis (DTA-TGA), to study the thermal decomposition mortar under investigation; consequently, a series of age of the various types of carbonates, based on the experi- control investigations are made for comparison with other mental evidence that lower temperatures are required to techniques, which may, however present other types of prob- decompose pyrogenic CaCO than geogenic CaCO (see, lems. In the second category, it is possible to list: 3 3 e.g. Moropoulou et al. 1995; Anastasiou et al. 2006; Tof- folo et al. 2017b); – TL and OSL, to compare mortar dating with that obtained – Sequential dissolution, to provide representative C O on silica-based fragments such as those included in coc- samples of the binder, excluding contaminants (Lindroos ciopesto or, specifically, to date their last exposure to et al. 2007; Ringbom et al. 2014). This is a fundamental light before being put “in the dark” inside a mortar (OSL preparation stage of the AMS dating technique (Tubbs applications in Moropoulou et al. 2018 and Ergenç et al. and Kinder 1990; Hale et al. 2003; Regev et al. 2017), 2021); chiefly because the sequentiality of carbonates dissolu- – Dendrochronology and palaeoenvironmental studies, to tion (secondary carbonates → pyrogenic CaCO → geo- compare mortar dating with that obtained on other con- genic aggregates) allows for C O isolation during textual materials; hydrolysis. The times with which the sequential dissolu- – Archaeological stratigraphy for a relative frame. tion takes place are not known with certainty. Therefore, further investigations are needed to verify the reliability Despite all these limitations, researchers are certainly of the selected sample and, consequently, of the proposed not discouraged. Future research will surely solve the raised dating. In an attempt to solve this problem, Toffolo et al. issues; however, it was necessary to describe the criticalities (2017a) and Toffolo and Boaretto ( 2014) started from of the method to not misguide researchers on chronological the observations made on aragonite and developed a issues. new procedure involving the thermal decomposition (at 500 °C) of pyrogenic calcite. Mortars and plasters alteration and degradation Among all these methods, there is no one that, taken The alteration and degradation of stone materials, mortars individually, guarantees a correct selection of the sample. and plasters represent a critical issue in the field of restora- The combined use of two or more techniques is always tion and conservation of built heritage (Price and Doehne necessary, even if not necessarily decisive. As clearly 2011). Alteration and degradation are often used as syno- demonstrated by experimental and cross-laboratory test- nyms, but they are not. The term alteration refers to a phys- ing, different or analogous treatments can yield very dif- ico-chemical change that generally occurs on the surfaces, ferent dates (Hayen et al. 2017; Michalska 2019). In the for example, a colour change. It can be seen, but hardly can former study (Hayen et al. 2017), a detailed compositional it be linked to something that threatens the integrity and the analysis of the samples was preliminarily performed and function of the stone. On the contrary, the degradation of the same four mortars were investigated using different a mortar/plaster represents something that must be taken techniques and laboratories. The dates obtained on dif- into account to preserve the material itself. In other words, ferent parts of the sample and/or with different analytical degradation implies a physico-chemical modification of the techniques varied by many centuries between them and materials, which leads to a worsening of its properties and, if there is no possibility of establishing which is the “right” it has a great entity, a restoration/conservation intervention is one, except with a combined investigation of a numeri- needed. It is important to establish a common language and cally high and compositionally heterogeneous collection let the researchers and stakeholders understand each other. of samples (e.g. lime lumps, charcoal fragments, specific For this reason, national and international organizations grain fractions, etc.). In the latter study, Michalska (2019) have set up common glossaries to give the same terminol- compared the C results obtained for 37 mortars with ogy to alteration and degradation forms. One of the most different compositions and enucleated three groups: (1) used is the glossary issued by ICOMOS (Vergès-Belmin air-hardening and slightly hydraulic mortars that can be 2008). Chemical, physical and biological agents can cause successfully dated, (2) highly hydraulic mortars affected the alteration of mortars and plasters; however, it is common by rejuvenation effects and (3) mortars with very high con- that an alteration/degradation pattern is due to a combination tent of ageing components, for which “obtaining the true of the abovementioned agents. age of mortar production is not possible with the current The characterisation of the alteration and degrada- state of knowledge”. tion forms, both on a macroscopic and microscopic level, 1 3 Archaeol Anthropol Sci (2021) 13:193 Page 21 of 30 193 requires a multidisciplinary approach since chemical, min- from each other. On the other hand, the great compositional eralogical, and biological skills are needed (La Russa and homogeneity of certain other pigments, such as cinnabar, Ruffolo 2021). The interventions are planned based on the makes it difficult to discriminate between geographically materials’ conservation state (Caroselli et al. 2021). That is close resources. In practice, it all depends on the composi- why alteration/degradation analyses should be carried out tion and/or the information we have available for a compo- before restoration. sitional comparison (see the specific contributions, pigment When we deal with archaeometric analysis of mortar and by pigment, of this TC). plaster samples, a particular attention has to be paid to their degradation since it can induce wrong interpretations. For The pictorial technique example, when the dating of mortars is performed on degraded samples, it is easy to observe a rejuvenation of the C age Understanding “how was the pigment applied?” and/or because of the generation of secondary carbonates (Michalska “how was the surface prepared?” means investigating the 2019). Generally speaking, the degradation of mortars induces link between the pigment and its support. Each technique a change in terms of elemental and mineralogical composi- corresponds to different archaeometric evidence that must be tion, as well as microstructural (i.e. porosity, binder-aggregate investigated and unravelled (see Salvadori and Sbrolli 2021 ratio), which must be taken into account when approaching an and Murat 2021 in this TC). archaeometric study based on such parameters. For example, some pigments, such as ochers, lend them- selves to being used with the a fresco technique, while for C. The characterisation of pigments others, such as those based on lead, a secco technique is preferred. Therefore, investigating the painting technique The first questions concern pigments identification (“what means investigating the palette and technique of the painter pigment is it?”) and nature (“is it a natural or synthetic and verifying the appropriateness and effectiveness of some pigment?”). techniques compared to others. Most of the time, answering the first question also pro- Moreover, we know that some pigments, such as those vides the answer to the second question. For example, if based on arsenic, are particularly sensitive to light and the answer is “Egyptian blue” or “lime white”, we already sometimes obliged to adopt particular techniques for their know from previous studies that these compounds were arti- application. Therefore, the information we obtain from the ficially produced (see Gliozzo and Burgio 2021; Gliozzo painting technique analysis provides useful information for and Ionescu 2021; Cavallo and Riccardi forthcoming, and a correct display of the artefact and its conservation. Švarcová et al. 2021 in this TC for futher examples). Finally, the information that we can obtain from the study Conversely, if the answer is “yellow ochre” or “green of a single artefact or artwork represents a piece of the wider earth”, the natural origin will be straightforwardly deter- mosaic that describes the evolution of painting techniques mined (see Mastrotheodoros et al. forthcoming in this TC). and is, therefore, of value in the broad field of art history. On the other hand, however, even pigments of natural ori- gin, such as ochers, could be processed to vary their colour Linking pigments with the social status or consistency. For example, it is well known that goethite- of the clients based yellow ochres could turn to hematite-red upon heating. Still another case is presented by arsenic-based pigments Up to this point, the questions posed to pigments are all for which, after a certain period, both natural and artificial technical. As we have seen, the answer we can get depends compounds of realgar and orpiment were in use (Gliozzo largely on the composition of the pigment itself and the qual- and Burgio 2021 in this TC). ity of the database available for comparison. Nevertheless, Therefore, if the answer to the second question is “natu- there are also other questions related to the use of specific ral”, we should proceed further with the other research ques- pigments that go beyond technological choices. For example, tions. On the contrary, if the answer is “processed” or “arti- “is it possible to establish whether the type of pigment ficial”, the next question will be aimed at investigating “how used corresponds to a desire for representation?” or, in was the pigment processed and produced”. other words, “can the low/high cost of a pigment faithfully reflect the social status of the clients?”. The provenance of minerals and pigments The use of cinnabar instead of ocher, for example, has often been referred to as a desire for social self-represen- The provenance study is often very complicated and not tation on the part of the clients rather than to a different— always conclusive. On the one hand, the great heteroge- more vivid and brilliant—rendering of cinnabar compared neity of widespread raw materials, such as ochers, makes to ocher. Similar assumptions have been made regarding the it difficult to distinguish supply areas that are very distant 1 3 193 Page 22 of 30 Archaeol Anthropol Sci (2021) 13:193 use of Egyptian blue and lapis lazuli, although there were no A sampling that complies with the representativeness other choices in the ancient world with which to obtain blue. criterion is first and foremost guided by the definition of the To avoid possible anachronism, the study of the relation- construction techniques and the stratigraphic sequence. The ship between pigment and client may help clarify if pig- parameters to be evaluated are many and the resulting sam- ments were used as a demagogic and self-representative tool pling must thus be representative of (1) the types and charac- and cast an eye on the tastes of the time. teristics of all construction materials (e.g. lithotypes/bricks and mortars), (2) the way they were assembled, i.e. the con- struction techniques and (3) the construction phases in which they were employed as delineated by the stratigraphy. Sampling criteria and methods In the first case, a preliminary evaluation is essential to identify macroscopic similarities/differences between In the cases that are addressed below, the sampling aims building materials. A range of simple tests that can be per- to answer the questions that we have analysed in the previ- formed in the field (e.g. the effervescence in dilute hydro- ous chapters; however, the most important aspect to keep in chloric acid) and the use of a magnifying lens may help this mind concerns contextualisation. first phase. A preliminary catalogue of the mortars is also Mortars, plasters and pigments constitute a complex essential. It should take into account the broadest range of system of structural and aesthetic importance that visible characteristics, such as colour, texture, clasts types, must be contextualised within the various phases of a dimensions and distribution, state of aggregation, and the historic building. Attributing a specific context to the possible presence of cracks and traces of weathering. These various components is essential for correct sampling characteristics, combined with the parameters used for the for archaeometric purposes. To this end, it is necessary definition of construction techniques (e.g. structure, equip- to apply the method of “reading” historical buildings ment, size of elements, percentage of pieces placed at the established precisely by the BA to correct extrapolation tip or end, size distribution of segments, the possible pres- of the components according to their space–time ence of plasters), provide the framework on which to set the dimension. selection of representative samples based on the first two As anticipated above, we must also consider that the parameters. research in progress may raise new questions not stated from In the third case, the stratigraphic sequence may suggest the beginning. For these reasons, the sampling and, above duplicating the selection of some types of materials or con- all, the quantity of material to be taken should always be struction techniques, in order to evaluate any changes or per- slightly greater than what we establish at the table, as long sistence on a diachronic basis. For example, the acquisition as the conservation requirements are respected. of two samples of the same lithotype from structures dated to chronologically different periods could demonstrate slight How to obtain a representative sample set variations in raw materials supply. of mortars and plasters Finally, the criterion of representativeness must guide a selection that accounts for all types of materials used, with The first assumption is that there is no arbitrary number of all construction techniques, for each period considered. samples that can be considered representative tout court. A sampling that conforms to the functionality criterion Although the most obvious and frequent question is “how is guided by the research questions. The representativeness many samples are needed?”, not only is there no straight- criterion has previously provided useful samples to answer forward answer but it is worth bypassing this question with some questions such as “what materials were used for con- another type of question: “what samples are needed and structing the walls?” but it cannot be considered sufficient. from where should they be taken?”. If, for example, we want to know where the workers took The main characteristics of sampling must be. the raw materials from, it is frequently necessary to expand the sampling outside the building under investigation. The a. Representativeness, which is given by stratigraphic, investigation moves to the surrounding territory to an extent typological and chronological criteria; that the researcher can establish, based on geological maps b. Functionality, given by sample collection’s intrinsic and historical information. Since a detailed characterisation ability to answer established research questions (e.g. of the lithotypes is required to focus the search for supply production technology, provenance, alteration and deg- basins, it is also foreseeable that sampling will be a multi- radation); step activity conducted on several occasions. c. Suitability, which concerns the suitability of the material For other types of questions, such as those aimed at recon- sample to be investigated by the analytical techniques structing the production technology of mortars, the “repre- (Gliozzo 2020a, Fig. 4). sentative sampling” already considers numerous features 1 3 Archaeol Anthropol Sci (2021) 13:193 Page 23 of 30 193 derived from the production processes and their eventual quick, cheap and non-destructive data that cannot pro- diachronic evolution. Still, only laboratory analyses can vide unquestionable answers and, on the other hand, data decide if the sampled collection is sufficient, or an expan- inaccuracy can lead to a waste of time and money. sion is necessary. Furthermore, evaluating the production Therefore, the choice of analytical techniques should processes and the mortars’ physical–mechanical character- first find a balance between the accuracy requirements istics may require a series of experimental reproductions for of quality research and the conservation needs and then which further sampling of raw materials may be necessary. guide sampling to the selection of suitable samples. In Similarly, it may be possible to answer questions relat- this regard and based on what has been said previously, ing to alteration and degradation through the set of samples it is good to underscore that sampling does not necessar- obtained with the first criterion. In these cases, however, it is ily involve the physical taking of material. In the case of more frequent to carry out targeted sampling. In fact, while non-destructive techniques, sampling may simply address unaltered samples are preferred for the characterisation of the selection of the analysis spots. building materials, altered specimens are also required to To conclude, while before proceeding with sampling, evaluate their conservation state. it is necessary to collect all available data from any field As far as the dating question is concerned, it was previ- of study; during sampling, it is mandatory to properly ously explained why the sampling issue is crucial for radiocar- record and document all phases and, after sampling, it bon dating and why it may be necessary to carry out multiple is required to remain open and f lexible to answer new samplings before obtaining a suitable sample for analysis. questions that may arise. Before deciding and after tak- In summary, while the “representative sampling” may ing the samples, the overall research should record the already provide the necessary material to answer some various actions and use a critical survey. This survey research questions, the functionality criterion aims precisely should be equipped with specific signs that provide a to test the completeness of the overall sampling from the cartographic mapping functional to sampling. Regard- point of view of research questions. less of the use of new technologies for the graphic rep- The research methodology guides sampling compliant resentation of historical buildings, we should create with the third criterion, i.e. the suitability. In addition to standardised protocols to record sampling through, for the example recalled above on selecting suitable samples example, a precision topographic positioning. for radiocarbon dating, another clarifying example con- cerns visibly altered mortars. By adding all the three cri- How to obtain a representative sample set teria together, the sample must be (a) representative of of pigments both the original composition and the altered composition; (b) functional to solve research questions, such as those A sampling of pigments is carried out primarily based on relating to the recipe and those relating to the evaluation colours. In this way, by counting the number of colours vis- of degradation; and, finally, (c) suitable for the analytical ible in the painting, the number of samples to be investi- techniques that the researcher will use to investigate it. gated is also obtained. In large-scale paintings such as wall In this case, it is well known that (1) the thickness of paintings, the sampling of a single sample per colour may the surface alteration layer is variable, (2) the preparation prove insufficient. Therefore, it will be necessary to carry out of a thin section requires a small volume but a suitable multiple samplings for each colour to evaluate any changes. surface and (3) an accurate chemical analysis may require This reasoning is valid both in the case of natural and arti- a variable volume depending on the chosen technique. ficial pigments, as it provides the possibility of identifying Therefore, the sampling methods will be crucial to obtain a different technologies and/or provenances. suitable sample while observing the fundamental principle In complex structures with several decoration phases, the of conservation, or in other terms, of minimum sampling. stratigraphic sequence of superimposed layers provides the Archaeometric analyses can move along the path of tool for guiding sampling. non-destructiveness or destructiveness. Between these When the analysis involves non-destructive instrumenta- two end-members, different levels of destructivity tion, it is undoubtedly desirable to consider a large increase and accuracy can be achieved (e.g. highly destructive, in the spots to be analysed. When, on the other hand, the minimally-destructive). Provided the same quality of research question requires the use of destructive investiga- the results obtained, the most obvious approach would tions, the number of samples will adapt to the minimum nec- be that of non-destructiveness; however, it should be essary and will possibly be taken in the least exposed portion. emphasised that, often, non-destructive techniques can- For the study of the painting technique, there is an addi- not provide sufficiently accurate or comparable results tional difficulty, as the analysis needs to go beyond the pic- to those obtained with destructive methods. Therefore, torial film, in order to reach the preparatory layer. In this on the one hand, the researcher risks collecting a lot of case, the researcher may choose either to analyze the gaps or 1 3 193 Page 24 of 30 Archaeol Anthropol Sci (2021) 13:193 take a sample that shows the cross-section. The first method on the workers and the construction site's organisation does not necessarily damage the artefact but is not always (“cantiere”). resolutive, while the second has the advantage of ensuring The integration with other historical information on eco- an exhaustive response but is destructive. The researcher nomic trades sheds light on the commercial dynamics and on must, therefore, choose the first or second method based on the routes of trade, as well as on the status of the clients and conservation needs. A third option is to be considered when the functional destination of some buildings. the analysis concerns mobile artwork and consists of instru- Finally, the integration with technological knowledge can add ments with a higher penetration capacity than conventional useful pieces of information to reconstruct the history of technolo- techniques, such as Raman or portable X-ray fluorescence. gies, in a diachronically and geographically wide perspective. In any case, while it is not acceptable that the availabil- The main assumption that derives from these examples must ity/absence of specific instrumentation or funds decides the be that of a plurality of questions that follow one another, rather analytical protocol, the diversity offered by the artworks and than a rigid and pre-set list of questions. This last method is their needs (wall painting/mobile artefact, alteration issues functional to the start of the research but must then open up and conservation needs, the impossibility of transferring the to the unexpected how, when and why that arise during the find to the laboratory, etc.) always guide the selection of the advances/progress of the research. most suitable sampling and analytical techniques. Consequently, sampling must remain flexible and, subject to conservation requirements, must answer both pre-set and unexpected questions. Mortar sampling should be guided by Concluding summary of key concepts three main parameters: representativeness, functionality and suitability. Pigment's sampling should be based on colours and, The archaeometric study of mortars, plasters and pigments when applicable, guided by stratigraphy. provides information about: Non-destructive methodologies are certainly an advantage in the study of pigments. Still, they cannot always provide – The origin of the raw materials used as aggregate, addi- sufficiently accurate measurements and, being superficial tives or pigments or as raw material for the production analyses, they can mislead due to surface alterations. On of lime; the other hand, destructive methodologies are almost always – The production technology of lime, mortars, plasters and preferable (if not indispensable) in the study of mortars and pigments as well as the painting technique; plasters. A reasoned sampling can minimise the damage. – The state of alteration and/or degradation of both build- Finally, three important aspects should be born in mind. First, ings and paintings, as well as the yield of products for the results obtained on a single sample or a poorly selected sam- restoration; ple collection cannot be extended to entire rooms or buildings. – The chronology of some components of the mortar with Secondly, mortars/plasters that are compositionally different levels of likelihood depending on whether they very different from each other may show comparable are siliceous materials, datable through TL-OSL (relatively performance, at least adopting ancient standards. We accurate results), or carbonaceous, datable through radio- should avoid anachronisms putting ourselves in the shoes carbon (results not trustable as received but to be verified). of people who do not necessarily think with our param- eters. Leaving aside qualitative evaluations (that we can- From the first series of questions addressing the “study’s not estimate except with current criteria) is likely the best object” directly, the integration of laboratory results with the approach in most cases. geological knowledge of the territory provides information about: Thirdly, there is an unquantifiable level of randomness that we cannot in any way reconstruct; therefore, we must – The exploitation of the territory itself over the centuries be careful in not falling into attractive overinterpretations. or in a specific period; Some choices or variations of the “products” we analyse – The relationship between knowledge of the raw materials should not necessarily be interpreted as a decline in quality available and their exploitation for production purposes; or an evolution of the technique. They may (simply) repre- – The tools to understand better certain technical choices sent the best choice at that given moment. relating to the use of certain materials compared to oth- ers. Authors’ contributions Building Archaeology by AP; Mortars and plas- ters alteration and degradation by MFLR; Concluding summary of key The integration with the stratigraphy allows the observa- concepts AP-MFLR-EG; other contents by EG. English professional tion of changes during the same phase and/or specify the revision provided by MFLR. chronological seriation and/or provide indirect information 1 3 Archaeol Anthropol Sci (2021) 13:193 Page 25 of 30 193 Code availability Not applicable Cementitious materials: Composition, properties, application. De Gruyter, Berlin, pp 147–158 Ashurst J, Ashurst N (1988) Practical Building Conservation 3: Mor- Declarations tars, Plasters and Renders. (English Heritage technical hand- book). 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Journal

Archaeological and Anthropological SciencesSpringer Journals

Published: Nov 1, 2021

Keywords: Mortars and plasters; Pigment analysis; Building archaeology; Archaeometry and archaeology; Research questions; Sampling criteria

References