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Production, transport and on-site organisation of Roman mortars and plasters

Production, transport and on-site organisation of Roman mortars and plasters This paper examines the role of mortars and plasters in the construction process during the Roman period and seeks to elucidate the chaîne opératoire from the production of the main ingredients — lime and aggregate — to their application in structures, based on archaeological, visual and archaeometric data. As well as looking at the actual processes involved, it also considers the functional requirements of the mortars and plasters and the economic implications of their use, especially the nature and cost of transport, which may have led to particular choices being made by Roman builders; it also considers the supply of materials in terms of the logistics of construction. The emphasis is on mortared rubble construction, which was a particular development of the Roman period from the second century BCE and required much larger quantities of lime than previous building techniques. Attention is also paid to the human actions involved and the tools employed. Keywords Lime · Aggregate · Mortar · Roman construction · Chaîne opératoire Premise The second group of contributions is focused on pig- ments, starting from a philological essay on terminology This Topical Collection (TC) covers several topics in the (Becker 2021). Three archaeological reviews on prehistoric field of study, in which ancient architecture, art history, (Domingo Sanz and Chieli 2021), Roman (Salvadori and archaeology and material analyses intersect. The chosen Sbrolli 2021) and Mediaeval (Murat 2021) wall paintings perspective is that of a multidisciplinary scenario, capable clarify the archaeological and historical/cultural frame- of combining, integrating and solving the research issues work. A series of archaeometric reviews illustrate the state raised by the study of mortars, plasters and pigments (Gli- of the art of the studies carried out on Fe-based red, yellow ozzo et al. 2021). and brown ochres (Mastrotheodoros et al. 2021); Cu-based The first group of contributions explains how mortars greens and blues (Švarcová et al. 2021); As-based yellows have been made and used through the ages (Arizzi and and reds (Gliozzo and Burgio 2021); Pb-based whites, reds, Cultrone 2021; Ergenç et al. 2021; Lancaster 2021; Vitti yellows and oranges (Gliozzo and Ionescu 2021); Hg-based 2021). An insight into their production, transport and on-site red and white (Gliozzo 2021) and organic pigments (Aceto organisation is further provided by this paper. Furthermore, 2021). An overview of the use of inks, pigments and dyes several issues concerning the degradation and conservation in manuscripts, their scientific examination and analysis of mortars and plasters are addressed from practical and protocol (Burgio 2021) as well as an overview of  glass- technical standpoints (La Russa and Ruffolo 2021; Caro- based pigments (Cavallo and Riccardi 2021) are also pre- selli et al. 2021). sented. Furthermore, two papers on cosmetic (Pérez-Aran- tegui 2021) and bioactive (antibacterial) pigments (Knapp et al. 2021) provide insights into the variety and different uses of these materials. This article is part of the Topical Collection on Mortars, plasters and pigments: Research questions and answers * Janet DeLaine janet.delaine@classics.ox.ac.uk Wolfson College, University of Oxford, Oxford, UK Vol.:(0123456789) 1 3 195 Page 2 of 17 Archaeol Anthropol Sci (2021) 13:195 at a microscopic level (e.g. Jackson et al. 2007; Marra and Introduction D’Ambrosio 2013; Wehby Murgatroyd 2016; Columbu et al. 2019; Coutelas 2019; Dilaria et al. 2019), all of which have Since mortars and plasters are in themselves composite implications for the construction process and the relation materials, and mortar itself is an essential part of another between the specific recipes and the function of the mortar composite material, their role in the construction process is or plaster. These have been matched with archaeological necessarily complex. First, Roman architects or builders had studies focusing on the organisation of mortar production to identify suitable sources of raw materials for the binder and use on building sites (e.g. Loustaud 1983; Coulthard (usually lime in the Roman period but also gypsum) and 1999; Coutelas 2005; Coutelas and Hourcade 2016; Guyard aggregate. These then had to be extracted and given any et al. 2008), although the evidence for this is generally less preliminary processing at the extraction site, before their frequently preserved. transport to the construction site, where any further on- Overall, evidence for mortar in the ancient construction site preparation occurred, before the mortar or plaster was process is heavily weighted towards what can be deduced mixed and then finally used in construction or decoration. from the finished product, with direct evidence in the archae- Malacrino (2010, 61–76) gives a brief recent introduction ological record of the actual processes surprisingly rare in to the subject, while Coutelas (2019) and Traini (2013) both relation to the volume of construction which took place. This provide excellent detailed overviews, with Traini providing is true even in the case of lime kilns, which provide the bulk the more detailed bibliography but with a narrower focus of the direct evidence for production. Because the processes on the lime component; Coutelas is mainly concerned with and practices continued through the mediaeval period and ancient Gaul and Traini with Roman Italy, although both into the nineteenth — and in some places the twentieth — range more widely over the area of the Roman empire and century, historic images and records plus ethnographic stud- its successors in Europe, North Africa and the Near East. ies are often used to fill out or interpret the ancient evidence. General considerations Physical requirements Although the use of mortars and plasters in the ancient In the period under question, there is ample evidence that the and early mediaeval worlds has long been recognised, the mortars used for a binder in mortared rubble construction, detailed processes which formed the chaîne opératoire for for mortar floors and for wall plaster, deployed the same construction and decoration have only relatively recently range of basic raw and processed materials. The differences become an important focus for archaeologists (see Coutelas came at the final stages of use, where the choice of recipe 2008 for a good overview). The general monograph of Adam and the manner of applying the material related to its spe- (1984) provided an accessible summary for a wider audience cific function. It is clear from the ancient written sources while also giving a general account of the processes of con- (see Lancaster 2021) that at least by the first century BCE, struction and rendering using lime-based mortars and plas- ancient builders had developed some firm ideas about the ters; the work has been translated into several languages and qualities and nature of the raw materials required for mak- still provides a starting point for many students. Lamprecht ing the best mortars and plasters for different functions, and (1984) is an early study of Roman mortared rubble construc- of the effects of using substitutes. Their preference for lime tion including archaeometric studies of the mortars. The made from pure calcitic limestone, identified by the white- operation of limekilns and the production of lime in general ness and hardness of the source limestone, meant that it was have been a particular area of interest since the pioneering often brought from considerable distance, while the aggre- study by Baradez (1957), followed up by Sölter (1970) and gate was most likely to be local, both inert sands and natural especially in the 1980s by Dix (1982) and Adam and Varene pozzolans as well as crushed terracotta, although imported (1985), who showed the importance of ethnographic studies pozzolans were also used. All of these had different implica- for understanding the production cycle. tions in terms of sourcing, preparation and transport. Interest in the logistics of supply and the economics of We should remember that these preferences were not use developed in the late 1990s following the publication based on any scientific analysis as we know it but on empiri- of this author’s study of the Baths of Caracalla in Rome cal observation, experience and experimentation. It is also (1997), although few concentrate on mortar (see Camporeale clear that the reality, as identified through modern scientific 2011 for a rare example). Since the 1990s, there has also analysis of mortars and plasters, does not always match up been an increasing number of scientific studies of ancient to the prescriptions of the ancient sources (Coutelas 2019), mortars and plasters which have contributed greatly to our something that in itself throws interesting light on ancient understanding of the sources of raw materials, the types of building practices and the role of economics in the construc- recipes used and the interaction between the components tion process. 1 3 Archaeol Anthropol Sci (2021) 13:195 Page 3 of 17 195 requirements: function and economy, the latter dictated Scheduling the supply of materials for mortar largely by accessibility and available transport routes. The most economical way of sourcing aggregate was to The need to assemble all the constituents for making mor- tar or plaster on site at the appropriate time has substantial use whatever local materials were the most abundant and the easiest to extract but still fulfilled the need for fine- implications for the logistics of the construction process. This is especially the case where mortar forms a key element grained material essential to the creation of the mortar (for an overview Coutelas 2009, 64–70). The use of local sands in creating the structural envelope, as was common from the second century BCE onwards with the development and and gravels in mortars for bulk construction, even when the quality of resultant mortar is poor, reflects this economic extensive use of mortared rubble construction (usually but misleadingly called ‘concrete’) using lime-based mortars in mentality; examples are discussed by Coutelas for several sites in Gaul (Coutelas 2005; 2011; 2012b), by Dilaria and Rome and central Italy (see Vitti 2021). Since mortar occu- pies about a quarter to a third of the volume of good-quality Secco (2018) for Aquileia and by Cardoso et al. (2014) for Ammaia in Lusitania (Portugal). It may also reflect, at a mortared rubble construction (DeLaine 2001; Camporeale 2011), this development brought about a large increase in distance, the tradition, previously common throughout the area of the Roman empire, of earth construction and mortars the amount of lime needed in urban areas, some of which — like Rome — did not have suitable limestones locally. Since which were made from local soils and clays. In some cases, further processing may have been necessary, including siev- mortar or plaster was needed at all stages of construction from the foundations to finishing, establishing and main- ing to remove coarser (or sometimes finer) particles or wash- ing to remove clay and soil impurities (Coutelas 2009), but taining a supply of lime may have been one of the priorities at the very inception of any building project and vital for careful choice of natural deposits would have avoided this in many cases, depending on the use to which the mortar extra-large ones like city walls, permanent military camps (e.g. at Tipasa, Baradez 1957), infrastructure projects like was being put. The use of local materials for mortars employed in bulk aqueducts or harbour works and major public buildings (e.g. the Baths of Caracalla in Rome, DeLaine 1997, 111–114 construction appears to have been the origin of the use in Rome of the pozzolanic pit sands (harena fossicia), which and 189–191). Once construction began, all the materials for mortar had to be available separately on site and already had such important repercussions for Roman architecture and engineering (Jackson et al. 2007); the famous pulvis prepared, with a chain of supply established to replenish them, since once the mortar itself was mixed, it needed to puteolanus from the Bay of Naples presumably had a similar origin, as in both cases there was a local volcanic geology be used within a very short space of time; this is particularly true for mortars using pozzolans in the aggregate. Changes and a relative dearth of ordinary inert sands. An interesting study by Marra et al. (2016) of some of the earliest mortared to the composition of the mortar within a single building project can therefore reflect not only technical requirements rubble structures identified in and around the city shows that the aggregates in the earliest mortars came from nearby and different phases of construction, but also problems with or alterations to the supply of materials, substantial breaks in outcrops or the construction site itself and included debris from working the rubble, itself a tuff of volcanic origin. In construction or changes in the workforce (Coutelas 2012a). Ec ffi ient sourcing of the ingredients of the mortar, combined other examples, the pit sand appears to have become mixed with tuff debris as a result of the two strata interfacing at with the relatively rapid curing times for the pozzolanic mor- tars of Rome, allowed an impressive speed of construction; the quarry. Towards the end of the second century BCE, there appears to have been a change to using a different — masons’ marks on the brick-faced walls of the early second century Baths of Trajan in Rome indicate that 15 m of ver- and more effective — pit sand deposit (pozzolana rossa) on the outskirts of the city, which fits with the revised date for tical wall were erected in only 2 months (Volpe and Rossi 2012). the start of wide-spread construction in mortared rubble in Rome (Mogetta 2015), requiring a more reliable and pre- dictable supply than simply using what was closest to hand. Nevertheless, Wehby Murgatroyd’s study (2016) of closely Sources, production and transport of primary materials contemporary mortars from the second century CE Ostia, the port of Rome, shows that even so close to Rome aggre- Aggregates gates in otherwise similar mortars used different mixtures of materials of volcanic origins, and not just the pozzolana Of the two main components which make up lime mortars rossa, with no effect on the mechanical quality of the result- ant mortars. and plasters, it is the aggregate that provides much of the volume and is the more variable component. The vari- The use of pozzolans has long been one of the main interests in scientific research into Roman mortars, and it is ability arises from two distinct and at times contradictory 1 3 195 Page 4 of 17 Archaeol Anthropol Sci (2021) 13:195 increasingly the case that the use of sources besides those and crops in rural ones. Siddall (2011) has demonstrated in of Rome or the Bay of Naples can be identified. Lancaster the case of the baths at Corinth that the ground terracotta (2015, 22–26; 2019) gives an overview of other places in came from pottery, particularly low-fired ceramics such as the Roman empire where local sources of volcanic materi- those used for amphorae and coarsewares, rather than from als were used in mortar, from the Rhineland in the west to bricks or roof tiles which tended to be fired at a higher tem- eastern Turkey, for example, at Sagalassos (Callebaut et al. perature and were both more difficult to process and less 2000), while Brandon et al. (2014) discuss this in relation reactive in the mortar. The overwhelming use of the material to the pozzolans used in harbour works around the Mediter- was in surface treatments, including pointing the joints of ranean. Other naturally occurring pozzolans of volcanic ori- masonry, wall plasters and the fixing base for marble and gin in Italy not included in Lancaster’s survey include those other wall veneer, mortar floors and the lining of water fea- from the Euganean Hills west of Padua, used for the theatre tures including aqueduct channels, cisterns, fountains and and amphitheatre of the Roman city (Bonetto et al. 2021), basins in baths. and from the Vulsini volcanic district in Tuscany (Marra and Even this volumetrically limited use of crushed terracotta D’Ambrosio 2013). At Nora in Sardinia, ground obsidian would have required, in all but the smallest applications, was also used as a pozzolan, which Columbu et al. (2019) an organised system for collecting and possibly marketing argue came not from its primary source but from abundant the discarded ceramics. This would not be surprising given deposits of obsidian tool-making waste from prehistoric con- that amphorae had other uses in the construction industry; texts around the city. whole, they were used to build enclosure walls (Serlorenzi In the Roman period, it is usually hard to identify the pre- 2010) or in drainage schemes (Carbonara et al. 2018), while cise extraction sites of these materials, when abundant local broken pieces were common as part of fills on building sites; sources were used and/or small quantities were required for they were also used as containers for building materials (see minor projects. In areas of continued habitation, larger quar- below). The waste ceramic for mortar then had to be crushed ries for aggregates were also commonly obliterated by later or ground, although there is not the evidence to say whether construction, as was happening already in antiquity. Exca- this happened at the collection point or the building site; vations in the House of Amaranthus at Pompeii revealed much may have depended on the quantities required. For a series of pits in the natural volcanic soil some at least of large quantities at least, it is feasible that animal- or even which appear to have been quarries to extract aggregate for water-powered mills were employed, using similar technol- either mud brick or (more likely given the first century BCE ogy to that used for milling grain. Although crushed ter- date) for mortar (Fulford and Wallace-Hadrill 1999), but this racotta was sometimes used in mortar for construction, for is a rare find. At Rome, because the pozzolan deposits are example, in the later second century CE bouleuterion at intercalated between those of lithoidal tuffs, the aggregate Smyrna (Turkey) (Felekoğlu et al. 2016) where it was clearly was frequently obtained from underground galleries which used for its structural properties, the difficulty of obtaining can still be identified, with examples from the Aventine the necessary quantities of material and the cost of process- (Marra et al. 2016), the Esquiline outside Servian walls from ing may have been limiting factors for more general use in the second century BCE (Serlorenzi 2014) and from the areas which did not have natural pozzolans. The continued south-east of city (Buccellato and Coletti 2014). Large piers use of crushed terracotta in plasters and floors, either on of the material were left to support the strata above, and little their own or combined with local pozzolans, perhaps reflects extra shoring seems to have been used. These pozzolans can the inherent conservatism of builders working in an empiri- be quarried simply using a pick, and given that the galleries cal tradition despite the hydraulic properties of the natural appear to have been at least 2 m wide, the material could be pozzolans (e.g. Rispoli et al. 2020). loaded directly into carts or panniers on pack animals at the quarry face. Some idea of the process of working of these Lime quarries in the Roman period can be deduced from much later sources, in particular from De Marchi (1894), when it There is overwhelming scientific evidence that in the main appears that very similar methods were in use. Roman period, when the use of lime-based mortars for rub- In contrast to these naturally occurring materials, the ble construction was at its height and very widely spread other main types of aggregates used in lime mortars — the over the empire, the preference was to use the purest cal- crushed terracotta and plant ash — were both waste products citic limestones available to burn for lime as recommended which were used for their pozzolanic actions particularly, by Vitruvius (see Lancaster 2021). At Ammaia, a Roman but not universally, to exploit their ability to resist mois- town in Portugal, Cardoso et al. (2014) have shown active ture (Coutelas 2019; Lancaster 2012). These two materials selection for this material on the part of Roman builders. would have been very different to source; waste terracotta Although in the earliest buildings a clay-based mortar was products were more abundant in urban situations, manure used, builders soon turned to using calcitic limestone, even 1 3 Archaeol Anthropol Sci (2021) 13:195 Page 5 of 17 195 though this had to be imported from an as yet unknown with late antiquity and the mediaeval period; Traini (2013, source, rather than making the lime from the local dolomitic 49–82) provides a useful catalogue with bibliography for the limestone, the latter only being employed in the post-Roman area of the Roman empire, while new finds are constantly period. Nevertheless, examples have been identified where extending the evidence (e.g. Traxler et al. 2018). in the absence of better material, limestone marls with a high While quicklime could be produced by the simple burn- clay content and even the shells of marine and riverine mol- ing of limestone in an open space, the resultant temperatures luscs were employed (Suméra 2009; Dilaria et al. 2019). In were not high enough to result in a complete calcination, and late antiquity and the mediaeval period, lime kilns often used the lime produced was contaminated by ash and charcoal debris from ruined or surplus buildings, including marble from the fuel (Traini 2013, 32–33). As the process leaves from statuary, funerary monuments, inscriptions and veneer. few visible traces, it was probably more wide-spread in the These appear frequently in urban centres (see e.g. Venditelli Roman period than is often assumed (for an example, see and Ricci (2015) for Rome; Lenzi (1998) for Ostia; Bonnie Coulthard 1999). The most common form of lime-kiln in (2016) for Galilee; Del Moro 2008 for Cyrene) but are also the archaeological record is that described by the Roman found in decommissioned Roman rural villas, exploiting senator Cato the Elder in the mid-second century BCE (see both marble and limestone building elements, as part of a Lancaster 2021), taking the form of a broad truncated cone broader practice of late antique and early mediaeval recy- of circular or elliptical plan, with one, or occasionally two, cling, often for church building (Munro 2016). accesses to the outside at ground level for adding fuel and The general process of lime-burning in antiquity has removing ashes, often with a ledge running around the inside been well-described in the modern literature (e.g. Adam at the base, and sometimes a pit in the centre for collect- 1984, 65–71; Dix 1982; Traini 2013, 31–48; Suméra 2009), ing the ashes (Fig. 2). The lowest stones (referred to as the and there is ample evidence, both archaeological and from charge) were laid initially over some kind of formwork or depictions, for example, on the mid-twelfth century mosaic corbelled to form a rough vaulted firing space which would from the south side of the nave of the Cappella Palatina be self-supporting once the props burnt away during firing, in Palermo (Fig.  1), to show that the techniques did not and then the rest of the charge was gradually added over this. vary enormously in later periods, even persisting until very recently in traditional lime-burning areas (for examples, see Adam 1984, 68–70; Wurch-Kozelj and Kozelj 1999). The archaeological evidence for actual limekilns is however rela- tively thin for the early- to mid-Roman periods compared Fig. 1 Mid-twelfth century mosaic from the south side of the nave Fig. 2 Late second century CE lime kiln no. 9, Lauriacum, Emms of the Cappella Palatina in Palermo (Sicily), showing the building of (Austria), aerial view. The kiln is built into the natural conglomer- the Tower of Babel. To the left, a worker loads fuel into a lime kiln. ate, and the combustion chamber (top) and access to the stoke hole At the bottom right, a man works lime in a lime-slaking basin, while (below) are built from rubble. The edge of the ash pit can be seen just beside him, another worker shovels lime into a basket being raised to above the stole hole, and the floor of the access corridor is discol- the second level of the structure. At the third level, the man on the oured by ash and charcoal. From Traxler et  al., 2018, 103, courtesy right holds a mason’s trowel. Photo author S. Traxler 1 3 195 Page 6 of 17 Archaeol Anthropol Sci (2021) 13:195 Excavated examples have a lower diameter ranging from 2 to strongly suggesting that lime producing on this military 7 m, with 3–4 m being the most common. Cato’s kiln, with scale would have been needed to serve the requirements of a diameter of just under 3 m, would have had a volume of a city the size of Rome. about 20 m3, producing perhaps 14–15 m of usable quick- All these kilns are of the periodic kind, that is, they need lime, while Baradez (1957) estimated that the largest of the to be filled, fired and then emptied before the cycle can begin kilns at Tipasa (Algeria), an ellipse with long axis around again. While historic figures for firing cycles vary greatly 6 m in diameter, had a volume of 90 m able to produce 65 (see DeLaine 1997, 112 note 48 for examples), Baradez m of quicklime. (1957) suggested that only three firings per month would Many of the excavated examples of the Roman period be feasible in his study of the kilns at Tipasa. This fits with are of single kilns, mainly in rural areas and arguably serv- experimental firings at Iversheim, which took 6–7  days ing the needs of villas or other rural settlements, and this is including the cooling, plus 3 days for loading and unloading the context of Cato’s kiln. Some at least of the lime would (Sölter 1970, 35–40); larger kilns would have taken longer have been used in agriculture rather than for construction to load, fire and unload, so that perhaps 12–14 firings were (Dix 1982). These are unlikely, however, to have been able possible in the drier months of the year (cf. Delaine 1997, to serve the needs of large urban centres or major infra- 112–114). The longer the firing, the more likely that the structure projects, yet relatively few limekilns have been limestone would be completely calcined and the resultant found in Roman cities; those for the baths at Vieil-Evreux quicklime of better quality. At some sites, notably Lucus (France) are an exception (Guyard et  al. 2008). Rather, Feroniae (Fontana 1995) and Tipasa (Baradez 1957), there they tend to be located at or near the limestone quarries is an arrangement of several normal-sized kilns of 3–4 m in and in rural areas where fuel was more easily found. The diameter plus one much larger one, which suggests some hilly area around Lucus Feroniae, along the Tiber north of kind of distinction in the production cycle, but whether this Rome, has provided several examples of sites with multiple was for different destinations for the lime or simply to create kilns directly adjacent to the quarries, in one case organised a more flexible structure of supply can no longer be deter - in pairs to allow for a more efficient firing cycle (Fontana mined. Relatively few known production sites based at quar- 1995; Savi Scarponi 2013); most show evidence of use over ries have facilities for slaking lime (see below), which seems a long period of time and can be interpreted as commer- to have been done mainly at the construction site (Suméra cial enterprises serving the needs of Rome. The existence 2009, 58). of subsidiary structures connected to the kilns, as at Lucus Lime burning also has a large fuel requirement, which Feroniae, reinforces the interpretation of these as perma- has been roughly estimated at 1.5–3 tonnes of wood per nent installations producing for a market rather than for m of lime, depending on the type and degree of moisture occasional or intermittent use supplying the needs of a rural (DeLaine 1997, 113; Suméra 2009, 40–42). The general estate. A similar phenomenon has been found in the Swiss use of oak and other hardwoods has been confirmed by Jura (Coutelas 2009, 56) and at several sites in Gaul, one Vaschalde et al. (2013) for Roman and mediaeval lime kilns at Touffréville with 16 permanent kilns of different peri- in southern France, based on a study of charred wood from ods demonstrating a long period of exploitation (Coulthard kilns. Local availability, however, appears to have informed 1999), although the particular market for the lime is far from the choice of species used, and there is evidence that smaller clear. Large groups of kilns have also been found near mili- materials, including typical Mediterranean shrubby plants tary sites or at least operated by Roman legionaries, along like rosemary and juniper, and even chaff or dried animal the northern limes and at Tipasa (Algeria); the best known dung, were also used at some point in the firing cycle. The are those at Iversheim (Sölter 1970), with a row of five kilns gathering of fuel must have been an important part of the plus one slightly later, while the most recently discovered, at lime-burning operation for the sites with large banks of Lauriacum/Emms (Traxler et al. 2018, 2019), had 12 kilns kilns, while for individual smaller kilns on rural estates, each 3–4 m in diameter (Fig. 2). While these have usually some at least could be accumulated from general arboricul- been interpreted as serving the needs of the Roman army in ture or other estate activities. the building of fortresses, the group of at least 10 kilns at Krivina, in northern Bulgaria, dating to the later first century CE has no clear relation to any major military sites of that Transport date using mortar in construction (Vagalinski 2011), and as at Touffréville no obvious market can be determined. This It has been estimated that one of the major elements in the author (1997, 189) has calculated that at least 21 large kilns cost of construction was the transport of building materials each producing 60 m quicklime and operating 14 cycles per (DeLaine 1997, 219), but this depended on the weight and year over four years were needed for the Baths of Caracalla, volume to be transported, the distance, the form of trans- port and the available infrastructure. Russell (2013) provides 1 3 Archaeol Anthropol Sci (2021) 13:195 Page 7 of 17 195 a thorough analysis of the problem in relation to building The situation was rather different for lime, which in the stone, much of which also applies to the components of early- to mid-Roman periods was generally produced at mortar and plaster, in particular for mortared rubble con- the quarry site where fuel could also be easily acquired structions where the volumes of mortar were considerable. and often had to be transported longer distances than This made the burning of limestone at source an obvious aggregates (for an overview, see Suméra 2009, 58–60). It solution, as it reduces the weight by about 44%. The natural is not therefore surprising that many of the known lime- pozzolans of Rome and the Bay of Naples weigh roughly 1 kilns are situated very close to major rivers such as the tonne per m , coarse dry sands and loose gravel about 1.5 Tiber (Fontana 1995; Savi Scarponi 2013), Rhine (Sölter tonnes per m and quicklime 1.1–1.5 tonnes, depending on 1970) and Danube (Vagalinski 2011). Since neither pro- the limestone used (Giuliani 1990, Table 7.1). duction nor construction sites were generally directly Land transport was much more expensive than water, and beside a river, some transport by cart would have been there was a closer relation between cost and weight over necessary at each end of the journey, with the additional land than by water. Although both aggregates and quick- transhipments involved. At the same time, river transport lime could be carried by pack animals, the volumes required for lime was not always possible; the lime for the Villa of suggest that wagons, drawn by oxen or mules, were the the Quintilii via Appia south of Rome has been shown to usual mode for overland transport, with a maximum load come from the Monti Cornicolani to the east of the city of 1–2 tonnes for standard carts. This did, however, require (Fichera et al. 2015), where river transport via the Tiber a fairly smooth and level surface, so that proximity to good would involve two sections of road transport scarcely if roads was important for any materials that had to be moved any shorter than direct road transport, plus the extra tran- over any distance. River transport downstream may have shipments. Transporting quicklime, as seems generally to cost about a tenth of land transport or a fifth if movement have been the case, saved on weight but posed other prob- upstream was involved, while sea transport may have cost lems. The material is both caustic and had to be protected 30–40 times less and involved much larger loads. Since from moisture to prevent premature slaking which would transferring loads from one means of transport to another make it unusable for construction. It therefore needed to added extra labour and cost, solutions involving the fewest be transported in some form of container, such as heavy transhipments may have been preferred. sacks or large baskets. Things could go wrong; the Greek Direct evidence of transport solutions for aggregates philosopher Theophrastos, writing in the early third cen- is almost non-existent, but the relatively short distances tury BCE, recounts the tale of a ship carrying lime and involved in most cases indicate that the norm would have textiles which caught fire after the lime became wet (cf. been to use carts, with the material being loaded directly Traini 2013, 83). at the quarry and unloaded at the building site without any need for transhipment. One of the pozzolan quarries identi- fied by Buccellato and Coletti (2014) south of Rome and just 1 km from Tiber might have used the river, but this On‑site storage and preparation of materials would have required two extra transhipments and an up-river journey, making the 12-km land journey along the major Efficient use of manpower in construction requires that all highway of the via Ostiensis a more likely solution. Some the necessary materials are at hand. Depots of building aggregates made longer journeys. The natural pozzolans materials are therefore a normal feature of construction found loose as ballast in the hold of a sunken ship at Pisa sites, including modern ones, but can be hard to identify (Marra and D’Ambrosio 2013), dated to the first decades of in the archaeological record. This is partly due to the fact the first century CE (Camilli 2012), came from the Vulsini that piles of dry materials, including aggregates, may leave volcanic district in Tuscany via the river Fiora, along an little or no trace once incorporated in the structure, and established commercial water trade route with a river port partly due to their location in open areas which are less and a sea port connected to the important town of Vulci. likely to be the focus of modern excavations (Guyard et al. According to Jacopo Bonetto, unpublished research by the 2008; cf. Spera et al. 2011 for a mediaeval example). The University of Padua has demonstrated that these pozzolans bulk of the archaeological evidence relates to lime-slaking from the Bay of Naples were used in the construction of pits and basins, which were generally dug into the working the large baths and theatre at Aquileia. The closeness of the surface and preserved by being buried at the end of the various sources of these pozzolans to the sea must have been building project and/or reused as rubbish pits (see the cata- an important factor in their exploitation. Trade in pozzolans logue in Traini 2013, 95–102). The villa at Brachaud, near from the Bay of Naples, used for harbour works, is a special Limoges, provides a rare example of a work depot with case discussed in Brandon et al. (2014, 223–226), who argue both lime-slaking pits and an area for aggregate marked for a major trade in the material across the Mediterranean. out by an edge of tile, the two separated by only a couple 1 3 195 Page 8 of 17 Archaeol Anthropol Sci (2021) 13:195 of metres (Loustaud 1983, Fig. 6). Pompeii also provides (c. 24.5 mm) in mortar floors and foundations, digitus (c. examples on a small scale, for example, in what appears 18.5  mm) in wall mortars and semiuncia (12.3  mm) in to be interrupted building works at the House of Amaran- plasters. At Ostia, from personal observation, the largest thus, with a pile of pozzolanic aggregate and lime, mortar aggregate in mortar for ordinary facing brickwork would and crushed terracotta stored in amphorae (Fulford et al., also pass through a digitus screen, while in fine-work used 1995-96). The final necessary element is water for slaking for decorative brick facing the maximum is generally less lime and mixing mortar, yet this is rarely discussed in the than 2 mm, possibly derived from a tenth of a digitus sieve archaeological context. (Fig. 3). In the Villa of the Quintilii just outside of Rome, the largest aggregate in the mortar for the core would have Aggregates fit through an uncia screen (Fichera et al. 2015, Fig.  4b). A similar distinction between core and facing can be seen in Since the proportion of aggregate to lime in most Roman the Baths of Caracalla in Rome (DeLaine 1997, 140). In the mortars was around 3:1 by volume, considerable areas amphitheatre at Aquileia, a semiuncia sieve seems likely needed to be set aside for storing aggregates on large build- for general construction and a very fine one for laying brick ing sites. Alternatively, and particularly for major build- (Dilario and Secco 2018, 183–185). ing projects, stocks of aggregate needed to be replenished on a regular basis if the construction schedule was not to Lime slaking be delayed. The variability of composition in mortars and plasters using the same basic materials, whether sands, Both Suméra (2009, 50–58) and Traini (2013, 83–91) natural pozzolans or crushed terracotta (e.g. Di Benedetto give good overviews of the process of turning quicklime et al. 2018), presupposes that some degree of processing into slaked lime through the addition of water, to make the and sorting of aggregates took place on site, which would essential component in lime mortars and plasters. The lim- have added another level to the supply chain, and required ited evidence available suggests that the usual process of space for working the different materials and storing them slaking lime was by fusion, involving adding water gradu- separately. ally to a layer of quicklime while mixing it continually with The evidence that aggregates for mortar and especially a long-handled draw-hoe (one with a blade at right angles plaster were sorted for size is clear, although little dis- to the handle) until the lumps of quicklime disintegrated. cussed in terms of the construction process. Davey (1974) This process appears to be what is being shown on a late is an exception; he grouped the maximum size of aggre- antique mosaic from Oued R’mel (Tunisia, now in the Bardo gates in construction mortars from the province of Britan- Museum; Adam 1984, Fig. 164); although the mosaic has nia into sizes that would have fit through sieves of uncia considerable lacunae, it clearly shows a man pouring water Fig. 3 Construction detail, Casa a Giardino, Ostia (Italy), built in the 120  s CE. On the right, part of wall showing standard brick facing, with mortar having a pozzolan aggregate which would go through a digitus sieve; on the left, the adjacent decorative pilaster, an integral Fig. 4 Lime-slaking basin, Molesme ‘Sur-les-Creux’ (Côte-d’Or, part of the same structure, with very narrow joints requiring the poz- France), early first century CE. The floor of the basin is made of zolan to be put through a fine sieve. Photo courtesy S. Camporeale planks of fir. From Coutelas 2005, Fig. 1, courtesy A. Coutelas 1 3 Archaeol Anthropol Sci (2021) 13:195 Page 9 of 17 195 from an amphora onto a pile of white material which is being Putting in place worked with a draw hoe by a largely lost figure. Fully slaked lime, if kept away from air, is more stable than quicklime, The process of building with mortar and plaster varied to and if covered, it can be stored for long periods. The benefit some degree with the size, nature and importance of the of storing lime before use was to make sure that the process project. We should not expect to find the same qualities of was complete and no lumps of quicklime remained which mortar or the same care in applying it on small domestic might react explosively when further water was added in the restoration works as on large public buildings. Nevertheless, making of mortar and in particular plaster. the basic organisation of work was presumably similar and The limited archaeological evidence for the Roman period dictated by the physical requirements of the site and the pro- indicates that on the whole, lime slaking took place at the ject. The wall painting from the Tomb of Trebius Justus in building site, whether this was the location of the lime kilns Rome (Fig. 5; Marucchi 1911) is the only surviving example or not. Lime slaking/storage pits have generally not been from the Roman world of builders working on a brick-faced well-recorded or published. Loustaud (1983) was one of the mortared rubble wall. The scene is remarkably modern: first to discuss the phenomenon, examining 8 basins from there are five workmen, wearing the short tunic of the slave ancient Limoges and a nearby villa; although the evidence or labourer, with two bricklayers standing on scaffolding has grown since then, Traini (2013, 95–102) only catalogues on either side of a wall, while one labourer carries a load 22 from Roman and late Roman sites, not all entirely con- of mortar up a ladder in half an amphora, another carries a vincing, although he notes that this is not an exhaustive list. load of bricks or rubble in a basket, and a third mixes mortar Most of the reliable examples are from domestic contexts using a long-handled draw-hoe. The scene can be broken and are fairly small, with a longest dimension of about 1 m. down into three essential actions: mixing the mortar, trans- The largest of those discussed by Loustaud (1983) measured porting the materials to the workface and putting in place. 1.4 × 2.4 m, with a volume of 1.58 m , and relates to villa baths. Some of these basins were pits dug into the earth and Mortar and plaster mixing lined with tiles and/or wooden planks (Fig. 4); others were of masonry and lined with cocciopesto. Traini and Man- While any aggregate processing and lime slaking needed to nelli (2013) report a much larger basin, 1.88 × 2.65 m and be carried out in advance of construction, mortar and plaster 1.82 m deep and lined with cocciopesto, which served the preparation had to take place concurrently with construction construction of the late second/early third century CE baths or decoration, especially when pozzolans were being used on the slopes of the Palatine in Rome, although there is no as such mortars begin the curing and hardening processes direct evidence to show that it was in fact used for slaking very quickly. or storing lime. A basin of this capacity could hold 5 m of slaked lime, enough to make c. 20 m of mortar at the nor- mal proportion of 1:3. To put this in perspective, the amount of slaked lime required for the Baths of Caracalla in Rome was over 100,000 m , which would have needed a basin of this size being filled over 20,000 times over the course of construction. It is clear that we have not yet found any major installations for lime-slaking on imperial building sites. A number of sites recorded by Loustaud (1983) have two lime-slaking basins, one larger than the other; he argues, on the basis of ethnographic parallels, that the larger was used for the primary slaking and the second for refining the result- ant product to ensure complete slaking and to remove impu- rities such as fragments of carbonated lime, which could be caused by the reuse of lime-slaking pits and basins; such fragments have been identified in archaeometric studies of mortars. Alternatively, the basins could have been used for different qualities of slaked lime, the larger for mortar and Fig. 5 Wall painting showing a scene of builders at work from the the smaller for plaster, where the complete slaking of the Tomb of Trebius Justus, Via Latina (Rome), fourth century CE. Two men on scaffolding either side of a brick-faced wall are laying bricks lime was more critical. The fact that such basins have been using masonry trowels. To the left, a man carries a basket full of rub- found still containing lime has led Loustaud (1983, 149) to ble or bricks, while to his right, another man climbs a ladder with a suggest that this was a deliberate act to preserve lime for halved amphora of mortar on his shoulder. To the right, a man mixes later maintenance. mortar with a draw-hoe. From Marucchi 1911, Fig. 5 1 3 195 Page 10 of 17 Archaeol Anthropol Sci (2021) 13:195 The recipes for mortar mixes in the written sources (see no evidence that Romans used wheelbarrows, but some form Lancaster 2021) are given as whole ratios such as 1:2 or of simple two-wheeled cart may have been used instead; 1:3 lime to aggregate by volume, and these proportions are nevertheless, the same reservations as for pack animals or roughly confirmed by archaeometric analyses (e.g. Wehby larger wagons apply. Murgatroyd 2016) although there is much variation. Such In the wall painting from the Tomb of Trebius Justus, a proportions could be achieved simply on site by using a bas- distinction is made between the man on the ground, who ket-load as a measure, which would account for at least some appears to have a basket load of bricks or rubble, and the of the variability in the results. Baskets appear on both the man on the ladder, who appears rather to be carrying the Oued R’mel mosaic and the Trebius Justus wall painting, as mortar in half an amphora. The caustic nature of both lime well as on Trajan’s Column in Rome, as standard elements in and mortar would make this a sensible choice; the use of construction scenes, usually as containers for moving mate- amphorae to store lime and mortar observed at Pompeii (see rials, but there is no reason that they could not be used at the above) suggests that this was common practice. In addition, same time for measuring. Romans used a modius for measur- the open nature of a longitudinally halved amphora would ing grain, but these would not have been feasible to use on make the mortar easier to access for the mason than the more building sites; this author (1997, 107) however has estimated closed form of a basket. that the baskets on Trajan’s Column would have contained around two modii or roughly a cubic Roman foot (0.026 m ). Using mortar in construction The process of mortar mixing is well described by Traini (2013, 103–112; cf. Coutelas, 2008, 2009, 71–72). It can be Mortared rubble masonry seen at the lower right-hand corner of the scene from the Tomb of Trebius Justus (Fig. 5) and reappears frequently The construction process for mortared rubble is generally in mediaeval representations of building sites. A measured well-understood; Adam (1984, 125–150) provides a useful quantity of aggregate was arranged on a flat piece of ground if brief introduction. It is, however, rarely if ever discussed in a rough circle with a well in the centre, the appropriate specifically from the point of view of how the mortar is used. quantity of slaked lime is placed in the centre, and the aggre- Overall, the construction process can be divided into three gate is mixed into the lime using the same kind of draw hoe stages: laying foundations, laying the facing and core of the as used in lime slaking, with limited water used as required walls and laying the vaults (if used). At each stage, the work to obtain the right consistency of the paste (see Adam 1984, could be done well or done poorly. Generally, what has been Fig. 163, for a reconstruction drawing). In order to produce preserved has been so by virtue of being at the better end of the best mortar or plaster, the mixing needed to be thorough. the scale; the very poor quality of some of the construction Coutelas (2008) provides examples from a number of sites at Pompeii, and in particular the weakness of much of the to show that mortar-mixing areas were often reused, and this mortar, is a reminder that not all Roman construction was of may have been the way in which small fragments of old mor- high quality. The following applies to the higher end of the tar found their way into fresh (for an example, see Dilaria construction spectrum. and Secco 2018). There is no evidence that Roman build- The fundamental principle of Roman mortared construc- ers used the kind of mortar-mixing machines which have tion is that the rubble is laid in mortar, rather than being been identified for mediaeval building sites (Bianchi 2011), mixed into the mortar before being put in place. Even in although this may simply be due to the absence of evidence. foundations, where the sides of the foundation trench, whether just earth or timber shoring, served to contain the Moving mortar and plaster around the construction mass of rubble and mortar, all the evidence points to the site rubble being either roughly laid or thrown into the trench in layers alternating with mortar and then rammed down to Over the short distances involved in most building sites, compact the mass and remove air pockets (cf. Dilaria and human portage was the most basic and most flexible way Secco 2018, 184). The mortar therefore was not liquid and of moving the different ingredients from the stockpiles and could not be poured like modern concrete. lime-slaking or storage pits to where the mortar was mixed The walls above ground were generally constructed as and then moving the mortar to the workface (for a sum- two faces with a central core, either with short vertical sec- mary, see DeLaine 1997, 107). Except possibly on very tions of the external faces being put in place and then the large building sites such as the imperial baths, pack animals core or the two rising together (Fig. 6). The ability of the and wagons would require extra manpower for loading and pozzolanic mortars of Rome and central Italy to cure and unloading and might simply clutter the site unnecessarily. gain strength rapidly, and the use of relatively narrow mortar Over short distances, a man can carry a load of around 50 kg joints, meant that the work could proceed rapidly without the or even more, around the same as a small donkey. There is need for any supporting formwork, although this may not 1 3 Archaeol Anthropol Sci (2021) 13:195 Page 11 of 17 195 Fig. 6 Construction detail, Grandi Horrea, Ostia (Italy), late second Fig. 7 Brick facing, Grandi Horrea, Ostia (Italy), late second cen- century CE phase. Section and face of wall, from left to right: section tury CE phase. Detail showing vertically struck joints (indicated by through neatly coursed brick facing; section through roughly coursed arrows); elsewhere, the surface of the mortar has deteriorated. Photo core; neatly coursed brick facing. Photo author author have been the case everywhere (cf. Camporeale 2011). As with the foundations, the core was laid more or less roughly double-struck joints which slope towards in both directions in courses alternating with mortar but generally without any away from a central ridge of mortar. In mortared rubble con- further consolidation, and it is not uncommon to see gaps struction using irregular pieces or where the quality of the between individual pieces of rubble where the mortar has bulk of the mortar in the core was poor, the joints between not penetrated (cf. DeLaine 2001, 234). Laying the facing the facing pieces could be finished at a second stage called required more careful work than laying the core and some- pointing, using a different quality of mortar and even almost times used a less coarse mortar, especially for finely finished pure lime putty to protect the structural integrity of the core work. The detail depended on the shape and form of the and/or to establish a smooth outer face, sometimes for deco- facing materials used, but the basic process was to bed each rative purposes (Coutelas 2005, 332–334; 2009, 78–82). facing element into a horizontal layer of mortar; for larger Mortar was used in vaulting with the shape defined by elements, such as rectangular blocks or reticulate pieces, wooden formwork which also supported the structure of mortar was first applied to the side of the element which the vault against gravity until it had cured sufficiently to was to be in contact with the piece previously put in place be self-supporting (see Lancaster 2005). Although in early as in modern bricklaying, while the triangular bricks used Roman vaults the rubble was set radially, in later buildings in mid-imperial construction in Rome and Ostia were often in central Italy where strong pozzolanic mortar was used, laid simply on the mortar bed, creating narrower horizontal the rubble was laid horizontally in the mortar exactly like than vertical joints (see Fig. 6). The mortar had to be of a the wall-core, and it is often difficult to say where the wall consistency that would give way slightly when the facing ended and the vault started. The main difficulty in using element was put in place, ensuring good contact between mortared rubble for vaults appears to have been separating the two, without flowing too much out of the joint. In the the supporting formwork from the underside of the vault best construction, the horizontal joints are of fairly uniform once it had cured. One solution used in Rome and Ostia was thickness while still allowing for variations in the thickness to cover the formwork with one or two layers of brick, which of the facing element; this suggests that the workmen main- remained attached to the underside of the vault when the tained a uniform consistency of mortar and used a specific formwork was removed; elsewhere the mortar skin on the amount for each course, even though this amount could vary vaults has often preserved the size and shape of the timbers both within a single building for different functions (Fig.  3) used for formwork (Lancaster 2005, 22–50; Coutelas and and from one building to another. Hourcade 2016). The mortar in the joints was then finished in various ways, being struck either vertically to be flush with the plane of the Trowels wall (Fig. 7) or at an angle or slightly hollowed out. Coutelas (2012b, 173–175) provides examples of both weather-struck The main tool for laying and finishing mortar in construc- joints, where the mortar slopes in from top to bottom and tion was the leaf- to rhomboid-shaped builder’s trowel, 1 3 195 Page 12 of 17 Archaeol Anthropol Sci (2021) 13:195 which has remained in use to the present day. Relatively between pointing and plasterers’ trowels can be made few actual examples have survived from antiquity, and there (Fig. 8). Gaitzsch found no examples from the Greek period are a small number of depictions, the earliest of which is and associated the development of the tool, and of local on the relief of the structor (builder) Diogenes at Pompeii. and specialised variants, to the growth of mortared rubble Gaitzsch (1980,133–147) provides a catalogue of 83 exam- construction in the Roman period. The range of shapes and ples from Italy and the north-western provinces, to which sizes can be paralleled in early twentieth century trowels, for can be added one from the Roman veteran colony of Cuicul example, those advertised in the catalogue of the specialist (Tunisia) now in the museum of Djemila (Belhout 2019, cat. company Forges de Mutzig (1929) which gives the place of No. and Fig. 14); imprints of a pointed trowel have also been use as well as the function of the different types. The wall found in mortar pointing at Cassinomagus (Coutelas 2012b, painting from the Tomb of Trebius Justus in Rome (Fig. 5) Fig. 50). Gaitzsch divides them into six types based on the is to my knowledge the only surviving representation from basic shape, with variants defined mainly by the different antiquity which shows a trowel being used by builders, but length to width ratios, while within each type, the trow- such scenes are relatively common in mediaeval construc- els vary also in length and breadth. This rather masks the tion scenes (Fig. 1). functional aspect of his types, which he otherwise divides into masons’ trowels (54 out of the 83), pointing trowels — Harbour works some also used in plasterwork (26 out of the 83), and three ‘pointed’ (or ‘English’) trowels, the latter being the familiar The production of mortar and laying of mortared rubble for form currently used by archaeologists. A further separation harbour works were rather different to those for land-based Fig. 8 A selection of Roman masonry, pointing and plaster- ers’ trowels, first to fourth centuries CE. A2 and B1 are the most common forms, while the pointing and plasterers’ trowels are relatively rare. Adapted from Gaitzsch 1980, 139 1 3 Archaeol Anthropol Sci (2021) 13:195 Page 13 of 17 195 structures, as described in the ancient sources (see Lancas- 1984, Fig. 522), while a few wooden floats, looking remark - ter 2021) and discussed in Brandon et al. (2014, especially ably like modern ones, have been found in excavations as 143–222). The success of mortared rubble harbour works well as examples of scrapers used by plasterers (Gaitzsch depended on the mortar. Nearly all of those tested by Bran- 1980, 11–18). For utilitarian purposes, the aggregate was don et al. (2014, 161) fall in the range of 55–60% mortar often crushed terracotta, and there was no fine finishing coat, by volume, compared with around 35% for land structures. although, as can be seen in many places at Pompeii, red pig- Jackson (in Brandon et al. 2014, 164–166) argues that in ment was sometimes applied to the surface. some harbour works either quicklime or aged slaked lime, Mortar floors, made with or without crushed terracotta in the form of small granules, may have been mixed with the aggregate, generally included more and larger aggregate pozzolan either dry or with a small amount of water, only which helped give the mortar more strength than similar fully hydrating in the seawater once put in place, although plasters; they were also often compressed to improve density many questions remain. In two out of the three types of form- and resistance, especially important for cisterns and other work used to create harbour structures discussed by Bran- water features. Bedding layers for veneer, on the other hand, don (in Brandon et al. 2014, 191–222), success depended were often little different to the mortars used in masonry, on the use of hydraulic pozzolanic mortar which would cure while those for mosaic usually had a top layer of almost pure and continue to gain strength when completely submerged. lime, sometimes with added fine aggregate, into which the Alternatively, harbour works needed to use cofferdams cre- individual mosaic tesserae were set. ated in situ, which allowed the structure to be built in a dry environment. Otherwise, the process appears to have been very similar to that used in the construction of foundations, Concluding summary of key concepts although in at least some cases, the degree of compaction is less, reducing the strength of the mortared rubble, presum- The focus of this paper is the chaîne opératoire of the con- ably due to the difficulties of working underwater. struction process from the production of the main ingre- dients — lime and aggregate — to their application in Plastering and finishing structures in the Roman period. First, Roman architects or builders had to identify suitable sources of raw materials The final stage of construction often involved adding a sepa- for the binder (usually lime) and aggregate. These then had rate coating to built surfaces, all of which involved some to be extracted and given any preliminary processing at the form of lime-based plaster or mortar. This includes mortar extraction site, before their transport to the construction site, floors, rendering coats for walls, the supporting base for where further preparation occurred, before the two compo- marble and stone veneer and mosaics on floors and walls, nents were mixed to form the mortar or plaster which was and special hydraulic treatments designed to waterproof a then used in construction or decoration. The emphasis here variety of structures especially aqueducts, cisterns, fountains is on mortared rubble construction, which was a particular and industrial basins. Coutelas (2009, 87–121, cf. Büttner development of the Roman period from the second century and Coutelas 2011) provides a good starting point for under- BCE and required much larger quantities of lime than previ- standing how mortar was adapted for these different types ous building techniques. of use in the context of ancient and mediaeval Gaul, and see The processes which formed the chaîne opératoire for Adam (1984, 216–234) for a more general account. construction have only recently become the focus of aca- Wall coverings nearly always consist of at least two lay- demic interest, and evidence for identifying the details of ers, and more normally three, rather than the seven recom- this sequence of events is uneven. It requires an interdisci- mended by Vitruvius (see Lancaster 2021); Di Benedetto plinary approach, and much has to be deduced by working et al. (2018) provide a good example from central Italy. The back from the finished product, while direct archaeologi- first (anchorage) layer was the thickest, as much as 50 mm cal evidence of the actual processes is relatively rare, and in some cases, as it had to even out the face of the wall. The the detail has not always been recorded or published. As mortar was applied to the wall using a trowel, with the sur- well as archaeological and archaeometric evidence, literary face left rough to help the second layer to adhere; the large sources, ancient representations and ethnographic studies particles of poorly sorted aggregate in the mix encouraged all contribute. The production of lime is the best understood this further. The second layer usually contained smaller and and has been the focus of considerable literature, as have the better-sorted aggregate and was given a flat finish probably components and working of pozzolanic mortars, both those using a float, as was the very thin final layer composed of using natural pozzolans and those using pozzolanic waste lime with little or no aggregate, which could be left plain products, especially crushed terracotta. or painted. The application of this final layer is shown in a The archaeometric identification of the ingredients of relief from Sens (Uffler 1971; reconstruction drawing Adam surviving mortars and plasters provides evidence for the 1 3 195 Page 14 of 17 Archaeol Anthropol Sci (2021) 13:195 need to obtain permission directly from the copyright holder. To view a choices by builders, in terms of the functional requirements copy of this licence, visit http://cr eativ ecommons. or g/licen ses/ b y/4.0/ . but also, and in many cases perhaps primarily, in terms of the economics of construction. Local geological settings, the physical environment especially the ready availability of fuel and their relation to the building site in terms of References transport routes, appear to have been primary factors affect- ing choice. In terms of extraction and preparation of materi- Adam J-P (1984) La construction romaine: matériaux et techniques. Picard, Paris als, the archaeological evidence shows that well-established Adam J-P, Varene P (1985) Fours à chaux artisanaux dans le basin technologies were used, especially notable in relation to the méditerranéen. 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Production, transport and on-site organisation of Roman mortars and plasters

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Abstract

This paper examines the role of mortars and plasters in the construction process during the Roman period and seeks to elucidate the chaîne opératoire from the production of the main ingredients — lime and aggregate — to their application in structures, based on archaeological, visual and archaeometric data. As well as looking at the actual processes involved, it also considers the functional requirements of the mortars and plasters and the economic implications of their use, especially the nature and cost of transport, which may have led to particular choices being made by Roman builders; it also considers the supply of materials in terms of the logistics of construction. The emphasis is on mortared rubble construction, which was a particular development of the Roman period from the second century BCE and required much larger quantities of lime than previous building techniques. Attention is also paid to the human actions involved and the tools employed. Keywords Lime · Aggregate · Mortar · Roman construction · Chaîne opératoire Premise The second group of contributions is focused on pig- ments, starting from a philological essay on terminology This Topical Collection (TC) covers several topics in the (Becker 2021). Three archaeological reviews on prehistoric field of study, in which ancient architecture, art history, (Domingo Sanz and Chieli 2021), Roman (Salvadori and archaeology and material analyses intersect. The chosen Sbrolli 2021) and Mediaeval (Murat 2021) wall paintings perspective is that of a multidisciplinary scenario, capable clarify the archaeological and historical/cultural frame- of combining, integrating and solving the research issues work. A series of archaeometric reviews illustrate the state raised by the study of mortars, plasters and pigments (Gli- of the art of the studies carried out on Fe-based red, yellow ozzo et al. 2021). and brown ochres (Mastrotheodoros et al. 2021); Cu-based The first group of contributions explains how mortars greens and blues (Švarcová et al. 2021); As-based yellows have been made and used through the ages (Arizzi and and reds (Gliozzo and Burgio 2021); Pb-based whites, reds, Cultrone 2021; Ergenç et al. 2021; Lancaster 2021; Vitti yellows and oranges (Gliozzo and Ionescu 2021); Hg-based 2021). An insight into their production, transport and on-site red and white (Gliozzo 2021) and organic pigments (Aceto organisation is further provided by this paper. Furthermore, 2021). An overview of the use of inks, pigments and dyes several issues concerning the degradation and conservation in manuscripts, their scientific examination and analysis of mortars and plasters are addressed from practical and protocol (Burgio 2021) as well as an overview of  glass- technical standpoints (La Russa and Ruffolo 2021; Caro- based pigments (Cavallo and Riccardi 2021) are also pre- selli et al. 2021). sented. Furthermore, two papers on cosmetic (Pérez-Aran- tegui 2021) and bioactive (antibacterial) pigments (Knapp et al. 2021) provide insights into the variety and different uses of these materials. This article is part of the Topical Collection on Mortars, plasters and pigments: Research questions and answers * Janet DeLaine janet.delaine@classics.ox.ac.uk Wolfson College, University of Oxford, Oxford, UK Vol.:(0123456789) 1 3 195 Page 2 of 17 Archaeol Anthropol Sci (2021) 13:195 at a microscopic level (e.g. Jackson et al. 2007; Marra and Introduction D’Ambrosio 2013; Wehby Murgatroyd 2016; Columbu et al. 2019; Coutelas 2019; Dilaria et al. 2019), all of which have Since mortars and plasters are in themselves composite implications for the construction process and the relation materials, and mortar itself is an essential part of another between the specific recipes and the function of the mortar composite material, their role in the construction process is or plaster. These have been matched with archaeological necessarily complex. First, Roman architects or builders had studies focusing on the organisation of mortar production to identify suitable sources of raw materials for the binder and use on building sites (e.g. Loustaud 1983; Coulthard (usually lime in the Roman period but also gypsum) and 1999; Coutelas 2005; Coutelas and Hourcade 2016; Guyard aggregate. These then had to be extracted and given any et al. 2008), although the evidence for this is generally less preliminary processing at the extraction site, before their frequently preserved. transport to the construction site, where any further on- Overall, evidence for mortar in the ancient construction site preparation occurred, before the mortar or plaster was process is heavily weighted towards what can be deduced mixed and then finally used in construction or decoration. from the finished product, with direct evidence in the archae- Malacrino (2010, 61–76) gives a brief recent introduction ological record of the actual processes surprisingly rare in to the subject, while Coutelas (2019) and Traini (2013) both relation to the volume of construction which took place. This provide excellent detailed overviews, with Traini providing is true even in the case of lime kilns, which provide the bulk the more detailed bibliography but with a narrower focus of the direct evidence for production. Because the processes on the lime component; Coutelas is mainly concerned with and practices continued through the mediaeval period and ancient Gaul and Traini with Roman Italy, although both into the nineteenth — and in some places the twentieth — range more widely over the area of the Roman empire and century, historic images and records plus ethnographic stud- its successors in Europe, North Africa and the Near East. ies are often used to fill out or interpret the ancient evidence. General considerations Physical requirements Although the use of mortars and plasters in the ancient In the period under question, there is ample evidence that the and early mediaeval worlds has long been recognised, the mortars used for a binder in mortared rubble construction, detailed processes which formed the chaîne opératoire for for mortar floors and for wall plaster, deployed the same construction and decoration have only relatively recently range of basic raw and processed materials. The differences become an important focus for archaeologists (see Coutelas came at the final stages of use, where the choice of recipe 2008 for a good overview). The general monograph of Adam and the manner of applying the material related to its spe- (1984) provided an accessible summary for a wider audience cific function. It is clear from the ancient written sources while also giving a general account of the processes of con- (see Lancaster 2021) that at least by the first century BCE, struction and rendering using lime-based mortars and plas- ancient builders had developed some firm ideas about the ters; the work has been translated into several languages and qualities and nature of the raw materials required for mak- still provides a starting point for many students. Lamprecht ing the best mortars and plasters for different functions, and (1984) is an early study of Roman mortared rubble construc- of the effects of using substitutes. Their preference for lime tion including archaeometric studies of the mortars. The made from pure calcitic limestone, identified by the white- operation of limekilns and the production of lime in general ness and hardness of the source limestone, meant that it was have been a particular area of interest since the pioneering often brought from considerable distance, while the aggre- study by Baradez (1957), followed up by Sölter (1970) and gate was most likely to be local, both inert sands and natural especially in the 1980s by Dix (1982) and Adam and Varene pozzolans as well as crushed terracotta, although imported (1985), who showed the importance of ethnographic studies pozzolans were also used. All of these had different implica- for understanding the production cycle. tions in terms of sourcing, preparation and transport. Interest in the logistics of supply and the economics of We should remember that these preferences were not use developed in the late 1990s following the publication based on any scientific analysis as we know it but on empiri- of this author’s study of the Baths of Caracalla in Rome cal observation, experience and experimentation. It is also (1997), although few concentrate on mortar (see Camporeale clear that the reality, as identified through modern scientific 2011 for a rare example). Since the 1990s, there has also analysis of mortars and plasters, does not always match up been an increasing number of scientific studies of ancient to the prescriptions of the ancient sources (Coutelas 2019), mortars and plasters which have contributed greatly to our something that in itself throws interesting light on ancient understanding of the sources of raw materials, the types of building practices and the role of economics in the construc- recipes used and the interaction between the components tion process. 1 3 Archaeol Anthropol Sci (2021) 13:195 Page 3 of 17 195 requirements: function and economy, the latter dictated Scheduling the supply of materials for mortar largely by accessibility and available transport routes. The most economical way of sourcing aggregate was to The need to assemble all the constituents for making mor- tar or plaster on site at the appropriate time has substantial use whatever local materials were the most abundant and the easiest to extract but still fulfilled the need for fine- implications for the logistics of the construction process. This is especially the case where mortar forms a key element grained material essential to the creation of the mortar (for an overview Coutelas 2009, 64–70). The use of local sands in creating the structural envelope, as was common from the second century BCE onwards with the development and and gravels in mortars for bulk construction, even when the quality of resultant mortar is poor, reflects this economic extensive use of mortared rubble construction (usually but misleadingly called ‘concrete’) using lime-based mortars in mentality; examples are discussed by Coutelas for several sites in Gaul (Coutelas 2005; 2011; 2012b), by Dilaria and Rome and central Italy (see Vitti 2021). Since mortar occu- pies about a quarter to a third of the volume of good-quality Secco (2018) for Aquileia and by Cardoso et al. (2014) for Ammaia in Lusitania (Portugal). It may also reflect, at a mortared rubble construction (DeLaine 2001; Camporeale 2011), this development brought about a large increase in distance, the tradition, previously common throughout the area of the Roman empire, of earth construction and mortars the amount of lime needed in urban areas, some of which — like Rome — did not have suitable limestones locally. Since which were made from local soils and clays. In some cases, further processing may have been necessary, including siev- mortar or plaster was needed at all stages of construction from the foundations to finishing, establishing and main- ing to remove coarser (or sometimes finer) particles or wash- ing to remove clay and soil impurities (Coutelas 2009), but taining a supply of lime may have been one of the priorities at the very inception of any building project and vital for careful choice of natural deposits would have avoided this in many cases, depending on the use to which the mortar extra-large ones like city walls, permanent military camps (e.g. at Tipasa, Baradez 1957), infrastructure projects like was being put. The use of local materials for mortars employed in bulk aqueducts or harbour works and major public buildings (e.g. the Baths of Caracalla in Rome, DeLaine 1997, 111–114 construction appears to have been the origin of the use in Rome of the pozzolanic pit sands (harena fossicia), which and 189–191). Once construction began, all the materials for mortar had to be available separately on site and already had such important repercussions for Roman architecture and engineering (Jackson et al. 2007); the famous pulvis prepared, with a chain of supply established to replenish them, since once the mortar itself was mixed, it needed to puteolanus from the Bay of Naples presumably had a similar origin, as in both cases there was a local volcanic geology be used within a very short space of time; this is particularly true for mortars using pozzolans in the aggregate. Changes and a relative dearth of ordinary inert sands. An interesting study by Marra et al. (2016) of some of the earliest mortared to the composition of the mortar within a single building project can therefore reflect not only technical requirements rubble structures identified in and around the city shows that the aggregates in the earliest mortars came from nearby and different phases of construction, but also problems with or alterations to the supply of materials, substantial breaks in outcrops or the construction site itself and included debris from working the rubble, itself a tuff of volcanic origin. In construction or changes in the workforce (Coutelas 2012a). Ec ffi ient sourcing of the ingredients of the mortar, combined other examples, the pit sand appears to have become mixed with tuff debris as a result of the two strata interfacing at with the relatively rapid curing times for the pozzolanic mor- tars of Rome, allowed an impressive speed of construction; the quarry. Towards the end of the second century BCE, there appears to have been a change to using a different — masons’ marks on the brick-faced walls of the early second century Baths of Trajan in Rome indicate that 15 m of ver- and more effective — pit sand deposit (pozzolana rossa) on the outskirts of the city, which fits with the revised date for tical wall were erected in only 2 months (Volpe and Rossi 2012). the start of wide-spread construction in mortared rubble in Rome (Mogetta 2015), requiring a more reliable and pre- dictable supply than simply using what was closest to hand. Nevertheless, Wehby Murgatroyd’s study (2016) of closely Sources, production and transport of primary materials contemporary mortars from the second century CE Ostia, the port of Rome, shows that even so close to Rome aggre- Aggregates gates in otherwise similar mortars used different mixtures of materials of volcanic origins, and not just the pozzolana Of the two main components which make up lime mortars rossa, with no effect on the mechanical quality of the result- ant mortars. and plasters, it is the aggregate that provides much of the volume and is the more variable component. The vari- The use of pozzolans has long been one of the main interests in scientific research into Roman mortars, and it is ability arises from two distinct and at times contradictory 1 3 195 Page 4 of 17 Archaeol Anthropol Sci (2021) 13:195 increasingly the case that the use of sources besides those and crops in rural ones. Siddall (2011) has demonstrated in of Rome or the Bay of Naples can be identified. Lancaster the case of the baths at Corinth that the ground terracotta (2015, 22–26; 2019) gives an overview of other places in came from pottery, particularly low-fired ceramics such as the Roman empire where local sources of volcanic materi- those used for amphorae and coarsewares, rather than from als were used in mortar, from the Rhineland in the west to bricks or roof tiles which tended to be fired at a higher tem- eastern Turkey, for example, at Sagalassos (Callebaut et al. perature and were both more difficult to process and less 2000), while Brandon et al. (2014) discuss this in relation reactive in the mortar. The overwhelming use of the material to the pozzolans used in harbour works around the Mediter- was in surface treatments, including pointing the joints of ranean. Other naturally occurring pozzolans of volcanic ori- masonry, wall plasters and the fixing base for marble and gin in Italy not included in Lancaster’s survey include those other wall veneer, mortar floors and the lining of water fea- from the Euganean Hills west of Padua, used for the theatre tures including aqueduct channels, cisterns, fountains and and amphitheatre of the Roman city (Bonetto et al. 2021), basins in baths. and from the Vulsini volcanic district in Tuscany (Marra and Even this volumetrically limited use of crushed terracotta D’Ambrosio 2013). At Nora in Sardinia, ground obsidian would have required, in all but the smallest applications, was also used as a pozzolan, which Columbu et al. (2019) an organised system for collecting and possibly marketing argue came not from its primary source but from abundant the discarded ceramics. This would not be surprising given deposits of obsidian tool-making waste from prehistoric con- that amphorae had other uses in the construction industry; texts around the city. whole, they were used to build enclosure walls (Serlorenzi In the Roman period, it is usually hard to identify the pre- 2010) or in drainage schemes (Carbonara et al. 2018), while cise extraction sites of these materials, when abundant local broken pieces were common as part of fills on building sites; sources were used and/or small quantities were required for they were also used as containers for building materials (see minor projects. In areas of continued habitation, larger quar- below). The waste ceramic for mortar then had to be crushed ries for aggregates were also commonly obliterated by later or ground, although there is not the evidence to say whether construction, as was happening already in antiquity. Exca- this happened at the collection point or the building site; vations in the House of Amaranthus at Pompeii revealed much may have depended on the quantities required. For a series of pits in the natural volcanic soil some at least of large quantities at least, it is feasible that animal- or even which appear to have been quarries to extract aggregate for water-powered mills were employed, using similar technol- either mud brick or (more likely given the first century BCE ogy to that used for milling grain. Although crushed ter- date) for mortar (Fulford and Wallace-Hadrill 1999), but this racotta was sometimes used in mortar for construction, for is a rare find. At Rome, because the pozzolan deposits are example, in the later second century CE bouleuterion at intercalated between those of lithoidal tuffs, the aggregate Smyrna (Turkey) (Felekoğlu et al. 2016) where it was clearly was frequently obtained from underground galleries which used for its structural properties, the difficulty of obtaining can still be identified, with examples from the Aventine the necessary quantities of material and the cost of process- (Marra et al. 2016), the Esquiline outside Servian walls from ing may have been limiting factors for more general use in the second century BCE (Serlorenzi 2014) and from the areas which did not have natural pozzolans. The continued south-east of city (Buccellato and Coletti 2014). Large piers use of crushed terracotta in plasters and floors, either on of the material were left to support the strata above, and little their own or combined with local pozzolans, perhaps reflects extra shoring seems to have been used. These pozzolans can the inherent conservatism of builders working in an empiri- be quarried simply using a pick, and given that the galleries cal tradition despite the hydraulic properties of the natural appear to have been at least 2 m wide, the material could be pozzolans (e.g. Rispoli et al. 2020). loaded directly into carts or panniers on pack animals at the quarry face. Some idea of the process of working of these Lime quarries in the Roman period can be deduced from much later sources, in particular from De Marchi (1894), when it There is overwhelming scientific evidence that in the main appears that very similar methods were in use. Roman period, when the use of lime-based mortars for rub- In contrast to these naturally occurring materials, the ble construction was at its height and very widely spread other main types of aggregates used in lime mortars — the over the empire, the preference was to use the purest cal- crushed terracotta and plant ash — were both waste products citic limestones available to burn for lime as recommended which were used for their pozzolanic actions particularly, by Vitruvius (see Lancaster 2021). At Ammaia, a Roman but not universally, to exploit their ability to resist mois- town in Portugal, Cardoso et al. (2014) have shown active ture (Coutelas 2019; Lancaster 2012). These two materials selection for this material on the part of Roman builders. would have been very different to source; waste terracotta Although in the earliest buildings a clay-based mortar was products were more abundant in urban situations, manure used, builders soon turned to using calcitic limestone, even 1 3 Archaeol Anthropol Sci (2021) 13:195 Page 5 of 17 195 though this had to be imported from an as yet unknown with late antiquity and the mediaeval period; Traini (2013, source, rather than making the lime from the local dolomitic 49–82) provides a useful catalogue with bibliography for the limestone, the latter only being employed in the post-Roman area of the Roman empire, while new finds are constantly period. Nevertheless, examples have been identified where extending the evidence (e.g. Traxler et al. 2018). in the absence of better material, limestone marls with a high While quicklime could be produced by the simple burn- clay content and even the shells of marine and riverine mol- ing of limestone in an open space, the resultant temperatures luscs were employed (Suméra 2009; Dilaria et al. 2019). In were not high enough to result in a complete calcination, and late antiquity and the mediaeval period, lime kilns often used the lime produced was contaminated by ash and charcoal debris from ruined or surplus buildings, including marble from the fuel (Traini 2013, 32–33). As the process leaves from statuary, funerary monuments, inscriptions and veneer. few visible traces, it was probably more wide-spread in the These appear frequently in urban centres (see e.g. Venditelli Roman period than is often assumed (for an example, see and Ricci (2015) for Rome; Lenzi (1998) for Ostia; Bonnie Coulthard 1999). The most common form of lime-kiln in (2016) for Galilee; Del Moro 2008 for Cyrene) but are also the archaeological record is that described by the Roman found in decommissioned Roman rural villas, exploiting senator Cato the Elder in the mid-second century BCE (see both marble and limestone building elements, as part of a Lancaster 2021), taking the form of a broad truncated cone broader practice of late antique and early mediaeval recy- of circular or elliptical plan, with one, or occasionally two, cling, often for church building (Munro 2016). accesses to the outside at ground level for adding fuel and The general process of lime-burning in antiquity has removing ashes, often with a ledge running around the inside been well-described in the modern literature (e.g. Adam at the base, and sometimes a pit in the centre for collect- 1984, 65–71; Dix 1982; Traini 2013, 31–48; Suméra 2009), ing the ashes (Fig. 2). The lowest stones (referred to as the and there is ample evidence, both archaeological and from charge) were laid initially over some kind of formwork or depictions, for example, on the mid-twelfth century mosaic corbelled to form a rough vaulted firing space which would from the south side of the nave of the Cappella Palatina be self-supporting once the props burnt away during firing, in Palermo (Fig.  1), to show that the techniques did not and then the rest of the charge was gradually added over this. vary enormously in later periods, even persisting until very recently in traditional lime-burning areas (for examples, see Adam 1984, 68–70; Wurch-Kozelj and Kozelj 1999). The archaeological evidence for actual limekilns is however rela- tively thin for the early- to mid-Roman periods compared Fig. 1 Mid-twelfth century mosaic from the south side of the nave Fig. 2 Late second century CE lime kiln no. 9, Lauriacum, Emms of the Cappella Palatina in Palermo (Sicily), showing the building of (Austria), aerial view. The kiln is built into the natural conglomer- the Tower of Babel. To the left, a worker loads fuel into a lime kiln. ate, and the combustion chamber (top) and access to the stoke hole At the bottom right, a man works lime in a lime-slaking basin, while (below) are built from rubble. The edge of the ash pit can be seen just beside him, another worker shovels lime into a basket being raised to above the stole hole, and the floor of the access corridor is discol- the second level of the structure. At the third level, the man on the oured by ash and charcoal. From Traxler et  al., 2018, 103, courtesy right holds a mason’s trowel. Photo author S. Traxler 1 3 195 Page 6 of 17 Archaeol Anthropol Sci (2021) 13:195 Excavated examples have a lower diameter ranging from 2 to strongly suggesting that lime producing on this military 7 m, with 3–4 m being the most common. Cato’s kiln, with scale would have been needed to serve the requirements of a diameter of just under 3 m, would have had a volume of a city the size of Rome. about 20 m3, producing perhaps 14–15 m of usable quick- All these kilns are of the periodic kind, that is, they need lime, while Baradez (1957) estimated that the largest of the to be filled, fired and then emptied before the cycle can begin kilns at Tipasa (Algeria), an ellipse with long axis around again. While historic figures for firing cycles vary greatly 6 m in diameter, had a volume of 90 m able to produce 65 (see DeLaine 1997, 112 note 48 for examples), Baradez m of quicklime. (1957) suggested that only three firings per month would Many of the excavated examples of the Roman period be feasible in his study of the kilns at Tipasa. This fits with are of single kilns, mainly in rural areas and arguably serv- experimental firings at Iversheim, which took 6–7  days ing the needs of villas or other rural settlements, and this is including the cooling, plus 3 days for loading and unloading the context of Cato’s kiln. Some at least of the lime would (Sölter 1970, 35–40); larger kilns would have taken longer have been used in agriculture rather than for construction to load, fire and unload, so that perhaps 12–14 firings were (Dix 1982). These are unlikely, however, to have been able possible in the drier months of the year (cf. Delaine 1997, to serve the needs of large urban centres or major infra- 112–114). The longer the firing, the more likely that the structure projects, yet relatively few limekilns have been limestone would be completely calcined and the resultant found in Roman cities; those for the baths at Vieil-Evreux quicklime of better quality. At some sites, notably Lucus (France) are an exception (Guyard et  al. 2008). Rather, Feroniae (Fontana 1995) and Tipasa (Baradez 1957), there they tend to be located at or near the limestone quarries is an arrangement of several normal-sized kilns of 3–4 m in and in rural areas where fuel was more easily found. The diameter plus one much larger one, which suggests some hilly area around Lucus Feroniae, along the Tiber north of kind of distinction in the production cycle, but whether this Rome, has provided several examples of sites with multiple was for different destinations for the lime or simply to create kilns directly adjacent to the quarries, in one case organised a more flexible structure of supply can no longer be deter - in pairs to allow for a more efficient firing cycle (Fontana mined. Relatively few known production sites based at quar- 1995; Savi Scarponi 2013); most show evidence of use over ries have facilities for slaking lime (see below), which seems a long period of time and can be interpreted as commer- to have been done mainly at the construction site (Suméra cial enterprises serving the needs of Rome. The existence 2009, 58). of subsidiary structures connected to the kilns, as at Lucus Lime burning also has a large fuel requirement, which Feroniae, reinforces the interpretation of these as perma- has been roughly estimated at 1.5–3 tonnes of wood per nent installations producing for a market rather than for m of lime, depending on the type and degree of moisture occasional or intermittent use supplying the needs of a rural (DeLaine 1997, 113; Suméra 2009, 40–42). The general estate. A similar phenomenon has been found in the Swiss use of oak and other hardwoods has been confirmed by Jura (Coutelas 2009, 56) and at several sites in Gaul, one Vaschalde et al. (2013) for Roman and mediaeval lime kilns at Touffréville with 16 permanent kilns of different peri- in southern France, based on a study of charred wood from ods demonstrating a long period of exploitation (Coulthard kilns. Local availability, however, appears to have informed 1999), although the particular market for the lime is far from the choice of species used, and there is evidence that smaller clear. Large groups of kilns have also been found near mili- materials, including typical Mediterranean shrubby plants tary sites or at least operated by Roman legionaries, along like rosemary and juniper, and even chaff or dried animal the northern limes and at Tipasa (Algeria); the best known dung, were also used at some point in the firing cycle. The are those at Iversheim (Sölter 1970), with a row of five kilns gathering of fuel must have been an important part of the plus one slightly later, while the most recently discovered, at lime-burning operation for the sites with large banks of Lauriacum/Emms (Traxler et al. 2018, 2019), had 12 kilns kilns, while for individual smaller kilns on rural estates, each 3–4 m in diameter (Fig. 2). While these have usually some at least could be accumulated from general arboricul- been interpreted as serving the needs of the Roman army in ture or other estate activities. the building of fortresses, the group of at least 10 kilns at Krivina, in northern Bulgaria, dating to the later first century CE has no clear relation to any major military sites of that Transport date using mortar in construction (Vagalinski 2011), and as at Touffréville no obvious market can be determined. This It has been estimated that one of the major elements in the author (1997, 189) has calculated that at least 21 large kilns cost of construction was the transport of building materials each producing 60 m quicklime and operating 14 cycles per (DeLaine 1997, 219), but this depended on the weight and year over four years were needed for the Baths of Caracalla, volume to be transported, the distance, the form of trans- port and the available infrastructure. Russell (2013) provides 1 3 Archaeol Anthropol Sci (2021) 13:195 Page 7 of 17 195 a thorough analysis of the problem in relation to building The situation was rather different for lime, which in the stone, much of which also applies to the components of early- to mid-Roman periods was generally produced at mortar and plaster, in particular for mortared rubble con- the quarry site where fuel could also be easily acquired structions where the volumes of mortar were considerable. and often had to be transported longer distances than This made the burning of limestone at source an obvious aggregates (for an overview, see Suméra 2009, 58–60). It solution, as it reduces the weight by about 44%. The natural is not therefore surprising that many of the known lime- pozzolans of Rome and the Bay of Naples weigh roughly 1 kilns are situated very close to major rivers such as the tonne per m , coarse dry sands and loose gravel about 1.5 Tiber (Fontana 1995; Savi Scarponi 2013), Rhine (Sölter tonnes per m and quicklime 1.1–1.5 tonnes, depending on 1970) and Danube (Vagalinski 2011). Since neither pro- the limestone used (Giuliani 1990, Table 7.1). duction nor construction sites were generally directly Land transport was much more expensive than water, and beside a river, some transport by cart would have been there was a closer relation between cost and weight over necessary at each end of the journey, with the additional land than by water. Although both aggregates and quick- transhipments involved. At the same time, river transport lime could be carried by pack animals, the volumes required for lime was not always possible; the lime for the Villa of suggest that wagons, drawn by oxen or mules, were the the Quintilii via Appia south of Rome has been shown to usual mode for overland transport, with a maximum load come from the Monti Cornicolani to the east of the city of 1–2 tonnes for standard carts. This did, however, require (Fichera et al. 2015), where river transport via the Tiber a fairly smooth and level surface, so that proximity to good would involve two sections of road transport scarcely if roads was important for any materials that had to be moved any shorter than direct road transport, plus the extra tran- over any distance. River transport downstream may have shipments. Transporting quicklime, as seems generally to cost about a tenth of land transport or a fifth if movement have been the case, saved on weight but posed other prob- upstream was involved, while sea transport may have cost lems. The material is both caustic and had to be protected 30–40 times less and involved much larger loads. Since from moisture to prevent premature slaking which would transferring loads from one means of transport to another make it unusable for construction. It therefore needed to added extra labour and cost, solutions involving the fewest be transported in some form of container, such as heavy transhipments may have been preferred. sacks or large baskets. Things could go wrong; the Greek Direct evidence of transport solutions for aggregates philosopher Theophrastos, writing in the early third cen- is almost non-existent, but the relatively short distances tury BCE, recounts the tale of a ship carrying lime and involved in most cases indicate that the norm would have textiles which caught fire after the lime became wet (cf. been to use carts, with the material being loaded directly Traini 2013, 83). at the quarry and unloaded at the building site without any need for transhipment. One of the pozzolan quarries identi- fied by Buccellato and Coletti (2014) south of Rome and just 1 km from Tiber might have used the river, but this On‑site storage and preparation of materials would have required two extra transhipments and an up-river journey, making the 12-km land journey along the major Efficient use of manpower in construction requires that all highway of the via Ostiensis a more likely solution. Some the necessary materials are at hand. Depots of building aggregates made longer journeys. The natural pozzolans materials are therefore a normal feature of construction found loose as ballast in the hold of a sunken ship at Pisa sites, including modern ones, but can be hard to identify (Marra and D’Ambrosio 2013), dated to the first decades of in the archaeological record. This is partly due to the fact the first century CE (Camilli 2012), came from the Vulsini that piles of dry materials, including aggregates, may leave volcanic district in Tuscany via the river Fiora, along an little or no trace once incorporated in the structure, and established commercial water trade route with a river port partly due to their location in open areas which are less and a sea port connected to the important town of Vulci. likely to be the focus of modern excavations (Guyard et al. According to Jacopo Bonetto, unpublished research by the 2008; cf. Spera et al. 2011 for a mediaeval example). The University of Padua has demonstrated that these pozzolans bulk of the archaeological evidence relates to lime-slaking from the Bay of Naples were used in the construction of pits and basins, which were generally dug into the working the large baths and theatre at Aquileia. The closeness of the surface and preserved by being buried at the end of the various sources of these pozzolans to the sea must have been building project and/or reused as rubbish pits (see the cata- an important factor in their exploitation. Trade in pozzolans logue in Traini 2013, 95–102). The villa at Brachaud, near from the Bay of Naples, used for harbour works, is a special Limoges, provides a rare example of a work depot with case discussed in Brandon et al. (2014, 223–226), who argue both lime-slaking pits and an area for aggregate marked for a major trade in the material across the Mediterranean. out by an edge of tile, the two separated by only a couple 1 3 195 Page 8 of 17 Archaeol Anthropol Sci (2021) 13:195 of metres (Loustaud 1983, Fig. 6). Pompeii also provides (c. 24.5 mm) in mortar floors and foundations, digitus (c. examples on a small scale, for example, in what appears 18.5  mm) in wall mortars and semiuncia (12.3  mm) in to be interrupted building works at the House of Amaran- plasters. At Ostia, from personal observation, the largest thus, with a pile of pozzolanic aggregate and lime, mortar aggregate in mortar for ordinary facing brickwork would and crushed terracotta stored in amphorae (Fulford et al., also pass through a digitus screen, while in fine-work used 1995-96). The final necessary element is water for slaking for decorative brick facing the maximum is generally less lime and mixing mortar, yet this is rarely discussed in the than 2 mm, possibly derived from a tenth of a digitus sieve archaeological context. (Fig. 3). In the Villa of the Quintilii just outside of Rome, the largest aggregate in the mortar for the core would have Aggregates fit through an uncia screen (Fichera et al. 2015, Fig.  4b). A similar distinction between core and facing can be seen in Since the proportion of aggregate to lime in most Roman the Baths of Caracalla in Rome (DeLaine 1997, 140). In the mortars was around 3:1 by volume, considerable areas amphitheatre at Aquileia, a semiuncia sieve seems likely needed to be set aside for storing aggregates on large build- for general construction and a very fine one for laying brick ing sites. Alternatively, and particularly for major build- (Dilario and Secco 2018, 183–185). ing projects, stocks of aggregate needed to be replenished on a regular basis if the construction schedule was not to Lime slaking be delayed. The variability of composition in mortars and plasters using the same basic materials, whether sands, Both Suméra (2009, 50–58) and Traini (2013, 83–91) natural pozzolans or crushed terracotta (e.g. Di Benedetto give good overviews of the process of turning quicklime et al. 2018), presupposes that some degree of processing into slaked lime through the addition of water, to make the and sorting of aggregates took place on site, which would essential component in lime mortars and plasters. The lim- have added another level to the supply chain, and required ited evidence available suggests that the usual process of space for working the different materials and storing them slaking lime was by fusion, involving adding water gradu- separately. ally to a layer of quicklime while mixing it continually with The evidence that aggregates for mortar and especially a long-handled draw-hoe (one with a blade at right angles plaster were sorted for size is clear, although little dis- to the handle) until the lumps of quicklime disintegrated. cussed in terms of the construction process. Davey (1974) This process appears to be what is being shown on a late is an exception; he grouped the maximum size of aggre- antique mosaic from Oued R’mel (Tunisia, now in the Bardo gates in construction mortars from the province of Britan- Museum; Adam 1984, Fig. 164); although the mosaic has nia into sizes that would have fit through sieves of uncia considerable lacunae, it clearly shows a man pouring water Fig. 3 Construction detail, Casa a Giardino, Ostia (Italy), built in the 120  s CE. On the right, part of wall showing standard brick facing, with mortar having a pozzolan aggregate which would go through a digitus sieve; on the left, the adjacent decorative pilaster, an integral Fig. 4 Lime-slaking basin, Molesme ‘Sur-les-Creux’ (Côte-d’Or, part of the same structure, with very narrow joints requiring the poz- France), early first century CE. The floor of the basin is made of zolan to be put through a fine sieve. Photo courtesy S. Camporeale planks of fir. From Coutelas 2005, Fig. 1, courtesy A. Coutelas 1 3 Archaeol Anthropol Sci (2021) 13:195 Page 9 of 17 195 from an amphora onto a pile of white material which is being Putting in place worked with a draw hoe by a largely lost figure. Fully slaked lime, if kept away from air, is more stable than quicklime, The process of building with mortar and plaster varied to and if covered, it can be stored for long periods. The benefit some degree with the size, nature and importance of the of storing lime before use was to make sure that the process project. We should not expect to find the same qualities of was complete and no lumps of quicklime remained which mortar or the same care in applying it on small domestic might react explosively when further water was added in the restoration works as on large public buildings. Nevertheless, making of mortar and in particular plaster. the basic organisation of work was presumably similar and The limited archaeological evidence for the Roman period dictated by the physical requirements of the site and the pro- indicates that on the whole, lime slaking took place at the ject. The wall painting from the Tomb of Trebius Justus in building site, whether this was the location of the lime kilns Rome (Fig. 5; Marucchi 1911) is the only surviving example or not. Lime slaking/storage pits have generally not been from the Roman world of builders working on a brick-faced well-recorded or published. Loustaud (1983) was one of the mortared rubble wall. The scene is remarkably modern: first to discuss the phenomenon, examining 8 basins from there are five workmen, wearing the short tunic of the slave ancient Limoges and a nearby villa; although the evidence or labourer, with two bricklayers standing on scaffolding has grown since then, Traini (2013, 95–102) only catalogues on either side of a wall, while one labourer carries a load 22 from Roman and late Roman sites, not all entirely con- of mortar up a ladder in half an amphora, another carries a vincing, although he notes that this is not an exhaustive list. load of bricks or rubble in a basket, and a third mixes mortar Most of the reliable examples are from domestic contexts using a long-handled draw-hoe. The scene can be broken and are fairly small, with a longest dimension of about 1 m. down into three essential actions: mixing the mortar, trans- The largest of those discussed by Loustaud (1983) measured porting the materials to the workface and putting in place. 1.4 × 2.4 m, with a volume of 1.58 m , and relates to villa baths. Some of these basins were pits dug into the earth and Mortar and plaster mixing lined with tiles and/or wooden planks (Fig. 4); others were of masonry and lined with cocciopesto. Traini and Man- While any aggregate processing and lime slaking needed to nelli (2013) report a much larger basin, 1.88 × 2.65 m and be carried out in advance of construction, mortar and plaster 1.82 m deep and lined with cocciopesto, which served the preparation had to take place concurrently with construction construction of the late second/early third century CE baths or decoration, especially when pozzolans were being used on the slopes of the Palatine in Rome, although there is no as such mortars begin the curing and hardening processes direct evidence to show that it was in fact used for slaking very quickly. or storing lime. A basin of this capacity could hold 5 m of slaked lime, enough to make c. 20 m of mortar at the nor- mal proportion of 1:3. To put this in perspective, the amount of slaked lime required for the Baths of Caracalla in Rome was over 100,000 m , which would have needed a basin of this size being filled over 20,000 times over the course of construction. It is clear that we have not yet found any major installations for lime-slaking on imperial building sites. A number of sites recorded by Loustaud (1983) have two lime-slaking basins, one larger than the other; he argues, on the basis of ethnographic parallels, that the larger was used for the primary slaking and the second for refining the result- ant product to ensure complete slaking and to remove impu- rities such as fragments of carbonated lime, which could be caused by the reuse of lime-slaking pits and basins; such fragments have been identified in archaeometric studies of mortars. Alternatively, the basins could have been used for different qualities of slaked lime, the larger for mortar and Fig. 5 Wall painting showing a scene of builders at work from the the smaller for plaster, where the complete slaking of the Tomb of Trebius Justus, Via Latina (Rome), fourth century CE. Two men on scaffolding either side of a brick-faced wall are laying bricks lime was more critical. The fact that such basins have been using masonry trowels. To the left, a man carries a basket full of rub- found still containing lime has led Loustaud (1983, 149) to ble or bricks, while to his right, another man climbs a ladder with a suggest that this was a deliberate act to preserve lime for halved amphora of mortar on his shoulder. To the right, a man mixes later maintenance. mortar with a draw-hoe. From Marucchi 1911, Fig. 5 1 3 195 Page 10 of 17 Archaeol Anthropol Sci (2021) 13:195 The recipes for mortar mixes in the written sources (see no evidence that Romans used wheelbarrows, but some form Lancaster 2021) are given as whole ratios such as 1:2 or of simple two-wheeled cart may have been used instead; 1:3 lime to aggregate by volume, and these proportions are nevertheless, the same reservations as for pack animals or roughly confirmed by archaeometric analyses (e.g. Wehby larger wagons apply. Murgatroyd 2016) although there is much variation. Such In the wall painting from the Tomb of Trebius Justus, a proportions could be achieved simply on site by using a bas- distinction is made between the man on the ground, who ket-load as a measure, which would account for at least some appears to have a basket load of bricks or rubble, and the of the variability in the results. Baskets appear on both the man on the ladder, who appears rather to be carrying the Oued R’mel mosaic and the Trebius Justus wall painting, as mortar in half an amphora. The caustic nature of both lime well as on Trajan’s Column in Rome, as standard elements in and mortar would make this a sensible choice; the use of construction scenes, usually as containers for moving mate- amphorae to store lime and mortar observed at Pompeii (see rials, but there is no reason that they could not be used at the above) suggests that this was common practice. In addition, same time for measuring. Romans used a modius for measur- the open nature of a longitudinally halved amphora would ing grain, but these would not have been feasible to use on make the mortar easier to access for the mason than the more building sites; this author (1997, 107) however has estimated closed form of a basket. that the baskets on Trajan’s Column would have contained around two modii or roughly a cubic Roman foot (0.026 m ). Using mortar in construction The process of mortar mixing is well described by Traini (2013, 103–112; cf. Coutelas, 2008, 2009, 71–72). It can be Mortared rubble masonry seen at the lower right-hand corner of the scene from the Tomb of Trebius Justus (Fig. 5) and reappears frequently The construction process for mortared rubble is generally in mediaeval representations of building sites. A measured well-understood; Adam (1984, 125–150) provides a useful quantity of aggregate was arranged on a flat piece of ground if brief introduction. It is, however, rarely if ever discussed in a rough circle with a well in the centre, the appropriate specifically from the point of view of how the mortar is used. quantity of slaked lime is placed in the centre, and the aggre- Overall, the construction process can be divided into three gate is mixed into the lime using the same kind of draw hoe stages: laying foundations, laying the facing and core of the as used in lime slaking, with limited water used as required walls and laying the vaults (if used). At each stage, the work to obtain the right consistency of the paste (see Adam 1984, could be done well or done poorly. Generally, what has been Fig. 163, for a reconstruction drawing). In order to produce preserved has been so by virtue of being at the better end of the best mortar or plaster, the mixing needed to be thorough. the scale; the very poor quality of some of the construction Coutelas (2008) provides examples from a number of sites at Pompeii, and in particular the weakness of much of the to show that mortar-mixing areas were often reused, and this mortar, is a reminder that not all Roman construction was of may have been the way in which small fragments of old mor- high quality. The following applies to the higher end of the tar found their way into fresh (for an example, see Dilaria construction spectrum. and Secco 2018). There is no evidence that Roman build- The fundamental principle of Roman mortared construc- ers used the kind of mortar-mixing machines which have tion is that the rubble is laid in mortar, rather than being been identified for mediaeval building sites (Bianchi 2011), mixed into the mortar before being put in place. Even in although this may simply be due to the absence of evidence. foundations, where the sides of the foundation trench, whether just earth or timber shoring, served to contain the Moving mortar and plaster around the construction mass of rubble and mortar, all the evidence points to the site rubble being either roughly laid or thrown into the trench in layers alternating with mortar and then rammed down to Over the short distances involved in most building sites, compact the mass and remove air pockets (cf. Dilaria and human portage was the most basic and most flexible way Secco 2018, 184). The mortar therefore was not liquid and of moving the different ingredients from the stockpiles and could not be poured like modern concrete. lime-slaking or storage pits to where the mortar was mixed The walls above ground were generally constructed as and then moving the mortar to the workface (for a sum- two faces with a central core, either with short vertical sec- mary, see DeLaine 1997, 107). Except possibly on very tions of the external faces being put in place and then the large building sites such as the imperial baths, pack animals core or the two rising together (Fig. 6). The ability of the and wagons would require extra manpower for loading and pozzolanic mortars of Rome and central Italy to cure and unloading and might simply clutter the site unnecessarily. gain strength rapidly, and the use of relatively narrow mortar Over short distances, a man can carry a load of around 50 kg joints, meant that the work could proceed rapidly without the or even more, around the same as a small donkey. There is need for any supporting formwork, although this may not 1 3 Archaeol Anthropol Sci (2021) 13:195 Page 11 of 17 195 Fig. 6 Construction detail, Grandi Horrea, Ostia (Italy), late second Fig. 7 Brick facing, Grandi Horrea, Ostia (Italy), late second cen- century CE phase. Section and face of wall, from left to right: section tury CE phase. Detail showing vertically struck joints (indicated by through neatly coursed brick facing; section through roughly coursed arrows); elsewhere, the surface of the mortar has deteriorated. Photo core; neatly coursed brick facing. Photo author author have been the case everywhere (cf. Camporeale 2011). As with the foundations, the core was laid more or less roughly double-struck joints which slope towards in both directions in courses alternating with mortar but generally without any away from a central ridge of mortar. In mortared rubble con- further consolidation, and it is not uncommon to see gaps struction using irregular pieces or where the quality of the between individual pieces of rubble where the mortar has bulk of the mortar in the core was poor, the joints between not penetrated (cf. DeLaine 2001, 234). Laying the facing the facing pieces could be finished at a second stage called required more careful work than laying the core and some- pointing, using a different quality of mortar and even almost times used a less coarse mortar, especially for finely finished pure lime putty to protect the structural integrity of the core work. The detail depended on the shape and form of the and/or to establish a smooth outer face, sometimes for deco- facing materials used, but the basic process was to bed each rative purposes (Coutelas 2005, 332–334; 2009, 78–82). facing element into a horizontal layer of mortar; for larger Mortar was used in vaulting with the shape defined by elements, such as rectangular blocks or reticulate pieces, wooden formwork which also supported the structure of mortar was first applied to the side of the element which the vault against gravity until it had cured sufficiently to was to be in contact with the piece previously put in place be self-supporting (see Lancaster 2005). Although in early as in modern bricklaying, while the triangular bricks used Roman vaults the rubble was set radially, in later buildings in mid-imperial construction in Rome and Ostia were often in central Italy where strong pozzolanic mortar was used, laid simply on the mortar bed, creating narrower horizontal the rubble was laid horizontally in the mortar exactly like than vertical joints (see Fig. 6). The mortar had to be of a the wall-core, and it is often difficult to say where the wall consistency that would give way slightly when the facing ended and the vault started. The main difficulty in using element was put in place, ensuring good contact between mortared rubble for vaults appears to have been separating the two, without flowing too much out of the joint. In the the supporting formwork from the underside of the vault best construction, the horizontal joints are of fairly uniform once it had cured. One solution used in Rome and Ostia was thickness while still allowing for variations in the thickness to cover the formwork with one or two layers of brick, which of the facing element; this suggests that the workmen main- remained attached to the underside of the vault when the tained a uniform consistency of mortar and used a specific formwork was removed; elsewhere the mortar skin on the amount for each course, even though this amount could vary vaults has often preserved the size and shape of the timbers both within a single building for different functions (Fig.  3) used for formwork (Lancaster 2005, 22–50; Coutelas and and from one building to another. Hourcade 2016). The mortar in the joints was then finished in various ways, being struck either vertically to be flush with the plane of the Trowels wall (Fig. 7) or at an angle or slightly hollowed out. Coutelas (2012b, 173–175) provides examples of both weather-struck The main tool for laying and finishing mortar in construc- joints, where the mortar slopes in from top to bottom and tion was the leaf- to rhomboid-shaped builder’s trowel, 1 3 195 Page 12 of 17 Archaeol Anthropol Sci (2021) 13:195 which has remained in use to the present day. Relatively between pointing and plasterers’ trowels can be made few actual examples have survived from antiquity, and there (Fig. 8). Gaitzsch found no examples from the Greek period are a small number of depictions, the earliest of which is and associated the development of the tool, and of local on the relief of the structor (builder) Diogenes at Pompeii. and specialised variants, to the growth of mortared rubble Gaitzsch (1980,133–147) provides a catalogue of 83 exam- construction in the Roman period. The range of shapes and ples from Italy and the north-western provinces, to which sizes can be paralleled in early twentieth century trowels, for can be added one from the Roman veteran colony of Cuicul example, those advertised in the catalogue of the specialist (Tunisia) now in the museum of Djemila (Belhout 2019, cat. company Forges de Mutzig (1929) which gives the place of No. and Fig. 14); imprints of a pointed trowel have also been use as well as the function of the different types. The wall found in mortar pointing at Cassinomagus (Coutelas 2012b, painting from the Tomb of Trebius Justus in Rome (Fig. 5) Fig. 50). Gaitzsch divides them into six types based on the is to my knowledge the only surviving representation from basic shape, with variants defined mainly by the different antiquity which shows a trowel being used by builders, but length to width ratios, while within each type, the trow- such scenes are relatively common in mediaeval construc- els vary also in length and breadth. This rather masks the tion scenes (Fig. 1). functional aspect of his types, which he otherwise divides into masons’ trowels (54 out of the 83), pointing trowels — Harbour works some also used in plasterwork (26 out of the 83), and three ‘pointed’ (or ‘English’) trowels, the latter being the familiar The production of mortar and laying of mortared rubble for form currently used by archaeologists. A further separation harbour works were rather different to those for land-based Fig. 8 A selection of Roman masonry, pointing and plaster- ers’ trowels, first to fourth centuries CE. A2 and B1 are the most common forms, while the pointing and plasterers’ trowels are relatively rare. Adapted from Gaitzsch 1980, 139 1 3 Archaeol Anthropol Sci (2021) 13:195 Page 13 of 17 195 structures, as described in the ancient sources (see Lancas- 1984, Fig. 522), while a few wooden floats, looking remark - ter 2021) and discussed in Brandon et al. (2014, especially ably like modern ones, have been found in excavations as 143–222). The success of mortared rubble harbour works well as examples of scrapers used by plasterers (Gaitzsch depended on the mortar. Nearly all of those tested by Bran- 1980, 11–18). For utilitarian purposes, the aggregate was don et al. (2014, 161) fall in the range of 55–60% mortar often crushed terracotta, and there was no fine finishing coat, by volume, compared with around 35% for land structures. although, as can be seen in many places at Pompeii, red pig- Jackson (in Brandon et al. 2014, 164–166) argues that in ment was sometimes applied to the surface. some harbour works either quicklime or aged slaked lime, Mortar floors, made with or without crushed terracotta in the form of small granules, may have been mixed with the aggregate, generally included more and larger aggregate pozzolan either dry or with a small amount of water, only which helped give the mortar more strength than similar fully hydrating in the seawater once put in place, although plasters; they were also often compressed to improve density many questions remain. In two out of the three types of form- and resistance, especially important for cisterns and other work used to create harbour structures discussed by Bran- water features. Bedding layers for veneer, on the other hand, don (in Brandon et al. 2014, 191–222), success depended were often little different to the mortars used in masonry, on the use of hydraulic pozzolanic mortar which would cure while those for mosaic usually had a top layer of almost pure and continue to gain strength when completely submerged. lime, sometimes with added fine aggregate, into which the Alternatively, harbour works needed to use cofferdams cre- individual mosaic tesserae were set. ated in situ, which allowed the structure to be built in a dry environment. Otherwise, the process appears to have been very similar to that used in the construction of foundations, Concluding summary of key concepts although in at least some cases, the degree of compaction is less, reducing the strength of the mortared rubble, presum- The focus of this paper is the chaîne opératoire of the con- ably due to the difficulties of working underwater. struction process from the production of the main ingre- dients — lime and aggregate — to their application in Plastering and finishing structures in the Roman period. First, Roman architects or builders had to identify suitable sources of raw materials The final stage of construction often involved adding a sepa- for the binder (usually lime) and aggregate. These then had rate coating to built surfaces, all of which involved some to be extracted and given any preliminary processing at the form of lime-based plaster or mortar. This includes mortar extraction site, before their transport to the construction site, floors, rendering coats for walls, the supporting base for where further preparation occurred, before the two compo- marble and stone veneer and mosaics on floors and walls, nents were mixed to form the mortar or plaster which was and special hydraulic treatments designed to waterproof a then used in construction or decoration. The emphasis here variety of structures especially aqueducts, cisterns, fountains is on mortared rubble construction, which was a particular and industrial basins. Coutelas (2009, 87–121, cf. Büttner development of the Roman period from the second century and Coutelas 2011) provides a good starting point for under- BCE and required much larger quantities of lime than previ- standing how mortar was adapted for these different types ous building techniques. of use in the context of ancient and mediaeval Gaul, and see The processes which formed the chaîne opératoire for Adam (1984, 216–234) for a more general account. construction have only recently become the focus of aca- Wall coverings nearly always consist of at least two lay- demic interest, and evidence for identifying the details of ers, and more normally three, rather than the seven recom- this sequence of events is uneven. It requires an interdisci- mended by Vitruvius (see Lancaster 2021); Di Benedetto plinary approach, and much has to be deduced by working et al. (2018) provide a good example from central Italy. The back from the finished product, while direct archaeologi- first (anchorage) layer was the thickest, as much as 50 mm cal evidence of the actual processes is relatively rare, and in some cases, as it had to even out the face of the wall. The the detail has not always been recorded or published. As mortar was applied to the wall using a trowel, with the sur- well as archaeological and archaeometric evidence, literary face left rough to help the second layer to adhere; the large sources, ancient representations and ethnographic studies particles of poorly sorted aggregate in the mix encouraged all contribute. The production of lime is the best understood this further. The second layer usually contained smaller and and has been the focus of considerable literature, as have the better-sorted aggregate and was given a flat finish probably components and working of pozzolanic mortars, both those using a float, as was the very thin final layer composed of using natural pozzolans and those using pozzolanic waste lime with little or no aggregate, which could be left plain products, especially crushed terracotta. or painted. The application of this final layer is shown in a The archaeometric identification of the ingredients of relief from Sens (Uffler 1971; reconstruction drawing Adam surviving mortars and plasters provides evidence for the 1 3 195 Page 14 of 17 Archaeol Anthropol Sci (2021) 13:195 need to obtain permission directly from the copyright holder. To view a choices by builders, in terms of the functional requirements copy of this licence, visit http://cr eativ ecommons. or g/licen ses/ b y/4.0/ . but also, and in many cases perhaps primarily, in terms of the economics of construction. Local geological settings, the physical environment especially the ready availability of fuel and their relation to the building site in terms of References transport routes, appear to have been primary factors affect- ing choice. In terms of extraction and preparation of materi- Adam J-P (1984) La construction romaine: matériaux et techniques. Picard, Paris als, the archaeological evidence shows that well-established Adam J-P, Varene P (1985) Fours à chaux artisanaux dans le basin technologies were used, especially notable in relation to the méditerranéen. 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Journal

Archaeological and Anthropological SciencesSpringer Journals

Published: Nov 1, 2021

Keywords: Lime; Aggregate; Mortar; Roman construction; Chaîne opératoire

References