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This paper presents observations and analyses on seven slag pieces from two third-millennium cal BC (Late Copper Age/ Early Bronze Age) rock shelters in the Trentino, north-eastern Italy: La Vela di Valbusa and the Riparo di Monte Terlago. We review previous work on contemporary slags from the region and show that the smelting did not follow the well-known ‘Timna’, ‘Eibner’ or so-called ‘Chalcolithic’ copper smelting processes. We show that ethnographic accounts of copper smelting in the Himalayas (Sikkim and Nepal) illuminate the smelting process, in particular the lack of preliminary roast- ing or ore beneficiation by washing, the use of slags as fluxes for the first smelt (matte smelting) and the use of wooden (?) implements to lift the hot slags from the furnace during the smelt. The rock inclusions in the slag are consistent with an ore origin from mines at Calceranica or Vetriolo, as previously reported in the literature. Keywords Trentino · Copper smelting · Slag · Fluxes · Ethnography · Archaeometallurgy Introduction such study which we have used with profit is Nils Anfinset’s (2001), Social and technological aspects of mining, smelt- As a by-product of smelting, metallurgical slags represent ing and casting copper: an ethnoarchaeological study from an ideal record of the technological processes undertaken Nepal. We shall call the process he describes the ‘Himala- in producing metal from ore, and they have long been seen yan’ method as it is substantially the same as that observed as such (review in Hauptmann 2014, 2020: 222–280). Slags by Blanford (1861) in Sikkim. are artificial and are the result of human choice and fur - The south Alpine region of Trentino–Alto Adige/Südtirol, nace firing conditions, but two important aspects should be in northern Italy, has extensive evidence for prehistoric cop- remembered (1) that they represent a specific moment in the per smelting, which is attested in two main periods (Perini smelting process—when the slag solidified—i.e. was ‘frozen 1989). The first phase, discussed in this paper, dates to the in time’, and (2) that the primary aim of the metalworker is third millennium cal BC, corresponding to part of the Cop- to make metal, not slags. per Age (CA; 3500–2200 cal BC) and Early Bronze Age As will be seen in this paper, the interpretation of slags (EBA; 2200–1650 cal BC). The dating of the second phase is not always self-evident, and we suggest that ethnographic (Marzatico 1997; Pearce 2007: 73–81, 2020; Cierny 2008; accounts of smelting can provide useful analogies, illustra- Silvestri et al. 2019) is controversial: Pearce et al. (2020, tions of possible ways in which metal may be smelted. One 2021) have argued, on the basis of radiocarbon dates and the isotopic signal of Trentino copper in artefacts, that the second phase begins in the fifteenth, perhaps even the * Mark Pearce sixteenth, century cal BC and continues until the ninth firstname.lastname@example.org century cal BC, i.e. from the Middle Bronze Age (MBA; Department of Classics and Archaeology, University 1650–1350 cal BC) to the Early Iron Age (EIA; 950–390 cal of Nottingham, Nottingham NG7 2RD, UK BC), while Marzatico (2021) argues, on the basis of mate- Deutsches Bergbau-Museum, Am Bergbaumuseum 31, rial culture at smelting sites, that it is limited to the Recent 44791 Bochum, Germany (RBA; 1350–1150 cal BC) and Final Bronze Ages (FBA; Ufficio Beni Archeologici, Soprintendenza per i Beni 1150–950 cal BC). It is also not impossible that produc- Culturali, Provincia autonoma di Trento, via Mantova 67, tion continues between the two periods (Pearce et al. 2021: 38122 Trento, Italy Vol.:(0123456789) 1 3 10 Page 2 of 21 Archaeol Anthropol Sci (2022) 14:10 193–196; cf. Mottes et al. 2012). A number of CA and EBA EBA tumulus and inhumation burial, close to the rock face, a sites are known (Fig. 1), and their main features are sum- spread of some hundreds of slag pieces was found. Because marised in Table 1; bibliographies for their archaeological of the nature of the excavation, it is not clear how much features are provided in the online resource (supplementary of the slag was recovered, and it is therefore unfortunately materials). Most of these sites are situated in the valley of impossible to quantify how much was originally present at the Adige/Etsch, particularly around Trento, with two hilltop the site. At the centre of this slag spread and overlying it, sites in the Fersina valley ore district to the east (Montesei di there was a roughly oval bowl-shaped area of baked clay Serso and Croz del Cius), and there is another cluster further associated with three tuyères (Fig. 2; Fasani 1990: 165–75); north in the Adige/Etsch valley around Bressanone/Brixen. a further area of baked earth was found 4 m away, again One recently discovered EBA site, Riparo di Monte Terlago, associated with slag (Perini 1989: 378, Fig. 11); these areas is located west of the Adige/Etsch valley, across the water- are probably best interpreted as roasting beds. The tomb is shed in the Valley of the Lakes, which drains southwards dated to the Early Bronze Age, so providing a terminus ante into Lake Garda. or ad quem for the smelting activity (Fasani 1990: 175). In this paper, we present observations and analyses on The Riparo (rock shelter) di Monte Terlago was discov- slags from two rock shelters in the Trentino: La Vela di Val- ered in 2009, at an altitude of c. 900 m a.s.l. in a calcare- busa (late CA or EBA) and the Riparo di Monte Terlago ous breccia on the slopes of the Paganella massif, and has (EBA). been excavated since 2010 (Dalmeri et al. 2011: 327). Slag The rock shelter of La Vela di Valbusa, in the Adige/ was found in layers 8 and 9 (EBA and MBA), which were Etsch valley just north of Trento, was discovered by local sealed between sterile layers (Dalmeri et al. 2011: 329); the enthusiasts in 1969 as a result of aggregates quarrying for slag presented in this paper comes from level 8; a radiocar- motorway construction and recorded in an emergency rescue bon date for layer 9 of 2470–2200 cal BC at 95.4% (LTL- excavation (Fasani 1990). Unfortunately, much of the stra- 15090A: 3862 ± 45; calibration using OxCal 4.4: Bronk tigraphy was damaged by the amateurs, but underneath an Ramsey 2009; Reimer et al. 2020), at the transition between Fig. 1 (1) Grotte di Castel Corno; (2) San Rocco; (3) Acq- uaviva di Besenello; (4) Romag- nano Loch; (5) Romagnano Maso Monache; (6) Romagnano Tof de la Val; (7) Romagnano Angeli; (8) Riparo del Santu- ario; (9) Montesei di Serso; (10) Croz del Cius; (11) La Vela di Valbusa; (12) Riparo Gaban; (13) Riparo Marchi; (14) Riparo di Monte Terlago; (15) Gudon/ Gufidaun, propr. Plank; (16) Tanzgasse; (17) Albes/Albeins, propr. Notflatscher; (18) Bres- sanone/Brixen Circonvallazione ovest; and (19) Millan/Milland 1 3 Archaeol Anthropol Sci (2022) 14:10 Page 3 of 21 10 1 3 Table 1 Evidence for Copper and Early Bronze Age smelting slags in Trentino–Alto Adige/Südtirol. All radiocarbon determinations calibrated at 95.4% using OxCal v.4.4 (Bronk Ramsey 2009; Reimer et al. 2020) Commune Site type Date 14C date Finds Ore Slag analyses Alto Adige/Südtirol (Bolzano province) Albes/Albeins, propr. Bressanone/Brixen Open CA/EBA Terminus ante quem Slag Notflatscher 2450–2020 cal BC (ETH-27484: 3770 ± 55 BP) Circonvallazione ovest Bressanone/Brixen Open CA/EBA Slag Angelini et al 2012; Artioli et al. 2015 Gudon/Gufidaun, Chiusa/Klausen Open Late CA Terminus ante quem Slag (including platy Chalcopyrite Colpani et al. 2009; Artioli propr. Plank 2580–2290 cal BC slag), 2 tuyères, 2 et al. 2009, 2015 (ETH-31825: 3950 ± 50 furnaces BP) Millan/Milland Bressanone/Brixen Open Late CA 2880–2480 cal BC (ETH- Slag, 4 tuyère frags, Chalcopyrite Artioli et al. 2005, 2009, 2015; Cremante and Storti 26698: 4090 ± 50 BP) fragment of copper, 2005; Colpani et al. 2009 grinding-stone and ham- merstone (?) Tanzgasse Velturno/Feldthurns Open Late CA/EBA 2580–2210 cal BC (ETH- Slag, grinding stone and Chalcopyrite 12287: 3945 ± 55 BP); hammerstone (?) 2840–2340 cal BC (ETH-12288: 3995 ± 50 BP) Trentino (Trento province) Acquaviva Besenello Rock shelter Terminus post quem (?) Furnace and slag; second Chalcopyrite D’Amico et al. 1998; Cattoi 3340–2900 cal BC furnace and tuyère et al. 2001; Anguilano (ETH-12497: 4410 ± 70 nearby et al. 2002; Metten 2003; BP) Artioli et al. 2009, 2015 Croz del Cius Pergine Valsugana Open; hilltop EBA Furnace, slag Artioli et al. 2009 Grotte di Castel Corno Isera Cave EBA Terminus post quem (ear- Tuyère and a slag lier burials) 2580– 2290 cal BC (DSH- 9256_GE: 3946 ± 40) Montesei di Serso Pergine Valsugana Open; hilltop CA-EBA Furnace, slag (including Chalcopyrite Metten 2003; Artioli et al. platy slag); mould 2009, 2015 Riparo del Santuario Lasino Rock shelter EBA Roasting floor(?), slag Riparo di Monte Terlago Rock shelter EBA Slag Terlago Riparo Gaban Trento Rock shelter Terminus ante quem Furnace, slag, tuyères and Chalcopyrite D’Amico et al. 1998; Cattoi 2630–2300 cal BC (Bln- crucible et al. 2001; Anguilano 1776: 3985 ± 50 BP) et al. 2002, Artioli et al. 2009, 2015 10 Page 4 of 21 Archaeol Anthropol Sci (2022) 14:10 1 3 Table 1 (continued) Commune Site type Date 14C date Finds Ore Slag analyses Riparo Marchi Trento Rock shelter End CA, beginning EBA Roasting structures (?), Mottes et al. 2014 tuyères and slag (includ- ing platy slag) Romagnano Angeli Trento Open EBA? Slag (including platy slag) Metten 2003 Romagnano Loch Trento Rock Shelter CA-EBA Crucible in layer Q: Furnace, slag (including Chalcopyrite Cattoi et al. 2000, 2001; between layers R, platy slag), crucible, Metten 2003; Artioli et al. 3710–3380 cal BC tuyères 2009, 2015 (R-775: 4810 ± 50) and P, 2290–1960 cal BC (R-769: 3720 ± 50 BP) Romagnano Maso Trento Open CA Slag Monache Romagnano Tof de Trento Open CA Roasting floor, slag and Chalcopyrite Cattoi et al. 2000, 2001; la Val tuyères Metten 2003 San Rocco Volano Foot of a cliff EBA Contemporary burials are Copper ingot, tuyère, dated 2340–2140 cal BC hammerstones and slag (KIA-12455: 3798 ± 26) La Vela di Valbusa Trento Rock shelter EBA? Roasting floors, tuyères Chalcopyrite Storti 1991; Metten 2003; and slag (including a Artioli et al. 2009, 2015 glassy slag) Archaeol Anthropol Sci (2022) 14:10 Page 5 of 21 10 Fig. 2 Site plan of La Vela di Valbusa showing the two roasting beds, the larger one, which underlies the kerb of an EBA tumulus, overlay a large spread of slag (after Fasani 1990: Fig. 9 and Perini 1989: Fig. 11) the Copper Age and the Bronze Age, provides a terminus and Bohemia, describing copper ore roasting (1912 : post quem for layer 8, which diagnostic pottery suggests 273–275, 279, 349–351) and copper smelting (1912 : dates to the EBA (Paolo Bellintani, pers. com. 28 Sept. 353–354, 374, 384–386, 388–390, 401–408). In Eibner’s 2016). Because of the small size of the test pit, it is not (1982) model, the ore was first roasted and then smelted; possible to quantify the amount of slag in the rock shelter, the resulting matte was then roasted and smelted to metallic but it would seem that the excavated area is marginal to the copper (matte conversion). focus of smelting activity, and no smelting facilities have been identified to date. ‘Free‑silica’ slags A number of models have been put forward for the smelt- ing process in the Bronze Age of the Alps, and we shall Another influential idea was introduced by Beno Rothenberg briefly review these. and Antonio Blanco-Freijeiro in 1981, in their interpreta- tion of slags found in Huelva province, south-eastern Spain. The ‘Eibner’ roasting and smelting model They noted the presence of quartz inclusions in many LBA and Phoenician slags, which they suggested might have been As we shall see, interpretation of the technological process added intentionally ‘to make the solidified slag more easily of smelting in the Alps has been strongly influenced by a breakable’ (Rothenberg & Blanco-Freijeiro 1981: 68, 176 model proposed by Clemens Eibner (1982), which in the note 28). Tylecote (1987: 306–7) later suggested that the 1990s replaced the ‘Timna model’ which had until then quartz was added at the end of the smelt to ‘thicken’ the been the dominant paradigm for understanding prehistoric slag, enabling it to be extracted easily from the furnace, lift- smelting processes. The ‘Timna model’ was based on the ing it off the melt. Craddock (2013: 245 note 13) argued the evidence for the Late Bronze Age (LBA)–EIA smelting of quartz was unreacted and that it was therefore added at the the largely oxidised sedimentary copper ore deposits of the end of the process to ‘quickly cool the slag making it much Negev, which are rather different from the complex copper stiffer’. With specific reference to the presence of quartz in sulphides of Europe (Craddock 2009: 3–4). Eibner (1982) CA-EBA slags from the Trentino, D’Amico et al. (1998: on the other hand interpreted the smelting process on the 34) maintained that, since it was thermically altered, quartz basis of the sixteenth-century AD account of Georgius was added as a silica flux to aid the smelting process, and Agricola (1912 ), who documented the matte smelt- this idea has been influential for the interpretation of the ing of complex sulphides in the Erzgebirge between Saxony slags from the Trentino. Hauptmann et al. (1993, 2003: 205) 1 3 10 Page 6 of 21 Archaeol Anthropol Sci (2022) 14:10 and Hauptmann (2000: 108–109; 2020: 240–243), however, nodules (D’Amico et al. 1998: 37); this important deposit explained unmelted remains of host rock and quartz inclu- is situated to the east of Riparo Gaban and Acquaviva in the sions as restites, i.e. unmelted parts of a given charge. Valsugana. The same minerals are also present at Vetriolo (Levico Terme), close to Calceranica. Finally, they argued The ‘Chalcolithic copper smelting model’ for a single smelt, made possible by the addition of a quartz flux. It has been observed that early smelting was different to fully Cattoi et al. (2000) present three CA-EBA slags, two developed Bronze Age technologies (Hauptman 2000: 102). from Tof de la Val and one from Romagnano Loch, along It is argued that all of the very earliest smelting processes with slags from later Bronze Age sites in the Trentino. They were small scale and utilised very rich ores, typically oxide argued that the abundant quartz in the slags was typical of ore or mixed oxide-sulphide ore. No flux was added to the the phyllites of the south Alpine basement (Cattoi et al. smelt, and the process was likely self-fluxing. The smelting 2000: 129), so that the ore was metamorphic in nature; the produced small, nut-sized slags that were not fully liquefied high K O and Al O suggested micaceous-chloritic phyl- 2 2 3 and therefore non-optimised (from a modern viewpoint). As lites. This paper too argues that the quartz was added to a result of the poor mastery of the smelting process, the slags the smelt as a flux and that Calceranica was the most likely show a poor level of reduction and separation and were thus provenance of the chalcopyrite ore. They further suggest that wasteful, which meant that the slag often required manual the CaO content is greater than is to be expected (4.41% in beneficiation to separate out the prills of metallic copper the Romagnano Loch slag, 2.33% and 3.26% in those from (Hauptmann 2000: 101–116; Hauptmann et al. 2003; Bour- Tof de la Val), indicating the addition of small amounts of garit 2007). carbonatic flux. In a later article in English, Cattoi et al. (2001) summarise Previous analyses of CA/EBA slags from Trentino the results of D’Amico et al. (1998) and Cattoi et al. (2000). They confirm the presence of phyllites and quartz-bearing Storti (1991) published bulk quantitative analyses of four metamorphic rocks and again argue that the abundance of slags from La Vela di Valbusa. He hypothesised the use of quartz in the slag is a result of its addition as a flux and that chalcopyrite ore and suggested that three of the slags were calcite too must have been added as its abundance is ‘not matte smelting slags (his n.1, found to the west of the burial, compatible with phyllitic chemical compositions’ (Cattoi and n.s 2 and 3, found to the north of the burial), while he et al. 2001: 152). suggested that a more glassy slag (n.4), found to the north Anguilano et al. (2002) describe thin sections of CA, EBA of the burial, was produced while smelting matte to metallic and later Bronze Age slags, noting the stages through which copper (matte conversion), on the grounds of its higher com- the chalcopyrite is reduced to metallic copper. Following position of Fe-oxides and ZnO (Storti 1991: 351–3, tab1). Donaldson (1976), they suggest on the basis of the fayalitic He further argued that the area of fire-hardened clay was a olivine crystals identified that the coarse slags cooled at a roasting bed, noting that similar structures had been found rate of 1–50 °C per hour, which they claim ‘would indicate at Tof de la Val, at Acquaviva di Besenello and at Montesei a total of at least 20 h for the cooling of the coarse slags to di Serso (Storti 1991: 354). room temperature’ (Anguilano et al. 2002: 636). They con- D’Amico et al. (1998) studied 12 slags from Riparo firm the hypothesis of a two-stage smelting, hypothesising Gaban and ten from Acquaviva. They found phyllites and that there may also have been preliminary roasting, but note other quartz-bearing metamorphic rocks in the slags, and that according to D’Amico et al. (1998), where no platy slag presence of relicts of chalcopyrite (D’Amico et al. 1998: is present (for example, at CA-EBA sites), the process was 34). SEM-EDAX analysis of the glassy matrix reflected the one-step. composition of the phyllites (SiO , Al O, K O), with the As part of her major study of RBA and FBA slags, 2 2 3 2 addition of Fe and Zn, which presumably originated in the Beate Metten (2003: 8, 74–7, Figs. 34–6, pictures 33–6, ore, and CaO, which they suggested indicated the addition tables 28–32) studied coarse slags from Acquaviva, La Vela of a limestone flux. They found that the slags from Acq - di Valbusa, Montesei di Serso, Romagnano Loch, Romag- uaviva were more homogeneous. Noting that Zn minerals nano Tof de la Val and Romagnano Angeli, plus a platy slag were present, they argued that the chalcopyrite ore body from Romagnano Angeli. She noted that chalcopyrite and was metamorphic in origin, that quartz was added as a flux pyrite were smelted but suggested that the presence of mag- and that differences in composition suggested different ore netite agglomerations associated with copper prills might bodies which were exploited by the two sites. They con- indicate that at least part of the charge was secondary ores. cluded that the ore was most probably mined at Calceranica, Colpani et al. (2009) present analyses of slags from the a metamorphic deposit with pyrites, chalcopyrite and zinc Alto Adige/Südtirol province, 25 from Millan/Milland and blende, which is co-genetic with phyllites rich in quartz 12 from Gudon/Gufidaun—propr. Plank. Both sites have 1 3 Archaeol Anthropol Sci (2022) 14:10 Page 7 of 21 10 both coarse and dense slags; the latter are more homoge- at this site and the Alto Adige/Südtirol sites (Millan, Gudon, neous, have fewer vesicles and less gangue material, but Bressanone/Brixen Circonvallazione Ovest) (Artioli et al. more copper droplets, and so are argued to represent a more 2015: 81). Summarising previous work, they state that ‘All advanced stage in the smelting process. Platy slags (often the investigated smelting slags from Trentino (Romagnano called Plattenschlacken in the literature) are reported only Loc, La Vela, Gaban, Acquaviva di Besenello, Montesei di at Gudon and show low viscosity. The ore is identified as Serso) and Alto Adige/Sud Tyrol (Millan, Gudon, Bressa- chalcopyrite with accessory sphalerite and galena, and it is none Circonvallazione Ovest) have been recently character- argued on the basis of the fayalitic olivine crystals that the ized … and demonstrated to be the product of copper smelt- coarse slags cooled at a slower rate (< 100 °C per hour) than ing activities of chalcopyrite-based mineral charges, with an the dense slags (up to 500 °C per hour). immature technological extraction process referred as the Artioli et al. (2009) summarise investigations of early “Chalcolithic” smelting process’ (Artioli et al. 2015: 78). slags, including the sites of Gudon, Millan, Acquaviva, Rip- aro Gaban, La Vela di Valbusa, Croz del Cius, Romagnano The Himalayan smelting method: an ethnographic Loch and Montesei di Serso in our study area. While they model found only coarse slags at Millan, Acquaviva, Riparo Gaban and La Vela, they report platy slags from Gudon, Monte- Blanford (1861) describes in detail a traditional method of sei di Serso and Romagnano Loch, with wüstite present in smelting he observed in Sikkim, which is similar to the pro- platy slags from Montesei di Serso and Romagnano Loch. cess observed by Nils Anfinset (2001: 47–61, figs 10-20, Some of the coarse slags showed ‘a more advanced degree tabs 5–7) in Okharbot, western Nepal. The two accounts of transformation’ (Artioli et al. 2009: 17), with 30–40 wt% appear to describe the same process, but the two perceived magnetite and no quartz inclusions. Dense coarse slags with and interpreted what they observed differently. Some details high magnetite content were found at Gudon, Millan and that were noted by one are ignored by the other. For exam- Romagnano Loch. A fragment of chalcocite-rich matte was ple, the multiple cycles of ore charging and the continual identified from Montesei di Serso (Artioli et al. 2009: 17). removal of slag cakes during the first smelt, clearly described They were unsure whether the quartz was added as a flux by Blanford (1861: 390), are not mentioned explicitly by or present as gangue material, and whether smelting was Anfinset but can be inferred from the information supplied one-step or two. They suggested that the chalcopyrite ores in the tables and by the mention that during smelting the were associated with sphalerite (Millan) and tennantite-tet- bellowers take short breaks to take out slag (Anfinset 2001: rahedrite (Montesei di Serso and Romagnano Loch). They 51). According to the descriptions, the copper ore is first commented that most of the CA ‘coarse slags are unbroken enriched by washing. The bottom of the furnace is filled and slowly cooled in the furnace indicates that the molten with a bed of crushed charcoal in order to stop the copper matte was already efficiently removed from the slags dur - sinking to the bottom of the furnace. Air is delivered from ing the high-temperature operations, without the need of above through long tuyères reaching into the centre of the extracting matte prills from the cold slags by breaking them’ furnace. Blanford (1861: 390) describes the first smelt as (Artioli et al. 2009: 18). a fusion process where charcoal is used to melt the ‘crude Angelini et al. (2013) provide a brief review, noting that metal’ or copper matte out of the ore and the waste accumu- the CA and EBA slags they analysed fit with the ‘Chalco- lates as slag cakes, which are lifted from the furnace with lithic copper smelting model’ (Hauptmann 2000; Haupt- pincers. The cyclical process of charging and slag lifting was mann et al. 2003; Bourgarit 2007), in as much as they are repeated continuously until enough matte was collected in mineralogically heterogeneous, contain large quantities of the bottom of the furnace. Anfinset adds that matte conver - non-reacted phases and do not have a homogeneous liquidus sion slag from previous smelts is continuously added to the phase, showing no evidence for the addition of fluxes. r fi st smelt to accelerate the melting and instead of using pin - Artioli et al. (2015) present Pb-isotope analyses for CA cers, simple wooden sticks are used to stir and lift the slag slags from the study area, arguing that the Trentino smelting from the furnace. After around 45 min, ‘the smith takes a sites of Acquaviva, Riparo Gaban, La Vela di Valbusa and wooden stick and stirs roughly in the furnace to separate the Romagnano Loch in the Adige/Etsch Valley show ‘affinity slag and the molten metal even more, while the bellows are to the ores of the Pre-Variscan massive deposits related to continually worked’; after a few minutes, the charcoal is then the Hercynian basement’ and the ores are therefore likely removed, and the slag is allowed to cool (Anfinset 2001: to have been mined at Calceranica (Artioli et al. 2015: 81; 52). Once the slag has hardened, it can be lifted out of the thereby confirming D’Amico et al.’s ( 1998) considerations furnace using wooden sticks, leaving the accumulated cop- on the mineralogy of the slag inclusions). Although Monte- per matte in the bottom of the furnace to cool. The matte is sei di Serso is close to Calceranica, ‘Post-Variscan sulphidic then crushed and ground and mixed with cow dung to form ores related to Permian and Triassic volcanics’ were smelted small balls, which are then roasted using tree bark as fuel. 1 3 10 Page 8 of 21 Archaeol Anthropol Sci (2022) 14:10 The matte and cow dung balls are then placed in the furnace been previously studied. We therefore chose to investigate for a second, matte conversion smelt. The furnace is stirred in detail five slags from the Riparo di Monte Terlago and occasionally by the smith with a wooden stick in order to two slags from La Vela di Valbusa. It should be stressed speed ‘up the separation of the slag from the molten metal’ that we looked at complete profiles of the slags, which has (Anfinset 2001: 53–54). The slag is then removed at regular not been done before for slags of this period from the area, intervals using two wooden sticks, until only copper is left rather than thin sections from a larger number of slags. in the furnace. Slag from the second smelt ‘is reused in the The wide cross-section of three slag cakes required mul- next first smelt because it is said to speed up the smelting tiple mounted thin sections, each of which was separately process’ (Anfinset 2001: 55). analysed (MT13: 3 thin sections; VB2: 4 thin sections; Since 2001, researchers have begun to apply these ethno- VB 10: 4 thin sections—see Fig. 4). All the slags are visu- graphic examples to the study of Late Bronze Age Alpine ally and morphologically similar, and all appear to contain smelting technology, particularly through experimental the same constituents (Fig. 3). They belong to a relatively archaeology (Goldenberg et al 2011; Hanning 2012; Della standardised technological process. The overall sample Casa et al. 2016; Reitmaier-Naef 2019; Rose et al. 2021), but number therefore reflects the heterogeneity of the assem- this has yet to be applied to evidence from the earliest phase blage—the more heterogeneous it is, the more samples of copper production in the Chalcolithic/Early Bronze Age. need to be taken. The slags were first sawn and slices were mounted as polished thin sections, and fragments were cut Research questions and milled for X-ray diffraction (XRD) and inductively coupled-plasma mass spectrometry (ICP-MS) at the labo- In this paper, on the basis of our observations and analyses ratory of the Deutsches Bergbau-Museum Bochum. The of slag cakes from La Vela di Valbusa and the Riparo di profiles of three slag cakes (MT13, VB2 and VB10) were Monte Terlago, we seek to reconstruct the smelting process mounted in three or four thin sections to better assess the used to produce copper in the third millennium cal BC Tren- overall heterogeneity of the material (Fig. 4). tino, north-eastern Italy. The thin sections were prepared by grinding to a thickness of 30 µm and polishing the surface with abrasives and lead. This allows the slides to be viewed with transmitted and reflected Methods light microscopy and scanning electron microscopy (SEM). A Zeiss Axiophot optical microscope was used as well as a Zeiss Seven slag pieces were investigated to determine their Gemini SEM with energy dispersive X-ray spectroscopy (EDS) mineralogical and chemical composition and to describe capabilities (Thermo UltraDry Silicon Drift X-ray Detector). the petrographic texture of the slags (Fig. 3; Table 2). As The slides were not coated with carbon and were thus analysed noted above (cf. Table 1), a number of CA slags from the under low vacuum conditions (30–50 Pascal). An energy of Trentino have previously been analysed by other work- 20 kV and a working distance of 13.3–16.4 mm were used. The ers, including slags from La Vela di Valbusa. On the other EDS NSS software (Noran System Seven) uses a fitted standard hand, slags from the Riparo di Monte Terlago have not calibration and can be viewed as semi-quantitative. Approximately 20 g from each slag sample were crushed using steel implements and milled in portions with an agate ball mill to grain sizes under 30 µm. The XRD was performed on milled samples using a PANalytical X’Pert instrument (PRO MPD) with X’Celerator detector and HighScore Plus software for analytical interpretation. The analysis requires about 100 mg of homogenised powdered sample (< 0.063 mm fraction size), and the samples were analysed with ADS (automatic divergence slit) Cu-Kα-radiation of 1.54178 Å at 45 kV (40 mA) with angle array set to 5–70° 2-theta at a rate of 0.017°/10 s. The digestion of the slag for quantitative ICP-MS was carried out with a μPREP-A microwave using concentrated acids. The sample size was 100 mg of homogenised pulver- ised material. The sample material was digested in PTFE pressure vessels with a mixture of concentrated acids (6 ml HCl: 1.75 ml HF: 4.8 ml HN O ) for 40 min at 250 °C. In a second step, 10 ml of boric acid (50 g/l) was added, and Fig. 3 Detail of slag VB5, showing charcoal (note the cell structure the samples were then heated to 200 °C for 20 min to avoid of the wood) and whiteish quartz inclusions 1 3 Archaeol Anthropol Sci (2022) 14:10 Page 9 of 21 10 Table 2 List of slag investigated in this study. Seven slag samples were selected for sampling (*) Riparo di Monte Terlago, Terlago TN, Italy Arch. ID Lab ID Material Weight (g) l × w × th (mm) MT2 4364/16 Slag 20 43 × 33 × 18 MT3 4365/16 Slag 41 44 × 38 × 16 MT4 4366/16 Slag 48 58 × 38 × 21 MT5 4367/16 Slag 123 84 × 69 × 26 MT6 4368/16 Slag 27 49 × 35 × 28 MT7* 4369/16 Slag 60 49 × 43 × 22 MT8* 4370/16 Slag 16 36 × 29 × 12 MT9 4371/16 Slag 14 31 × 27 × 10 MT10* 4372/16 Slag 22 50 × 36 × 21 MT11* 4373/16 Slag 15 37 × 24 × 15 MT12 4374/16 Slag 13 32 × 26 × 24 MT13* 4375/16 Slag 158 82 × 59 × 27 La Vela di Valbusa, Trento TN, Italy Arch. ID Lab ID Material Weight (g) l × w × th (mm) Comments VB1 4376/16 Slag 436 140 × 117 × 43 Mostly complete; impression from pole and tong marks on upper surface VB2* 4377/16 Slag 418 146 × 115 × 25 Mostly complete; impression from pole and tong marks on upper surface VB3 4378/16 Slag 298 123 × 77 × 31 VB4 4379/16 Slag 16 50 × 25 × 16 VB5 4380/16 Slag 359 117 × 75 × 37 Impression from wooden pole and tong marks on underside VB6 4381/16 Slag 307 122 × 84 × 26 Tong marks on upper surface VB7 4382/16 Slag 197 95 × 72 × 30 VB8 4383/16 Slag 262 93 × 85 × 29 Tong marks on upper surface VB9 4384/16 Slag 219 100 × 71 × 29 VB10* 4385/16 Slag 280 129 × 92 × 26 Mostly complete, tong marks on upper surface VB11 4386/16 Slag 250 122 × 69 × 31 MT Monte Terlago, VB La Vela di Valbusa, l length, w width, and th thickness the precipitation of calcium fluoride and aluminium fluo- 436 g. They had relatively flat faces, with bubbles and pro- ride. Finally, digestions were diluted with ultra-pure water tuberances. Some showed signs of having been broken in up to 100 ml. The ICP-MS analyses were performed using a antiquity (e.g. VB4). Indentations were noted on a number Thermo Scientific ELEMENT XR with ISDS software. The of slags; for example slag VB1, which is virtually whole, analyses were carried out with a FAST SC-system, ST 5532 has a v-shaped indentation on one face (Fig. 8), while the PFA μ-FLOW nebuliser, Peltier-cooled PFA spray chamber other face has a depression corresponding to the indenta- and 1.8 mm sapphire injector in triple detector mode at all tion, with protuberances around it. Most of the slag frag- three different mass resolutions (m/Δm) depending on the ments had inclusions visible to the naked eye, such as quartz elements of interest. Twenty-four elements were measured and chalcopyrite ore. Figure 3 shows a detail of slag VB5, (Na, Mg, Al, Si, P, S, K, Ca, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, with quartz and charcoal inclusions clearly visible. A large As, Ag, Sn, Sb, Ba, Pb, Bi and U). area of matte was visible on the lower face of fragment VB6. The thirteen slag fragments from the Riparo di Monte Terlago included in the study are much more fragmentary Results than those from La Vela di Valbusa and generally had much fewer and smaller inclusions but are more likely to General observations have visible charcoal impressions. Inclusions visible to the naked eye include light-coloured fragments of rock and Eleven slag fragments from La Vela di Valbusa were avail- quartz. able for study (VB1-11). The whole slag cakes seem to have Sections of three slag cakes can be seen in Fig. 4; they had an original diameter of 12–14 cm and weighed up to show a coarse breccia texture with numerous bubbles and 1 3 10 Page 10 of 21 Archaeol Anthropol Sci (2022) 14:10 Fig. 4 Sections of three slag cakes from top to bottom: MT13, VB2 and VB10. All slags show a pronounced het- erogeneous breccia-like texture with large and small, partially angular inclusions of rock frag- ments and numerous bubbles imbedded in a black slag laminated and fractured angular rock fragments imbedded optical microscopy (Fig. 6a) and XRD of two quartz inclusions in a black slag. (MT13, VB2), and between the bands of quartz, the phyllosili- The XRD results and the bulk slag compositions deter- cates are almost entirely vitrified exhibiting a glassy, bubbly mined by ICP-MS can be found in Tables 3 and 4. structure. In very few cases can feldspars be observed, but one example is in MT8 where microcline (K-feldspar) is present Microscopy: inclusions and phases in gneiss together with quartz and mica. In addition to foliated rock inclusions, fine-grained rock inclusions mostly consisting Rock inclusions of ferromagnesian aluminosilicates with compositions similar to iron-rich chlorite were found in all slag samples. These rock The slag cakes contain abundant rock inclusions that are com- inclusions are relatively homogeneous agglomerations of small monly associated with chalcopyrite and pyrrhotite (Fig. 5a). (< 100 µm) flake-like crystals and often pyrrhotite and chal- The large rock inclusions were heavily altered during smelt- copyrite fill the porosity. Chlorites can be formed by hydro - ing. They often have foliated texture consistent with meta- thermal alteration of igneous rocks and during metamorphism, morphic rock, resembling schist, and consist primarily of i.e. chloritic schist (Deer et al. 1962: 131), and some iron-rich quartz, relics of phyllosilicates and ore minerals. Cristobalite, chlorite minerals can be associated with sulphide ore minerals a high-temperature form of SiO , could be identified both by (Doelter 1921: 324–5), as appears to be the present case. Table 3 X-ray diffraction results MT7 MT8 MT10 MT11 MT13 VB2 VB10 MT13* VB2* of the slag samples and two rock inclusions extracted from Quartz x x x x x x x x x the slag (*) Tridymite x x x x x Cristobalite x x x x x Fayalite (magnesian) x x x x x x x Magnetite x x x Hercynite x x x x x x 1 3 Archaeol Anthropol Sci (2022) 14:10 Page 11 of 21 10 Table 4 ICP-MS bulk slag MT7 MT8 MT10 MT11 MT13 VB2 VB10 compositions. Values of main % % % % % % % and minor components are given in weight percent (%) SiO 46.5 43.4 47.7 51.7 43.8 45.9 51.7 and trace elements in parts per TiO 0.22 0.29 0.22 0.29 0.21 0.35 0.27 million (ppm) 2 Al O 6.29 7.94 7.45 8.41 6.97 9.06 8.13 2 3 FeO 28.9 32.2 29.8 26.4 30.6 27.1 26.8 MnO 0.17 0.21 0.21 0.24 0.20 0.21 0.15 MgO 3.04 3.94 3.94 4.31 3.61 4.35 3.88 CaO 4.84 7.12 6.32 2.62 3.22 2.49 1.52 BaO 0.01 0.02 0.01 0.01 0.005 0.01 0.01 ZnO 1.66 1.27 2.35 1.19 1.79 0.79 0.42 K O 0.87 0.69 0.60 0.46 0.75 0.85 0.54 Na O 0.05 0.08 0.06 0.06 0.05 0.06 0.19 P O 0.41 0.30 0.37 0.16 0.18 0.16 0.12 2 5 S 1.16 1.08 0.93 0.59 1.30 1.29 1.12 Cu 1.41 0.87 0.46 0.57 1.92 1.81 1.37 Total 95.5 99.4 100.4 97.0 94.6 94.3 96.3 ppm ppm ppm ppm ppm ppm ppm Pb 1300 690 1100 1200 2300 840 300 Ag 25 5 6 7 20 20 9 As 45 95 120 40 30 55 35 Sb 1860 250 2200 610 1400 110 180 Bi 15 4 20 6 30 8 15 Co 170 150 170 90 210 90 110 Ni < 5 < 5 15 < 5 < 5 < 5 < 5 Cr 15 20 40 20 11 30 15 Sn 30 35 25 15 35 20 40 U 1.7 1.8 1.8 1.7 1.9 2.0 2.8 Olivine material. The olivine crystals in the slag show diverse morphological crystallised features, but the most promi- One of the most commonly encountered phases in the slag nent forms are hoppers that range from euhedral crystals is olivine. The olivines in the slag are in the forsterite (Mg- to hollow elongated structures with marked edge growth rich, Fo)–fayalite (Fe-rich, Fa) system and are exclusively (Fig. 5b). It is clear that olivine did not achieve equilibrium on the fayalite side of the spectrum. The olivine crystals in with the surrounding liquid melt because of the presence slag from both smelting sites often exhibit zonation with of secondary olivine phases that quickly crystallised out of a core richer in magnesium oxide a phenomenon that has solution as the melt reached the olivine solidus tempera- been described in later Bronze Age slags from Trentino ture. The hoppers are normally under 100 µm in much of (Addis et al. 2017: 990). The high magnesium oxide con- the slag, but there is a perceptible tendency for the olivine tents (Fo ≈ 28%) were found in some cores, but most of hoppers at the bottoms of the slag cakes to be up to twice the 28 olivine cores analysed were much richer in iron (Fo the size of those on the upper parts of the slag, reaching ≈ 18%, or 57 wt% FeO). Following the work of Bowen sizes up to 200 µm. and Schairer (1935), within the pure system, the cores of the olivine richest in magnesium oxide would crystal- Spinels and iron‑rich regions lise at a temperature around 1320 °C and finalise with the crystallisation of olivine richer in iron between 1220 and Spinels of the hercynite-magnetite series are found in every 1240 °C. This means that a c. 80–100 °C temperature dif- slag, and both end members are present. Aluminium oxide- ference existed between the initial crystallisation and the rich spinels are distributed fairly homogeneously throughout termination of crystal growth. In reality, i.e. in our slags, the slag. Rims of hercynite can be found surrounding chlo- the temperatures are lower due to the interaction of oli- rite-like rock inclusions that were being attacked by the slag. vine crystallisation with other constituents in the charged The presence of this phase reflects the content of aluminium 1 3 10 Page 12 of 21 Archaeol Anthropol Sci (2022) 14:10 Fig. 5 Scanning electron microscope backscatter images. a Slag present throughout. The right edge shows a quartz-rich inclusion. c VB10, slide 2: fragment of ore and gangue minerals decomposing Massive magnetite in slag VB2-3. The inclusion contains a range and vitrifying inside the slag. The gangue exhibits a laminated texture of copper-bearing inclusions from chalcopyrite to metallic copper with quartz (1) and decomposed phyllosilicates (2). The metallifer- with an emphasis on covellite. d Slag MT8, slide 2: iron-rich inclu- ous phases (3) primarily consist of chalcopyrite together with lesser sion containing dendritic iron oxides, metallic copper prills and large amounts of bornite, magnetite and pyrrhotite. b Slag VB10, slide 4: tabular fayalite crystals in slag. The rounded morphology of the iron fayalite crystals in glassy slag with small sulphide inclusions (white) oxide dendrites points to wüstite or a wüstite/magnetite mixture oxide in the melt. Iron oxide-rich spinels, though present Iron-rich agglomerates tend to be between 0.5 and in all slags, are much rarer and tend to be in the form of 3 mm in size with a few exceptional examples being larger. agglomerates or bands. The iron oxide-rich spinels are either There is significant variation in the sizes, shapes and remnants of iron oxide-bearing minerals from the charge or compositions of these inclusions. They are almost always formed from the oxidation of the matte or the slag surface. associated with sulphides and/or metallic copper. Some Bands of magnetite formed by brief exposure to oxygen on are massive with small sulphide inclusions (Fig. 5c), and the surface of the slag, particularly as oxidation rims of sul- others are reminiscent of slag with dendritic wüstite/mag- phides at the slag’s surface: these bands, or oxidation rims, netite, fayalite and metallic copper (Fig. 5d). A number of appear sometimes within the slag (Fig. 6b–c). These rims this later type of inclusion exist and all contain metallic mark former upper surfaces of the slag, and their displace- copper, which is found almost nowhere else in the slags. ment within the slag is the result of movement possibly from The fayalite morphology in these inclusions is discordant stirring the slag with a wooden pole, a practice observed by with the surrounding slag; in these inclusions, the fayalite Anfinset (2001: 52) aimed at facilitating the separation of the crystals are typically larger and more fully developed than slag and molten metal. the surrounding slag. 1 3 Archaeol Anthropol Sci (2022) 14:10 Page 13 of 21 10 Copper and iron sulphides islands of sulphides remain (Fig. 6f); however, the mag- netite tends to be stable and shows extensive exsolution Copper-iron and iron sulphides are present in all slag and/or eutectic patterns with sulphides that are no longer specimens. The slag fragments have all been subjected to present due to corrosion. The matte layer is almost always weathering causing many of the sulphides to corrode or to rimmed with a continuous 10–20 µm thick magnetite be replaced with other minerals such as covellite, cuprite, crust where the hot matte would have been exposed to hematite and iron-copper sulphate minerals. Sulphides air. Given the range of compositions, the melting point can be divided into three categories: sulphides trapped in of the sulphides is between c. 941 and 1100 °C (Schlegel gangue, sulphides trapped in slag and the matte sulphide and Schüller 1952). layer that adheres to the bottoms of the slag cakes. The sulphides in gangue rock are almost exclusively pyrrhotite X‑ray diffraction (XRD) and chalcopyrite, often with little visible decomposition to bornite and magnetite due to smelting (Fig. 6d). The The XRD analyses help to confirm the mineral and phase sulphides in the melted regions of the slag are typically identifications and show broadly similar characteristics. All of spherical form, sometimes adjoined to bubbles, and contain quartz and fayalitic olivine and most contain size- show a much larger compositional variability in the Cu:Fe able quantities of high temperature silica phases (tridymite ratio; however, chalcopyrite and pyrrhotite are nearly and cristobalite) and aluminium oxide-rich spinels (hercyn- always dominant, with subsidiary bornite and magnetite ite). Magnetite is less frequently found in significant quan- (Fig. 6e). The intermittent matte layers on the bottom of tities, and as the microscopy shows, it tends to be found in the slag cakes are typically between 100 µm and 1 mm small agglomerations or as oxidised crusts on the upper and thick and are heavily impacted by corrosion. Only small lower surfaces of the slags. Fig. 6 Optical microscope images. a Quartz-rich inclusion in slag bar 200 µm. e Melted sulphide inclusion in slag VB10-4 adhering to MT11 showing high temperature alteration and the formation of cris- a bubble. The inclusion is dominated by chalcopyrite and pyrrhotite tobalite (Crs). Note the so-called ballen texture (Flörke 1959). Trans- with lesser amounts of bornite (Bn) and magnetite (Mag). The sur- mitted light, scale bar 50 µm. b Magnetite rim on the upper surface rounding slag is glassy with crystals of fayalite olivine (Ol). Reflected of slag MT13 and remnants of magnetite rims within slag. Reflected light, scale bar 100 µm. f Matte layer on the base of slag VB10 light, scale bar 200 µm. c Magnetite rim within slag MT13 with approximately 0.25 mm thick. The layer represents a mixed sulphide/ trapped sulphide layer above. Reflected light, scale bar 500 µm. d oxide melt with relics of chalcopyrite with minimal bornite exsolu- Sulphide inclusion containing chalcopyrite (Ccp) and pyrrhotite (Po) tion and magnetite eutectic and exsolution phases. The continuous in quartz-rich gangue (Qz) fragment in slag VB10-3 which are likely magnetite rim is visible in the lower quarter of the image. Reflected to reflect the type of ore used during smelting. Reflected light, scale light, scale bar 100 µm 1 3 10 Page 14 of 21 Archaeol Anthropol Sci (2022) 14:10 smaller inclusions than those from La Vela di Valbusa, Inductively coupled plasma mass spectrometry (ICP‑MS) but no significant differences were noted in the nature of the inclusions, and we suggest that the macroscopic dif- The elemental analyses indicate that the slags contain ca. ferences observed reflect the heterogeneous nature of the slags rather than any difference in process between the 0.5–2 wt% copper trapped in the slag. The copper content is directly correlated with sulphur (r = 0.8), and thus, this two sites. loss is almost exclusively as matte in the slag. The slag com- positions are dominated by silica (43–53 wt%), followed by Ore processing iron oxide (ca. 30 wt%) and aluminium oxide (6–9 wt%) and calcium oxide (1.5–7 wt%) and magnesium oxides (3–4 The ore must have been only coarsely crushed and ben- eficiated. In the slag cakes, there are centimetre-sized wt%). The compounds titanium oxide and magnesium oxide are strongly correlated to aluminium oxide (r = 0.9) and pieces of gangue rock inter-grown with ore minerals, of sizes that would have been selected by hand. Pulverisa- must have entered the system together, probably as chloritic rock, but these are only weakly correlated with iron oxide, tion and wet beneficiation steps, as described by Blan- ford (1861), were not performed. Anfinset (2001:48–9) so there must be additional sources contributing iron oxide to the slag, either from other minerals accessory to the ore also describes wet beneficiation preceded by hand sort- ing but comments that wet processing was not needed if and/or an iron oxide-bearing flux. There are slight differ - ences between the two sites in the relative quantities of some the ore was of good quality. Both of these ethnographic accounts indicate that there was no roasting step prior to metalliferous impurities from accessory minerals in the ore. The slags from Riparo di Monte Terlago are often richer smelting, and the condition of the sulphides in ore frag- ments in the slag cakes we studied also shows no signs in zinc oxide and antimony than the slags from La Vela di Valbusa. These are not correlated with the copper content, of it being first roasted. so they represent heterogeneities in the mineral assemblage in the ore. The only trace element that is consistently associ- Cooling rates ated with copper is silver (r = 0.7). Following the work of Donaldson (1976), the types of oli- vine hoppers in the slag suggest cooling rates of 15–40 °C Discussion per hour (over the c. 80 °C span of the initial crystallisation and the cessation of crystal growth between the olivine liq- Comparison between the two sites uidus and solidus temperatures). However, the cooling rate described by Donaldson was based on forsterite-rich olivine Some differences were observed between the slags from the and appears not be representative of the iron-rich systems found in archaeological slags (Ettler et al. 2009). Ettler et al. Riparo di Monte Terlago and La Vela di Valbusa. While some slag pieces at the Riparo di Monte Terlago are mac- (2009) argue that, based on the work of Faure et al. (2003), the olivine morphology progression of fayalite occurs more roscopically indistinguishable, others are more gracile and thinner than the slags from La Vela di Valbusa. All the slags quickly, with hopper-type crystals forming at cooling rates in the low hundreds of degrees per hour and chains at high from the Riparo di Monte Terlago are fractured/fragmented, so none reflects a complete slag cake. MT13 is exactly like hundreds of degrees per hour. The wooden (?) implement impressions and the fact that matte adhering to the under- the La Vela di Valbusa slags in terms of shape, thickness, morphology and texture. MT4 and MT6 are also similar to side of the slag formed magnetite crusts upon exposure to air indicate that the slag cakes were lifted while hot and the La Vela di Valbusa samples but are thinner. The rest of the Riparo di Monte Terlago samples are generally more were likely semi-solid. It is not unreasonable to assume that the slag solidified over the course of 10 min in the furnace compact and vitreous than the La Vela di Valbusa samples, and the rock inclusions are smaller. This difference, however, before lifting. The olivine crystals do not tell us anything about the rate of cooling below their crystallisation tempera- may simply be a bias of the sample selection. The edges of the large La Vela di Valbusa slags are sometimes of a similar ture (contra Anguilano et al. 2002: 636). thickness to the majority of the Riparo di Monte Terlago samples. It is not possible to undertake a proper comparison Flux or no flux as the Riparo di Monte Terlago samples are fragments of slag cakes, whereas whole slag cakes were available from The metamorphic rock inclusions in the slag are consistent with the other prehistoric slag known from Trentino–Alto La Vela di Valbusa. The slag fragments from the Riparo di Monte Ter- Adige/Südtirol and described in the literature (full refer- ences in Table 1) and may have an origin in the pre-Variscan lago included in the study generally had much fewer and 1 3 Archaeol Anthropol Sci (2022) 14:10 Page 15 of 21 10 basement (D’Amico et al. 1998; Artioli et al. 2015). If cal- cium carbonates or iron carbonates were associated with the ore, we would expect to find relicts in the ore-bearing rock inclusions, either in the form of calcium oxide or iron oxide-rich melts or as iron oxide concentrations within the rock fragments, but these were not observed. The iron-rich chlorite minerals of the quartz-phyllite-dominated rocks appear to form the bulk of the slag, and one could argue that a mixture of gangue minerals was desired; however, they are still too iron-poor to produce a liquid slag at the temperatures used, but it is unlikely that a full liquefaction of slag was the main goal of the process. The slag phases show that they were capable of reaching the temperatures needed to produce a fully liquid slag had they crushed the rock into smaller pieces and slightly raised the iron content. The calcium oxide contents of the slag can easily be Fig. 7 Pseudo-ternary diagram SiO -Al O -(FeO + MgO + CaO + 2 2 3 explained by a contribution of charcoal ash (Tylecote et al. MnO + ZnO) adapted from the Slag Atlas (Verein Deutscher Eisen- 1977: 310–1), and it is thus likely that they do not result hüttenleute 1981: 11, Fig. 3.183). The theoretical melting tempera- tures of the bulk slag steeply incline because of excessive silica, sig- from an intentional addition of flux (contra Cattoi et al. nificant amounts of which remain unmelted in the slag. The melted 2000, 2001). parts of the slag consisting of primarily glass and fayalite conform Until now the possibility has been neglected that the slag to the lowest melting temperature region and centre at the juncture was recycled as a flux in CA-EBA Trentino–Alto Adige/ of olivine with the quartz-hercynite-iron cordierite trough, which is the lowest area in the entire system. Alternatively, the iron-rich Südtirol. The gangue material supplied several compounds, agglomerations containing wüstite and/or magnetite inclusions follow some of which having ‘fluxing’ effects: silica, aluminium the slope of the wüstite trough. Abbreviations: Crn, corundum; Fa, oxide, potassium oxide, calcium oxide, magnesium oxide fayalite; Hc, hercynite; Mul, mullite; ‘Qz’, quartz; Wüs, wüstite. The and, to some extent, iron oxide. It is clear from the pseudo- ellipses represent actual analyses carried out by the authors and indi- cate the distribution of SEM–EDS analyses. Bulk analyses represent ternary diagram (Fig. 7) that the aluminium oxide content, the average composition of each of the seven slags (multiple mm highest in the chloritic rock fragments, played an important area analyses of powdered and homogenised slag). The glass ellipse role in the melting system, pulling the liquid regions of the represents the average glass composition of each of the seven slags slag into the low-melting but steeply inclined iron-cordierite (comprised of 39 analyses of inclusion-free areas). The iron-rich areas represent 29 area analyses from the seven different slags trough. Technical ceramic can be a source of aluminium oxide in slag, but these slags were never in contact with ceramic and must have formed in a bed of charcoal. There which he interpreted as recycling of slag to help melt sil- is no evidence that ceramics may have influenced in the composition of the slag. Saturation of silica and aluminium ica-rich rock. It has been suggested for Late Bronze Age copper production in Trentino following the same line of oxide and the deficiency of iron oxide (FeO) controlled the slag melting temperature. It is traditionally thought that in reasoning (Hauptmann 2020: 236, 314). Two types of slag are used as fluxes in Nepal, but Anfinset (2001 : 50–51) the so-called Eibner Process, the sulphides, being rich in iron, would oxidise during roasting and smelting and supply notes that the last slag layer from the previous smelt (by which he seems to mean the matte conversion slag) is con- a flux for the silicates, but in these slags, the oxidation of iron-bearing sulphides is not in an advanced stage. Most of sidered to be necessary for a successful smelt. It would make sense that the slag from the second smelting step to the sulphides trapped in the gangue are not associated with iron oxides, and in the slag as well as the matte layers on the produce metallic copper (matte conversion) was collected and was re-used in the first smelting step (matte smelting) bottoms of the slag cakes, the sulphides are still iron rich, i.e. chalcopyrite and pyrrhotite. to help smelt the silica-rich gangue of the ore. During the matte conversion process, both sulphur and iron are In order to flux the system, a material rich in iron oxide (FeO) would have been most beneficial. In describing oxidised, and the iron oxides are then absorbed in a sili- cate slag, forming a platy slag, or Plattenschlacke, which the Himalayan copper smelting process, Blanford (1861) does not mention the addition of a flux, but Anfinset is iron-rich and fully liquefied at smelting temperatures. Some of the inclusions we observed resembling iron-rich (2001:50–7) mentions the addition of old slag to accelerate the smelting process. Herdits (1997: 40–41) noted pieces slag, such as those containing globular and dendritic iron oxides and metallic copper prills, may in fact be remnants of semi-smelted platy slag in a slag cake from the Brenner- wald smelting site at the Mitterberg (Salzburg, Austria), of intentional slag flux, though it was not possible to 1 3 10 Page 16 of 21 Archaeol Anthropol Sci (2022) 14:10 conclusively prove this. As we have seen, virtually all the slag found in CA/EBA contexts in Trentino–Alto Adige/ Südtirol region, are coarse, with platy slags only reported as present in small numbers at Gudon, Montesei di Serso, Riparo Marchi, Romagnano Loch and Romagnano Angeli, and a glassy slag at La Vela di Valbusa (Table 1). Rather than arguing that matte conversion, a vital step in the cop- per production process, was not practised (which would mean no copper metal), the relative absence of platy slag may be an indication of the re-use of slag as a flux in the ore smelting stage. It is possible that this rarity of platy slags and indeed their absence on most sites (13/19, cf. Table 1) indicates that the two stages of matte production and matte conversion were spatially separated, but it is striking that no evidence for sites that were specialised in matte conversion (which would be evidenced by large amounts of platy rather than coarse slag) have been found to date. We would ask why the two stages of smelting Fig. 8 Slag VB1, indicating impressions: oblique impression caused would be undertaken at two different places: certainly, by a wooden pole (dashed line) and from the (probable) wooden tongs used from the opposite side to extract it from the furnace (dot- there is no evidence for different specialists, and the scale ted line). The slag was compressed on the upper surface of the cake of production is not sufficient to warrant such a strategy. and began to pull apart/tear in half (from the stress of lifting). The We therefore prefer the more obvious explanation, which tong compressed the poking mark, showing the sequence of events is evidenced in our study, that the platy slags were mostly recycled as flux. It is likely, however, that the creation of a fully liquid can generally only be seen on the top of the slag cake, but in one case, an impression is attested on the underside slag in the first smelting stage was not desired, but rather the intention was to fuse the waste rock together so that (VB5). It could be that the slag was hotter on the upper surface than the bottom because the source of heat (char- it could be lifted from the matte, which was liquid, as described by Anfinset (2001: 53, Fig. 14) in Nepal and coal and airf low) was above the slag. Blanford (1861: 390) in Sikkim. Lifting out the fused slag cake would enable the continuation of the smelting pro- The process reconstruction model and its implications cess by allowing the further charging of ore to the hot fur- nace, thus considerably raising efficiency (Fig. 8). Once We have therefore shown that smelting in the third mil- a certain amount of matte accumulated in the furnace, the matte could be left to cool to be processed and converted lennium cal BC Trentino, in north-eastern Italy, did not follow the ‘Timna’, ‘Eibner’ or ‘Chalcolithic cop- to copper in subsequent production steps. The practice of lifting the slag from the furnace is attested by marks left per smelting model’ processes, but closely resembled ethnographically observed smelting practice in Sikkim by (presumably wooden) implements noted by Reitmaier- Naef (2019: 239–240, Figs. 2 & 3) on the underside of and Nepal. We have termed this process the ‘Himalaya’ model. It appears to be a controlled process that was Late Bronze Age/Early Iron Age copper smelting slags from the Oberhalbstein (Grisons, Switzerland), and the standardised in the Trentino and the adjoining Alto Adige/Südtirol. use of a wooden pole to poke and maybe lift large slag cakes is evidenced in the later Bronze Age at the Mit- Ore was crushed into centimetre-sized pieces and hand sorted and then smelted (matte smelting) with the addition terberg (Fig. 9; Klose 1918: II.30, Fig. 39). In our case, much smaller, finger-width, wooden sticks have been used of matte conversion slags as a flux. The slag was lifted out of the furnace using implements that were likely made of to stir or test the consistency of the slag by insertion at an oblique angle into the upper surface of some of the slags wood. This removal of slag, as documented in the Himala- yas, allows multiple charges of ore to be processed in short (VB1, VB2, VB5; Fig. 8). Faint finger-width channels and compressed folds and bubbles can be seen on some repetition, increasing the efficiency of the process. After the final slag removal, the matte was left to cool. The matte was slag cakes (VB1, VB2, VB5, VB6, VB8, VB10); gener- ally they are only visible on the cakes that are mostly then roasted and re-smelted (matte conversion) to produce copper; the resulting slags were mostly reused as flux for complete. These likely attest the use of wooden tongs for lifting these slags out of the furnace. The impressions matte smelting. 1 3 Archaeol Anthropol Sci (2022) 14:10 Page 17 of 21 10 Fig. 9 Bronze Age slag cakes from the Mitterberg, attesting the use of wooden poles to lift them out of the furnace. Photo 4 shows the original wooden pole in place; photo 3 shows a pole placed in the hole left in the slag by a pole (source: Klose 1918: Fig. 39) An important aspect, which is a new observation for minimal amount of labour and organisation involved, this CA and EBA Trentino, is that the semi-liquid coarse slag small-scale technology may have been optimal in the con- was nothing more than an intermediate product to facilitate text in which it was practised. The Himalayan process was the matte-smelting process. Whereas Bourgarit (2007: 5–7) still practised and economically viable until quite recently. suggests (implicitly from a modern viewpoint) that all early Previous descriptions of the Trentino slags, discussed slags that did not achieve fully liquid eutectic compositions above, have differed widely on their interpretation of the are ‘immature’ and ‘inefficient’ or indicate a ‘poor mas- flux used. Some say quartz, some calcium oxide, others tery of the slagging process’, we suggest that a fully liquid none. In this regard, it is important to look closely at the slag would have been both wasteful and a hindrance to the composition of the ores, gangue and host rock components smelting process. In our example, it was actually advan- and ask ‘What is missing that would help the process and tageous that the slags could be removed in a semi-solid what is restricting the process?’ We see that the slag is state, and we note that removing slag mid-smelt would deprived of iron oxide, and this is the only thing that could have permitted multiple charges per smelt. It is important be added to improve melting, but too much of this would when talking about technology to consider the economics have melted everything. The addition of matte conversion of scale. Some processes only make sense above a cer- slag, in the right amount, may have in effect made a slag tain level of metal demand. Squeezing out every ounce of like ‘ice cubes in water’, aiding in the separation of liquid metal, as done with ‘mature’ smelting technology in shaft matte without fully liquefying all the components. furnaces with low viscosity fully liquid tap slags, requires Having reconstructed the smelting process, it becomes tremendous amounts of energy and a clear financial incen- possible to interpret the archaeological evidence found at La tive to do so. Considering the low level demand and the Vela di Valbusa (Fig. 2). The two roughly oval, bowl-shaped 1 3 10 Page 18 of 21 Archaeol Anthropol Sci (2022) 14:10 areas of baked clay are likely roasting beds, used for roasting Although we cannot know this for certain, the fact that matte. It is likely that the smelting furnaces were removed hot slag cakes were lifted from the furnace, just as in the by quarrying prior to the rescue excavation; roughly quad- Himalayan examples, makes it highly likely that multiple rangular furnaces, made of three slabs of rock (Fig. 10), are slag cakes were produced per smelt. Each slag cake probably known at other sites in the Trentino and Alto Adige/Südti- represents one or two handful size charges of ore. Bringing rol (Gudon, Acquaviva, Riparo Gaban, Romagnano Loch, a furnace up to > 1200 °C to smelt one handful of ore would Montesei di Serso and Croz del Cius—Table 1). The internal have been exceptionally wasteful, not so if the process was dimensions of these furnaces vary; for example, the structure continual, with cycles of ore charging and slag lifting, tak- at Acquaviva is 14 × 16 cm, while that at Romagnano Loch ing advantage of the accumulated heat. If this was the case, is 30 × 40 cm. Given that the La Vela di Valbusa slag cakes the technology could be as standardised and controlled as were 12–14 cm in diameter, the furnace at the site is likely the Himalayan examples of the nineteenth and twentieth to have been closer in size to that at Acquaviva. centuries. Fig. 10 Plan and section of EBA smelting furnace, Monte- sei di Serso (after Perini 1989: Fig. 15a) 1 3 Archaeol Anthropol Sci (2022) 14:10 Page 19 of 21 10 Data availability All relevant data is included in the paper and in the Conclusions Online resource (supplementary materials). On the basis of observations with the naked eye and micros- Code availability Not applicable. copy, mineralogical and elemental analyses of seven slag pieces from two rock shelters, La Vela di Valbusa and the Declarations Riparo di Monte Terlago, we were able to shed new light on the smelting of copper in the third millennium cal BC Competing interests The authors declare no competing interests. Trentino, north-eastern Italy. Our observations do not dif- fer markedly from those of previous studies of slags from Open Access This article is licensed under a Creative Commons the Trentino noted above, but we have been able to interpret Attribution 4.0 International License, which permits use, sharing, them in a new way. By analogy with ethnographic accounts adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the of copper smelting in the Himalayas (Sikkim and Nepal), source, provide a link to the Creative Commons licence, and indicate we were able to elucidate aspects of the smelting process, in if changes were made. The images or other third party material in this particular the lack of ore beneficiation by crushing and wash- article are included in the article's Creative Commons licence, unless ing or preliminary roasting, the use of matte conversion slags indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended as fluxes for the first smelt (matte smelting) and the use of use is not permitted by statutory regulation or exceeds the permitted implements that were likely made of wood to lift the hot slags use, you will need to obtain permission directly from the copyright from the furnace during the smelt. The sulphide ore melts to a holder. To view a copy of this licence, visit http:// creat iveco mmons. highly fluid sulphide matte at a much lower temperature than org/ licen ses/ by/4. 0/. achieved during the smelting process, allowing it to separate from the silicates. It was the smelters’ intention to produce a semi-liquid coarse slag in order to facilitate the smelting pro- References cess as this enabled the slags to be removed from the smelt in a semi-solid state. 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The authors thank Giampaolo Dalmeri and Stefano Neri (MUSE Angelini I, Gallo F, Artioli G, Nimis P, Tecchiati U, Baumgarten B – Museo delle Scienze, Trento) for kindly providing slag from the Riparo (2012) Mineralogical and isotopic characterization of the late di Monte Terlago and generously providing stratigraphical information chalcolithic slags from Bressanone/Brixen (Northern Italy). In: and an unpublished radiocarbon date. The authors acknowledge Thomas Braekmans D, Honings J, Degryse P (eds) 39th International Stöllner (Ruhr-Universität Bochum), Thomas Kirnbauer (Technische Symposium on Archaeometry: 28 May - 1 June 2012: Leuven, Hochschule Georg Agricola), Elena Silvestri, Paolo Bellintani, Elisa- Belgium: Programme and Abstracts. 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Archaeological and Anthropological Sciences – Springer Journals
Published: Jan 1, 2022
Keywords: Trentino; Copper smelting; Slag; Fluxes; Ethnography; Archaeometallurgy
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