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Early copper smelting at Itziparátzico, Mexico
Journal of Archaeological Science 36 (2009) 1998–2006
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Early copper smelting at Itzipara´tzico, Mexico Blanca Maldonado a,1, Thilo Rehren b, * a b
Department of Anthropology, Pennsylvania State University, 409 Carpenter Building, University Park, PA 16802, USA Institute of Archaeology, University College London, 31-34 Gordon Square, London WC1H 0PY, United Kingdom
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 October 2008 Received in revised form 14 April 2009 Accepted 18 May 2009
Archaeological fieldwork and archaeometallurgical analysis have identified a Late Postclassic regional centre of copper production in western Mexico. The total output from a single unit of production at the site of Itzipara´tzico is an estimated ten tons of copper and nearly forty tons of slag over the lifetime of the installation. It is argued that this smelting was based on slag-tapping furnaces, a technology previously unknown from Mesoamerican archaeological sites. The smelting site is in dense woodland with ample fuel supply, some 125 km from the next mining area with documented contemporary ore extraction; the copper produced would have been passed on to the capital, another 60 km away, for further distribution and working. The scale of production at Itzipara´tzico indicates that copper smelting was done by parttime specialists embedded in a predominantly agricultural economy, and formed part of a centrally organized network of mining, smelting and processing of copper to supply the Tarascan state. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Tarascan Copper smelting Mexico Yield Postclassical Mesoamerica
1. Introduction Early West Mexican metallurgy and metalworking, especially among the Tarascans and their neighbours, was based mainly on copper and its alloys (Hosler, 1994: 127; Pollard, 1987: 743). PreHispanic copper exploitation reached its height during what Pollard (1982, 1987, 1993, 1997) (see also Gorenstein and Pollard, 1983) has called the Protohistoric period (AD 1450–1530) in the Tarascan domain. The technological processes that native metalworkers used for the extraction of copper from their ore and the organization of the craft itself remain, however, poorly understood. Archaeological investigations at the archaeological zone of Itzipara´tzico have for the first time documented evidence for important aspects of precontact extractive copper metallurgy in the Tarascan area. Itzipara´tzico is located on the south-central part of the Lake Zirahu´en Basin in north central Michoaca´n, lying among the modern agricultural fields of Santa Clara del Cobre and Opopeo (Fig. 1). Artisans from Santa Clara del Cobre, a modern Tarascan municipality in Michoaca´n, have maintained traditional techniques for working copper to this day. Information provided by written sources, along with the factual presence of smelting waste products
* Corresponding author. Tel.: þ44 20 7679 4757; fax: þ44 20 7383 2572. E-mail address: [email protected] (T. Rehren). 1 Current address: Curt-Engelholm-Zentrum fu¨r Archa¨ometrie, Universita¨t Tu¨bingen, D6, 3, D-68151 Mannheim, Germany. 0305-4403/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jas.2009.05.019
(i.e. slag) on the surface of archaeological areas near Santa Clara, suggest that the ongoing production of copper has its roots in preHispanic traditions (Horcasitas de Barros, 1981; Maldonado, 2002, 2006). This is particularly significant since, in the absence of mines in the area, ore had to be carried some 125 km from the region of La Huacana (see Fig. 2). The access to pine-oak uplands where forests provided high-quality charcoal seems to have been a major factor for locating the smelters here. While this situation may not seem economically advantageous, the movement of ore from mine to settlement is well represented in the archaeological record elsewhere. Several early examples from the Old World can be cited, including Chalcolithic sites such as Abu Matar (Gilead and Rosen, 1992) and Shiqmim (Golden et al., 2001; Shalev and Northover, 1987) in Israel, which lie some 150 km from the nearest ore source and yet present substantial evidence of smelting activities. Similarly, evidence from the Early Bronze Age in Crete has indicated that smelting operations were being carried out at Chrysokamino, a site located in the northeast of the island, far away from any known ore sources. It has been suggested that the beneficiated ore was brought in by ship for smelting (see Betancourt, 2006). Ethnohistorical evidence indicates that prior to the arrival of the Spanish the bulk of the metal that moved into the Pa´tzcuaro Basin came in the form of tribute delivered with regularity (see Paredes 1984; Pollard, 1982, 1987). The primary supplier of copper was the central Balsas Basin, in the southern portion of the Tarascan territory. Paredes (1984) and Pollard (1987) have suggested that during the last century of the Tarascan Empire the state took a more direct
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Fig. 1. Location of Itzipara´tzico within the Zirahue´n Basin and the state of Michoaca´n (Modified from Davies et al., 2004, Fig. 1).
control of the copper resources of this particular region than simple tribute. This idea is largely based on accounts in the Legajo 1204 (Warren, 1968), which suggest that the Cazonci (the paramount ruler of the Tarascan state, residing in the Tarascan capital, Tzintzuntzan, roughly 60 km NE from Itzipara´tzico) sent workers to extract copper from the mines of La Huacana (see Fig. 3) to meet his needs (Pollard, 1987: 748; Warren, 1968: 47, 48). Most mines,
however, were exploited by the local population, who provided the copper as tribute to the state (Pollard, 1987: 748). According to the Legajo 1204 (in Pollard, 1982: 258, 1987: 748; see also Warren, 1968), the schedule for payments of copper, of either every 40 days or on demand, substantially exceeds that of any other tributary item for the central authority. The Legajo further indicates that mining activities and smelting operations often took
Fig. 2. Pre-Hispanic and Colonial mining centres in the Central Balsas Basin (Adapted from Barrett, 1987: Map 2).
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Fig. 3. Location of Itzipara´tzico with respect to Tzintzuntzan and the mining sources (Modified from Roskamp et al., 2003: Fig. 4).
place at separate locations within the central Balsas Basin (see Fig. 2). While mining centres were concentrated at Churumuco, Sinagua, Cutzian and La Huacana, among others, smelting operations were carried out at La Huacana, Cutzian, and Huetamo-Cutzio (Pollard, 1987; Warren, 1968). At the Cutzian mine alone, there had been up to 50 miners and 40 other workers, some of whom were moving the dirt and mineral out of the mines and some were processing the ore (Warren, 1968: 49). Accounts in the Legajo also state that the metalworkers from the La Huacana region owned and cultivated the fields at the foot of the hill where the copper veins were mined. This suggests that mining and metallurgy (at least at this particular location) represented part-time activities, undertaken mostly during the slack period in the agricultural season. The great climatic variation between the rainy and dry seasons in the region supports this assumption. During the rainy season the mines were probably flooded, while during the dry season agricultural production must have fallen dramatically due to the extreme dryness in the region. It is likely that the smelting operations followed the same seasonal patterns as the mining operations, and were therefore carried out during the dry season. The miners/smelters probably alternated between metalworking and farming, according to the seasons as well as to royal demands (Grinberg, 1996: 433). ´ n and the ´ n de Michoaca According to accounts in the Relacio Legajo 1204, processed ingots were transported for final manufacture on the backs of tamemes (the Nahuatl term for human carriers), each load consisting of 20–30 ingots (Warren, 1968: 47, 49) which together weighed about 32–72 kg (Pollard, 1987: 748). Based on information found in the Legajo, Pollard (1987: 750) has estimated that the major smelting centres of La Huacana and Cutzian were a two days’ journey from Tzintzuntzan. These figures make the transport of ore to a relatively intermediate point like Itzipara´tzico seem plausible, in particular considering the demand for fuel and labour required to sustain a significant smelting operation.
Referring to the Churumuco region, the Legajo 1204 states that twenty labourers worked in each mine, and each of them collected one-half celemı´n (Warren, 1968: 37), that is 2.25 litres (Brand, 1951: 132) of mineral in the form of rock fragments and dust. The sources do not specify whether this is a daily production, but it appears reasonable to assume so. After grinding the mineral and mixing it with ground charcoal, the metal-workers smelted it into an ingot one hand long, one hand wide, and two fingers high (Grinberg, 1996: 433; Pollard, 1987: 748; Warren, 1968: 37). According to calculations by Grinberg (1996: 433) the twenty labourers together produced one carga (load) of copper per day, ´ n (charge) per month. One carga therefore was 20 and one monto ´ n was the ingots, and probably weighted about 90 kg. A monto production achieved in 20 days, the length of a Mesoamerican month, which amounted to 400 ingots, weighting around 1800 kg. These figures, however, seem quite high, particularly if they refer to a sustained production output, and might require re-evaluation. Overall, the ethnohistorical evidence indicates that large-scale production of copper metal took place before the arrival of the Spanish, involving a spatially separated and well-organized labour system. Nevertheless, nearly nothing is known about the technical practicalities of this copper smelting activity, or the reaction vessels in which it took place. Pictorial evidence of metalworking from the early contact period relates mostly to the melting and casting of gold and copper or bronze, and therefore provides little guidance for understanding the extraction process. 1.1. Research background The purpose of the archaeological research carried out at the zone of Itzipara´tzico was to define the extent and nature of the area, to identify and characterize its chronology, and to locate and record evidence for metallurgical activity in order to distinguish traits, or clusters of traits, associated with different stages of pre-Hispanic
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metallurgy. The results would provide the basis for comparing archaeological evidence for metallurgical production with descriptions of pre-Hispanic metallurgy found in ethnohistorical sources. These comparisons were intended to examine both the technology and organization of production. This work represents the first fully reported study of copper smelting in Mesoamerica, as well as the first systematic archaeological investigation carried out in the entire Zirahue´n Basin. Surface survey carried out during the Itzipara´tzico Archaeological Research Project (IARP) in 2003–2004 identified and mapped three major sectors of the site, divided according to the variability of their archaeological materials and features (Fig. 4). While ceramic and lithic artefacts were common throughout the research area, slag concentrations were located almost exclusively in one particular sector (Sector 1). The presence of smelting by-products indicates that smelting activities took place in or around this zone, an indication possibly further supported by the proximity of this sector to water, indispensable for many metalworking processes. Other materials include moderate amounts of potsherds and lithics (mainly grayblack obsidian prismatic blades), as well as a set of stylistically diverse Tarascan pipes. These pipes and several polychrome ceramic fragments have been identified as Late Postclassic in date (see Table 1). Fieldwork at Itzipara´tzico also involved archaeological test excavations in the three major sectors of the area. The results of these excavations were consistent with the observations on the surface. No substantial evidence for occupation before or after the Late Postclassic Period (c. AD 1350–AD 1521) was found. There were a few glazed/European-style ceramics but only very near the surface and not substantial in quantity. All of the ceramic and lithic artefacts excavated appear to belong to a Late Postclassic Tarascan occupation. Although no identifiable metalworking structures (furnaces, hearths, and pits) were found at Itzipara´tzico during the test-pitting, large amounts of slag were recovered from the excavation, together with lithics and ceramics as found elsewhere on site. The absence of metallurgical materials other than slag (i.e. hearth structures, crucible fragments, mould fragments, stock metal, metal prills, failed castings, part-manufactured objects and
Fig. 4. Three main sectors of the Itzipara´tzico area (Map created by van Rossum).
2001
Table 1 Established chronology for the Pa´tzcuaro Basin according to Pollard (2005: 9). Period
Local phases
Late postclassic Middle postclassic Early postclassic Epiclassic Middle classic Early classic Middle preclassic
Tariacuri (A.D. 1350–1525) Urichu Tardı´o (A.D. 1000/1100–1350) Urichu Temprano (A.D. 900–1000/1100) Lupe-La Joya (A.D. 600/700–900) Jara´cuaro (A.D. 500–600/700) Loma Alta 3 (A.D. 350–500) Loma Alta 2 (100/50 B.C.–A.D. 350)
spillages, etc.) around Itzipara´tzico indicates that only primary copper production was being carried out at this location. 2. The slag A representative sample of 2.1 kg of slag from the excavations at Itzipara´tzico was selected from several hundreds of kilograms and exported for archaeometallurgical analysis. Part of this material has been examined at the Wolfson Archaeological Science Laboratories at the UCL Institute of Archaeology, London, UK. The samples were analyzed for microstructure and composition using light microscopy, X-ray fluorescence spectrometry (XRF), and scanning electron microscopy with energy-dispersive X-ray spectrometry (SEM/EDS). A brief summary of the results of the analyses is presented below. Detailed descriptions of these results have been reported and published elsewhere (see Maldonado et al., 2005). Two major types of slag from Itzipara´tzico were identified macroscopically and labelled as either lumpy or platy, according to their general morphology. Lumpy slags are commonly characterized by plano-convex to irregular surfaces, with sizes ranging from gravel-size up to 2 kg. The specimens often present smooth surfaces on one or both of their faces: porosity is always apparent, and the fracture is uneven (Fig. 5). Platy slags are usually flat, with minor amounts of porosity and signs of ropey flow on one or more surfaces, and the impression of soil particles on the opposite side. The thickness ranges from approximately 2–9 mm. Generally, one of the surfaces exhibits a metallic to glassy finish, while the other shows a more resinous lustre. Breakage normally produces a conchoidal fracture (Fig. 6). In both cases the slags consisted of crystalline phases in a glassy matrix. The distinction between the two slag types is fluid, and numerous macroscopically and chemically transitional pieces exist. Similar slag morphologies have been observed in Old World copper slags from Bronze Age to medieval contexts using a variety of furnaces (see e.g. Bachmann, 1982; Metten, 2003; Meyerdirks et al., 2004).
Fig. 5. Lumpy slag fragment from Itzipara´tzico.
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Fig. 6. Platy slag fragment from Itzipara´tzico.
The slags from Itzipara´tzico are relatively consistent in terms of their mineralogy, presenting various newly-formed phases, partially reacted minerals, and metallic prills. The dominant phases in both slag types were fayalite, magnetite, quartz, glass and various sulphides together with small prills of virtually pure copper. The main difference between the two slag types is in their bulk composition. Platy slags tend to have c. 45–50 wt% FeO and around 35 wt% SiO2, while lumpy specimens have only c. 30 wt% FeO and 50–60 wt% SiO2 (see Table 2); samples 2-1b, 1-3b and 3-1b show intermediate compositions. The lumpy slags contain a high concentration of partly reacted quartz grains, thought to be residual host rock of the ore mineral, and resulting in the elevated bulk silica values/lower iron oxide values of this slag type. In contrast, most platy slags contained little or no residual quartz (Fig. 7). Analyses of melt phase and newly-formed slag phases such as fayalite and magnetite by SEM–EDS are consistent between the two types, further indicating that the main difference between the lumpy and the platy slag is in the mechanical incorporation of residual quartz in the former. Significantly, almost all slag fragments have well developed tap slag morphology, with the platy slags having been more fluid, probably due to the absence of quartz inclusions. The two slag types may thus represent sequential waste products of the same continuous smelting process, possibly relating to consecutive tapping events (Maldonado et al., 2005). The regular and large-scale presence of tap slag is important here, since it is typically associated
with a smelting process carried out in a furnace rather than in a crucible (Craddock, 2001: 157). Bulk analyses derived from XRF analyses of platy slag samples from Itzipara´tzico have been plotted on a phase diagram illustrated in Fig. 8. The bulk composition and the ubiquitous presence of magnetite and fayalite match other known examples of fayalitic copper slag. The mineral ore being processed has been shown to be sulfidic, most likely chalcopyrite (CuFeS2), in a silica-rich matrix (Maldonado et al., 2005). During smelting under reducing conditions the iron from the ore would react with silica from the quartz to form fayalite, while the copper is first enriched in a sulphide phase (‘matte’) and then reduced to metal. The dominance of wellcrystallised fayalite and magnetite in a homogenous slag matrix, and the presence of copper prills indicate: 1) a consistent smelting temperature of around 1200 C, and 2) the occurrence of smelting under strongly reducing conditions. This temperature estimate is based on the position of most platy slags near the eutectic region of the ternary diagram in Fig. 8, with liquidus temperatures of around 1100 C, and allowing for some necessary overheating to facilitate full melt conditions throughout. The actual liquidus temperature of the slag is likely lower than indicated by the diagram, due to the cumulative effect of additional minor oxides such as lime, magnesia, and alkalis, all of which exert a further reducing effect on real melting temperatures by an estimated 50–100 C. A single platy slag sample (2-1b) falls outside this low-temperature region; this is most likely due to some residual quartz inclusions, occasionally seen not only in the lumpy slag but also in the platy variety. This underlines on the one hand their close technological relationship, and on the other hand will lead to a distortion of the bulk chemical composition towards higher silica values, resulting in apparently higher liquidus temperatures. A reducing atmosphere at such high temperature is normally considered inconsistent with mouth-blown smelting operations (e.g. Rehder, 1994). The maximum attainable furnace temperature with blowpipes was sufficient to smelt copper and produce a siliceous slag, as indicated by Early Bronze Age evidence from the Old World (e.g. Bartelheim et al., 2002; Mu¨ller et al., 2004) as well as experimental reconstruction. However, the amount of slag produced under these conditions is typically small, in the order of several grams to tens of grams, and the slag mineralogy is very heterogeneous, reflecting the highly variable redox conditions of the process. While fayalite does occur frequently in crucible smelting slags, it has not been reported as a major or even predominant phase in those slags. The combination of temperature and reducing atmosphere necessary for consistent fayalite formation on a scale of several kilograms liquid slag at a time, as evidenced from the larger slag lumps from Itzipara´tzico (Maldonado et al., 2005), is very strong evidence for furnace smelting.
Table 2 XRF analysis of copper slag from Itzipara´tzico. In the slag column, ‘‘p’’ stands for platy and ‘‘l’’ for lumpy, referring to the two major types of slag identified in the sample. The higher and more variable silica values of the lumpy slags are due to residual quartz inclusions in the slag. Analyses were done on powder pellets using a calibration method for iron-rich materials developed by Veldhuijzen (2003). Context Slag SiO2 (%) Al2O3 (%) FeO (%) TiO2 (%) MnO (%) CaO (%) MgO (%) Na2O (%) K2O (%) P2O5 (%) SO3 (%) CuO (%) PbO (ppm) ZnO (ppm) Total (%) 1-1c 1-2b 1-4a 1-4c 2-1b
p p p p p
34.1 32.4 35.0 35.2 40.0
7.20 10.86 9.65 6.40 3.44
49.7 46.9 43.7 50.3 46.2
0.23 0.35 0.29 0.21 0.04
0.06 0.13 0.09 0.09 0.13
0.63 1.46 1.72 2.44 4.29
1.88 2.06 2.30 1.62 0.61
0.36 0.56 0.61 0.58 0.56
0.92 0.82 1.11 0.82 0.50
0.05 0.11 0.07 0.09 0.01
0.13 0.27 0.26 0.59 0.24
2.41 1.29 1.48 0.73 1.44
0 225 0 400 1500
500 1050 650 700 1100
97.4 97.2 96.2 99.0 97.7
1-2a 1-3a 1-3b 2-1a 3-1a 3-1b 3-1c
l l l l l l l
56.4 55.9 41.2 54.4 57.0 41.6 61.7
7.78 7.59 7.56 11.06 5.04 7.12 3.96
30.5 30.3 42.8 27.5 32.7 44.0 32.6
0.27 0.23 0.25 0.23 0.15 0.24 0.12
0.07 0.06 0.09 0.08 0.06 0.07 0.04
0.89 1.71 0.93 2.50 0.59 2.23 0.48
1.29 1.30 0.96 1.44 0.52 1.46 0.03
0.41 0.44 0.56 0.35 0.21 0.57 0.08
1.47 1.34 0.96 1.95 0.84 1.06 0.87
0.05 0.05 0.10 0.04 0.03 0.07 0.06
0.48 0.42 0.46 0.71 0.35 0.71 0.54
1.57 1.23 1.11 0.77 1.87 3.28 0.93
0 0 380 0 400 150 300
150 150 1350 250 600 2800 2550
101.0 100.5 97.0 101.1 99.4 102.5 101.4
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Fig. 7. Low magnification images of a typical ‘‘lumpy’’ slag sample. a) Region of slag that contains a fayalitic solidification structure (at A), together with a polycrystalline aggregate of un-reacted quartz grains (at B). A duplex copper/copper sulfide prill (C) is seen in association with a gas bubble (D). b) Regions of the slag that contain fayalitic slag (A), together with an un-reacted aggregate of chalcopyrite (E).
3. Copper production at Itzipara´tzico The data above raise important questions about the chronological context and development of the primary production of copper in the area. The evidence of tap slag at Itzipara´tzico has significant implications for Mesoamerican archaeometallurgy, as it suggests that the smelting process took place in a furnace rather than in a crucible. The technology employed in the process involved a consistent highly reducing smelting environment, which would be difficult to achieve using lung-powered blowpipes and crucibles
(see also Rehder, 1994). If the smelting activities at Itzipara´tzico involved the use of furnaces powered by hand-worked bellows, we could be dealing with a relatively early post-Contact production area which greatly overlapped with pre-Hispanic traditions, as indicated by the predominance of Tarascan pottery and lithics and the near-absence of colonial wares (see above). Alternatively, if the operations at the site are entirely Late Postclassic in date, as we believe based on the ceramic and lithic evidence and further supported by ethnohistoric sources, this would indicate that Tarascan metalworkers had developed a copper smelting method which
Fig. 8. Triangular diagram showing phase relationships in the system FeO–Al2O3–SiO2 under reducing conditions (after Muan, 1957: Fig. 10). The three shaded ellipses correspond to the compositions obtained for the platy slags (lowermost), intermediate platy to lumpy (central) and lumpy slags (uppermost) (see Table 2), indicating liquidus temperatures of around 1100 C for the platy slags. The higher temperatures indicated for the intermediate group and particularly the lumpy slags are unrealistic; their bulk compositional data is distorted by free quartz particles embedded in the slag.
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most likely involved some form of natural draft, assuming that bellows were not known in pre-Contact Mesoamerica. In either case, it seems clear that copper ore (chalcopyrite) was brought a considerable distance (see Fig. 3) to Itzipara´tzico for smelting, and copper ingots were probably transported elsewhere for their final processing (Maldonado et al., 2005). The choice of smelting site away from the mining area may reflect the availability of large quantities of fuel in the densely wooded area around Itzipara´tzico, missing from the more arid environment near the mining sites. This scenario is consistent with ethnohistorical data, and would conform with a multi-dimensional production model which involved a considerable degree of control from the Tarascan state on the macro-economic scale. Its main organizational characteristic is the division of an industry into small segments that could be easily controlled, separating mining, smelting, and final processing, and probably including a range of subdivisions within these main activities. With no single section performing all the needed manufacturing steps, centralized control was both necessary and more easily established and maintained. While many outstanding questions regarding metallurgical production at Itzipara´tzico and its organizational setting within the wider Tarascan economy are still in the process of being resolved, the existing data provide an initial base for further archaeological and archaeometallurgical studies. One of the aspects of this production that can be explored based on existing research refers to its scale. It is difficult to accurately quantify metallurgical production at Itzipara´tzico based on the information currently available, and no smelting facilities were located during the survey or testpitting. The highest slag concentrations, however, are located within a relatively limited area, away from what appears to be the residential terraces (Maldonado, 2006). As outlined above, the mineral raw materials (copper ores) were not locally available and had to be imported, bringing up issues of transport and organization. It is worth noting that the slag tapping technology employed in the smelting process involved a highly reducing smelting environment, which together with the wide distribution of slag suggests a complex organization of the craft both between geographically disparate sites engaged respectively in mining, smelting, and further processing, and within the individual smelting sites. This would support the idea that part of the production was in the hands of highly skilled smelters who were part-time to full-time specialists, producing copper in excess of their own community’s needs. 3.1. Scale of production Systematic quantification of slag can be used to obtain rough estimates of the copper production at Itzipara´tzico, based on the quantity of slag produced and an estimate of the ratio of slag to metal produced. Such estimates are made by calculating the relative and the absolute copper yield: the relative yield describes the proportion of copper in the ore that was extracted by the smelt and is thus of mostly technical interest, while the absolute yield describes the amount of metal produced, and is of major socioeconomic significance. Slag analysis has determined that the ore being processed was chalcopyrite (CuFeS2) in a quartz matrix, resulting in a fayalitic slag with only 1–2 wt% copper (Maldonado et al., 2005). Because the chemical composition of the slag reflects predominantly that of the ore, and since the chemical composition of the copper mineral is known, a good estimate of the copper yield may be obtained from the composition and total weight of the slag. The difference in copper content between the assumed ore and the analyzed slag indicates a relative yield in excess of 90 percent, in keeping with the efficiency generally assumed for a tap slag technology. For the following discussion we will concentrate on the
absolute yield in order to understand the potential significance of Itzipara´tzico for the metal supply of the Tarascan state. Slag concentrations were located almost exclusively at Sector 1 of Itziparatzico. Intensive surface collections and preliminary testpitting operations were conducted in three of the tractor-ploughed agricultural fields in this sector, parcels 10, 11, and 12, the latter being the most important in terms of the data that have been collected. No solid slag heaps were located during the explorations. Parcel 12 (approximately 40 30 m) covers 1200 m2. Within this parcel, a 5 m radius circular ‘‘dog leash’’ collection was made of 100% of all slag on surface within the circle, resulting in the recovery of 703 fragments, weighing 14.5 kg. Projected to the entire parcel area, this equals c. 225 kg of slag being present directly on the surface, most likely brought up from lower layers by ploughing which penetrates up to 50 cm into the soil below. For an initial volumetric quantification of the slag present within this parcel, and to obtain ceramic from undisturbed archaeological layers, a test pit (Unit 3) of 2 by 2 m was excavated to about 1 m depth, and its content quantified. Most slag was recovered from stratigraphic levels 1, 2, and 3 (0–20 cm, 20–40 cm, and 40–60 cm respectively); level 4, at 60–80 cm, had only little slag, and was generally poor in cultural remains, transitioning to sterile soil beneath. The amount of slag recovered from this test pit totalled 13,857 fragments, weighing 127 kg. The slag was relatively evenly distributed within the total excavated volume, and mixed with some quantities of domestic ceramic; however, neither furnace nor crucible fragments were found. The domestic pottery was predominantly pre-contact; the only post-contact potsherds were found in the uppermost horizon, disturbed by agricultural activity. The quantification from Unit 3 allows us a first estimate of the total slag quantity within this parcel, and a rough calculation of the possible copper metal production from it. The following calculation assumes a closed smelting system using pure chalcopyrite ore, where all iron oxide in the slag originates from the iron content of the chalcopyrite, and all iron from the ore goes into the slag, fluxed by the necessary quantity of silica from the host rock to form fayalite and an additional glassy phase incorporating the fuel ash generated in the process. Pure chalcopyrite (CuFeS2) contains about 30.5 wt% iron and 34.5 wt% copper, i.e. by weight about 10% relative more copper than iron. However, as shown above, not all the copper was extracted, with some being left in the slag, and most chalcopyrite contains varying amounts of pyrite (FeS2) as well as other sulphide minerals. For ease of calculation and to underline the preliminary character of this calculation, round values will be used throughout this estimate, and an effective ratio of 1 will be assumed for copper to iron in the ore. Unit 3 produced about 125 kg of slag from an excavated area of 4 m2; taking this as representative for the entire parcel area of 1200 m2 gives a total slag mass present of 37,500 kg. This slag contains on average slightly less than 40 wt% iron oxide, or about just under 30 wt% iron metal. Thus, the 37.5 tons of slag contain about 10 tons of iron; using the parameters given above this equals about 10 tons of copper metal being produced from the smelting operations represented by the slag volume in parcel 12. 4. Discussion The data and estimates presented above enable some preliminary observations concerning the nature and scale of copper production at Itzipara´tzico. It is obvious that the copper smelting took place in some kind of furnace, and not in crucibles. Both the nature and scale of the slag make this a virtual certainty, providing the first – albeit indirect – indication for a developed furnace technology for copper smelting in pre-Contact Mesoamerica. It is also apparent that there was a substantial and sustained production at the location, running
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for a small unit (Parcel 12) to around 10 tons of metal. Surface survey has identified at least two more parcels with similar slag intensity on the surface, Parcels 10 and 11 (Maldonado, 2006). At present, it is impossible to determine over which length of time this amount of copper was being produced; the ceramic evidence from the test pit is restricted to the Late Postclassical Period (c. AD 1350–AD 1521). Taking an arbitrary estimate of 100 years of copper production being represented in this parcel would result in an average annual production of 100 kg of copper; certainly more than would be required for the local consumption of the group of artisans working in Parcel 12, but considerably less than the 1.8 tons of copper which Grinberg (1996) estimated as a month’s output for a mine in the Churumuco region, being worked by 20 miners (see above). An annual production of 100 kg of copper is, on the other hand, probably not enough to justify a full-time occupation for a group of people. For the Churumuco mines, Grinberg (1996) assumes a daily smelt per miner producing an ingot of just under 5 kg in weight. It is therefore reasonable to assume an average yield of 5 kg of copper for a single smelt, and a total time input for mining, ore preparation and smelting of not more than a few man-days per smelt. This equals about twenty smelts for the estimated 100 kg of annual copper production, and would take a small group of artisans not more than one or two months to achieve, even allowing for the necessary preparation of charcoal and repairs to the furnaces. The absence of any traces of further processing of the smelted copper and the nearabsence of copper tools and objects from the site indicate that the copper production at Itzipara´tzico was embedded in a wider network, linking mining of chalcopyrite in the copper-rich regions around La Huacana with the administrative (and probably consumption) centre of the Tarascan state at Tzintzuntzan. These production figures and the apparent labour separation are consistent with the information on pre-Hispanic copper production deduced from Colonial documents (see above). The smelting activity could be accommodated within the seasonal cycle of agricultural production dictated by climatic conditions, and it would only take two or three people to carry the annual copper yield from this particular parcel to Tzintzuntzan. The presence of several such parcels at Itzipara´tzico indicates that the annual copper production at this area was probably larger than we estimated here, and places Itzipara´tzico among the other smelting sites mentioned in the ethnohistorical documents. The wide spatial and technological separation of mining at La Huacana, smelting at Itzipara´tzico, and further processing at Tzintzuntzan demonstrate a thoroughly organized nature of the metallurgical activities within the Tarascan state, underlining the ideological, political and economic importance of copper production for the ruling elite. It is important to stress the preliminary nature of our calculations. The furnace-based character of the smelting operation is beyond doubt (Maldonado et al., 2005), although we still do not know enough technical details for a full reconstruction of the technology. In particular, details of furnace design and wind provision are entirely lacking, and the size of the furnace and its daily yield can only be very roughly estimated based on ethnohistorical production figures. However, it is reasonable to assume that the furnaces employed in and around Itzipara´tzico were probably semi-permanent structures requiring only limited maintenance and repair during periods of use, since both slag and copper metal were likely removed by tapping, making it unnecessary to break open parts of the furnace for retrieval after each smelt. As indicated by the archaeological observations slag disposal occurred in a relatively wide scatter around the smelting site, probably in order to keep the working space around the furnaces free, while the furnaces themselves were most likely left to decay in situ once copper production at the site had ceased. This, and the general fragility of furnace wall fragments in the archaeological
2005
record, may explain the absence of recognisable furnace wall material in the test-pitting areas. The assumed very limited footprint of the furnaces, of probably not more than one metre square per furnace, makes them very difficult to locate without large area excavation or targeted geophysical prospection. Furthermore, we still do not know the chronological extent of the operation, or its spatial distribution and full scale within the region of Itzipara´tzico. Are there other such slag scatters present beyond the three parcels identified so far? What is the socio-economic relationship between the three copper-producing parcels and the overall economic activity at the site? A wider surface and geophysical survey and further test-pitting to establish representative slag quantities are necessary before the true scale of the operation at Itzipara´tzico can be more closely narrowed down. Similarly, further efforts are necessary to date the slag; the high content of quartz inclusions in the lumpy slags makes them particularly suitable for luminescence dating, which might give us a closer determination of the antiquity and longevity of this smelting technology. 5. Conclusions The archaeometallurgical investigation of fayalitic tap slags from Itzipara´tzico in West Mexico has identified the first recorded indigenous furnace-based copper smelting technology in Mesoamerica, using chalcopyrite as the main ore (Maldonado et al., 2005). Archaeological fieldwork combining surface survey with test excavations has identified three areas or parcels characterized by high slag densities in layers dominated by Late Postclassic pottery; we are confident that the smelting activity is substantially precontact in date even if it may have continued for a short while after the Spanish conquest. Quantification and archaeometallurgical analysis of the excavated slag demonstrated that the scale of production for a single parcel of c. 30 by 40 m extension was of the order of 10 tons of copper metal. Even spreading this over a period of one century indicates an annual production of around 100 kg of metal, probably more than the local demand would have been. On the other hand, this scale of production is less than would be expected of a group of full-time specialists working here; rather, we assume that this amount of copper could have been produced by a small group of people within one or two months. The smelting activity at Itzipara´tzico would be part of a subsistence strategy of a farming community including part-time specialists, with the smelting set within the annual farming cycle, and fully integrated in a wider network of mining, long-distance transport of ore and metal, and tax collection. The absence of evidence for the further processing of the metal at Itzipara´tzico indicates that it was probably delivered as regular tribute to the administrative centre at Tzintzuntzan, as reported by ethnohistorical sources for other communities within the Tarascan state. Acknowledgements The work presented here was done as part of PhD research at the Department of Anthropology at Penn State University, with supervision by Ken Hirth and Paul Howell. We are grateful to the Foundation for the Advancement of Mesoamerican Studies, Inc. (FAMSI) for providing funds for the fieldwork research, and the Copper Museum in Santa Clara del Cobre, for assistance with the weighing and measuring of slag samples. The XRF analyses and metallurgical interpretations were done at the Wolfson Archaeological Science Laboratories at the UCL Institute of Archaeology during a Marie Curie Fellowship for Early Stage Researchers. We thank Simon Groom for his help in the laboratory. Funding by the European Union for the Marie Curie action project IoASCA, under contract MEST-CT-2004-514509, is gratefully acknowledged.
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References Bachmann, H.G., 1982. The Identification of Slags from Archeological Sites. Institute of Archaeology, London. Occasional Publication No. 6. Barrett, E.M., 1987. The Mexican Colonial Copper Industry. University of New Mexico Press, Albuquerque. Bartelheim, M., Eckstein, K., Huijsmans, M., Krause, R., Pernicka, E., 2002. Kupfer¨ sterreich. In: Bartelheim, M., zeitliche Metallgewinnung in Brixlegg, O Pernicka, E., Krause, R. (Eds.), Die Anfa¨nge der Metallurgie in der Alten Welt (The Beginnings of Metallurgy in the Old World). Forschungen zur Archa¨ometrie und Altertumswissenschaft, 1, pp. 33–82. Rahden (Westf.). Betancourt, Ph, 2006. Chrysokamino I: the Metallurgy Workshop and its Territory. Hesperia Supplement 36. American School of Classical Studies, Athens. Brand, D., 1951. Quiroga: a Mexican Municipio. Smithsonian Institution, Institute of Social Anthropology No. 11. U.S. Govt. Print. Off., Washington. Craddock, P.T., 2001. From hearth to furnace: evidences for the earliest metal smelting technologies in the Eastern Mediterranean. Pale´orient 26 (2), 151–165. Davies, S.J., Metcalfe, S.E., MacKenzie, A.B., Newton, A.J., Endfield, G.H., Farmer, J.G., 2004. Environmental changes in the Zirahue´n Basin, Michoaca´n, Mexico, during the last 1000 years. Journal of Paleolimnology 31 (1), 77–98. Gilead, I,. Rosen, S., 1992. New Archaeo-metallurgical Evidence for the Beginnings of Metallurgy in the Southern Levant: Excavations at Tell Abu Matar, Beersheba (Israel) 1990/1. IAMS Newsletter 18, 11–14 Golden, J., Levy, T.E., Hauptmann, A., 2001. Recent discoveries concerning chalcolithic metallurgy at Shiqmim, Israel. Journal of Archaeological Science 28, 951–963. Gorenstein, S., Pollard, H.P., 1983. The Tarascan Civilization: a Late Prehispanic Cultural System, 28. Publications in Anthropology, Nashville. Grinberg, D.M.K., 1996. Te´cnicas Minero-Metalu´rgicas en Mesoame´rica. In: Cervantes, M. (Ed.), Mesoamec´rica y los Andes. Centro de Investigaciones y Estudios Superiores de Antropologic´a Social, Me´xico, pp. 427–471. Horcasitas de Barros, M.L., 1981. Una Artesanı´a con Raı´ces Prehispa´nicas en Santa Clara del Cobre. Coleccio´n Etnologı´a, INAH, Me´xico. Hosler, D., 1994. The Sounds and Colors of Power. MIT Press, Cambridge. Maldonado, B.E., 2002. Modern Metallurgy, Prehispanic Roots: Coppersmithing in Me´xico. Paper presented at the SAA Meetings, Denver, unpublished manuscript. Maldonado, B.E., 2006. Preindustrial Copper Production at the Archaeological Zone of Itziparatzico. Ph.D. dissertation. Department of Anthropology, The Pennsylvania State University, University Park. Maldonado, B.E., Rehren, Th., Howell, P.R., 2005. Archaeological copper smelting at Itzipara´tzico. In: Vandiver, P.B., Mass, J.L., Murray, A. (Eds.), Materials Issues in
Art and Archaeology VII. MRS Proceedings, vol. 852. Michoacan, Mexico, pp. 231–240. Warrendale. Metten, B., 2003. Beitrag zur spa¨tbronzezeitlichen Kupfermetallurgie im Trentino (Su¨dalpen) im Vergleich mit anderen pra¨historischen Kupferschlacken aus dem Alpenraum. Metalla 10. Deutsches Bergbau-Museum, Bochum. Meyerdirks, U., Rehren, Th., Harvey, A., 2004. Reconstructing the early medieval copper smelting at Ross Island. In: O’Brien, W. (Ed.), Ross Island. Mining, Metal and Society in Early Ireland. National University of Ireland, Galway, pp. 651–664. Mu¨ller, R., Rehren, Th., Rovira, S., 2004. Almizaraque and the early copper metallurgy of southeast Spain: new data. Madrider Mitteilungen 45, 33–56. Muan, A., 1957. Phase equilibria at liquidus temperatures in the system iron oxide– Al2O3–SiO2 in air atmosphere. Journal of the American Ceramic Society 40 (4), 121–133. Paredes, C., 1984. El Tributo Indı´gena en la Regio´n del Lago de Pa´tzcuaro. In: Paredes, C. (Ed.), Michoaca´n en el Siglo XVI. Coleccio´n Estudios Michoacanos VII. FIMAX Publicistas, Morelia, pp. 21–104. Pollard, H.P., 1982. Ecological variation and economic exchange in the Tarascan State. American Ethnologist 9 (2), 250–268. Pollard, H.P., 1987. The political economy of prehispanic Tarascan metallurgy. American Antiquity 52 (4), 741–752. Pollard, H.P., 1993. Tariacuri’s Legacy: the Prehispanic Tarascan State. University of Oklahoma Press, Norman. Pollard, H.P., 1997. Recent research in west Mexican archaeology. Journal of Archaeological Research 5 (4), 345–384. Pollard, H.P., 2005. Desarrollo del Estado Tarasco: Proyecto Erongarı´cuaro. Informe Te´cnico Parcial de la Temporada 1 (Campo), 2001 y la Temporada 2 (Laboratorio), 2002–2004. Rehder, J.E., 1994. Blowpipes versus bellows in ancient metallurgy. Journal of Field Archaeology 21, 345–350. Roskamp, H., Retiz, M., Cuellar, A., Ca´rdenas, E., 2003. Pre-hispanic and Colonial Metallurgy in Jicala´n, Michoaca´n, Me´xico: an Archaeological Survey Reports Submitted to FAMSI: http://www.famsi.org/reports/02011/index.html. Shalev, S., Northover, P.J., 1987. Chalcolithic metal and metalworking from Shiqmim. In: Levy, T.E. (Ed.), Shiqmim I: Studies Concerning Chalcolithic Societies in the Northern Negev Desert, Israel (1982–1984), pp. 357–371. BAR International Series 356, Oxford. Veldhuijzen, H.A., 2003. ‘Slag_Fun’, a new tool for archaeometallurgy: development of an analytical (P)ED-XRF method for iron-rich materials. Papers from the Institute of Archaeology 14, 102–118. Warren, J.B., 1968. Minas de Cobre de Michoaca´n, 1533. Anales del Museo Michoacano 6, 35–52.
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Journal of Archaeological Science journal homepage: http://www.elsevier.com/locate/jas
Early copper smelting at Itzipara´tzico, Mexico Blanca Maldonado a,1, Thilo Rehren b, * a b
Department of Anthropology, Pennsylvania State University, 409 Carpenter Building, University Park, PA 16802, USA Institute of Archaeology, University College London, 31-34 Gordon Square, London WC1H 0PY, United Kingdom
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 October 2008 Received in revised form 14 April 2009 Accepted 18 May 2009
Archaeological fieldwork and archaeometallurgical analysis have identified a Late Postclassic regional centre of copper production in western Mexico. The total output from a single unit of production at the site of Itzipara´tzico is an estimated ten tons of copper and nearly forty tons of slag over the lifetime of the installation. It is argued that this smelting was based on slag-tapping furnaces, a technology previously unknown from Mesoamerican archaeological sites. The smelting site is in dense woodland with ample fuel supply, some 125 km from the next mining area with documented contemporary ore extraction; the copper produced would have been passed on to the capital, another 60 km away, for further distribution and working. The scale of production at Itzipara´tzico indicates that copper smelting was done by parttime specialists embedded in a predominantly agricultural economy, and formed part of a centrally organized network of mining, smelting and processing of copper to supply the Tarascan state. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Tarascan Copper smelting Mexico Yield Postclassical Mesoamerica
1. Introduction Early West Mexican metallurgy and metalworking, especially among the Tarascans and their neighbours, was based mainly on copper and its alloys (Hosler, 1994: 127; Pollard, 1987: 743). PreHispanic copper exploitation reached its height during what Pollard (1982, 1987, 1993, 1997) (see also Gorenstein and Pollard, 1983) has called the Protohistoric period (AD 1450–1530) in the Tarascan domain. The technological processes that native metalworkers used for the extraction of copper from their ore and the organization of the craft itself remain, however, poorly understood. Archaeological investigations at the archaeological zone of Itzipara´tzico have for the first time documented evidence for important aspects of precontact extractive copper metallurgy in the Tarascan area. Itzipara´tzico is located on the south-central part of the Lake Zirahu´en Basin in north central Michoaca´n, lying among the modern agricultural fields of Santa Clara del Cobre and Opopeo (Fig. 1). Artisans from Santa Clara del Cobre, a modern Tarascan municipality in Michoaca´n, have maintained traditional techniques for working copper to this day. Information provided by written sources, along with the factual presence of smelting waste products
* Corresponding author. Tel.: þ44 20 7679 4757; fax: þ44 20 7383 2572. E-mail address: [email protected] (T. Rehren). 1 Current address: Curt-Engelholm-Zentrum fu¨r Archa¨ometrie, Universita¨t Tu¨bingen, D6, 3, D-68151 Mannheim, Germany. 0305-4403/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jas.2009.05.019
(i.e. slag) on the surface of archaeological areas near Santa Clara, suggest that the ongoing production of copper has its roots in preHispanic traditions (Horcasitas de Barros, 1981; Maldonado, 2002, 2006). This is particularly significant since, in the absence of mines in the area, ore had to be carried some 125 km from the region of La Huacana (see Fig. 2). The access to pine-oak uplands where forests provided high-quality charcoal seems to have been a major factor for locating the smelters here. While this situation may not seem economically advantageous, the movement of ore from mine to settlement is well represented in the archaeological record elsewhere. Several early examples from the Old World can be cited, including Chalcolithic sites such as Abu Matar (Gilead and Rosen, 1992) and Shiqmim (Golden et al., 2001; Shalev and Northover, 1987) in Israel, which lie some 150 km from the nearest ore source and yet present substantial evidence of smelting activities. Similarly, evidence from the Early Bronze Age in Crete has indicated that smelting operations were being carried out at Chrysokamino, a site located in the northeast of the island, far away from any known ore sources. It has been suggested that the beneficiated ore was brought in by ship for smelting (see Betancourt, 2006). Ethnohistorical evidence indicates that prior to the arrival of the Spanish the bulk of the metal that moved into the Pa´tzcuaro Basin came in the form of tribute delivered with regularity (see Paredes 1984; Pollard, 1982, 1987). The primary supplier of copper was the central Balsas Basin, in the southern portion of the Tarascan territory. Paredes (1984) and Pollard (1987) have suggested that during the last century of the Tarascan Empire the state took a more direct
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Fig. 1. Location of Itzipara´tzico within the Zirahue´n Basin and the state of Michoaca´n (Modified from Davies et al., 2004, Fig. 1).
control of the copper resources of this particular region than simple tribute. This idea is largely based on accounts in the Legajo 1204 (Warren, 1968), which suggest that the Cazonci (the paramount ruler of the Tarascan state, residing in the Tarascan capital, Tzintzuntzan, roughly 60 km NE from Itzipara´tzico) sent workers to extract copper from the mines of La Huacana (see Fig. 3) to meet his needs (Pollard, 1987: 748; Warren, 1968: 47, 48). Most mines,
however, were exploited by the local population, who provided the copper as tribute to the state (Pollard, 1987: 748). According to the Legajo 1204 (in Pollard, 1982: 258, 1987: 748; see also Warren, 1968), the schedule for payments of copper, of either every 40 days or on demand, substantially exceeds that of any other tributary item for the central authority. The Legajo further indicates that mining activities and smelting operations often took
Fig. 2. Pre-Hispanic and Colonial mining centres in the Central Balsas Basin (Adapted from Barrett, 1987: Map 2).
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Fig. 3. Location of Itzipara´tzico with respect to Tzintzuntzan and the mining sources (Modified from Roskamp et al., 2003: Fig. 4).
place at separate locations within the central Balsas Basin (see Fig. 2). While mining centres were concentrated at Churumuco, Sinagua, Cutzian and La Huacana, among others, smelting operations were carried out at La Huacana, Cutzian, and Huetamo-Cutzio (Pollard, 1987; Warren, 1968). At the Cutzian mine alone, there had been up to 50 miners and 40 other workers, some of whom were moving the dirt and mineral out of the mines and some were processing the ore (Warren, 1968: 49). Accounts in the Legajo also state that the metalworkers from the La Huacana region owned and cultivated the fields at the foot of the hill where the copper veins were mined. This suggests that mining and metallurgy (at least at this particular location) represented part-time activities, undertaken mostly during the slack period in the agricultural season. The great climatic variation between the rainy and dry seasons in the region supports this assumption. During the rainy season the mines were probably flooded, while during the dry season agricultural production must have fallen dramatically due to the extreme dryness in the region. It is likely that the smelting operations followed the same seasonal patterns as the mining operations, and were therefore carried out during the dry season. The miners/smelters probably alternated between metalworking and farming, according to the seasons as well as to royal demands (Grinberg, 1996: 433). ´ n and the ´ n de Michoaca According to accounts in the Relacio Legajo 1204, processed ingots were transported for final manufacture on the backs of tamemes (the Nahuatl term for human carriers), each load consisting of 20–30 ingots (Warren, 1968: 47, 49) which together weighed about 32–72 kg (Pollard, 1987: 748). Based on information found in the Legajo, Pollard (1987: 750) has estimated that the major smelting centres of La Huacana and Cutzian were a two days’ journey from Tzintzuntzan. These figures make the transport of ore to a relatively intermediate point like Itzipara´tzico seem plausible, in particular considering the demand for fuel and labour required to sustain a significant smelting operation.
Referring to the Churumuco region, the Legajo 1204 states that twenty labourers worked in each mine, and each of them collected one-half celemı´n (Warren, 1968: 37), that is 2.25 litres (Brand, 1951: 132) of mineral in the form of rock fragments and dust. The sources do not specify whether this is a daily production, but it appears reasonable to assume so. After grinding the mineral and mixing it with ground charcoal, the metal-workers smelted it into an ingot one hand long, one hand wide, and two fingers high (Grinberg, 1996: 433; Pollard, 1987: 748; Warren, 1968: 37). According to calculations by Grinberg (1996: 433) the twenty labourers together produced one carga (load) of copper per day, ´ n (charge) per month. One carga therefore was 20 and one monto ´ n was the ingots, and probably weighted about 90 kg. A monto production achieved in 20 days, the length of a Mesoamerican month, which amounted to 400 ingots, weighting around 1800 kg. These figures, however, seem quite high, particularly if they refer to a sustained production output, and might require re-evaluation. Overall, the ethnohistorical evidence indicates that large-scale production of copper metal took place before the arrival of the Spanish, involving a spatially separated and well-organized labour system. Nevertheless, nearly nothing is known about the technical practicalities of this copper smelting activity, or the reaction vessels in which it took place. Pictorial evidence of metalworking from the early contact period relates mostly to the melting and casting of gold and copper or bronze, and therefore provides little guidance for understanding the extraction process. 1.1. Research background The purpose of the archaeological research carried out at the zone of Itzipara´tzico was to define the extent and nature of the area, to identify and characterize its chronology, and to locate and record evidence for metallurgical activity in order to distinguish traits, or clusters of traits, associated with different stages of pre-Hispanic
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metallurgy. The results would provide the basis for comparing archaeological evidence for metallurgical production with descriptions of pre-Hispanic metallurgy found in ethnohistorical sources. These comparisons were intended to examine both the technology and organization of production. This work represents the first fully reported study of copper smelting in Mesoamerica, as well as the first systematic archaeological investigation carried out in the entire Zirahue´n Basin. Surface survey carried out during the Itzipara´tzico Archaeological Research Project (IARP) in 2003–2004 identified and mapped three major sectors of the site, divided according to the variability of their archaeological materials and features (Fig. 4). While ceramic and lithic artefacts were common throughout the research area, slag concentrations were located almost exclusively in one particular sector (Sector 1). The presence of smelting by-products indicates that smelting activities took place in or around this zone, an indication possibly further supported by the proximity of this sector to water, indispensable for many metalworking processes. Other materials include moderate amounts of potsherds and lithics (mainly grayblack obsidian prismatic blades), as well as a set of stylistically diverse Tarascan pipes. These pipes and several polychrome ceramic fragments have been identified as Late Postclassic in date (see Table 1). Fieldwork at Itzipara´tzico also involved archaeological test excavations in the three major sectors of the area. The results of these excavations were consistent with the observations on the surface. No substantial evidence for occupation before or after the Late Postclassic Period (c. AD 1350–AD 1521) was found. There were a few glazed/European-style ceramics but only very near the surface and not substantial in quantity. All of the ceramic and lithic artefacts excavated appear to belong to a Late Postclassic Tarascan occupation. Although no identifiable metalworking structures (furnaces, hearths, and pits) were found at Itzipara´tzico during the test-pitting, large amounts of slag were recovered from the excavation, together with lithics and ceramics as found elsewhere on site. The absence of metallurgical materials other than slag (i.e. hearth structures, crucible fragments, mould fragments, stock metal, metal prills, failed castings, part-manufactured objects and
Fig. 4. Three main sectors of the Itzipara´tzico area (Map created by van Rossum).
2001
Table 1 Established chronology for the Pa´tzcuaro Basin according to Pollard (2005: 9). Period
Local phases
Late postclassic Middle postclassic Early postclassic Epiclassic Middle classic Early classic Middle preclassic
Tariacuri (A.D. 1350–1525) Urichu Tardı´o (A.D. 1000/1100–1350) Urichu Temprano (A.D. 900–1000/1100) Lupe-La Joya (A.D. 600/700–900) Jara´cuaro (A.D. 500–600/700) Loma Alta 3 (A.D. 350–500) Loma Alta 2 (100/50 B.C.–A.D. 350)
spillages, etc.) around Itzipara´tzico indicates that only primary copper production was being carried out at this location. 2. The slag A representative sample of 2.1 kg of slag from the excavations at Itzipara´tzico was selected from several hundreds of kilograms and exported for archaeometallurgical analysis. Part of this material has been examined at the Wolfson Archaeological Science Laboratories at the UCL Institute of Archaeology, London, UK. The samples were analyzed for microstructure and composition using light microscopy, X-ray fluorescence spectrometry (XRF), and scanning electron microscopy with energy-dispersive X-ray spectrometry (SEM/EDS). A brief summary of the results of the analyses is presented below. Detailed descriptions of these results have been reported and published elsewhere (see Maldonado et al., 2005). Two major types of slag from Itzipara´tzico were identified macroscopically and labelled as either lumpy or platy, according to their general morphology. Lumpy slags are commonly characterized by plano-convex to irregular surfaces, with sizes ranging from gravel-size up to 2 kg. The specimens often present smooth surfaces on one or both of their faces: porosity is always apparent, and the fracture is uneven (Fig. 5). Platy slags are usually flat, with minor amounts of porosity and signs of ropey flow on one or more surfaces, and the impression of soil particles on the opposite side. The thickness ranges from approximately 2–9 mm. Generally, one of the surfaces exhibits a metallic to glassy finish, while the other shows a more resinous lustre. Breakage normally produces a conchoidal fracture (Fig. 6). In both cases the slags consisted of crystalline phases in a glassy matrix. The distinction between the two slag types is fluid, and numerous macroscopically and chemically transitional pieces exist. Similar slag morphologies have been observed in Old World copper slags from Bronze Age to medieval contexts using a variety of furnaces (see e.g. Bachmann, 1982; Metten, 2003; Meyerdirks et al., 2004).
Fig. 5. Lumpy slag fragment from Itzipara´tzico.
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Fig. 6. Platy slag fragment from Itzipara´tzico.
The slags from Itzipara´tzico are relatively consistent in terms of their mineralogy, presenting various newly-formed phases, partially reacted minerals, and metallic prills. The dominant phases in both slag types were fayalite, magnetite, quartz, glass and various sulphides together with small prills of virtually pure copper. The main difference between the two slag types is in their bulk composition. Platy slags tend to have c. 45–50 wt% FeO and around 35 wt% SiO2, while lumpy specimens have only c. 30 wt% FeO and 50–60 wt% SiO2 (see Table 2); samples 2-1b, 1-3b and 3-1b show intermediate compositions. The lumpy slags contain a high concentration of partly reacted quartz grains, thought to be residual host rock of the ore mineral, and resulting in the elevated bulk silica values/lower iron oxide values of this slag type. In contrast, most platy slags contained little or no residual quartz (Fig. 7). Analyses of melt phase and newly-formed slag phases such as fayalite and magnetite by SEM–EDS are consistent between the two types, further indicating that the main difference between the lumpy and the platy slag is in the mechanical incorporation of residual quartz in the former. Significantly, almost all slag fragments have well developed tap slag morphology, with the platy slags having been more fluid, probably due to the absence of quartz inclusions. The two slag types may thus represent sequential waste products of the same continuous smelting process, possibly relating to consecutive tapping events (Maldonado et al., 2005). The regular and large-scale presence of tap slag is important here, since it is typically associated
with a smelting process carried out in a furnace rather than in a crucible (Craddock, 2001: 157). Bulk analyses derived from XRF analyses of platy slag samples from Itzipara´tzico have been plotted on a phase diagram illustrated in Fig. 8. The bulk composition and the ubiquitous presence of magnetite and fayalite match other known examples of fayalitic copper slag. The mineral ore being processed has been shown to be sulfidic, most likely chalcopyrite (CuFeS2), in a silica-rich matrix (Maldonado et al., 2005). During smelting under reducing conditions the iron from the ore would react with silica from the quartz to form fayalite, while the copper is first enriched in a sulphide phase (‘matte’) and then reduced to metal. The dominance of wellcrystallised fayalite and magnetite in a homogenous slag matrix, and the presence of copper prills indicate: 1) a consistent smelting temperature of around 1200 C, and 2) the occurrence of smelting under strongly reducing conditions. This temperature estimate is based on the position of most platy slags near the eutectic region of the ternary diagram in Fig. 8, with liquidus temperatures of around 1100 C, and allowing for some necessary overheating to facilitate full melt conditions throughout. The actual liquidus temperature of the slag is likely lower than indicated by the diagram, due to the cumulative effect of additional minor oxides such as lime, magnesia, and alkalis, all of which exert a further reducing effect on real melting temperatures by an estimated 50–100 C. A single platy slag sample (2-1b) falls outside this low-temperature region; this is most likely due to some residual quartz inclusions, occasionally seen not only in the lumpy slag but also in the platy variety. This underlines on the one hand their close technological relationship, and on the other hand will lead to a distortion of the bulk chemical composition towards higher silica values, resulting in apparently higher liquidus temperatures. A reducing atmosphere at such high temperature is normally considered inconsistent with mouth-blown smelting operations (e.g. Rehder, 1994). The maximum attainable furnace temperature with blowpipes was sufficient to smelt copper and produce a siliceous slag, as indicated by Early Bronze Age evidence from the Old World (e.g. Bartelheim et al., 2002; Mu¨ller et al., 2004) as well as experimental reconstruction. However, the amount of slag produced under these conditions is typically small, in the order of several grams to tens of grams, and the slag mineralogy is very heterogeneous, reflecting the highly variable redox conditions of the process. While fayalite does occur frequently in crucible smelting slags, it has not been reported as a major or even predominant phase in those slags. The combination of temperature and reducing atmosphere necessary for consistent fayalite formation on a scale of several kilograms liquid slag at a time, as evidenced from the larger slag lumps from Itzipara´tzico (Maldonado et al., 2005), is very strong evidence for furnace smelting.
Table 2 XRF analysis of copper slag from Itzipara´tzico. In the slag column, ‘‘p’’ stands for platy and ‘‘l’’ for lumpy, referring to the two major types of slag identified in the sample. The higher and more variable silica values of the lumpy slags are due to residual quartz inclusions in the slag. Analyses were done on powder pellets using a calibration method for iron-rich materials developed by Veldhuijzen (2003). Context Slag SiO2 (%) Al2O3 (%) FeO (%) TiO2 (%) MnO (%) CaO (%) MgO (%) Na2O (%) K2O (%) P2O5 (%) SO3 (%) CuO (%) PbO (ppm) ZnO (ppm) Total (%) 1-1c 1-2b 1-4a 1-4c 2-1b
p p p p p
34.1 32.4 35.0 35.2 40.0
7.20 10.86 9.65 6.40 3.44
49.7 46.9 43.7 50.3 46.2
0.23 0.35 0.29 0.21 0.04
0.06 0.13 0.09 0.09 0.13
0.63 1.46 1.72 2.44 4.29
1.88 2.06 2.30 1.62 0.61
0.36 0.56 0.61 0.58 0.56
0.92 0.82 1.11 0.82 0.50
0.05 0.11 0.07 0.09 0.01
0.13 0.27 0.26 0.59 0.24
2.41 1.29 1.48 0.73 1.44
0 225 0 400 1500
500 1050 650 700 1100
97.4 97.2 96.2 99.0 97.7
1-2a 1-3a 1-3b 2-1a 3-1a 3-1b 3-1c
l l l l l l l
56.4 55.9 41.2 54.4 57.0 41.6 61.7
7.78 7.59 7.56 11.06 5.04 7.12 3.96
30.5 30.3 42.8 27.5 32.7 44.0 32.6
0.27 0.23 0.25 0.23 0.15 0.24 0.12
0.07 0.06 0.09 0.08 0.06 0.07 0.04
0.89 1.71 0.93 2.50 0.59 2.23 0.48
1.29 1.30 0.96 1.44 0.52 1.46 0.03
0.41 0.44 0.56 0.35 0.21 0.57 0.08
1.47 1.34 0.96 1.95 0.84 1.06 0.87
0.05 0.05 0.10 0.04 0.03 0.07 0.06
0.48 0.42 0.46 0.71 0.35 0.71 0.54
1.57 1.23 1.11 0.77 1.87 3.28 0.93
0 0 380 0 400 150 300
150 150 1350 250 600 2800 2550
101.0 100.5 97.0 101.1 99.4 102.5 101.4
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Fig. 7. Low magnification images of a typical ‘‘lumpy’’ slag sample. a) Region of slag that contains a fayalitic solidification structure (at A), together with a polycrystalline aggregate of un-reacted quartz grains (at B). A duplex copper/copper sulfide prill (C) is seen in association with a gas bubble (D). b) Regions of the slag that contain fayalitic slag (A), together with an un-reacted aggregate of chalcopyrite (E).
3. Copper production at Itzipara´tzico The data above raise important questions about the chronological context and development of the primary production of copper in the area. The evidence of tap slag at Itzipara´tzico has significant implications for Mesoamerican archaeometallurgy, as it suggests that the smelting process took place in a furnace rather than in a crucible. The technology employed in the process involved a consistent highly reducing smelting environment, which would be difficult to achieve using lung-powered blowpipes and crucibles
(see also Rehder, 1994). If the smelting activities at Itzipara´tzico involved the use of furnaces powered by hand-worked bellows, we could be dealing with a relatively early post-Contact production area which greatly overlapped with pre-Hispanic traditions, as indicated by the predominance of Tarascan pottery and lithics and the near-absence of colonial wares (see above). Alternatively, if the operations at the site are entirely Late Postclassic in date, as we believe based on the ceramic and lithic evidence and further supported by ethnohistoric sources, this would indicate that Tarascan metalworkers had developed a copper smelting method which
Fig. 8. Triangular diagram showing phase relationships in the system FeO–Al2O3–SiO2 under reducing conditions (after Muan, 1957: Fig. 10). The three shaded ellipses correspond to the compositions obtained for the platy slags (lowermost), intermediate platy to lumpy (central) and lumpy slags (uppermost) (see Table 2), indicating liquidus temperatures of around 1100 C for the platy slags. The higher temperatures indicated for the intermediate group and particularly the lumpy slags are unrealistic; their bulk compositional data is distorted by free quartz particles embedded in the slag.
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most likely involved some form of natural draft, assuming that bellows were not known in pre-Contact Mesoamerica. In either case, it seems clear that copper ore (chalcopyrite) was brought a considerable distance (see Fig. 3) to Itzipara´tzico for smelting, and copper ingots were probably transported elsewhere for their final processing (Maldonado et al., 2005). The choice of smelting site away from the mining area may reflect the availability of large quantities of fuel in the densely wooded area around Itzipara´tzico, missing from the more arid environment near the mining sites. This scenario is consistent with ethnohistorical data, and would conform with a multi-dimensional production model which involved a considerable degree of control from the Tarascan state on the macro-economic scale. Its main organizational characteristic is the division of an industry into small segments that could be easily controlled, separating mining, smelting, and final processing, and probably including a range of subdivisions within these main activities. With no single section performing all the needed manufacturing steps, centralized control was both necessary and more easily established and maintained. While many outstanding questions regarding metallurgical production at Itzipara´tzico and its organizational setting within the wider Tarascan economy are still in the process of being resolved, the existing data provide an initial base for further archaeological and archaeometallurgical studies. One of the aspects of this production that can be explored based on existing research refers to its scale. It is difficult to accurately quantify metallurgical production at Itzipara´tzico based on the information currently available, and no smelting facilities were located during the survey or testpitting. The highest slag concentrations, however, are located within a relatively limited area, away from what appears to be the residential terraces (Maldonado, 2006). As outlined above, the mineral raw materials (copper ores) were not locally available and had to be imported, bringing up issues of transport and organization. It is worth noting that the slag tapping technology employed in the smelting process involved a highly reducing smelting environment, which together with the wide distribution of slag suggests a complex organization of the craft both between geographically disparate sites engaged respectively in mining, smelting, and further processing, and within the individual smelting sites. This would support the idea that part of the production was in the hands of highly skilled smelters who were part-time to full-time specialists, producing copper in excess of their own community’s needs. 3.1. Scale of production Systematic quantification of slag can be used to obtain rough estimates of the copper production at Itzipara´tzico, based on the quantity of slag produced and an estimate of the ratio of slag to metal produced. Such estimates are made by calculating the relative and the absolute copper yield: the relative yield describes the proportion of copper in the ore that was extracted by the smelt and is thus of mostly technical interest, while the absolute yield describes the amount of metal produced, and is of major socioeconomic significance. Slag analysis has determined that the ore being processed was chalcopyrite (CuFeS2) in a quartz matrix, resulting in a fayalitic slag with only 1–2 wt% copper (Maldonado et al., 2005). Because the chemical composition of the slag reflects predominantly that of the ore, and since the chemical composition of the copper mineral is known, a good estimate of the copper yield may be obtained from the composition and total weight of the slag. The difference in copper content between the assumed ore and the analyzed slag indicates a relative yield in excess of 90 percent, in keeping with the efficiency generally assumed for a tap slag technology. For the following discussion we will concentrate on the
absolute yield in order to understand the potential significance of Itzipara´tzico for the metal supply of the Tarascan state. Slag concentrations were located almost exclusively at Sector 1 of Itziparatzico. Intensive surface collections and preliminary testpitting operations were conducted in three of the tractor-ploughed agricultural fields in this sector, parcels 10, 11, and 12, the latter being the most important in terms of the data that have been collected. No solid slag heaps were located during the explorations. Parcel 12 (approximately 40 30 m) covers 1200 m2. Within this parcel, a 5 m radius circular ‘‘dog leash’’ collection was made of 100% of all slag on surface within the circle, resulting in the recovery of 703 fragments, weighing 14.5 kg. Projected to the entire parcel area, this equals c. 225 kg of slag being present directly on the surface, most likely brought up from lower layers by ploughing which penetrates up to 50 cm into the soil below. For an initial volumetric quantification of the slag present within this parcel, and to obtain ceramic from undisturbed archaeological layers, a test pit (Unit 3) of 2 by 2 m was excavated to about 1 m depth, and its content quantified. Most slag was recovered from stratigraphic levels 1, 2, and 3 (0–20 cm, 20–40 cm, and 40–60 cm respectively); level 4, at 60–80 cm, had only little slag, and was generally poor in cultural remains, transitioning to sterile soil beneath. The amount of slag recovered from this test pit totalled 13,857 fragments, weighing 127 kg. The slag was relatively evenly distributed within the total excavated volume, and mixed with some quantities of domestic ceramic; however, neither furnace nor crucible fragments were found. The domestic pottery was predominantly pre-contact; the only post-contact potsherds were found in the uppermost horizon, disturbed by agricultural activity. The quantification from Unit 3 allows us a first estimate of the total slag quantity within this parcel, and a rough calculation of the possible copper metal production from it. The following calculation assumes a closed smelting system using pure chalcopyrite ore, where all iron oxide in the slag originates from the iron content of the chalcopyrite, and all iron from the ore goes into the slag, fluxed by the necessary quantity of silica from the host rock to form fayalite and an additional glassy phase incorporating the fuel ash generated in the process. Pure chalcopyrite (CuFeS2) contains about 30.5 wt% iron and 34.5 wt% copper, i.e. by weight about 10% relative more copper than iron. However, as shown above, not all the copper was extracted, with some being left in the slag, and most chalcopyrite contains varying amounts of pyrite (FeS2) as well as other sulphide minerals. For ease of calculation and to underline the preliminary character of this calculation, round values will be used throughout this estimate, and an effective ratio of 1 will be assumed for copper to iron in the ore. Unit 3 produced about 125 kg of slag from an excavated area of 4 m2; taking this as representative for the entire parcel area of 1200 m2 gives a total slag mass present of 37,500 kg. This slag contains on average slightly less than 40 wt% iron oxide, or about just under 30 wt% iron metal. Thus, the 37.5 tons of slag contain about 10 tons of iron; using the parameters given above this equals about 10 tons of copper metal being produced from the smelting operations represented by the slag volume in parcel 12. 4. Discussion The data and estimates presented above enable some preliminary observations concerning the nature and scale of copper production at Itzipara´tzico. It is obvious that the copper smelting took place in some kind of furnace, and not in crucibles. Both the nature and scale of the slag make this a virtual certainty, providing the first – albeit indirect – indication for a developed furnace technology for copper smelting in pre-Contact Mesoamerica. It is also apparent that there was a substantial and sustained production at the location, running
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for a small unit (Parcel 12) to around 10 tons of metal. Surface survey has identified at least two more parcels with similar slag intensity on the surface, Parcels 10 and 11 (Maldonado, 2006). At present, it is impossible to determine over which length of time this amount of copper was being produced; the ceramic evidence from the test pit is restricted to the Late Postclassical Period (c. AD 1350–AD 1521). Taking an arbitrary estimate of 100 years of copper production being represented in this parcel would result in an average annual production of 100 kg of copper; certainly more than would be required for the local consumption of the group of artisans working in Parcel 12, but considerably less than the 1.8 tons of copper which Grinberg (1996) estimated as a month’s output for a mine in the Churumuco region, being worked by 20 miners (see above). An annual production of 100 kg of copper is, on the other hand, probably not enough to justify a full-time occupation for a group of people. For the Churumuco mines, Grinberg (1996) assumes a daily smelt per miner producing an ingot of just under 5 kg in weight. It is therefore reasonable to assume an average yield of 5 kg of copper for a single smelt, and a total time input for mining, ore preparation and smelting of not more than a few man-days per smelt. This equals about twenty smelts for the estimated 100 kg of annual copper production, and would take a small group of artisans not more than one or two months to achieve, even allowing for the necessary preparation of charcoal and repairs to the furnaces. The absence of any traces of further processing of the smelted copper and the nearabsence of copper tools and objects from the site indicate that the copper production at Itzipara´tzico was embedded in a wider network, linking mining of chalcopyrite in the copper-rich regions around La Huacana with the administrative (and probably consumption) centre of the Tarascan state at Tzintzuntzan. These production figures and the apparent labour separation are consistent with the information on pre-Hispanic copper production deduced from Colonial documents (see above). The smelting activity could be accommodated within the seasonal cycle of agricultural production dictated by climatic conditions, and it would only take two or three people to carry the annual copper yield from this particular parcel to Tzintzuntzan. The presence of several such parcels at Itzipara´tzico indicates that the annual copper production at this area was probably larger than we estimated here, and places Itzipara´tzico among the other smelting sites mentioned in the ethnohistorical documents. The wide spatial and technological separation of mining at La Huacana, smelting at Itzipara´tzico, and further processing at Tzintzuntzan demonstrate a thoroughly organized nature of the metallurgical activities within the Tarascan state, underlining the ideological, political and economic importance of copper production for the ruling elite. It is important to stress the preliminary nature of our calculations. The furnace-based character of the smelting operation is beyond doubt (Maldonado et al., 2005), although we still do not know enough technical details for a full reconstruction of the technology. In particular, details of furnace design and wind provision are entirely lacking, and the size of the furnace and its daily yield can only be very roughly estimated based on ethnohistorical production figures. However, it is reasonable to assume that the furnaces employed in and around Itzipara´tzico were probably semi-permanent structures requiring only limited maintenance and repair during periods of use, since both slag and copper metal were likely removed by tapping, making it unnecessary to break open parts of the furnace for retrieval after each smelt. As indicated by the archaeological observations slag disposal occurred in a relatively wide scatter around the smelting site, probably in order to keep the working space around the furnaces free, while the furnaces themselves were most likely left to decay in situ once copper production at the site had ceased. This, and the general fragility of furnace wall fragments in the archaeological
2005
record, may explain the absence of recognisable furnace wall material in the test-pitting areas. The assumed very limited footprint of the furnaces, of probably not more than one metre square per furnace, makes them very difficult to locate without large area excavation or targeted geophysical prospection. Furthermore, we still do not know the chronological extent of the operation, or its spatial distribution and full scale within the region of Itzipara´tzico. Are there other such slag scatters present beyond the three parcels identified so far? What is the socio-economic relationship between the three copper-producing parcels and the overall economic activity at the site? A wider surface and geophysical survey and further test-pitting to establish representative slag quantities are necessary before the true scale of the operation at Itzipara´tzico can be more closely narrowed down. Similarly, further efforts are necessary to date the slag; the high content of quartz inclusions in the lumpy slags makes them particularly suitable for luminescence dating, which might give us a closer determination of the antiquity and longevity of this smelting technology. 5. Conclusions The archaeometallurgical investigation of fayalitic tap slags from Itzipara´tzico in West Mexico has identified the first recorded indigenous furnace-based copper smelting technology in Mesoamerica, using chalcopyrite as the main ore (Maldonado et al., 2005). Archaeological fieldwork combining surface survey with test excavations has identified three areas or parcels characterized by high slag densities in layers dominated by Late Postclassic pottery; we are confident that the smelting activity is substantially precontact in date even if it may have continued for a short while after the Spanish conquest. Quantification and archaeometallurgical analysis of the excavated slag demonstrated that the scale of production for a single parcel of c. 30 by 40 m extension was of the order of 10 tons of copper metal. Even spreading this over a period of one century indicates an annual production of around 100 kg of metal, probably more than the local demand would have been. On the other hand, this scale of production is less than would be expected of a group of full-time specialists working here; rather, we assume that this amount of copper could have been produced by a small group of people within one or two months. The smelting activity at Itzipara´tzico would be part of a subsistence strategy of a farming community including part-time specialists, with the smelting set within the annual farming cycle, and fully integrated in a wider network of mining, long-distance transport of ore and metal, and tax collection. The absence of evidence for the further processing of the metal at Itzipara´tzico indicates that it was probably delivered as regular tribute to the administrative centre at Tzintzuntzan, as reported by ethnohistorical sources for other communities within the Tarascan state. Acknowledgements The work presented here was done as part of PhD research at the Department of Anthropology at Penn State University, with supervision by Ken Hirth and Paul Howell. We are grateful to the Foundation for the Advancement of Mesoamerican Studies, Inc. (FAMSI) for providing funds for the fieldwork research, and the Copper Museum in Santa Clara del Cobre, for assistance with the weighing and measuring of slag samples. The XRF analyses and metallurgical interpretations were done at the Wolfson Archaeological Science Laboratories at the UCL Institute of Archaeology during a Marie Curie Fellowship for Early Stage Researchers. We thank Simon Groom for his help in the laboratory. Funding by the European Union for the Marie Curie action project IoASCA, under contract MEST-CT-2004-514509, is gratefully acknowledged.
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