When the wind blows: environmental adaptability in current day silver production within the Bolivian...

WhEn ThE WInd bLoWs: EnvIronMEnTAL AdAPTAbILITy In CurrEnT dAy sILvEr ProduCTIon WIThIn ThE boLIvIAn AndEs

c.r. cohenth. rehrenm. VAn buren

1. InTroduCTIon

Recent ethnographic studies by Proyecto Arqueológico Porco-Potosí of current silver pro-duction has revealed the continuing tradition of using huayrachinas, peculiar small wind-blown furnaces. Historical literature cites the huayrachina as being an indigenous technique local to the Andes. This technology was believed to have become extinct at the turn of the 20th century, with the last published record of it by Robert J. Peele (1893). This project is for the first time in a hundred years recording and analysing how these current-day technologies function. The discovery of the current huayrachinas and other metallurgical activity still in use further highlights the continuation and significance of this technique within Andean metallurgy.

The much wider aim of this project is to use the ethnographic material as a comparable basis upon which archaeological material can be understood, and the influence of European methodology on indigenous smelting can be established. This paper will present data from current-day silver production: it will comment on the technological efficiency of the process and its relationship to the archaeological debris. Little is known about pre-colonial metallurgy, especially in the Porco-Potosí region, and the analysis of the current-day materials may lead to some interesting parallels between the historical and archaeological information.

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Offprint from Proceedings ISA 2006, Eds. JF Moreau, R Auger, J Chabot and A Herzog, Quebec 2009 ISBN 978 2 920576 98 8

466 Proceedings of the 36th internAtionAl symPosium on ArchAeometry

2. rELATIng EThnogrAPhIC sILvEr ProduCTIon To ThE hIsTo-rICAL rECord

The origins of the current-day technology appear to be a symbiosis of techniques; the use of the huayrachina indicates a link to Andean technology but, in contrast, the use of the cupellation chamber seems to have connotations of European Renaissance technology. Since the arrival of the Spanish in the Porco-Potosí region in the mid-16th century, evidence for the use of huayrachinas and associated refining technologies has been documented in the early chroniclers’ accounts: Cieza de Leon 1553, Capoche 1585, Acosta 1590, Ramírez 1597, and Alonso Barba 1640 (Oehm 1984). In conjunction with the use of the huayrachina (Fig. 1) early chroniclers also documented the use of refining furnaces; Oehm (1984) and Van Buren and Mills (2005) summarise the information documented by Spanish manuscripts, both concluding that refining features were used in antiquity to produce pure silver. The chroniclers’ accounts explain that these refining furnaces required: reeds or blowpipes made from copper or the use of bellows to aid the refining process. The use of bel-lows is unknown in the pre-colonial New World, so the documentation of this technology may infer transmission of technological knowledge from the Spanish to indigenous peoples. The significance of this refining technology within Andean history is still unknown; archaeometallurgical studies on material debris may be able to explain more about the origins of this type of technology and the technical expertise of the indigenous people of the Porco-Potosí region

figure 1. A huayrachina from the seventeenth century (Douglass R E and Mathewson E P (trans) 1923, 199).Alvaro Alonso Barba’s representation of the huayra or indigenous furnace, first published in 1640. He illustrates the column shaft of the furnace, flames jet out of the pierced holes used to aerate the furnace. The metal flows out of the base of the furnace (bottom left).

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467When the Wind bloWs

The influence of the Spanish in the mining communities was one of the most prevalent as the Spanish were motivated by their desire for precious metals (Steward 1946). This region has played a pivotal role in the mining and extraction of silver since pre-colonial times, and in particular for the Spanish since the discovery of the Cerro Rico (Rich Mountain) of Potosí in 1545. Analysis of trace elements in sediment layers of some regional lakes has shown that increased lead concentrations were found during Inca through to early colonial periods (Abbott & Woolfe 2003), indicating that the intensity of production was greatly increased during the early colonial period. Historical records show relatively consistent use of the huayrachina and we assume that other refining features were also in use, certainly after and most probably already before the arrival of the Spanish.

Robert J. Peele visited Potosí at the turn of the 20th century. His article on ‘A primitive smelting furnace’ (1893) is believed to have been the last published information on observations of working huayrachinas, indicating the continued use of the huayrachina throughout the colonial era. He considers the type of ore smelted (according to him an argentiferous galena) and notes that the fuel/ore ratio was 1:1. Peele explains that this ratio is 1:1 because the high altitude will require a much higher use of fuel, lack of galena would require a higher quantity of fuel and small furnaces do not promote economical function (Peele 1893, 9).

Pfordte (1893) describes smelting/refining technology from the 19th century in the Cerro de Pasco, Central Peru. The style of production is similar to the one documented from Bolivia, and is a useful comparison to the current-day silver production in Porco. Quite fittingly, he comments on this technology as follows: “Although the subject has no practical bearing on the metallurgy of the present day, it may not be entirely uninteresting to note how the art of silver-lead smelting has been, and in a few remote districts of Peru is still practised by an old method, and under conditions widely different from our own” (Pfordte 1893, 25).

Pfordte is clear to emphasise the lack of fuel available to Cerro de Pasco residents; he claims that lack of fuel prevented large-scale production necessary for the export of metals, this reduced availability also affecting the type of smelting technique. Pfordte describes a small reverberatory furnace that has a shallow hearth for cupellation. It is an enclosed structure built out of fire-resistant rock lined with clay and externally sealed with lime and sand. The furnace consists of a fire box, concave hearth and a chimney built 10-18 ft high out of adobe. Pfordte says that these furnaces are generally contained within an adobe-thatched house, and require the following tools: a rabble, scraper, poker and paddle-shaped spatula for extracting litharge. The furnace needs to be air-dried before use, and then a fire is started slowly, the heat increased over a four-day period using taquia (dried llama dung). “It [the furnace] is generally fed in such a manner that a small quantity is constantly being thrust rapidly through the fire-door with a thin board or sheet of tin; this operation having also the advantage of producing at the same time a slight draft” (Pfordte 1893, 27). Pfordte is specific regarding the way in which the furnace is charged: 100-125 pounds of lead metal and/or litharge is first put in, this is allowed to melt and reduce, then 400 pounds of an easily fused mate-rial such as a lead carbonate or lead oxide is added, and finally the ore (galena with c. 50 ounces

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silver per ton of grey copper (fahlore) or ruby silver (argentite) which has been crushed (3-8 inch screen) is added to the bath. The temperature of the furnace is slowly increased and any slag formed is removed; this process took up to 4 hours. Once the lead has begun to oxidise, the process takes a further 7-8 hours. If the charge is poor it may need further refining episodes. The litharge produced is saved and used in further smelts. This detailed account is one of the most important for grasping 19th century silver refining. The key features of this process are similar to the cupellation observed in Porco-Potosí: the use of a closed hearth, llama dung as fuel, and the charging of sulphidic ore onto the cupellation hearth. Only the scale differs by an order of magnitude.

The historical record shows that indigenous technologies have been continuously used for pro-ducing lead and silver within the Andes. Clearly, lack of fuel and the accessibility of minerals were some of the key constraints within antiquity. These limitations are still apparent in the current-day production.

3. CurrEnT LEAd sMELTIng usIng huayrachinas

In 2001 Proyecto Arqueológico Porco-Potosí personnel identified a retired miner, Carlos Cuiza, who still uses traditional smelting techniques to produce lead and silver, and who agreed to have the process documented. Cuiza said that he learned how to use the huayrachinas and cupellation hearth from his parents who used similar furnaces to produce silver and sold it to the mint in Potosí (Van Buren & Mills 2005); currently he sells his silver privately to jewellers in Potosí. It should be noted that Cuiza is more or less regularly smelting and producing silver using traditional techniques learnt from his parents; his activity is not an artificial reconstruction of an extinct technique but a continuation of traditional knowledge. His huayrachinas sit on a saddle above his house; they are used three to four times a year to supplement his normal income. They measure 86 cm high, have an internal height of 45 cm, and sit on top of four or five base stones (Van Buren 2003, 137). They have twelve small apertures and two larger openings on opposing sides that are facing into the wind.

It became apparent that Cuiza used a two-stage process to produce pure silver: initial smelting of galena (PbS) using huayrachinas, followed by processing/smelting of a high grade silver ore by cupellation using the lead produced in stage one in an indoor reverberatory hearth. Cuiza owns two huayrachinas, but one works better than the other and in 2002 and 2003 only one was selected by him for use, based on his experience with both.

A total of seven smelts were documented in 2001, 2002 and 2003. For the smelts to be recorded by the Proyecto Arqueológico Porco-Potosí, Cuiza requested that members of the project team pur-Cuiza requested that members of the project team pur-chase the silver ore, while he provided the lead ore. The following is a summary of the smelting practise.

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Initially Cuiza repaired any cracks or damage on the huayrachinas, which are made from local un-tempered clay and prone to cracking. It was not necessary to let these fillers dry before the smelt. Cuiza ameliorated his lead ore by crushing it and hand-sorting the pure galena from the gangue minerals. He also prepared lumps of litharge (lead oxide), which he obtained from his own refin-ing hearth, but also from other hearths elsewhere in the vicinity of Porco. For the smelting process he used a mixture of charcoal prepared by him from queñua wood, and churqui charcoal which he bought in Potosí. He loaded the charge (crushed galena wetted with urine, litharge and charcoal) into the top of the huayrachina. Once full the huayrachina was lit using ichu grass (a spiky grass that grows at high altitude), and the tap hole at the base of the huayrachina was plugged with a queñua stick. Once the furnace was ablaze, Carlos Cuiza attended to the smelt topping up with alternating layers of charcoal, beneficiated ore and litharge (Fig. 2).

Occasionally, molten lead was allowed to flow out of the tap hole, and Cuiza patiently waited while small quantities of lead metal were tapped from a hole at the base on to an iron dish (Fig. 3).

The smelting process lasted up to eight hours, but this was highly dependent on the weather conditions, as huayrachinas are reliant on strong constant winds to maintain them and to attain high temperatures in the smelting chamber. During the smelt, Cuiza performed ch’allas, symbolic ritual offerings using coca leaves that were fed into the furnaces, and libations of alcohol. Pieces of slag were removed from the two opposing mouths of the huayrachina; Cuiza continuously assessed the slag and decided whether it needed to be re-smelted and thrown back into the furnace or had had the majority of lead removed. The result of the huayrachina smelt was a quantity of pure lead tapped from the base of the furnace and larger quantities of irregular lumps of slag (Van Buren 2003, 138).

figure 2. Carlos Cuiza smelting using a huayrachina (courtesy of B Mills 2002).Carlos Cuiza attending to the huayrachina during a lead smelt recorded in 2002. Periodically he needs to check the slag that collects at the base of the furnace; this can be added to the top of the furnace to be re-smelted. The furnace is constantly topped up with charcoal and beneficiated ore.

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4. sILvEr rEfInIng vIA CuPELLATIon

The lead metal produced in the huayrachinas was then used to extract silver from rich ore via cupellation. Cuiza has two cupellation furnaces, one that has been used for three generations and the second that was built in the last twenty years during a dispute with his brother. The cupel-lation furnace consists of three chambers: the main chamber which holds a concave hearth, a fire box that is positioned to one side and slightly below this, and a chimney on the opposite side of the hearth. The furnace uses natural draft to carry the heat and flames from the firebox over the hearth and through the chimney out of the furnace structure. The hearth is lined with plant ash made from llareta (Azorella yareta) (Van Buren & Mills 2005), a dense woody plant of moss-like appearance native to the altiplano. The plant ash was wetted with urine and packed into the hearth, 2 cm thick. The ash acts like a sponge and absorbs the liquid litharge formed during the cupellation process. The fire box was prepared by loading with llama dung and thola wood. The lead was placed on the hearth through an opening at the top of the chamber, which was then closed and the fuel in the fire box lit. Once the lead had melted, Cuiza briefly removed the door from one of the side openings of the central chamber and added finely ground silver ore with an iron spoon. The whole process took up to 19 hours with Cuiza continuously adding llama dung and stoking the firebox and adding as much of the powdered ore onto the lead bath as possible. At the end a button of pure silver remained on top of a litharge cake (Fig. 4). This litharge can be removed from the hearth and used as part of the charge in the next huayrachina smelt.

figure 3. Lead smelting in the huayrachina (courtesy of B Mills 2002).When the metal is ready the twig at the base of the furnace is removed and the lead metal dribbles out on to an iron dish.

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5. AnALysIs of ThE LEAd sMELTIng

Here is not the place to present a detailed assessment of the smelting technique; for details, the reader is referred to Cohen et al. (in press) and Cohen et al. (forthcoming). However, a quick review of the lead smelting process indicates that it is not very efficient for metal yield, with only about 30 % of the lead metal available in the charge being actually extracted (Table 1), but the process may have other inherent benefits. Cuiza uses a mixture of galena and litharge as his charge; he is thus not only extracting the lead present in the galena but also recycling the lead within the litharge from previous smelts. However, almost two-thirds of the lead content of the charge are still lost in the process, either as fumes and dust, or absorbed onto the furnace wall, or discarded as solid slag lumps (Cohen et al. in press). We may assume that, ideally, the lead oxide from the litharge reacts with the lead sulphide from the ore to form lead metal and sulphur dioxide, but little slag and thus addition of litharge is inherently beneficial. While considering the charge we must review the fuel/ore ratio, which in the recorded smelts is typically 1:2 by weight. This is significantly less than the ratio usually encountered in traditional metal smelting, which ranges often from 1:1 to 3:1, or even higher (Anguilano et al. in press). This low ratio clearly does not facilitate high temperatures or very strongly reducing conditions. Here we can observe environmental adaptation, Cuiza needs to use charcoal for the smelt but charcoal is expensive to buy, since the altiplano region of Bolivia has scarce plant material and fuel is generally costly. Cuiza makes his own charcoal but this requi-res considerable time investments, of around one week to produce enough charcoal for a week of smelting. By reducing the amount of fuel used in the smelt Cuiza can economise on the cost of fuel

figure 4. The cupellation hut (Taken 2006 by C Cohen).This image shows one of Cuiza’s two cupellation huts (the hut is in the bottom left of the image). It is located in a ravine close to Cuiza’s property. It is very well camouflaged, totally hidden if you are standing next the house shown in the upper right of the image.

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and invest more time into the refining of his silver ore. The main consequence in economising on quantity of fuel is a reduction in the quantity of lead produced from the huayrachina smelts. This decreased yield is acceptable as Cuiza only needs sufficient lead metal to refine his rich silver ore in the cupellation chamber. These adaptations are driven by a necessity to create extra income; he only produces silver three to four times a year, so the investment of time needs to reflect the profit that results from selling the silver in Potosí.

6. AnALysIs of ThE CuPELLATIon ProCEss

Cupellation is a process used to extract the noble metals, gold and silver, from lead by oxidation. It requires temperatures above the melting point of silver (960 °C) and an oxidising environment. Lead is an ideal agent for cupellation: it becomes oxidised in conditions where noble metals do not, and when molten as lead oxide is immiscible with the noble metals (Bayley & Eckstein 1997, 108). Thus, during cupellation lead is oxidised to lead oxide (PbO/litharge), which then either flows out of the hearth or is absorbed via capillary action into the porous hearth lining (Rehren 2001, 67) to create cupellation hearth material (CHM). The noble metals do not oxidise and thus remain behind during the cupellation process. The hearth lining needs to be made from a material that does not contain high levels of silica, as silica will react with lead oxide to form a viscous lead silicate which clogs the necessary porosity in the lining (Bayley & Eckstein 1997, 108).

The ongoing scientific documentation of Cuiza’s refining process covers four different mate-rials, namely the llareta ash and the silver ore as starting materials, and the cupellation hearth mate-rial and silver metals as products. Here, we focus on the llareta ash and hearth lining to assess their suitability for the process.

Llareta ash: Llareta ash contains circa 40 % silica, 30 % lime, and c. 10 % potash as main constituents (Table 2). The high lime and relatively low silica contents make llareta ash an advan-tageous choice; it is preferable to grass or straw ash, or the ash of llama dung which is based prima-rily on grass or straw, because the high silicate levels typical of grass ashes would be detrimental to the cupellation. Trees are scarce in the Porco region, thus llareta ash is the most obvious plant ash available. It still contains relatively high levels of silica for cupellation and thus is not the ideal hearth lining.

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7. CuPELLATIon hEArTh MATErIAL AnALysEs

SEM imaging of hearth lining soaked in litharge shows that the llareta ash reacts with the litharge. The impregnated hearth lining is commonly porous with large inclusions of different mine-rals, but these were not documented in detail. They were merely categorised for proportion within the sample and with the clear aim of distinguishing where they had derived from. It is apparent that the llareta ash used to line the hearth has reacted and been partly dissolved within the main litharge matrix.

The analyses provide evidence that the llareta ash is not completely inert, but it is chemically reacting with lead oxide. The remaining high level of porosity in the used cupellation hearth mate-rial also indicates that the llareta does not act fully as a sponge soaking up the liquid litharge. The reaction between the ash and litharge probably creates a viscous lead silicate melt that is not liquid enough to flow through the residual ash.

The input and output quantities from the cupellation process (Table 3) show that the quantity of silver produced is sufficient to be sold on to jewellers in Potosí. In 2002 the cupellation process did not work, this is partially due to the Project team members who bought a poor/low grade silver ore, but also because the llareta ash hearth fractured and the molten metal mixture was lost in between the cracks. The analyses have indicated that the llareta ash is not an ideal material to be used for cupellation, but the selection of this rather than bone ash is an interesting technical adaptation. The use of llareta as fuel is well known in the Andes, it is highly probable that this ash was much more readily available than any other lower silica-based ash. The selection of llama dung rather than any other fuel also shows environmental adaptation, as an easily available fuel sources it would be pre-ferential to charcoal. Of greater interest is the clear separation between the lead smelting and silver refining. The use of two separate fuels could have allowed a much lower time investment into the overall process. Clearly Cuiza is using a process that is well adapted to the environment.

figure 5. Finally all the effort is worthwhile - silver!Silver is produced via cupellation; the silver bullion sits on a litharge cake/cupellation hearth material (CHM). The CHM is recycled back into the lead smelting. Cuiza sells the silver bullion to jewellers in Potosí.

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8. suMMAry of ThE CurrEnT sILvEr ProduCTIon ProCEss

The current-day silver production process indicates that there have been several environmental adaptations of the process. These include the selection of different fuels for smelting and cupella-tion, the use of local materials such as llareta ash for the refining process, and the remarkably low fuel/ore ration used in the huayrachina smelting. All indicate the preferential selection of available materials that are sufficient for the process, and ensuring the economic gains that drive the conti-nuation of this technology to this day.

Carlos Cuiza is using this process to gain extra income for his household. Peasant economy within the Andes is often driven by the need to bring in extra cash to the primary subsistence method, usually via agricultural activity, or trade and exchange. The necessity for cash means that Andean peasant households are reliant on a number of different economic strategies to survive. “Seasonal trading trips, migrant labour, artisan production, and animal husbandry may all be enga-ged in by a single household, but none is relied upon as the sole source of income” (Sillar 2000, 46). Carlos Cuiza and his family are a very obvious example of the different methods upon which extra income can be brought into the household; Cuiza has a number of different animal herds and other farming activities which he is reliant on for his primary income. The investment made into the production of silver is a time investment, but clearly this is supported by his ability to obtain the fuel and minerals at little or no cost (llama dung, llareta ash and minerals, all obtained with no or low monetary investment). His choices regarding the technology must have been made with strong economic factors in mind and the environmental adaptation that we can see from the process can bring another dimension into the understanding and variability of Andean metallurgy both past and present.

9. ConCLusIons

Analysis of the smelting process in the current-day huayrachinas has shown that these furna-ces have very low yield and are relatively inefficient. Very low quantities of fuel are necessary for a smelt, an obvious adaptation to having limited accessibility of fuel within the Andes (the Porco-Potosí region is 4000 m.s.a.l, vegetation is sparse and fuel is expensive and in limited supply).

The lead made in the huayrachinas is used to refine silver in cupellation furnaces. The cupel-lation process uses a hearth that has been lined with plant ash; lead metal produced in the huayra-chinas is melted and used to cupel a high grade silver ore. This cupellation process is not in itself very efficient, and metal is left behind in the waste material. But the ethnographic observation has shown that recycling of these waste materials in subsequent smelts can lead to an overall balanced process.

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rEfErEnCEs

Abbott MB, Wolfe AP, 2003. Intensive Pre-Incan Metallurgy Recorded by Lake Sediments from the Bolivian Andes. Science 301: 1893-1895.

Anguilano L, Timberlake S, Rehren Th, in press. Excavation, slag analysis and experimen-tal reconstruction of an Early Medieval lead smelting bole from Banc Tynddol, Cwmystwyth, Ceredigion (Wales UK). Historical Metallurgy special volume.

Bayley J, Eckstein K, 1997. Silver refining – production, recycling, assaying. In: A. Sinclair, E. Slater & J. Gowlett (eds), Archaeological Sciences 1995: 107-111.

Cohen CR, Rehren Th, Van Buren M, Mills BH, in press: Current silver smelting in the Bolivian Andes: a review of the technology employed. Historical Metallurgy special volume.

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Van Buren M, 2003. Un estudio etnoarqueológico de la tecnología de fundicíon en el sur de Potosí, Bolivia’. Textos Antropológicos 14(2) , UMSA, La Paz: 133-148.

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