Cupellation is a historic metallurgical refining process dating back to the Early Bronze Age, involving the treatment of ores or alloyed metals at high temperatures. The controlled operations aim to separate noble metals, such as gold and silver, from base metals like lead, copper, zinc, arsenic, antimony, or bismuth present in the ore. This process relies on the principle that precious metals oxidize or chemically react at higher temperatures than base metals. When heated, precious metals remain separate, while others react, forming slags or compounds. Cupellation was widely employed during the Middle Ages and the Renaissance for refining precious metals, and it continues to be used today in metallurgy.
Large-scale cupellation is a metallurgical process used for the primary production of silver from argentiferous lead ores. In this process, the alloy undergoes smelting and cupellation at high temperatures, around 960 °C to 1000 °C, in an oxidizing environment. The lead oxidizes to lead monoxide (litharge), which captures oxygen from other metals. The resulting liquid lead oxide is either removed or absorbed into the hearth linings, forming litharge cakes. The base of the hearth is designed with a calcareous lining to facilitate litharge absorption. Archaeologists analyze litharge cakes to gain insights into the ore, its components, and the production conditions during the Early Bronze Age.
Small-scale cupellation follows the same principles as large-scale cupellation but is applied to smaller quantities. The minerals are crushed, roasted, and smelted to concentrate metallic components for separating noble metals. This process was diverse during the Renaissance, used for assaying ores, testing silver content in jewelry or coins, and experimental purposes. Small-scale cupellation is conducted in shallow recipients known as cupels. The process involves carefully weighing the matter to be tested, using an assay furnace with windows and bellows to ensure lead oxidation. Pure lead is added to facilitate impurity separation, and after litharge absorption, buttons of silver form in the cupel. If the alloy contains gold, both metals need to be separated through parting.
Cupels.
Cupels are the essential tools for small-scale cupellation, designed as small vessels shaped like inverted truncated cones, traditionally made of bone ashes or alternative materials like burned antlers of deer or fish spines, according to Georg Agricola. These cupels are carefully manufactured, with ashes ground into a fine powder mixed with a sticky substance for molding. The molds, typically copper without bottoms, allow for easy removal of the cupels. Cupels feature a shallow depression in the center made with a rounded pestle, and their sizes vary based on the quantity of material to be assayed.
Historical evidence and archaeological findings suggest that cupels have been made from different materials, including mixtures of bones and wood ashes, or molded with a combination of such materials in the bottom layer and bone ashes on top. The choice of materials depends on the expertise of the assayer or the specific purpose, such as assays for minting, jewelry, or testing the purity of recycled materials or coins. Archaeological evidence indicates that, in the early stages of small-scale cupellation, potsherds or clay cupels were also used.
The history of cupellation
The history of cupellation, a refining process in metallurgy, dates back to the Early Bronze Age, with the first known use of silver in Anatolia and Mesopotamia during the 4th and 3rd millennium BC. Archaeological findings suggest silver extraction from lead ores or the addition of lead to collect silver from visible silver minerals. The process of cupellation played a crucial role during the Iron Age, where base metals were fused with surplus lead, and the resulting bullion was separated in cupellation furnaces.
Mines like Rio Tinto in Spain and Laurion in Greece became significant during the Iron Age, influencing political and economic landscapes. Athens gained advantage through control of Laurion mines, contributing to their victory over the Persians around 500 BC. In Roman times, the empire’s need for lead to support civilization led to the search for open lead-silver mines, further establishing silver coinage as a medium of exchange.
The use of cupellation for analysis has ancient origins, with one of the earliest references found in Theophilus Divers Ars in the 12th century AD. However, the process remained largely unchanged until the 16th century. Small-scale cupellation, considered a crucial fire assay and perhaps the origin of chemical analysis, gained prominence in the Renaissance. Writers like Vannoccio Biringuccio, Georg Agricola, and Lazarus Ercker documented cupellation’s role in mining, testing ores, minting, and jewelry testing. Cupellation became a standardized method, contributing to economic, political, and warfare spheres throughout ancient times, demonstrating its enduring efficiency.
Silver Metallurgy in the New World: Pre-Hispanic Mysteries and Colonial Mining
The abundance of Pre-Hispanic silver adornments in regions like Peru, Bolivia, and Ecuador raises questions about whether the civilizations obtained raw material from native ores or argentiferous-lead ores. While native silver is as rare in the New World as in the Old World, colonial texts reveal that silver mines were operational from Mexico to Argentina during colonial times. Key mines, such as those in Tasco, Mexico, and Potosí, Bolivia, were established by the Spaniards.
Colonial texts describe blast furnaces known as huayrachinas, utilized in Peru and Bolivia to smelt ores from Spanish-owned silver mines. There is speculation that these furnaces were in use before the Spanish Conquest, supported by ethnoarchaeological and archaeological findings in Porco Municipality, Potosí, Bolivia. Despite the absence of specific archaeological accounts of silver smelting or mining in the Andes prior to the Incas, silver and lead artifacts from the pre-Inca and Inca periods suggest cupellation, indicated by the presence of lead in silver artifacts.