Cupellation is a refining process in metallurgy, where ores or alloyed metals are treated under very high temperatures and have controlled operations to separate noble metals, like gold and silver, from base metals like lead, copper, zinc, arsenic, antimony or bismuth, present in the ore. The process is based on the principle that precious metals do not oxidise or react chemically, unlike the base metals; so when they are heated at high temperatures, the precious metals remain apart and the others react forming slags or other compounds.
Since the Early Bronze Age, the process was used to obtain silver from smelted lead ores. By the Middle Ages and the Renaissance, cupellation was one of the most common processes for refining precious metals. By then, fire assays were used for assaying minerals, that is, testing fresh metals such as lead and recycled metals to know their purity for jewellery and coin making. Cupellation is still in use today.
Native silver is a rare element, although it exists as such. It is usually found in nature combined with other metals, or in minerals that contain silver compounds, generally in the form of sulfides such as galena (lead sulfide) or cerussite (lead carbonate). So the primary production of silver requires the smelting and then cupellation of argentiferous lead ores.
Lead melts at 327°C, lead oxide at 888°C and silver melts at 960°C. To separate the silver, the alloy is melted again at the high temperature of 960°C to 1000°C in an oxidizing environment. The lead oxidises to lead monoxide, then known as litharge, which captures the oxygen from the other metals present. The liquid lead oxide is removed or absorbed by capillary action into the hearth linings. This chemical reaction may be viewed as:
The base of the hearth was dug in the form of a saucepan, and covered with an inert and porous material rich in calcium or magnesium such as shells, lime, or bone ash. The lining had to be calcareous because lead reacts with silica (clay compounds) to form viscous lead silicate that prevents the needed absorption of litharge, whereas calcareous materials do not react with lead. Some of the litharge evaporates, and the rest is absorbed by the porous earth lining to form "litharge cakes".
Litharge cakes are usually circular or concavo-convex, about 15 cm in diameter. They are the most common archaeological evidence of cupellation in the Early Bronze Age. By their chemical composition, archaeologists can tell what kind of ore was treated, its main components, and the chemical conditions used in the process. This permits insights about production process, trade, social needs or economic situations.
Small scale cupellation is based on the same principle as the one done in a cupellation hearth; the main difference lies in the amount of material to be tested or obtained. The minerals have to be crushed, roasted and smelted to concentrate the metallic components in order to separate the noble metals. By the Renaissance the use of the cupellation processes was diverse: assay of ores from the mines, testing the amount of silver in jewels or coins or for experimental purposes. It was carried out in small shallow recipients known as cupels.
As the main purpose of small scale cupellation was to assay and test minerals and metals, the matter to be tested has to be carefully weighed. The assays were made in the cupellation or assay furnace, which needs to have windows and bellows to ascertain that the air oxidises the lead, as well as to be sure and prepared to take away the cupel when the process is over. Pure lead has to be added to the matter being tested to guarantee the further separation of the impurities. After the litharge has been absorbed by the cupel, buttons of silver were formed and settled in the middle of the cupel. If the alloy also contained a certain amount of gold, it settled with the silver and both had to be separated by parting.
The primary tool for small scale cupellation was the cupel. Cupels were manufactured in a very careful way. They used to be small vessels shaped in the form of an inverted truncated cone, made out of bone ashes. According to Georg Agricola, the best material was obtained from burned antlers of deer although fish spines could work as well. Ashes have to be ground into a fine and homogeneous powder and mixed with some sticky substance to mould the cupels. Moulds were made out of brass with no bottoms so that the cupels could be taken off. A shallow depression in the centre of the cupel was made with a rounded pestle. Cupel sizes depend on the amount of material to be assayed. This same shape has been maintained until the present.
Archaeological investigations as well as archaeometallurgical analysis and written texts from the Renaissance have demonstrated the existence of different materials for their manufacture; they could be made also with mixtures of bones and wood ashes, of poor quality, or moulded with a mixture of this kind in the bottom with an upper layer of bone ashes. Different recipes depend on the expertise of the assayer or on the special purpose for which it was made (assays for minting, jewelry, testing purity of recycled material or coins). Archaeological evidence shows that at the beginnings of small scale cupellation, potsherds or clay cupels were used.
The first known use of silver was in the Near East in Anatolia and Mesopotamia during the 4th and 3rd millennium BC., the Early Bronze Age. Archaeological findings of silver and lead objects together with litharge pieces and slag have been studied in a variety of sites, and metallurgical analysis suggests that by then people were confidently extracting silver from lead ores so the method would have been known earlier.
During the following Iron Age, cupellation was done by fusing the debased metals with a surplus of lead, the bullion or result product of this fusion was then heated in a cupellation furnace to separate the noble metals. Mines such as Rio Tinto, near Huelva in Spain, started to be an important political and economic site for many people around the Mediterranean Sea, as well as Laurion in Greece. Around 500 B.C. control over the Laurion mines gave Athens political advantage and power in the Mediterranean so that they were able to defeat the Persians.
During the Roman times, the empire needed large quantities of lead in order to support the Roman civilization over a great territory; they searched for open lead-silver mines in whatever area they conquered. Silver coinage became the normalised medium of exchange, therefore silver production and mine control gave economic and political power. In Roman times it was worth mining lead ores if their content of silver was 0.01% or more.
The origin of the use of cupellation for analysis is not known. One of the earliest written references to cupels is Theophilus Divers Ars in the 12th century AD. The process then changed little until the 16th century.
Small-scale cupellation may be considered the most important fire assay developed in history, and perhaps the origin of chemical analysis. Most of the written evidence comes from the Renaissance in the 16th century. Georg Agricola and Lazarus Ercker, among others, wrote about the art of mining and testing the ores, as well as detailed descriptions of cupellation. Their descriptions and assumptions have been identified within diverse archaeological findings through Medieval and Renaissance Europe. By these times the amount of fire assays increased considerably, mainly because of testing the ores in the mines in order to identify the availability of its exploitation. A primary use of cupellation was related to minting activities, and it was also used in testing jewelry. Since the Renaissance, cupellation became a standardised method of analysis that has changed very little, demonstrating its efficiency. Its development certainly touched the spheres of economy, politics, warfare and power in ancient times.
The huge amount of Pre-Hispanic silver adornments known especially from Perú, Bolivia and Ecuador raises the question whether the pre-Hispanic civilizations obtained the raw material from native ores or from argentiferous-lead ores. Although native silver may be available in America, it is as rare as in the Old World. From colonial texts it is known that silver mines were open in colonial times by the Spaniards from Mexico to Argentina, the main ones being those of Tasco, Mexico and Potosí in Bolivia.
Some kind of blast furnaces called huayrachinas were described in colonial texts, as native technology furnaces used in Perú and Bolivia to smelt the ores that come from the silver mines owned by the Spaniards. Although it is not conclusive, it is believed that these kinds of furnaces may have been used before the Spanish Conquest. Ethnoarchaeological and archaeological work in Porco Municipality, Potosí, Bolivia has suggested pre-European use of huayrachinas.
There are no specific archaeological accounts about silver smelting or mining in the Andes prior to the Incas. However, silver and lead artefacts have been found in the Peruvian central highlands dated in the pre-Inca and Inca periods. From the presence of lead in silver artefacts, archaeologists suggest that cupellation may have occurred there.