Очистка благородных металлов методом купелирования

Refining process in metallurgy

Cupellation is a refining process in metallurgy in which ores or alloyed metals are treated under very high temperatures and subjected to 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 typically oxidise or react chemically at much higher temperatures than base metals. 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: testing fresh metals such as lead and recycled metals to determine their purity for jewellery and coin making. Cupellation is still in use today.

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 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 must 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 complete. Pure lead must 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.

The primary tool for small scale cupellation was the cupel. Cupels were manufactured in a careful manner. They used to be small vessels shaped in the form of an inverted truncated cone, made of bone ashes. According to Georg Agricola, the best material was obtained from burned antlers of deer, although fish spines could also work. 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 copper 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.

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. Although this has been interpreted as silver being extracted from lead ores, it has been also suggested that lead was added to collect silver from visible silver minerals embedded in host rock. In both cases silver would be retrieved from lead metal by cupellation. During the following Iron Age, cupellation was done by fusing the base metals with a surplus of lead. The bullion or 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, became an important political and economic site around the Mediterranean Sea, as well as Laurion in Greece. Around 500 BC 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 to support the Roman civilization over a great territory; they searched for open lead silver mines in areas they conquered. Silver coinage became the normalised medium of exchange, hence 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 changed little until 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 in diverse archaeological findings through Medieval and Renaissance Europe. By these times the amount of fire assays increased considerably, mainly because of testing ores in the mines 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. There are no specific archaeological accounts about silver smelting or mining in the Andes prior to the Incas. 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 occurred there.