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Archaeometallurgy

Archaeometallurgy is the study of the history and prehistory of metals and their use through humans. It is a sub-discipline of archaeology and archaeological science. After initial sporadic work, archaeometallurgy was more widely institutionalised in the 1960s and 70s, with research groups in Britain (The British Museum, the UCL Institute of Archaeology, the Institute for Archeo-Metallurgical Studies (iams)), Germany (Deutsches Bergbau Museum) and the US (MIT and Harvard). Specialisations within archaeometallurgy focus on metallography of finished objects, mineralogy of waste products such as slag and manufacturing studies.

Cupellation

Cupellation is a metallurgical process in which ores or alloyed metals are treated under high temperatures and carefully controlled operations in order to separate noble metals, like gold and silver, from base metals like lead, copper, zinc, arsenic, antimony or bismuth, that might be present in the ore.This process is based on the principle that precious metals do not oxidise or react chemically, contrary to what happens to the base metals; so that 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 of cupellation was used to obtain silver out of smelted lead ores; by the Middle Ages and the Renaissance was one of the most common and important processes used to refine metals. By then, fire assays were used for mineral assaying, testing recycled metals to know their purity (jewelry) and minting. Cupellation principles have always remained the same, they change only in the amount of material processed. It is still in use today based on the same principles.

Cupellation Process

Cupellation Hearths
Native silver is a rare element, although it exists as such. Most of the time it is found in nature combined with other metals and 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 must have been done by the smelting of argentiferous lead ores.
Lead melts at 327°C while silver melts at 960°C, when mixed, as in galena, the most common argentiferous lead ore, they have to be smelted at high temperatures in a reducing condition to produce argentiferous lead. The mineral then has to be smelted again at high temperatures of the order of 900°C or 1000°C in a hearth or blast furnace where air flow makes possible the oxidation of the lead. Lead transforms into lead oxide (PbO) known as litharge, a liquid substance when melted, which captures the oxides of the rest of the metals, while silver and gold remain floating on top of the liquid litharge. The latter is removed or absorbed by capillary action into the hearth linings.This chemical reaction may be view as (Ag+Cu) + Pb + O2 → (CuO+PbO) + Ag

The base of the hearth was dug in the form of a sauce pan and covered with an inert and porous material rich in calcium or magnesium (shells or lime) or by bone ash. The necessity of lining with calcareous materials was due to the fact that lead reacts with silica (clay compounds) to form lead silicate with a glassy appearance that will not allow the litharge to be absorbed in the correct way. Calcareous materials do not react with lead.[14] Some of the litharge fumes evaporate and the rest, in a liquid state are absorbed by the porous earth linings which form what it is known as litharge cakes.


Cupellation furnaces after Agricola, 1556/1950Litharge cakes are usually in the shape of circular, concavo-convex cakes of approximately 15 cm in diameter and are the most common archaeological evidence of the cupellation process in the Early Bronze Age.By their chemical composition (archaeometry), archaeometallurgists may know what kind of ore was treated, which were the main components in it or which steps might have been followed in the process. This information might give them insights about production process, trade, social needs or economic situations among others.

Small scale cupellation
Small scale cupellation or fire assay 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 samples 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, 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 oxides 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 have to be separated by parting.

Cupels

Bone ash cupel with silver prillThe primary tool for fire assay or cupellation was the cupel. Their manufacture was made 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 horns of deer although fish spines could work 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 plant ashes, which were not of a very high 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). It is thought that at the beginnings of small scale cupellation, potsherds or clay cupels were used