Assaying cupel

A cupel (small dish) of bone ash with a shallow depression containing droplets of litharge (lead oxide) or silver which have failed to coalesce. One of six.

The separation of precious metals (gold and silver) from lead ore was done by cupellation. This process was primarily used to test the purity of silver so that the metal could be refined or debased as required to produce an alloy of the required standard (ie. composition or fineness). In the cupellation process the metal to be refined was melted with oxidised lead forming a litharge (lead oxide) which dissolved any base metals present, and separated them from the precious metal component; leaving the latter on the surface. The litharge was continously removed; either in liquid form or by absorption in the hearth lining. Small cupels, usually made from bone ash, were used for the process of fire assay to determine the precious metal content of the ore or a metal sample. Cupels used for silver cupellation tend to be made from bone ash while those for gold fire assaying are usually made from more robust refractory fabrics. It is possible that some cupels might also have been used to hold small quantities of gold or silver filings when they were melted with a blow-pipe. Bone ash was predominately used for cupels from the post-medieval period, since it allows for better separation of the precious metal from the litharge. The use of bone ash, however, has far earlier antecedents and is mentioned by Theopilus. The earliest examples known in England date to the 16th century and these were recovered from the site of the Tower Mint. Large-scale silver refining used hearths lined with calcareous clays and crushed bones.