Assay Cupels From Washed Beech and Bone Ash, Rammed in a Brass Mould (Agricola, De Re Metallica, 1556)

"The cupels are made of ashes ... In these lead is separated from silver, and by them assays are concluded. Inasmuch as the assayers themselves make the cupels, something must be said about the material."

Pour water on the ash, skim off what floats (charcoal dust, fat), knead the ash with the hands into the water, and pour the muddy water into a second vessel, leaving the small stones and sand in the first to be thrown away. When the ash has settled and the water is clear and no longer tastes of lye, pour the water off and dry the ash in the sun or a furnace. Beech or other slow-growing wood gives the best ash; vine ash cracks and absorbs metal. If no such ash is to be had, roll the washed ash into little balls and burn them in a baker's or potter's oven to drive off fat and damp.

The older and drier the ash the better. Bone ash, especially from the heads of animals, deer-horn ash and fish-spine ash are the most suitable. Mix by his recipes (notebook): the usual way is beech ash sprinkled with beer or water to make it stick, ground in a small mortar, then ground again with skull or fish-spine ash; the more it is ground the better. Options he gives: sifted brick dust in the beech ash, so the lead does not eat the cupel; moistening with cow's milk again and again; deer horn alone, the best of all.

Ram the damp ash into a bottomless brass mould with a brass pestle whose round knob presses the hollow; the part next to the knob fits the top of the mould. Push the cupel out whole. Use the small mould for cupels that purge gold or silver from lead, the large for separating silver from copper and lead (Illustration 231). For ores of copper containing iron, some moisten the made cupels with white of egg, dry them in the sun and crush and mould them again.

Dry the cupels in the sun or in a furnace, then keep them a long time in a dry place; the older they are, the drier and better.

From Georgius Agricola, **De Re Metallica** (1556), Book VII, translated by Herbert C. and Lou H. Hoover, 1912 (Project Gutenberg 38015; checked on the archive.org scan georgiusagricola00agri, pp. 229-231). He gives proportions but no sizes or weights for cupels or moulds. The tools are ours.