Silver purification. Historical engraving showing workers using various furnaces to separate silver from its ores. The use of heat for the treatment of an ore is known as pyrometallurgy. The equipment used is numbered and includes: 1, an athanor furnace, which is self-feeding and maintains a uniform temperature. 6, a matras flask on a sand furnace. 7, a wind furnace with a blow pipe. 8, a furnace with a copper head and a receiver. 9, a furnace with an open fire. Taken from The Laws of Art and Nature, in Knowing, Judging, Assaying, Fining, Refining and Inlarging the Bodies of Confin'd Metals (1683) John Pettus, a translation of the work by Lazarus Erckern.