Preparing copper ore. Historical engraving of an assayer (right) and his assistant preparing copper ore for assaying. The assistant is pumping the bellows of an oven used to separate copper ore from copper stone. Assaying is the analysis of metal ores to determine the proportion of their contents. 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.