Crystallisation of saltpetre. Historical engraving of a worker crystallising saltpetre, or potassium nitrate. To extract saltpetre, soil is washed and the resulting water boiled. Distilled water is added to the evaporates, which are then filtered. Saltpetre is extracted from this solution by boiling. As it precipitates out it crystallises (zig-zags) on the pans. Saltpetre was used to make gunpowder. Taken from The Laws of Art and Nature, in Knowing, Judging, Assaying, Fining, Refining and Inlarging Bodies of Confin'd Metals (1683), John Pettus, a translation of the work by Lazarus Erckern.