Saltpetre manufacture. Engraving of saltpetre (potassium nitrate) manufacture in the 16th century. Saltpetre, a component of gunpowder, was made by first dissolving the encrustations on the walls of stables and cellars. The solution was then boiled over a furnace (at lower right) to concentrate it. The solution was then placed in open pans (upper frame) and the saltpetre crystallised out as it cooled. Image taken from a book by Lazarus Ercker, Beschreibung allerfurnemsten mineralischen Ertzt (1580).