Production of Salpetre (Nitre, Potassium Nitrate, or KN03). Nitre beds, heaps of manure mixed with chalky earth. These were watered with urine and manure water. Calcium nitrate crystallised on the surface and was scraped off and taken to shed for processing. Saltpetre is the principal ingredient in gunpowder, and is still used in the preservation of some foods. In medicine it was used internally as a diuretic, but now is only used externally for a number of conditions, such as asthma. From 1683 English edition of Beschreibung allerfurnemisten mineralischen Ertzt, Lazarus Ercker, (Prague, 1574). Copperplate engraving.