Saltpetre forms naturally wherever nitrogen rich organic matter, animal waste especially, decays in contact with alkaline soil or old mortar, a slow bacterial process early observers understood only as a mysterious efflorescence that appeared on cellar walls, stable floors and manure heaps in temperate climates. Saltpetre men, licensed in several European states specifically for the purpose, scraped these crusts, leached them with water, and boiled the liquor down until crystals of niter separated out, a laborious cottage industry that supplied gunpowder makers for centuries before industrial nitrate sources made it obsolete. In the chemical workshop, saltpetre distilled together with oil of vitriol or with alum and clay yielded aqua fortis, the strong acid that could dissolve silver but not gold, making saltpetre indirectly responsible for one of the standard early modern tests used to distinguish the two metals.
Mixed with sulfur and charcoal in the classic proportions refined across the medieval and early modern periods, saltpetre made gunpowder, whose military importance drove state interest in securing a reliable domestic saltpetre supply for centuries; England, France and other powers all regulated and taxed saltpetre production heavily. Purified saltpetre, refined by repeated dissolution and recrystallization, was called prunella salt or refined niter and was considered a distinct, higher grade product from the crude scraped crust.