Saltpetre

Saltpetre is potassium nitrate, KNO3, the potassium salt of nitric acid: a colourless to white crystalline substance with a sharp, salty, bitter taste. In nature it occurs as the mineral nitre, after which nitrogen was named, and saltpetre is also a shared name for several related nitrogen compounds. Its older names run back through Middle English nitre to Greek nitron, probably kin to Egyptian and Hebrew words; Arabs called it Chinese snow and Persians Chinese salt, and by the fifteenth century Europeans knew it as saltpetre, or Indian saltpetre. It was gathered as crystals from cave walls and from bat guano, and later made in nitraries and nitre beds, where excrement mixed with soil was left for microbes to turn into nitrate. It is best known as the oxidiser in gunpowder, which powered the world's firearms until the late 1880s, and it is still used in fireworks, smoke bombs, rocket propellants, fertilisers and tree-stump removers. It has cured meat since antiquity or the Middle Ages and still goes into some salami, dry-cured ham and corned beef, as the food additive E252; it is also added to cigarettes to keep them burning evenly. In incense, a nitrate oxidiser such as potassium nitrate keeps the charcoal or wood powder of self-burning incense alight. The old belief that it causes impotence, and the rumour that it is slipped into institutional food such as military rations, have no scientific support.