Potassium nitrate is a chemical compound with the chemical formula KNO3. A naturally occurring mineral source of Nitrogen, KNO3 constitutes a critical oxidizing component of black powder/gunpowder. In the past it was also used for several kinds of burning fuses, including slow matches. Since potassium nitrate readily precipitates, urine was a significant source, through various malodorous means, from the Late Middle Ages and Early Modern era through the 19th century.
Its common names include saltpetre, from Medieval Latin sal petrae: "stone salt" or possibly "Salt of Petra" (saltpeter in US English), nitrate of potash, and nitre (US niter). For specific information about the naturally occurring mineral, see nitre. The name Chile saltpeter is applied to sodium nitrate, a different nitrogen compound that is also used in explosives and fertilizers.
In more rural times, urine was collected and used in the manufacture of gunpowder. Stale urine was filtered through a barrel full of straw and allowed to continue to sour for a year or more. After this period of time, water was used to wash the resulting chemical salts from the straw. This slurry was filtered through wood ashes and allowed to dry in the sun. Saltpeter crystals were then collected and added to brimstone and charcoal to create black powder.
The earliest known complete purification process for potassium nitrate is described in 1270 by the Arab chemist and engineer Hasan al-Rammah of Syria in his book al-Furusiyya wa al-Manasib al-Harbiyya (The Book of Military Horsemanship and Ingenious War Devices), where he first described the use of potassium carbonate (in the form of wood ashes) to remove calcium and magnesium salts from the potassium nitrate.
During the 19th century and until around World War I, potassium nitrate was produced on an industrial scale, first by the Birkeland-Eyde process in 1905, and then later from ammonia produced by the much more efficient Haber process. The latter process came online during World War I, and supplied Germany with nitrates critical for the warfare that it otherwise had no access to because the deposits of natural nitrate in Chile were in British hands. It is assumed that this prolonged World War I. Today practically all nitrates are produced with ammonia from the Haber process.
In the process of food preservation, potassium nitrate has been a common ingredient of salted meat since the Middle Ages, but its use has been mostly discontinued due to health concerns over nitrates. Even so, saltpeter is still used in some food applications, such as being a key ingredient in the brine used to make Corned Beef brisket. Saltpeter gives Corned Beef a pink hue when cooked.
In the European Union, it is referred to as E252.
It is commonly used in pre-rolled cigarettes to maintain an even burn of the tobacco.
Potassium nitrate is also the main component (usually about 98%) of tree stump remover; it accelerates the natural decomposition of the stump.
Potassium Nitrate is also commonly used in the Heat Treatment of metals as a solvent in the Post-Wash. The oxidizing, water solubility and low cost make it an ideal short-term rust inhibitor.
It has also been used in the manufacture of ice cream and can be found in some toothpastes for sensitive teeth. Recently, the use of potassium nitrate in toothpastes for treating sensitive teeth has increased dramatically, despite the fact that it has not been conclusively shown to help dental hypersensitivity.
Potassium nitrate is also one of the three components of black powder, along with powdered charcoal (substantially carbon) and sulfur, where it acts as an oxidizer. It can also be burnt by itself (although it has a high flash point) producing a hot pink-orange flame.