text: Black powder is a mixture of saltpeter (potassium nitrate or, less frequently, sodium nitrate), charcoal and sulphur with a ratio (by weight) of approximately 15:3:2 respectively. The ratio has altered over the centuries of its use, and can be altered somewhat depending on the purpose of the powder. Unlike smokeless propellants, it acts more like an explosive since its burn rate is not affected by pressure, but it is a very poor explosive because it has a very slow decomposition rate and therefore a very low brisance. This same property that makes it a poor explosive makes it useful as a propellant — the lack of brisance keeps the black powder from shattering the barrel, and directs the energy to propelling the bullet. Historically, potassium nitrate was extracted from manure by a process superficially similar to composting. "Nitre beds" took about a year to produce crystallized potassium nitrate.