Gunpowder, also called black powder, is an explosive mixture of sulfur, charcoal and potassium nitrate, KNO3 (also known as saltpetre/saltpeter) that burns rapidly, producing volumes of hot solids and gases which can be used as a propellant in firearms and as a pyrotechnic composition in fireworks. The term gunpowder is also often used more broadly to describe any propellant powder used in firearms. Modern firearms do not use the traditional gunpowder (black powder) described in this article, but instead use smokeless powder. In this article, the terms gunpowder and black powder are used interchangeably.
Gunpowder is classified as a low explosive because of its slow decomposition rate and consequently low brisance. Low explosives produce a subsonic deflagration wave rather than the supersonic detonation wave produced by brisants, or high explosives. The gases produced by burning gunpowder generate enough pressure to propel a bullet, but not enough to destroy the barrel of a firearm. This makes gunpowder less suitable for shattering rock or fortifications, where high explosives such as TNT are preferred.
The term "black powder" was coined in the late 19th century to distinguish prior gunpowder formulations from the new smokeless powders and semi-smokeless powders. (Semi-smokeless powders featured bulk volume properties that approximated black powder in terms of chamber pressure when used in firearms, but had significantly reduced amounts of smoke and combustion products; they ranged in color from brownish tan to yellow to white. Most of the bulk semi-smokeless powders ceased to be manufactured in the 1920s.)
Black powder is a granular mixture of
a nitrate—typically potassium nitrate (KNO3)—which supplies oxygen for the reaction;
charcoal, which provides fuel for the reaction in the form of carbon (C);
sulfur (S), which, while also a fuel, lowers the temperature of ignition and increases the speed of combustion.
Potassium nitrate is the most important ingredient in terms of both bulk and function because the combustion process releases oxygen from the potassium nitrate, promoting the rapid burning of the other ingredients. To reduce the likelihood of accidental ignition by static electricity, the granules of modern black powder are typically coated with graphite, which prevents the build-up of electrostatic charge.
The current standard composition for black powder manufactured by pyrotechnicians was adopted as long ago as 1780. It is 75% potassium nitrate, 15% softwood charcoal, and 10% sulfur. These ratios have varied over the centuries and by country, and can be altered somewhat depending on the purpose of the powder. For instance, coarser grades of black powder, unsuitable for use in firearms but adequate for blasting rock in quarrying operations, is called blasting powder rather than gun powder; it is often made with the cheaper sodium nitrate substituted for potassium nitrate in proportions of 60% nitrate, 20% charcoal and 20% sulfur.
The burn rate of black powder can be changed by corning. Corning first compresses the fine black powder meal into blocks with a fixed density (1.7 g/cm³). The blocks are then broken up into granules. These granules are then sorted by size to give the various grades of black powder. In the United States, standard grades of black powder run from the coarse Fg grade used in large bore rifles and small cannons, through FFg (medium and smallbore arms such as muskets and fusils), FFFg (smallbore rifles and pistols), and FFFFg (extreme small bore, short pistols and most commonly for priming flintlocks). In the United Kingdom, the gunpowder grains are categorised by mesh size: the BSS sieve mesh size, being the smallest mesh size on which no grains were retained. Recognised grain sizes are Gunpowder 'G 7', 'G 20', 'G 40', and 'G 90'.
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