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 carbon and other fuel for the reaction, simplified as 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. Proportions by weight are 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, low power grades of black powder, unsuitable for use in firearms but adequate for blasting rock in quarrying operations, is called blasting powder rather than gunpowder with standard proportions of 70% nitrate, 14% charcoal and 16% sulfur; blasting powder may be made with the cheaper sodium nitrate substituted for potassium nitrate and proportions may be as low as 40% nitrate, 30% charcoal and 30% 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.
A simple, commonly cited, chemical equation for the combustion of black powder is
2 KNO3 + S + 3 C → K2S + N2 + 3 CO2.
A more accurate, but still simplified, equation is
10 KNO3 + 3 S + 8 C → 2 K2CO3 + 3 K2SO4 + 6 CO2 + 5 N2.
The burning of gunpowder does not take place as a single reaction, however, and the byproducts are not easily predicted. One study’s results showed that it produced (in order of descending quantities): 55.91% solid products: potassium carbonate, potassium sulfate, potassium sulfide, sulfur, potassium nitrate, potassium thiocyanate, carbon, ammonium carbonate. 42.98% gaseous products: carbon dioxide, nitrogen, carbon monoxide, hydrogen sulfide, hydrogen, methane, 1.11% water.
Black powder formulations where the nitrate used is sodium nitrate tend to be hygroscopic, unlike black powders where the nitrate used is saltpetre. Because of this, black powder which uses saltpetre can be stored unsealed and remain viable for centuries provided no liquid water is ever introduced. Muzzleloaders have been known to fire after hanging on a wall for decades in a loaded state, provided they remained dry. By contrast, powder that uses sodium nitrate, which is typically intended for blasting, must be sealed from moisture in the air to remain stable for long times.
In firearms, black powder allows loading by volumetric measure, whereas smokeless powder requires precise measuring of the charge by weight to prevent damage due to overloading, though damage by overloading is still possible with black powder.
In quarrying, high explosives are generally preferred for shattering rock. However, because of its low brisance, black powder causes fewer fractures and results in more usable stone compared to other explosives, making black powder useful for blasting monumental stone such as granite and marble.
Black powder is well suited for blank rounds, signal flares, burst charges, and rescue-line launches. Black powder is also used in fireworks for lifting shells, in rockets as fuel, and in certain special effects.
Black powder has low energy density compared to modern smokeless powders and produces a thick smoke that can impair aiming and reveal a shooter’s position.
Combustion converts less than half the mass of black powder to gas. The rest ends up as a thick layer of soot inside the barrel. In addition to being a nuisance, the residue from burnt black powder is hygroscopic and an anhydrous caustic substance. When moisture from the air is absorbed, the potassium oxide or sodium oxide turns into hydroxide, which will corrode wrought iron or steel gun barrels. Black powder arms must be well cleaned both inside and out to remove the residue.
The UN Model Regulations on the Transportation of Dangerous Goods and national transportation authorities, such as United States Department of Transportation, have classified Gunpowder (black powder) as a Group A: Primary explosive substance for shipment because it ignites so easily. Complete manufactured devices containing black powder are usually classified as Group D: Secondary detonating substance, or black powder, or article containing secondary detonating substance, such as firework, class D model rocket engine, etc., for shipment because they are harder to ignite than loose powder. As explosives, they all fall into the category of Class 1.
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