Gunpowder - Composition and Characteristics

Composition and Characteristics

The term black powder was coined in the late 19th century, primarily in the United States, to distinguish prior gunpowder formulations from the new smokeless powders and semi-smokeless powders, in cases where these are not referred to as cordite. Semi-smokeless powders featured bulk volume properties that approximated black powder, but had significantly reduced amounts of smoke and combustion products. One difference between them is that the older black powder burns at nearly the same rate in the open as when contained, while in smokeless powders the burn rate accelerates more rapidly within a closed chamber, making for a sharper rise in pressure which could rupture older weapons designed for black powder. Smokeless powders ranged in colour 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 serving as a fuel, lowers the temperature required to ignite the mixture, thereby increasing the rate 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.

Charcoal does not consist of pure carbon; rather, it consists of partially pyrolyzed cellulose, in which the wood is not completely decomposed. Carbon differs from charcoal. Whereas charcoal's autoignition temperature is relatively low, carbon's is much greater. Thus, a black powder composition containing pure carbon would burn similarly to a match head, at best.

The current standard composition for the black powders that are manufactured by pyrotechnicians was adopted as long ago as 1780. Proportions by weight are 75% potassium nitrate (known as saltpeter or saltpetre), 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, 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. In 1857 Lamont DuPont solved the main problem when using cheaper sodium nitrate formulations when he patented DuPont "B" Blasting powder; after manufacturing grains from press-cake in the usual way, the powder was tumbled with graphite dust for 12 hours, forming a coating on each grain which in this case reduced its ability to pick up moisture.

French war powder in 1879 used the ratio 75% saltpeter, 12.5% charcoal, 12.5% sulfur. English war powder in 1879 used the ratio 75% saltpetre, 15% charcoal, 10% sulfur. The British Congreve rockets used 62.4% saltpeter, 23.2% charcoal and 14.4% sulfur, but the British Mark VII gunpowder was changed to 65% saltpeter, 20% charcoal and 15% sulfur. The explanation for the wide variety in formulation relates to usage. Powder used for rocketry can use a slower burn rate since it will accelerate the projectile for a much longer period of time, whereas powders for use in weapons such as flintlocks, caplocks or matchlocks need a higher burn rate since they must accelerate the projectile in a much shorter distance. Cannons usually used lower burn rate powders because most would burst if higher burn rate powders are used.