Black powder, more commonly known as gunpowder, is one of the best known chemical explosives. It is a mixture of potassium nitrate, charcoal and sulfur, typically in a 75:15:10 ratio. The term black powder is usually preferred, to distinguish it from smokeless powder. It is classified as low order explosive.
Black powder will burn fast if ignited, even in vacuum, and will explode if ignited in a closed container.
No single equation covers the combustion of black powder and the products depend on the conditions but a general reaction can be considered:
- 7 KNO3 + C7H4O + 3 S → K2CO3 + K2SO4 + 3/2 K2S + 4 CO2 + 2 CO + SO2 + 2 H2O + 3 N2
Black powder, appears either as very fine powder or as pourable black granules, with a slight metallic taste and a weak smell, usually from the sulfur and/or charcoal type used. It is considered an insoluble material (sulfur and saltpeter have poor solubility in all of their mutual solvents and charcoal is completely insoluble in any solvents). Gunpowder has a pH of 6.0 - 8.0. Its specific gravity varies between 1.70 - 1.82 (via mercury method) and 1.92 - 2.08 (via pycnometer). The standard black powder has an autoignition temperature of 464°C.
In some countries, such as United States and the Russian Federation, black powder is sold in gun stores, though some might only sell smokeless powder labeled as black powder or grains. Purchasing black powder may or may not require a license (such as a license to own and use smoothbore firearms or to manufacture ammunition), depending on the jurisdiction in question. Black powder is generally sold in spark-free metal or wood containers.
In most of the world, the sale and own of black powder is forbidden or regulated, except under a certain amount, or is only permitted during New Year's Eve, as fireworks. Fireworks and firecrackers sold during holidays or celebrations generally contain black powder. While it's easy to extract the powder from them, as most firecrackers are generally made of cellulose which is easy to cut, this should not be tried unless you know what you're doing. Persons who work in the explosives domain may acquire a permit to use black powder.
Modern ammunition no longer uses gunpowder, and instead relies on smokeless powders. One exception is shotgun ammo: in certain countries black powder cartridges are still used for shotguns.
Potassium nitrate, sulfur and charcoal are first properly dried and grounded separately. In a mortar, a mixture of these three materials in a weight proportion of 75% for potassium nitrate, 15% for charcoal, and 10% sulfur is added. The mortar must be made of a spark-free material. The mixture is EXTREMELY CAREFULLY grounded until it becomes as fine as flour, as any spark can ignite the fine powder and cause an explosion. If a ball mill is used, spark free ball mills, such as lead-antimony are a must. Another tip would be to add a little water, to reduce the chance of ignition. Other volatile solvents can also be added, though preferably non-flammable ones. The best type of charcoal for making gunpowder is made from softwood, such as willow. Avoid hardwood as it contains too much ash. Pure carbon, such as amorphous carbon or graphite, requires a higher temperature for ignition than charcoal, and it's best to avoid using it.
If the black powder isn't milled after its three components are mixed, the resulting product is called green mixture. The green mixture burns much slower than real black powder, though it can still explode if ignited in a confined place.
Other and unconventional black powders
If potassium nitrate is unavailable, sodium nitrate can be used instead but is only practical in countries and locations with very low humidity.
Ammonium nitrate is a terrible choice to use instead of saltpeter, as it's hygroscopic and the resulting black powder will quickly turn into a mushy solid, which may not ignite at all. Even if you can somewhat keep this powder perfectly dry, it's simply not worth the effort.
Replacing the potassium nitrate with potassium chlorate was tried in several countries commercially shortly after the discovery of the latter and quickly resulted in deaths which lead to the banning of mixtures containing these substances which are sensitive to friction and shock and can detonate and/or ignite spontaneously. Using potassium perchlorate increases the explosive power of the gunpowder.
- Blasting caps
- Black match
- Historic firearm models (handcannon, arquebus, matchlock, etc.)
Gunpowder is a dangerous explosive and should only be handled by people who know what they're doing. The milling procedure is known to cause ignition of the powder, so this MUST NOT be done by any amateur. If gunpowder ever ignites, it is extremely difficult to extinguish. Water can be used to put out fires and to soak any spills to prevent fire. Burning black powder gives off sulfur dioxide, which is very toxic. NEVER SMOKE NEAR GUNPOWDER CONTAINERS! While is difficult to ignite most flammable materials with a lit cigarette, black powder contains its own oxidizer and can ignite with any hot body, even if it doesn't have a flame.
Storage of gunpowder is best done in wood or cardboard containers. The classic wooded keg is a good example. Gunpowder "kegs" should never be placed in the same room with flammable materials such as solvents, and instead should be kept in special rooms, away from anything and anyone. Metal containers and plastic are sometimes used to store black powder, such as military powder flasks and horns.
Black powder can be burned, though this will produce sulfur dioxide gases.
Gunpowder may be safely discarded in the ground, as its components are already used as fertilizers. Many gun owners have done that with old powder, claiming no negative effects on the land.