Gunpowder, or black powder (also called blasting powder), is not a chemical, but a mixture, though its decomposition, or deflagration, is a chemical reaction between potassium nitrate (saltpeter), sulfur, and charcoal mixed by weight roughly in the ratio 6:1:1.
Saltpeter is potassium nitrate (KNO(3)), a chemical formed by the excretions of bacteria (a microbial manure) after consuming ammonium salts in decaying animal and vegetable matter, which is why guano deposits have been so valued by militarily ambitious states. Most familiar to urban dwellers as a white crystalline excreta around urine stains in back alleys and subterranean tunnels, these bacteria form what one writer called "the salts of civilization." Saltpeter, from sal petrosum, so named for its salty taste, means "salt of the rock." In antiquity, saltpeter was used as a diuretic for the treatment of gout, high blood pressure and was particularly useful to elderly men with urinary tract blockages. Its first chemical use was in the preparation of nitric acid in the 9th Century. Imperfect chemical bonds between nitrogen and its three oxygen atoms make saltpeter the oxygen tanks of a black powder reaction.
Sulfur, element 16, was first encountered as a by-product in the smelting of copper and mercury ores. In the hit-and-miss methodology of ancient medicine, sulfur was found to be ideal as a salve for treating scabies. And possibly because ancient man didn't know what else to do with the stuff, it was also used for other skin ailments such as acne, itches and insect bites. The process for preparing sulfuric acid (Oil of Vitriol) wasn't known until the 1500s. But with an ignition point of 243 degrees Centigrade (half that of charcoal), sulfur kindles the reaction to get it going.
Charcoal is wood roasted in the absence of air to leach it of its resins and to make it highly electrovalent, or oxygen starved, which makes for a cleaner and hotter burning fire. Charcoal is necessary for the forging of iron and the making of steel which is 1.5 to 5 percent carbon. Charcoal also valued for its absorbance; it's estimated that one gram of charcoal, the mass of a postage stamp, has the surface area of over 10 square kilometers, an absorbance that trades saltpeter's shower of oxygen into a nearly instantaneous 2770 degrees Centigrade of heat and pressure.
BRIEF ORIGINS OF BLACK POWDER
In "A History of Greek Fire and Gunpowder," J. R. Partington suggests that black powder came about as a consequence of an arms race between the Chinese and the Byzantine Empire; which latter was expanding its trade routes as far as Hangzhou.
The Byzantines had developed the flame thrower as early as 670 C.E. Little is known of the propellant, but the "siphon" was undoubtedly a two-stroke hand pump (one excavated in England dates to 100 B.C.E.) mounted on the prows of their ships, and capable of shooting a stream of fire that burns on water. Unfortunately for us, the chemistry of it remains the best kept military secret in history.
Naturally, the Chinese were eager to duplicate it before the Byzantines turned their weapon on them.
The Chinese combined what was essentially medicines into a ground mixture to produce a "huo ch'iang," a "fire lance" (which today we recognize as a Roman candle) probably as early as the mid-10th Century. Unfortunately for the Song Dynasty, they turned the weapon on the Mongols (by 1127 C.E.), discovering perhaps too late that its military effectiveness was negligible. It's tempting to speculate that the Mongols praised the weapon for making the enemy easier to see.
But when they ground the mixture more finely (and in different ratios) the Chinese produced a "huo p'ao," or the "fire missile," perhaps between 998 to 1128.
When they ground the mixture finer still, and the Chinese developed the gunpowder grenade around the year 1000. By 1040, they stabilized the name to "huo yao," for "fire drug," which we know today as black powder.
By 1232, the Song developed a bomb so powerful they called it the "chen t'ien lei," or "heaven shaking thunder" bomb, and used it on the Mongols at the siege of Pien. It didn't do the Song much good. The last of the Song holdouts were crushed by the Mongols in 1271. However, as the 1200s saw the Mongols overrunning everything, including that most colossal of land grabs, the Middle East, it was there that the European Crusaders first learned of black powder, and it was this that inspired the Arabs to invent the first sidearm, the midfa, by 1383:
According to the French instructions (from Partington, p. 206), the midfa, or madfaa, was little more than a pole about the length of a table leg, with a cylinder of iron fixed to one end. The iron cylinder was bored longitudinally about two-thirds of its length to fit an iron crossbow bolt. Another smaller hole was bored into the end of this channel to fit a fuse. The instructions make no mention of a wick, so the first ones were probably fired with a separate torch. Though deadly, the midfa was too slow and unwieldy to use on anything other than an attacking lion or elephant or any other large animal oblivious to the weapon's crushing lethality. So fierce was its recoil, that the instructions end with the advisory that if the user intends to fire the weapon from horseback, he should take care to use a saddle with a cantle, or else "la fleche ne sorte pas" will knock him off on his back.
As a necessary caution, around 1280 in China, an accident while mixing black powder was reportedly so powerful that among its casualties it killed four tigers in a nearby zoo. Similar incidents were so common that it led the Chinese to warn that huo yao should never be mixed in a clay mortar with a metal pestle. Medieval man had yet to harness electricity, but he knew enough to avoid it.
CHEMISTRY OF EXPLOSION
Black powder is a low explosive and is said to deflagrate, or burn rapidly, at a speed of 500 m/s, which is still less than the speed of sound in the material itself.
Mixed together, the saltpeter and charcoal adhere (adsorb) to the surfaces of sulfur granules: Sulfur, with its relatively low ignition temperature (470 degrees Fahrenheit) is the primer; saltpeter, with its enormous oxygen content, is the oxidizer; as charcoal rapidly absorbs the oxygen, it liberates 2770 degrees Centigrade of heat (5018 degrees Fahrenheit) or 685 calories.
To say that black powder detonates is erroneous because "detonation" is descriptive of a high explosive reaction in which the shock wave is supersonic, moving faster than the speed of sound in the material; in high-explosives the speed of detonation is from 1500 to 7600 m/s.
Black powder is set-off by flame, sparks, friction, or impact. One gram of black power (the mass of a postage stamp) will expand to 271.3 cc, or over a quarter of a liter of space. Chemically the reaction may be idealized as:
2 KNO(3) + S + C ==> K(2)S + N(2) + 3 CO(2) + 685 calories (heat) + 11 Joules (K.E.)
Noting that I used carbon for "charcoal," which is Mother Nature's kitchen of compounds, the actual by-products are much more complex.
Alfred Nobel's 1880 analysis for a 75:12:13 mixture found the by-products to be (rounding up):
Gases - 43 %
Solids - 52 %
Water - 1 %
Gases produced as a percent of volume are:
Carbon dioxide, CO(2) - 49.3 %
Carbon monoxide, CO - 12.5 %
Nitrogen, N(2) - 32.9 %
Sulfuretted Hydrogen, H(2) - 2.6 %
Hydrogen, H(2) - 2.2 %
Methane, CH(4) - 0.4 %
Solids produced as a percent of volume are:
Potassium carbonate, KCO(3) - 61 %
Potassium sulfate, KSO(4) - 15.1 %
Potassium sulfide, K(2)S - 14.5 %
Sulfur, S(2) - 8.7 %
Potassium nitrate, KNO(3) - 0.3 %
Potassium sulfocyanide, KSCN - 0.2 %
Black powder is still used as an industrial explosive, though it's more frequently used as fireworks and in film and theatrical productions for its adaptability and easily controlled burn.
Its use in firearms and ordnance has been superseded by guncotton (nitrocellulose, 1846), smokeless powder (gelatinized nitrocellulose, 1865), double-base powder (Cordite: gelatinized nitrocellulose and nitroglycerine, 1888), and multiple-base powder (picric acid based, 1885).
J. R. Partington, "A History of Greek Fire and Gunpowder," W. Heffer & Sons, Ltd., Cambridge, England, 1960.
HARD DATA OBTAINED FROM
Robert P. Multhauf, "The Origins of Chemistry," Franklin Watts, Inc., NY, 1967.