Gunpowder

History

The first confirmed reference to gunpowder in China appears in the 9th century during the Tang dynasty. A formula from 808 in the Taishang Shengzu Jindan Mijue describes six parts sulfur, six parts saltpeter, and one part birthwort herb.

These Taoist texts suggest gunpowder was an accidental byproduct of alchemical experiments seeking an elixir of life, reflected in its Chinese name huoyao, meaning 'fire medicine'. Saltpeter was known in China by the mid-1st century AD, produced mainly in Sichuan, Shanxi, and Shandong. A Chinese alchemical text from 492 noted saltpeter burned with a purple flame, aiding its identification.

Components

Gunpowder is a granular mixture of a nitrate (typically potassium nitrate), charcoal, and sulfur. Potassium nitrate supplies oxygen for combustion, charcoal provides carbon fuel, and sulfur lowers the ignition temperature, increasing burn rate. The combustion process releases oxygen from potassium nitrate, promoting rapid burning of the other ingredients. Modern gunpowder granules are often coated with graphite to reduce electrostatic ignition risk.

Charcoal is partially pyrolyzed cellulose, not pure carbon; pure carbon would burn much less effectively. The standard composition adopted by 1780 is 75% potassium nitrate, 15% softwood charcoal, and 10% sulfur by weight. Proportions have varied historically and by purpose: blasting powder for quarrying may use 70% nitrate, 14% charcoal, and 16% sulfur, or even 40% nitrate, 30% charcoal, and 30% sulfur with cheaper sodium nitrate. In 1857, Lammot du Pont patented a process coating grains with graphite to reduce moisture absorption in sodium nitrate formulations.

Granularity

In 15th-century Europe, dry-compounded powder called 'Serpentine' was made by grinding ingredients together for up to 24 hours, producing a fine flour. Vibration during transport could separate components, requiring field remixing. Low-quality saltpeter or old powder needed re-drying in humid weather. Loading cannons with serpentine powder required skill: the powder chamber was filled only half full, neither too compressed nor too loose, with a wooden bung sealing the chamber.

A carefully determined empty space was needed for effective burning. When fired, turbulence from initial surface combustion exposed the rest of the powder to flame. The later invention of corned powder was more powerful and easier to use, but serpentine remained in use with older guns into the 17th century. For effective combustion, ingredients must be finely ground and thoroughly mixed, but the final product should form dense grains that spread fire quickly from grain to grain.

Production

For the most powerful black powder, wood charcoal from Pacific willow, alder, or buckthorn is used. In Great Britain, alder buckthorn charcoal was prized; the American Confederate States used cottonwood. Ingredients are reduced in particle size and intimately mixed, originally with mortar-and-pestle or stamping-mills using non-sparking materials, later with rotating ball mills. Historically, marble or limestone edge runner mills were used in Great Britain, but by the mid-19th century iron-shod stone or cast iron wheels on iron beds were common.

The mix was dampened with alcohol or water during grinding to prevent ignition and help saltpeter mix into charcoal pores. Around the late 14th century, European powdermakers began adding liquid during grinding to improve mixing and reduce dust. The resulting paste, mill cake, was shaped into grains to dry. Corned powder kept better and was more powerful.

Pressing mill cake under a screw press to half its volume produced press cake as hard as slate, which was broken and sorted by sieves. Éleuthère Irénée du Pont tumbled dried grains in rotating barrels to round edges and reduce dust during shipping.

Did You Know?

• Gunpowder is classified as a Group A primary explosive substance for shipment by the United Nations and U.S. Department of Transportation.
• In quarrying, gunpowder's low brisance causes fewer fractures than high explosives, making it useful for blasting fragile slate or monumental stone like granite and marble.
• A simplified chemical equation for gunpowder combustion is 2 KNO3 + S + 3 C → K2S + N2 + 3 CO2.