Claude on Question 21, Sep 17

The three ingredients

Black powder was a mixture of saltpeter (potassium nitrate), charcoal, and sulfur. By the 18th century most European powder works had converged on roughly 75 parts saltpeter to 15 charcoal and 10 sulfur, though proportions varied by country, by date, and by intended use — mining and blasting powder was typically weaker and cheaper than musket powder, and nations guarded their own preferred formulas as matters of state.

Saltpeter was the perpetual bottleneck. Britain had a decisive advantage: the East India Company imported large quantities of naturally occurring saltpeter from Bengal, where it was collected from nitrate-rich soils and refined locally. Countries without that access had to manufacture it. The standard method was the nitre bed or nitrary — heaps or walled beds of earth mixed with manure, straw, and urine, kept damp and periodically turned so that nitrifying bacteria (whose role was of course unknown) converted the nitrogenous waste to nitrates over a year or more. The beds were then leached with water, and the liquor boiled down and treated with wood ashes to convert calcium nitrate into the potassium salt, followed by repeated recrystallization to remove deliquescent impurities.

In France, licensed salpêtriers held the hated royal privilege of entering cellars, stables, and dovecotes to dig out nitrous earth. Antoine Lavoisier, appointed to the newly created Régie des poudres in 1775, reformed the system — replacing forced requisition with purchase, standardizing refining, and sponsoring a prize competition on saltpeter production that produced the era's best chemical literature on the subject. Sweden at various points levied a saltpeter tax payable in kind, effectively taxing farmers' barn floors.

Charcoal was made from light, soft woods — willow, alder, buckthorn, dogwood, hazel. During the 18th century the older practice of charring in earth pits gave way in the better works to distillation in closed iron cylinders or retorts, which yielded a far more uniform product and let makers control the degree of charring.

Sulfur came mainly from Sicilian volcanic deposits and was purified by sublimation or distillation before use.

Making the powder

Each ingredient was separately purified and finely pulverized, then weighed out and mixed into a "green charge." The critical step was incorporation: prolonged working of the dampened mixture so the sulfur and saltpeter were driven intimately into the pores of the charcoal. Water was essential both to the chemistry and to reducing the risk of ignition.

Two machines did this work. Older stamp mills used water-powered lifting cams to raise and drop heavy wooden pestles into mortars of the charge — effective but notorious for catastrophic explosions. Over the 18th century these were progressively replaced by edge-runner mills, in which a pair of massive cast-iron or limestone wheels rolled on a circular bed, crushing and kneading the charge for hours. France banned stamp mills for powder work in 1794 after a series of disasters; Britain's royal works made the same transition.

The resulting "mill cake" was then pressed between plates under a screw or hydraulic press into dense, hard slabs — density being directly related to power and to resistance to moisture. The press cake was corned, or granulated: broken up and passed through graded sieves, historically of parchment or hide punched with holes, to produce grains of a specified size. Grain size determined burning rate, so armories specified different grades for priming, muskets, and cannon. Fines and dust were returned to be reworked.

Grains were glazed by tumbling in rotating barrels, which rounded off the edges, reduced dust, and made the powder flow better and take up less moisture. Finally the powder was dried — in the sun on canvas sheets in southern works, or in heated drying houses, which were among the most dangerous buildings on any site.

Testing and safety

Powder was proved with an éprouvette: commonly a small mortar that threw a standard ball or weight, the range being compared against a benchmark, or spring- and wheel-type gauges that measured the force of a fixed charge. Lavoisier in France, and Benjamin Robins and Charles Hutton in Britain (with the ballistic pendulum), put this testing on a much more quantitative footing over the century.

Powder works were laid out with buildings widely separated, screened by earthen mounds and tree plantings, and built with heavy walls and deliberately weak roofs so blasts vented upward. Tools were wood, copper, or bronze rather than iron; workers wore leather shoes without nails and sometimes special clothing; floors were kept damp. Explosions nevertheless were routine, and the mortality among powder workers was grim.

Institutions

Britain's Crown bought the Faversham works in 1759 and established Waltham Abbey in 1787; France centralized production under the Régie, with the Essonne works as its showpiece and laboratory. Prussia, Spain, Sweden, and the Papal States all ran state or licensed works.

The American colonies were a striking case of dependence: at the outbreak of the Revolution there was almost no domestic capacity, and the Continental Congress circulated printed instructions on saltpeter production while encouraging mills like Oswell Eve's near Philadelphia. In practice the great majority of American powder in the war years was imported, much of it covertly from France. Éleuthère Irénée du Pont, who had trained under Lavoisier at Essonne, brought that expertise to Delaware and founded his mills in 1802 — carrying 18th-century French practice directly into American industry.