How Not to Make Saltpetre

Prof. David Hsiung’s “Making Saltpetre” seminar at the Massachusetts Historical Society last night was quite interesting, and I probably missed the most interesting part because I was in a committee meeting upstairs for the first half.

David’s paper collected eight sets of instructions for extracting saltpetre from soil promulgated in newspapers, pamphlets, and letters in 1775-76. Everyone at the seminar also got a copy of a ninth method published in Isaiah Thomas’s Royal American Magazine in August 1774, with the above engraving by Paul Revere as illustration (courtesy of the American Antiquarian Society).

Unfortunately, the Americans of 1775-76 weren’t simply wrestling with a shortage of saltpetre, and thus of gunpowder. They were also dealing with a very incomplete knowledge of chemistry. Joseph Priestley had just isolated oxygen, but Antoine Lavoisier and Humphrey Davy hadn’t isolated and named nitrogen and postassium yet. People did recognize nitre as the precursor to saltpetre, and that soil rich in nitre could produce saltpetre, so that was at least a start.

Unfortunately, none of the nine methods of finding nitre and turning it into saltpetre agreed with another. In fact, the instructions differed on fundamental questions like whether one should start with soil that’s been exposed to a lot of urine or not. The Royal American Magazine told its readers:

it has been often found by experiments made in England, that the mortar of old walls, moistened with urine, and exposed to the northeast wind, in a covered shed, will, in a few weeks, afford a considerable quantity of nitre.

But a report recommended by the Continental Congress and printed by Benjamin Edes in Watertown in 1775 recommended against starting with soil from “stables, and all other places, where the earths were impregnated with ruinous and excrementicious salts.”

Hsiung presented modern chemistry to show that bringing water close to a boil strongly increases the solubility of saltpetre without affecting the solubility of other mineral salts so much—so the hotter the water, the more saltpetre it will yield relative to undesired compounds. Yet some methods from 1775-76 say nothing about heating water, meaning they’re inefficient at best.

Early American saltpetre production was hampered by successful longtime makers’ reluctance to share their secrets, ignorance of which variables in different conditions really mattered, and the tendency of confident men (like Dr. Benjamin Rush) to drown out others whether or not they were correct.

Eventually the Continental authorities realized it was much more efficient just to smuggle in finished gunpowder from French, Dutch, and other territories, and the saltpetre instructions stopped appearing in print. But in 1775-76 making lots of people were working hard at making saltpetre—without a whole lot to show for that effort.

TOMORROW: Saltpetre and the Adams family.