When Military Might Relied on Urine

Focuses on the chemistry.

There's also a large social aspect. The king would give petermen the right to come to your land and dig for saltpeter. Oddly, Wikipedia doesn't have a page about petermen, but I found this mention at https://en.wikipedia.org/wiki/Gunpowder :

> Without access to cheap saltpeter (controlled by the British), for hundreds of years France had relied on saltpetremen with royal warrants, the droit de fouille or "right to dig", to seize nitrous-containing soil and demolish walls of barnyards, without compensation to the owners.[66] This caused farmers, the wealthy, or entire villages to bribe the petermen and the associated bureaucracy to leave their buildings alone and the saltpeter uncollected.

And from https://www.johndbrown.com/petermen-gunpowder-and-urine-laws... :

> The saltpeter supplier would send out teams of collectors who would locate promising places to dig (abandoned privies and dungheaps) by tasting the soil before digging it out and carting it off to be boiled, strained and evaporated to produce saltpeter of the required purity. It is said that throughout Europe no privy, stable, or dovecote was safe from saltpeter collectors or ‘petermen’.”

continuing from the original source, archived at https://web.archive.org/web/20080409082426/https://www-geolo... :

> The English set up a Parliamentary Commision in 1606 to look into abuses of property by petermen. However by 1624 King James I issued a proclamation complaining about citizens who were placing their selfish interest above that of their country by paving barns or putting down plank walkways, interfering with the accumulation and ripening of saltpeter in the dung and urine. Every kingdom was desperate for gunpowder, which probably accounts for the continuing use of swords, pikes, and bows, long after firearms had made them technically obsolete.

Also, during the US Civil War, "The South was so desperate for saltpeter for gunpowder that one Alabama official reportedly placed a newspaper ad asking that the contents of chamber pots be saved for collection" - https://en.wikipedia.org/wiki/Potassium_nitrate .