Saltpeter — scientifically named potassium nitrate — is the main ingredient of gunpowder. It can be purified from a mineral found underground or from certain kinds of excrements. Saltpeter is a rare material and was in high demand a few centuries ago because of its use in gunpowder. All that medieval people knew about getting saltpeter — which they called China snow — was that it was found almost anywhere where things were rotting, so the practice of saltpeter farming was born. Saltpeter farming consisted of filling pits with rot and waste and then waiting for the saltpeter to form. But even that was not enough for aspiring conquers so the saltpetermen were hired.