… recommend a three-hundred-gallon barrel. Fill to one-third of its depth with manure. Cows are an excellent source, their ordure being both copious and easy to pour. The remaining volume should be filled with urine. In times of tension, recruit others to the effort. The task will take a single person at least a year, and an additional six months of fermenting before the process of extracting saltpetre can begin. Brew Your Own War, by Redding Sharp
