Chapter 115: Development Always Takes Time

Guano is indeed usable for saltpeter production. React it with hydrochloric acid to turn the ammonium ions in the guano into ammonium salt, then pile it up like compost for curing, letting nitrifying bacteria convert the nitrogen in ammonium salts into nitrate under aerobic conditions. Add potash, and you'll precipitate potassium nitrate—saltpeter.

Harano already has hydrochloric acid on hand; the soy sauce workshop uses it all the time, so there's no shortage. Potash is easy to obtain, too—he's right by the sea. By repeatedly rinsing coarse salt using the leach salt flower technique, he can get potash. Or, in a more primitive way, he can slash-and-burn for huge amounts of wood ash, add brine to the ash solution, and get potash all the same.