At the cave, calcium nitrate, also called niter, is leached from dry soil.
en.wikipedia.org However, the process to obtain niter, the mineral form of potassium nitrate, was difficult, and he did not know how to prepare gunpowder from the raw materials.
en.wikipedia.org Niter for the manufacture of gunpowder, copper, lead, iron, coal, zinc, and other such materials as might be required for the prosecution of the war.
en.wikipedia.org Finally, the water was boiled off leaving niter crystals usable, after drying, for making gunpowder.
en.wikipedia.org When alum and niter and other chemicals are mixed together one gets "ching thung": green bronze.
en.wikipedia.org