Ancient civilizations through current day have used saltpeter for kindling by rubbing two sticks together. Black powder is another form of saltpeter. Recipes go all the way back to Greece and 15th century Germany. Saltpeter is also used for gun cotton, dynamite fuses and is for oxidation. Naturally forming, saltpeter can be relatively easy to make, as long as you have the ingredients.
Place cow manure into a large pile. Planting soil can also work.
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Mix the manure or planting soil with some green plant life, a little bit of ash from burned thistles, worm wood, ash from tree bark or wood ashes. If you have potato leaves, mix that in, too Your pile of half-and-half should be 6 to 7 feet tall.
Set your mixture on something that is waterproof.
Form a roof over your pile with tarp and cinder blocks or something similar.
Pour lant (stale cow urine) over the pile once a week for three to four months. Once you see yellowish crystals forming on top, you can stop the lant.
Scrape off the top layer when the potassium nitrate (crystals) that has formed has effervesced 6 to 10 centimeters thick. Keep up this practice until the saltpeter has been harvested.
Treat the saltpeter earth when all is harvested by using a bucket. Poke holes in the 5-gallon bucket, and put a paper (non-charcoal) filter on the bottom. Fill the bucket three-quarters high with the left over pile. Set a pan under the bucket, and slowly pour boiling water over the mixture. Continue with this until the pile is gone.
Take the mixture from the pan under the bucket, and pour it into containers. (Black works the best.) Continue this until all is gone.