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Saltpeter, or niter, is the common name for potassium nitrate. Biringuccio tels us how to extract it in his Pirotechnia:
As I told you in the chapter on salts, saltpeter is a mixture composed of many substances extracted with fire and water from arid and manurial soils, from that growth which exudes from new walls or from that loosened soil that is found in tombs or uninhabited caves where the rain cannot enter. It is my belief that it is engendered in these soils from an airy moisture that is drunk in and absorbed by the earthy dryness...
Biringuccio goes on to desrcibe a process for extracting saltpeter from this "manurial soil," i.e. soil that has formed from human or animal manure. On the surface of such soil or in certain caves there will be a white crust on the surface of the soil. The important feature of this soil is that it should contain organic material which contains nitrogen: chiefly proteins and their decomposition product, urea. Bacteria in the soil oxidize these nitrogen compounds to a family of nitrate salts: sodium nitrate, potassium nitrate, and calcium nitrate, depending on the other minerals present in the soil. These nitrate compounds are among the most soluble of all compounds. The solubilities of sodium, calcium, and potassium nitrate in boiling water are 952, 376, and 247 g / 100 mL. That is, boiling water will dissolve more than nine times its own weight of sodium nitrate.
The process for making saltpeter is very similar to that for making potash, the chief difference being the starting material. The soluble part of wood ash is mostly potassium carbonate and so when we purify it by recrystalization, the resulting product is purified potassium carbonate. The soluble part of these manurial soils are mostly mixed nitrates. The reason for this is that while the original animal waste may have had a wide variety of soluble materials, as the water wicked up through the soil and evaporated, the less soluble parts fell out of solution as it became more and more concentrated. Only the most soluble parts, in this case the nitrates, made it to the top of the soil and when the last bit of moisture evaporated, they were deposited as a white crust on the surface.
So the beginning of primitive saltpeter production is to collect this white crust leaving behind as much as possible the underlying soil. Of the three nitrates present, potassium nitrate is the one we need. To extract it, we use a metathesis reaction in much the same way that we have previously produced lye:
Ca(NO3)2(aq) + K2CO3(aq) -----> CaCO3(s) + 2 KNO3(aq)
This removes all the calcium as insoluble calcium carbonate leaving mostly potassium nitrate and a little sodium and potassium carbonate which can be further separated out by repeated recrystalizations in much the same way that we produced potash.
Saltpeter has been mined in Virginia, particularly in the dry caves of the Shenendoah valley. Most of the nitrate mined today comes from Chile and is called "Chile Saltpeter," which is chiefly sodium nitrate. However, most of the nitrate used in explosives and agriculture is today derived from nitrogen in the air

judgment_arms wrote:Hehehe, saltpeter, I can tell you how to make it from stuff found around your house, especially if you have pets…
You ever wondered were people got it 300 hundred years ago, In bat caves! (not the bat cave, but bat caves, as were bats live.) It’s made from dung, crap, poop, what ever you want to call it. I’m to lazy to type it when I can copy it…

Saltpeter, or niter, is the common name for potassium nitrate. Biringuccio tels us how to extract it in his Pirotechnia:
As I told you in the chapter on salts, saltpeter is a mixture composed of many substances extracted with fire and water from arid and manurial soils, from that growth which exudes from new walls or from that loosened soil that is found in tombs or uninhabited caves where the rain cannot enter. It is my belief that it is engendered in these soils from an airy moisture that is drunk in and absorbed by the earthy dryness...
Biringuccio goes on to desrcibe a process for extracting saltpeter from this "manurial soil," i.e. soil that has formed from human or animal manure. On the surface of such soil or in certain caves there will be a white crust on the surface of the soil. The important feature of this soil is that it should contain organic material which contains nitrogen: chiefly proteins and their decomposition product, urea. Bacteria in the soil oxidize these nitrogen compounds to a family of nitrate salts: sodium nitrate, potassium nitrate, and calcium nitrate, depending on the other minerals present in the soil. These nitrate compounds are among the most soluble of all compounds. The solubilities of sodium, calcium, and potassium nitrate in boiling water are 952, 376, and 247 g / 100 mL. That is, boiling water will dissolve more than nine times its own weight of sodium nitrate.
The process for making saltpeter is very similar to that for making potash, the chief difference being the starting material. The soluble part of wood ash is mostly potassium carbonate and so when we purify it by recrystalization, the resulting product is purified potassium carbonate. The soluble part of these manurial soils are mostly mixed nitrates. The reason for this is that while the original animal waste may have had a wide variety of soluble materials, as the water wicked up through the soil and evaporated, the less soluble parts fell out of solution as it became more and more concentrated. Only the most soluble parts, in this case the nitrates, made it to the top of the soil and when the last bit of moisture evaporated, they were deposited as a white crust on the surface.
So the beginning of primitive saltpeter production is to collect this white crust leaving behind as much as possible the underlying soil. Of the three nitrates present, potassium nitrate is the one we need. To extract it, we use a metathesis reaction in much the same way that we have previously produced lye:
Ca(NO3)2(aq) + K2CO3(aq) -----> CaCO3(s) + 2 KNO3(aq)
This removes all the calcium as insoluble calcium carbonate leaving mostly potassium nitrate and a little sodium and potassium carbonate which can be further separated out by repeated recrystalizations in much the same way that we produced potash.
Saltpeter has been mined in Virginia, particularly in the dry caves of the Shenendoah valley. Most of the nitrate mined today comes from Chile and is called "Chile Saltpeter," which is chiefly sodium nitrate. However, most of the nitrate used in explosives and agriculture is today derived from nitrogen in the air

I trust you found this interesting, to say the least.

Quote from “this old Caveman Chemistry website”