Historically, the term niter was not well differentiated from natron, both of which have been very vaguely defined but generally refer to compounds of sodium or potassium joined with carbonate or nitrate ions.

It is the mineral form of potassium nitrate, KNO3,[6] and is soft (Mohs hardness 2),[7] highly soluble in water,[6] and easily fusible.

[8] It occurs in the soils of arid regions and as massive encrustations and efflorescent growths on cavern walls and ceilings where solutions containing alkali potassium and nitrate seep into the openings.

A term (ἀφρόνιτρον, aphronitron or aphronitre) which translates as "foam of niter" was a regular purchase in a fourth-century AD series of financial accounts, and since it was expressed as being "for the baths" was probably used as soap.

In the "Cova del Rat Penat", guano (bat excrements) deposited over thousands of years became saltpeter after being leached by the action of rainwater.

Naturalists sent by academies from all Europe came in large number to visit the site; since niter is a fundamental ingredient in the production of gunpowder, these deposits were of considerable strategic interest.