Fig. 1: A Saltpeter brush (efflorescence, ca. 5 cm) on a cellar wall of an old mill. Outside the house there once was a dung pit of a former horse stable. There are many of these efflorences in this cellar. I harvested about 0.2 kg of saltpeter there.

	Fig 3: Saltpeter digger braking open the floor of a stable with a pick axe. This soil later will be processed according to Fig. 4

And by the way, saltpeter diggers were a sore in the eyes of farmers. But unfortunately the diggers were backed up by their governements.

Pictures 3 and 4 give a good impression of how the saltpeter was "mined" and later extracted from the soil dug in stables:

At first, the bottom of the extraction vat (Fig. 4) was covered with sand as a filter layer.
Then the vat was filled with saltpeter soil and leached with water. This lye was very turbid. This turbidity was flocculated by addition of some white of egg (A natural cationic polyelectrolyte a modern chemist would say today).
After the precipitation had settled, the cleared saltpeter solution was scooped into the reduction pan, a flat copper basin on a hearth. There, the solution boiled down to a concentrated saltpeter brine.

		Fig. 4: Georg Agricola's book:"Of mining and smelting" (1556) shows a saltpeter workshop. A) Pan for the reduction of the saltpeter brine B) Leeching of the saltpeter soil C) Plug D) Collector for the eluate E) Crystallization vat with copper sticks

Finally, the hot brine was poured into crystallization vats. The picture shows that these vats contain copper sticks, as crystallization nuclei. After crystallization, the saltpeter was scratched off the sticks and the rest of the brine went back to the reduction pan again. And so on.

Later, as the powder and saltpeter demand increased even more due to the continental-wide wars in Europe, stables didn't provide enough saltpeter. So, the next step was saltpeter plants.

		Fig. 5: Saltpeter plant: (16th century) Long rows of beds with porous walls(C), filled with a mixture of vegetable wastes, blood and dung as nitrogen source plus lime or ash to promote nitrification. Lime controls an optimum pH-value of 7.2 to 7.8. The porous walls of the beds allow an easy access of oxigen. In building (B) the contents of the ripe salpeter-beds were leached and saltpeter kristallised. (A) A vat, collecting rainwater from the roof.
		O. Guttmann:"Monumenta Pulveris Pyrii", London 1906. A collection of Reproductions of Ancient Pictures Concerning the History of Gunpowder

Only after 1890 was it understood, that bacteria played a roll in the oxidation of ammonium to nitrate. Never-the-less, from experience it was known that for an optimum yield, the saltpeter plants had to be inoculated with old saltpeter soil to prime the process and the manure had to be kept weakly alkaline by adding some lime or ash.

Today, this process isn't of interest anymore in regard to saltpeter production. But the process has been widely investigated in waste water treatment. Ammonium and especially ammonia are serious pollutants in creeks and rivers and, above all, toxic to most aquatic animals, especially fish. Hence it is the goal of any sewage treatment plant to oxidize the Ammonium of the raw waste water to harmless nitrate as far as possible..

The oxidation of ammonium to nitrate is done in two successive steps by two different kinds of bacteria

1st step: With the aid of nitrosomonas-germs ammonium is oxidized to nitrite

1. pH-value has to be within pH 6.8 and 7.8. Note the 2H⁺ in equation! 1).
   When this acid isn't neutralized, the process will soon come to an end.
2. The temperature is more than 10 °C
3. Nitrification bacts split once in 5 days at room temperature, so the process is very slow.
4. It's significantly quicker at 35° (summertime).
5. In wintertime there is no nitrification at all. Wait until the next summer!