LeConte Process

This is a process by which saltpeter may be produced from farm waste. To describe it as a single method is somewhat misleading, since it involves a three stage pipeline with a latency of eighteen to thirty months

Method

Mould Production

[...] thoroughly rotted manure of the richest kind, in the condition usually called mould, or black earth. [...] The process of preparation is so precisely similar to that of compost manure [...] the chief difference being the greater richness in nitrogenous matter in the case of compost intended for nitre-beds.

1. Prepare a watertight floor
2. Lay a set of three layers, each 20cm thick
   1. Common soil made slightly alkaline with ashes or lime
   2. Vegetable matter: straw, leaves, weeds
   3. Dung, optionally with small amounts of flesh, skin, etc.
3. Repeat to a total of 3 or 4 sets, making the stack 180-240cm tall
4. Erect light walls and/or roof to shield the heap from direct hot sun or direct rain

• Every week or two, wet the heap thoroughly with urine and/or liquified dung
• After eight months to a year, the result will be mould.

Nitrification

Two systems are described, each converting mould into nitrified earth which appears on the driest surface of the heap.

Niter Beds (French)

To make the bed, a floor is prepared of clay, well rammed, so as to be impervious to water. [...] An intimate mixture is then made of rotted manure, old mortar coarsely ground, or wood ashes (leached ashes will do), together with leaves, straw, small twigs, branches, &c. to give porosity to the mass, and a considerable quantity of common earth, if this has not been sufficiently added in the original manure-heap. The mixture is thrown somewhat lightly on the clay floor. [...] The whole is then covered by a rough shed to protect from weather, and perhaps protected on the sides in some degree from winds. The heap is watered every week with the richest kinds of liquid manure, such as urine, dung-water, water of privies, cess-pools, drains, &c. The quantity of liquid should be such as to keep the heap always moist, but not wet. Drains, also, should be so constructed as to conduct any superfluous liquid to a tank, where it is preserved and used in watering the heaps. [...] During the last few months of the process [...] the heaps must be kept moist by water only. [...] the nitre is brought to the surface by evaporation, and appears as a whitish efflorescence, detectible by the taste. When this efflorescence appears, the surface of the heap is removed, to the depth of two or three inches, and put aside.

Moving Heap (Prussian)

Five parts of black earth and one of spent ashes or broken mortar are mixed with barley straw, to make the mass porous. The mixture is then made into heaps six feet high and fifteen feet long with one side perpendicular (and hence called walls), and the opposite side sloping regularly by a series of terraces or steps. Straight sticks are generally introduced, and withdrawn when the mass is sufficiently firm. By this means air and water are introduced into the interior of the mass.

[...]

The whole is frequently watered with urine and dung-water.

[...] The liquid with which the heap is watered is drawn by capillarity and evaporation to this side, carrying the nitre with it, and the latter effloresces there as a whitish crust. The perpendicular wall is shaved off two or three inches deep as often as the whitish incrustation appears, and the material thus removed is kept for leaching.

Extraction

Leaching

Nitrified earth (the efflorescence and its base collected earlier) are leached to obtain the nitrates

A quantity of nitrified earth is thrown into a vat, or ash-tub, or barrel, or hogshead with an aperture below, closely stopped and covered lightly with straw. Water is added, about half as much in volume as the earth. After stirring, this is allowed to remain twelve hours. Upon opening the bung, about half the water runs through containing, of course, one-half the nitre. Pure water, in quantity half as much as first used, is again poured on, and after a few moments run through. This will contain one-half the remaining nitre, and therefore one-fourth of the original quantity. Thus the leys of successive leachings become weaker and weaker, until, after the sixth leaching, the earth is considered as sufficiently exhausted.

The leys are combined, then tested to see whether an egg will float on them. If the egg floats, the leys are of sufficient strength and pass to the next step. Otherwise they are reused as the wash for another 2 units of nitrified earth, as above. (At approximately 120g of dissolved salts per liter)

Conversion

The ley thus obtained contains, besides nitrate of potash (nitre), also nitrate of lime and magnesia, and chlorides of sodium and potassium. The object of the next process is to convert all other nitrates into nitrate of potash. This is done by adding wood ashes. The potash of the ashes takes all the nitric acid of the other nitrates forming the nitrate of potash (nitre), and the lime and magnesia are precipitated as an insoluble sediment. Sometimes the ashes is mixed with the nitrified earth and leached together, sometimes the saltpetre ley is filtered through wood ashes, sometimes the ley of ashes is added to the saltpetre ley. In either case the result is precisely the same.

Crystallization

The converted ley is separated from any precipitate and boiled in copper or iron containers. Scum floating to the surface is discarded. Once a steady boil is achieved, and every few minutes thereafter, take a drop of the boiling solution and drop it on a cool surface. If it immediately crystallizes, the solution is saturated. Discard any precipitate at this point (mostly NaCl and KCl) and the solution is set aside to cool. The bulk of the crude saltpeter (25% chlorides, 75% nitrates) crystallizes and is kept. The remaining fluid can be used as watering for mould production or nitrification.

Refining

Beginning with a repetition of the thermal separation of salts, followed by adding materials (glue or blood) to separate organics as flotsam. The crystals are gathered and kept in a fine state, then washed repeatedly with a saturated solution of saltpeter. The solution will take up additional salts but no more saltpeter. The wash is performed several times. Then 1 part by mass of water for every 53 parts of saltpeter is used as a final wash. The undissolved crystals are taken as purified saltpeter.

Analysis

Crude tests

1. Recrystallize a saturated solution very slowly. The crystals formed should be clear/white and needle-like: more than a dozen times longer than they are wide.
2. Melt a small sample, then push the end of a dry wooden splint into the liquid. It should burn readily at the interface between wood and liquid.

See also

• LeConte, Joseph (1862) "Instructions on the Manufacture of Saltpeter" (local copy)
  Charles P. Pelham, State Printer for South Carolina
  link courtesy University of North Carolina, last accessed 20-Jul-2014.
• Rains, Geo. W. (1861) "Notes on Making Saltpetre from the Earth of the Caves" (local copy)
  Daily Delta Job Office, New Orleans, Louisiana
  link courtesy Google, last accessed 20-Jul-2014.