Nitre.
1. Nitre.—Among the various articles made use of in the composition, none are of greater importance than salt-petre, for on the quantity and purity of this depends all the force and much of the brilliancy of the fire. The most common sort is that usually sold by the grocers, and is generally in large lumps formed of an assemblage of small crystals somewhat transparent, and often mixed with earthy matter and many other impurities. In its purest state it is in the form of small six-sided prismatic crystals, not apt to grow moist or powdery on exposure to the air. The pure nitre is now become very expensive, so it is of consequence to know how the common nitre, or nitre of commerce may be purified, for it is found to answer no purpose in Pyrotechny unless such change or purification in it have been effected.
Nitre is found, (like most of other saline bodies) to be much more soluble in boiling water, than in water of the ordinary temperature. If therefore the nitre of commerce be dissolved in a small quantity of boiling water, and the solution be properly strained, the liquor, when cold, will afford crystals that are very pure. The following is the most convenient method of proceeding: dissolve the nitre in boiling water, (which should be soft water,) in the proportion of about a quart to each pound of nitre; and that the solution may be more easily effected, let the nitre be reduced to a powder, previous to its being immersed, and let the vessel containing the nitre and water be kept at the boiling heat till all the salt is dissolved; then strain the liquor, while hot, through thick blotting paper, placed in a clean funnel; and set by the filtered liquor in a shallow vessel, in a cold place, to crystallize. The crystals thus obtained are to be dried, first in blotting-paper, and then before the fire, and kept for use. From the remaining solution, which is sometimes called mother-water, fresh crystals may be procured by boiling it in a clean tin vessel till a filming scum arises to the surface, then filtering it through paper, and setting it aside to crystallize as before.
Very pure nitre may also be obtained from damaged gunpowder, which may be sometimes procured at a cheap rate, at the shops where it is sold for this purpose. The damaged powder must be ground with a small quantity of hot water, in a large wooden or stone mortar, otherwise it may be boiled over a gentle fire, with as much water as will cover it, till as much as possible of the nitre is dissolved; the liquor is then to be strained through a thick flannel bag, afterwards filtered through blotting-paper while hot, the sediment to be boiled down till a film rises on the surface; again filtered and set by to cool and crystallize, as directed in the process for the former method.
As the nitre must always be reduced to fine powder, previous to mixing it with other substances, this is easily done by dissolving it in a little more than its own weight of boiling water, keeping the solution over a gentle fire, and continually stirring it with a flat stick till all the water is evaporated, when the powder is to be taken out and dried before a gentle fire; during which, care must be taken not to let it remain too long, or exposed to too great a heat, otherwise it will be melted into a firm cake. The drying may be completed by suffering it to remain a sufficient time on paper before the fire. For the purification of salt-petre, both these methods may (by attending to the foregoing instructions,) be practised with success; but of the two, we would more strongly recommend the former.