SALTPETRE, OR NITRE.
Nitre.
The principal ingredient in Gunpowder is an abundant production of Nature, and is a combination of nitric acid with the vegetable alkali. It is never found pure, being always contaminated with other salts and earthy matter. Where found.It is principally found in the East Indies, Ceylon, and South America, and is sometimes produced from decayed animal and vegetable matter. Unfit in natural state.It is totally unfit for Gunpowder until it has been refined; for, being combined with muriates of soda, lime, magnesia, and other salts, which absorb moisture, the close contact of the ingredients would be deranged by their presence, the strength of the powder weakened, and the power of resisting the action of the atmosphere greatly lessened. As for the efflorescent salts it may contain, they are noxious only inasmuch as, possessing no particular useful property, they interpose their atoms between the more combustible ingredients, and impede the rapidity of deflagration.
Two methods of refining.
There are two methods of refining saltpetre at Waltham Abbey:—1st, the Old Method, of re-crystallizing three times; and 2nd, the New Method, which has only just been adopted, both of which we shall here briefly describe.
OLD METHOD.
Old method.
About 35 cwt. of the grough saltpetre, as it is termed, viz., as it is imported in its impure state, is put into a copper capable of holding 500 gallons, with 270 gallons of water, in the proportion of about 11⁄2lbs. of nitre to 1lb. of water, (which proportion varies with the quality of the saltpetre). This is allowed to boil, and the impurities are skimmed off as they appear on the surface. Cold water is occasionally thrown in to precipitate portions of the chloride, which otherwise would remain on the top by the action of boiling. After being allowed to boil from three and a half to four hours, the furnace doors are thrown open, when the chlorides and salts fall to the bottom. In about two hours, a copper pump is lowered into the liquor, which is pumped out into a wooden trough, having four or five brass cocks, under which are suspended canvas filtering bags in the shape of a V. The solution is then filtered, and run off into pans, containing about 36 gallons, and allowed to remain for twenty-four hours, to crystallize, when they are set up on edge, to drain off the liquor which remains uncrystallized, and which is called mother liquor. The saltpetre thus obtained is called once-refined, and undergoes the same process twice again, the only difference being that there is a greater proportion to the water each time, viz. 13⁄4lb. to 1lb. of water the second time, and 2lb. to 1lb. of water the third time: moreover, the third time, a small quantity of ground charcoal is put into the solution, and it passes through double filters, which brings it to a very fine pure white colour when melted. The mother water which remains in the pans after each crystallization is conveyed away by gutters to cisterns under the building; it is then evaporated in iron pots to one quarter of its original bulk, filtered, and allowed to crystallize. The saltpetre obtained from the first mother water is considered one stage inferior to grough; that from the second, equal to grough; that from the treble-refined, equal to once-refined saltpetre. The water left from every stage is treated in the same way, so that actually nothing is lost of the pure material. Saltpetre treble-refined by this process is perfectly pure, and fit for the manufacture of Gunpowder; and in order to free it from moisture, as well as for the convenience of storage and transport, it is melted in iron pots holding about 4 cwt., Saltpetre fuzes at 600°.by raising it to a temperature of 600° Fahrenheit, and cast into gun-metal circular moulds holding about 38lbs. each. It must be observed that it requires about two hours to bring the saltpetre into a liquid state, and that, after this, the furnace doors are thrown open, to lower the heat to the proper temperature for casting into the moulds. When the cakes are cold, they are packed away in barrels containing 1 cwt., 1 qr. each, and put into store. Care must be taken, in melting the saltpetre, not to raise it to too high a temperature, as this would reduce the quantity of oxygen, and form nitrite of potash, which would render it unfit as an ingredient in the composition of Gunpowder.