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.
A neutral salt.
Saltpetre is a neutral salt, the constituents of which are 46.55 potash, and 53.45 nitric acid; the latter consisting of two volumes nitrogen and five of oxygen. It is white, and of a fresh, sharp, and slightly bitter taste. It crystallizes in six-sided prisms. Exposed to the air, it remains permanent unless impure, or that the atmosphere is very moist.
NEW METHOD.
New method.
Forty cwt. of the grough saltpetre is put into a copper with 270 gallons of water, and treated in precisely the same way as we have before described for the first refining; it is then filtered and run off into large troughs, about 10 feet long by 6 feet wide, and 9 inches deep, lined with sheet copper; this liquor is then kept in a state of agitation by a wooden rake, until nearly cold. By this process a large quantity of very minute crystals are formed, which are collected as they form by a wooden hoe, and shovelled with a spade on to a framework covered with copper sieving resting on the opposite sides of the trough, and allowed to drain. These fine white crystals, which have exactly the appearance of snow, when they have drained sufficiently, are raked over in a washing cistern adjoining, which is about 6 feet long, 4 feet wide, and 3ft. 6in. deep, and fitted with a false wooden bottom that can be removed at pleasure. Cold water is allowed to run on to the saltpetre in this cistern till it is nearly level with the top. After remaining for an hour it is drained off, and filled again with fresh water, which is drained off after about another hour. The saltpetre thus obtained is perfectly pure, and equal in every respect to the treble-refined by the old method. The water remaining in the cisterns after agitation, is left till the next morning, when a quantity of larger crystals are formed on the bottom and sides; these are equal to once-refined by the old method, and are used with grough; the mother-liquor is then drained off, and evaporated in the usual way. The water from each washing is conveyed into cisterns, and used with grough saltpetre instead of water; but, as it contains a small portion of saltpetre in solution, a lesser quantity of grough is used to make the proportions correct.
Drying.
The saltpetre flour, however, contains a certain degree of moisture, which has to be dried off in the following way: two large copper trays, about 10 feet by 6 feet, with a 3-inch rim, are fixed over flues heated by a furnace, 4 inches of sand being between the flues and the bottom of the trays; the saltpetre is spread about 2 inches deep all over, and raked about till dry; it is then barrelled up for use. It takes about two hours to dry 5 cwt.
Comparison of the two methods.
On comparing the two systems, there cannot for one moment be a doubt as to the immense advantages of the latter over the former. As an example, in the refinery where this new process is carried on, the result (that is to say, pure saltpetre) is obtained in one day instead of six, with less than one half the amount of labour and coals.
Why new method best.
On reflection, the reason of the great gain of time by this process will suggest itself. In the former method, when allowed to remain quiet, the crystals formed are very large, and the spaces left in them always contain a certain amount of mother-water, which necessitates its being crystallized three times to perfectly free it from the liquor. In the latter, the crystals are so minute that there is practically no space for the mother-water to collect; consequently, by careful washing, the saltpetre is obtained perfectly pure.