The saltpetre is, as has been previously stated, used moist as it comes from the refinery. It is generally sent up to the mixing house after standing four or five days in the store bins, and when used contains generally from 3 to 5 per cent. moisture. For this an allowance must of course be made in weighing, and to enable the mixer to make this accurately, the percentage of moisture in the quantity sent up each morning, generally amounting to 35-40 cwt., when the factory is in full work, is ascertained by the master-refiner by drying and fusing a sample, and comparing the weight before and after the operation. A ticket with the proper weight of the salt petre for the day'a charges is given to the master-mixer, who weighs out the saltpetre accordingly.
The subjoined Table shows the weight of moist saltpetre for a 50 lb. charge, with moisture from 8 to 6 per cent. ; thus, though the proportions used in England for gunpowder are 75 saltpetre, 15 charcoal, 10 sulphur, an extra 1 lb. of saltpetre is generally added to the 100 parts to cover loss in manufacture.
If dried refined saltpetre is employed, it is first ground under a pair of small atone edge-runnera fitted with scrapers, and then passed through a elope-reel covered with 28-mesh wire.
The sulphur is ground under a pair of iron edge-runners, also fitted with scrapers, and sifted in a slope-reel covered with 32-mesh wire.
Charcoal, after being carefully hand-pioked, to guard against the introduction of any fragments of foreign matter and underburnt knots of wood, is ground in a mill, Fig. 597. It consists of a cone working in a oylinder A, eaoh being furnished with diagonal ribs or teeth, which are widely apart at top but approaoh closely together at the bottom. The charcoal which is fed in at the top passes out at the bottom into a slope-reel II M covered with 32 mesh wire.
Charcoal, after having been burnt, should be allowed to stand for ten days to a fortnight before being ground, for when ground fresh after burning, the finely powdered charcoal absorbs and condenses oxygen so rapidly as to generate a great amount of heat, enough to cause spontaneous combustion. The relative proportions of the three ingredients are weighed out in quantities of 50 lb., that is to say, 37} saltpetre plus the extra weight for the moisture, 71 charcoal, and 5 sulphur, and transferred to the mixing machine, Figs. 598, 599. This consists of a hollow drum d of gun-metal, 1 ft. 6 in. wide by 2 ft. 9 in.
in diameter, which is made to revolve at a speed of 40 revolutions a minute. The bearings t are hollow to receive a shaft which passes tbrough them. This shaft carries in the interior of the drum a series of forty-four arms or filers, the points of which just clear the interior of the drum, and revolves at twice the speed of the drum, in the opposite direction. A 50 lb, bag of ingredients is emptied into the drum through a square opening at the top of it, and the drum and shaft carrying the fliers being set in motion for five minutes, the saltpetre, sulphur, and charcoal are'thoroughly mixed together. The mixed ingredients fall down the shoot into a tub, from which they are trans ferred to an 8-mesh wire sieve, placed over another shoot, having a composition bag placed beneath it. On the sieve the charge is carefully by hand, to guard against any foreign matters passing into it, and falls through into the bags, in which it is tied up tightly, and transferred to the expense magazine or charge-house ready for the incorporating mill.
A charge of fresh mixed ingredients is termed a green charge. Though not so easily ignited and slower in burning than gunpowder, it is, of course, explosive. Incorporation is unquestionably the most important of all the operations in the manufacture of gunpowder. The more thoroughly it is effected the stronger will he the resulting gunpowder. Provided the in corporating mill is sufficiently powerful, and is worked at a sufficient speed, a most thorough incorporation can be effected in a few hours, after which there is no object in continuing the process.
The old stamping mills, in which the ingredients were subjected to long-continued stamping under pestles work ing in small mortars, have been out of use for many years in Englaud, though they are still employed in other countries. The action of such mills has very little incorporating effect, and is moreover attended with even more risk than the mills used in this country. The same remark applies to the moulins a tonneaux, in which a partial incorporation is effected by placing the material in a revolving barrel or drum along with some large metal or wooden balls. Such mills are often used for pulverizing the ingredients, and occasionally for incorporation, but the most effectual and speedy incorporation can be obtained with mills such as shown.