Incorporation is the most dangerous operation in the manufacture of gunpowder. Accidents in the subsequent processes, where large quantities of powder are subjected to treatment at one time, are fortunately rare, but in the incorporating mills they may be expected from time to time. As such mills are generally built in groups, an explosion in 'one is very apt to spread amongst all the others round it, particularly as the roofs of all are saturated with powder dust. To prevent this, a drenching apparatus, of which Figs. 602, 603 are a representation, is erected over each pair of runners.
The apparatus consists of a large shutter C, balanced by a counterweight B, and pivoted on a spindle D which runs through the whole group of mills. To this spindle the shutter in each mill is attached and the spindle passes through bearings in the parti tion walls, being connected by couplings F, so that the lifting of one shutter lifts all the others. Balanced on the pivot edge of the shutter is a large copper vessel A full of water ; this is so arranged that the slightest lift of the shutter capsizes its contents into the bed of the mill beneath it.
An explosion in one mill there fore lifts the shutter above it, and throws down the water into the mill bed ; and though too late to do any good in the mill which has exploded, the movement of the shutter turns the spindle and drowns the charge in all the adjacent mills, and thus saves them from explosion. There is also an arrangement by which the millman can, in case of an explosion in the immediate neighbourhood, pull over the vessel of water from outside.
The explosion of a green charge does not, in some cases, do much damage to the structure of the mill or the machinery ; that of a worked charge is very violent, and leaves generally no part of the structure standing. Consequently all mills are made of a very strong framework covered with light boards, which can be quickly replaced if destroyed by an explosion. The cake leaves the mill partly soft and partly dust ; it hardens considerably if allowed to stand for a few days. In this form it would be, of course, quite unfit for use. The cake may indeed be broken up into grains, as is done in many countries, but powder made direct from mill-cake is dusty and irregular in action. It is also much more liable to absorb moisture, and therefore to cake and become lumpy.
To ensure uniformity and good keeping qualities, and freedom from dust, powder must be corned or converted into firm grains. The old process is to damp the incorporated materials till they acquire a certain consistence, and then force the mass through sieves, thus forming granular frag ments which are afterwards hardened by drying. There is no difficulty in thus forming good hard
grains, but the moisture added partly dissolves the saltpetre, which in drying forms a hard external surface. The best way is first to compress the soft material into hard masses by pressure alone, and then to crush up these masses into the description of grain required. The pressing is to convert the mill-cake into hard cakes, of the particular density which is found to give the best results when the powder is finished.
Gunpowder is generally pressed in layers, between plates of gun-metal or copper, in a hydraulic press. Screw presses are still in use in many factories, and there are different ways of placing the powder in the presses used. The best results are obtained by pressing in thin layers. The details of the process will be described further on, but it is merely necessary to say here that the gun-metal plates between which the powder is placed are, when the press is being filled, placed on their edges and the powder thrown in between them. As they stand closely together, inch apart, it follows that to ensure the powder finding its way down between them it must be reduced to a tolerably fine state of division, If taken direct to the press from the mills, the cake would not enter between the press plates ; a preliminary operation is therefore necessary, namely, breaking down. If the powder is to be pressed in very thick cakes it would not require breaking down. The breaking-down machine, Figs. 604, 605, consists of two pairs of gun-metal rollers placed in a frame, one pair directly above the other. They are grooved longitudinally to get a better hold of the mill-cake ; one roller of each pair works in a sliding bearing, and is held at a proper distance from the other by a weighted lever, to admit of each pair opening out a little if too large a quantity of the cake is drawn in between them. Attached to the machine is a large hopper ; an endless band of strong canvas, having transverse strips of leather sewed on to it, works through an opening at the bottom of the hopper up to the top of the maohioe, by means of which a regular enpply of the cake is carried up and dropped in between the rollers, which crush it up into fine meal ; this falls through the second pair of rollers, which reduce to dust any fragment which may have escaped the action of the first pair, and finally falls into wooden tubs placed underneath to receive it ; it is then ready for direct transfer to the press. Too long an interval must not elapse between the breaking down and pressing, for the meal, which should contain from 2 to 3 per cent. of moisture.