Figs. 606, 607 are the most convenient form of hydraulic gunpowder press ; the press box is made of gun-metal, lined inside and out with oak boards, and is of great strength. The bottom and one side are permanently attached to each other, the other three sides are hinged to the bottom, so that they can be opened out to facilitate unloading. When closed they are secured with short, very strong, coarse-threaded screws of gun-metal ; the box has two projecting gun-metal claws, which hinge on to a fixed horizontal rod of the same metal, so that the box can be turned on it, on to the table of the hydraulic press, when filled and ready for pressing, or outwards when it has to be unloaded. Being first turned down on its aide, the open top is closed temporarily with a piece of board which is fitted to it ; the uppermost aide is un covered and raised, and the other two sides are fastened in their places ; gun-metal racks to hold the press plates, having perpendicular grooves in them in. apart, are then slid in on each aide, and the plates being put in, the meal from the break ing-down machine is shovelled in and falls down readily be tween the plates till the box is full ; the racks are then drawn out, leaving the plates free with layers of powder between them, the excess of powder being carefully swept off the edge of the box ; the upper side is lowered and screwed to the other three, an overhead block and tackle is made fast to the gun-metal eye on the side of the box, and the box is turned over on to the press table.
The box now stands on its bottom, and the temporary board being lifted off, the powder and plates will be found to have settled down several inches by their own weight ; the vacant space at the top is filled up by shovelling in a few more layers of meal, placing a plate by hand on each in succession till the press is quite full ; the overhead block D is run into place directly over and nearly touching the contents of the box, to secure it there and to apply the pressure until the box rises to a sufficient height. The pumps are in another building, separated from the press house by large traverses ; in this building the workmen remain while the pumps are at work. The amount of pressure to be given to the contents of the press box is estimated by the distance to which the over head block enters the press box. Large-grain powder is compressed 13i in. and fine grain or thereabouts. A catch which can be adjusted to any height, and which holds back a kind of trigger, is fixed on the overhead block in such a way, that whenever the block has entered the box to the depth required, the catch comes in contact with the rising edge of the press box, and relieves the trigger, which rings a bell in the pump room. The pumps are then stopped, the escape valve opened, and the press table carrying the press box allowed to descend. The workmen then re-enter the press house and proceed to unload the box. Each plate, with a layer of hard cake adhering to it, is separated from the one beneath it, and being lifted into a wooden bin, a few knocks with a wooden mallet causes the cake to fall off in irregular fragments, which are broken into pieces of the size of a man's hand, shovelled into tubs, and removed to an expense magazine.
Powder is occasionally pressed without making use of the press box at all. This is only done when very great accuracy is required. If the pressure applied to powder be regulated according to the distance to which the meal is to be compressed, to ensure uniformity of results, not only must each pressing be carried on to the same distance, but there must be exactly the same quantity of material subjected to the process each time. There are other conditions also which affect the density of the resulting press cake. A sheet of canvas, the same size as a press plate, is laid on the table of the hydraulic press. On this is placed a wooden frame of the depth of the layers of meal which require to be pressed, generally 1-1i- in. ; powder meal is shovelled into the frame till it is full, and the excess carefully struck off by drawing a wooden rod over the edge of the frame. The frame is then lifted off, leaving a layer of meal of the required thickness on the press table ; on this layer is first placed another sheet of canvas, then a gun-metal press plate, and finally another sheet of canvas, on which a fresh layer is spread in a similar manner, and so on, till a pile of layers of powder, separated by canvas and gun-metal plates, is built up to the level of the under surface of the overhead block ; the pressure is applied in the usual way ; this system of pressing, though slower than the usual system, appears to offer some decided advantages.
It is a matter of considerable difficulty to ensure uniformity of results iu pressing powder. It is of the highest importance that the density obtained should be uniform, for the qualities and explo sive effect of gunpowder are materially affected by comparatively slight variations in density. For instance, a difference of • 05 in the density of the charge may affect the initial velocity of a 12-lb. shot, fired with a 1-1b. charge, to the extent of about 50 ft. a second ; no other difference between the two powders may be perceptible. It is necessary, when examining the densities of press cake, in ordor to ascertain if it is fitted for the manufacture of a particular powder, to have it previously dried.
It is found in practice that, though uniformity cannot be guaranteed in pressing, very tolerably close results can be obtained. To attain these in the finished powder, the density of every pressing, after it has been converted into grain, sometimes of every glazing, is taken, and the diffe rent pressings or glazings are then mixed in the proportions to give the density required. Thus if the density fixed for the powder be 1.67, and the densities of two pressings be found to be and 1 .64, they would be mixed in equal proportions, and would give a powder of 1.67 density; powders which, however, differ to a great extent in density are never mixed.