The mixed ingredients are put on this bed-stone in quantities not exceeding 40 or 50 pounds at a time, and moistened with just so much water, as will bring the mass in the grinding to a consistence considerably stiffer than paste, in which it is found by experience that the incorporation of the ingredients goes on with the most ease and accuracy. These mills are worked either by water or horses.
The composition is usually worked for about seven or eight hours before the mixture is thought to be sufficiently intimate, and even this time is often found, by the inferior quality of the powder, to be too little. The fine powder manufactured at Battle in Sussex, is still however made in large mortars or stamping mills, in the old way, with heavy lignum vitæ pestles. Only a very few pounds of the materials are worked at a time.
The composition is then taken from the mills and sent to the corning-house, to be corned or grained. This process is not essential to the manufacture of perfect gunpowder, but is adopted on account of the much greater convenience of using it in grains than in fine dust. Here the stiff paste is first pressed into hard lumps, which are put into circular sieves with parchment bottoms, perforated with holes of different sizes, and fixed in a frame connected with a horizontal wheel. Each of these sieves is also furnished with a runner or oblate spheroid of lignum vitæ, which being set in motion by the action of the wheel, squeezes the paste through the holes of the parchment bottom, forming grains of different sizes. The grains are then sorted and separated from the dust by sieves of progressive dimensions.
They are then glazed or hardened, and the rough edges taken off, by being put into casks, filling them somewhat more than half-full, which are fixed to the axis of a water-wheel, and in thus rapidly revolving, the grains are shaken against each other and rounded, at the same time receiving a slight gloss or glazing. Much dust is also separated by this process. The glazing is found to lessen the force of the powder from a fifth to a fourth, but the powder keeps much better when glazed, and is less liable to grow damp.
The powder being thus corned, dusted and glazed, is sent to the stove-house and dried, a part of the process which requires the greatest precautions to avoid explosion, which in this state would be much more dangerous than before the intimate mixture of the ingredients.
The stove-house is a square apartment, three sides of which are furnished with shelves or cases, on proper supports, arranged round the room, and the fourth contains a large cast-iron vessel called a gloom, which projects into the room, and is strongly heated from the outside, so that it is impossible that any of the fuel should come in contact with the powder. For greater security against sparks by accidental friction, the glooms are covered with sheet copper, and are always cool when the powder is put in or taken out of the room. Here the grains are thoroughly dried, losing in the process all that remains of the water added to the mixture in the mill, to bring it to a working stiffness. This Mr. Coleman finds to be from three to five parts in 100 of the composition. The powder when dry is then complete.
The government powder for ordnance of all kinds as well as for small arms, is generally made at one time, and always of the same composition; the difference being only in the size of the grains as separated by the respective sieves.
A method of drying powder by means of steam-pipes running round and crossing the apartment has been tried with success: by it all possibility of an accident from over-heating is prevented. The temperature of the room when heated in the common way by a gloom-stove is always regulated by a thermometer hung in the door of the stoves.
The strength of the powder is sometimes injured by being dried too hastily and at too great a heat, for in this case some of the sulphur sublimes out (which it will do copiously at a less heat than will inflame the powder) and the intimate mixture of the ingredients is again destroyed. Besides if dried too hastily, the surface of the grain hardens leaving the inner part still damp.
Mr. Coleman deduces from experiment the following inferences, namely: that the ingredients of gunpowder only pulverized and mixed have but a very small explosive force: that gunpowder granulated after having been only a short time on the mill, has acquired only a very small portion of its strength, so that its perfection absolutely depends on very long-continued and accurate mixture and incorporation of the ingredients: that the strength of gunpowder does not depend on granulation, the dust that separates during this process being as strong as the clean grains: that powder undried, is weaker in every step of the manufacture than when dried: and lastly, that charcoal made in iron cylinders in the way already mentioned, makes much stronger powder than common charcoal. This last circumstance is of so much consequence, and is so fully confirmed by experience, that the charges of powder now used for cannon of all kinds have been reduced one-third in quantity, when this kind of powder is employed.