The first process is that of simply mixing the ingredients. For this purpose the proper quantities of each are accurately weighed out, allowance being made for a certain amount of moisture in the saltpetre. The proportions vary in different countries, and according to the purpose for which the gunpowder is to he used. For English Government powder of every kind it is—saltpetre 75 parts, sulphur 10, charcoal 15, the sulphur being reduced almost to a minimum, as its chief use is only to ignite the charge and accelerate its action. In France and Prussia the quantity of sulphur is larger, the scale being saltpetre 75 and 12.5 parts each of sulphur and charcoal, while in Chinese powder the amount of sulphur is between 14 and 15. It is remarkable that in all countries the proportion of saltpetre remains about the same.
The ingredients, being weighed for a charge of 50 pounds, are poured into a "churn." This is a revolving drum, placed horizontally, and having within it an axis revolving in a different direction from the drum, and furnished with eight rows of projecting arms, or "fliers." So rapid is the action of this apparatus, that when the charge has been three minutes in the revolving churn the ingredients are thoroughly mixed together. It is then known as a "green charge," and is ready for the incorporating mills, the object of which, as the name indicates, is to incorporate the materials, or to make the mixture so intimate that a new substance is produced, namely gunpowder.
The incorporating-houses at Waltham contain at present thirty-two separate mills. Each mill consists of a pair of runners, coupled together by a strong axle. This axle rests in the socket of an upright shaft, which, passing down through the mill-bed, is connected by bevel-wheels with a revolving horizontal shaft, driven by steam or water-power. The runners are either of black Derbyshire limestone or of iron, and weigh from three and a half to four tons. Iron runners are now generally used, and their size varies from three and a half to seven feet in diameter. The mill-bed, a large circular vat with a flat bottom and sloping sides, is of stone or metal, according to the material of the runners. On this bed 50 pounds of the green charge is spread out and moistened with water, and the mill is then set going. The length of time required for the incorporation of the powder varies according to the use to which it is to be applied. Thus cannon-powder is left under the mills for three hours; while for rifle-powder, which requires a closer incorporation on account of its more rapid action, the time is five hours. The power of a gunpowder-factory is measured by the number of pairs of runners it possesses, for, as the law allows no more than 50 pounds to be placed in any mill at one time, the amount which can be incorporated in a year is easily calculated. A pair of iron runners, driven by steam and working day and night, will incorporate in a year nearly 100,000 pounds of cannon-powder, or about half that quantity of rifle-powder.
