The mode of drying gunpowder by the vapour of water, (confining it, however, in iron pipes or vessels,) was suggested in 1781, and 1787. See Mémoires de l'Académie des Sciences de Suede, 1781, the Journal des Savants, 1787, and the Transactions of the Society of Arts, vol. xxiv. Mr. Snodgrass, in the last work, gave an account of a method of communicating heat by steam, by using pipes of cast iron, for which the society of arts voted him forty guineas.[18] Chaptal (Elements de Chimie) has some judicious remarks on the exsiccation of powder.
The experiment made at Essonne near Paris, by M. Champy, in 1808, on a contrivance for the drying of powder, was satisfactory. This experiment may be seen in page 242 of Bottée and Riffault.
5. Dusting, (Epoussetage.) This operation is confined merely to the sifting. It is nothing more than the separation of the dust from the grain, which we have before noticed. The dust is put in the mortars, and worked over.
6. Barrelling &c. After the powder has gone through the several operations described, it is then put into barrels, and taken to the magazine.
After speaking of gunpowder under these heads, they describe the manner of treating the green, (verd) and dry meal powder; the police of powder establishments, for order and economy; the workmen necessary in a powder manufactory;[19] the process of making powder in the revolution; and for the manufacture of imperial powder (which contains 0.78 saltpetre 0.10 sulphur, and 0.12 charcoal); the process of Berne, where the powder is made of 0.76 saltpetre, 0.14 charcoal, and 0.10 sulphur; the process of Mr. Champy, noticed in this article; observations respecting different processes; on powder magazines; gunpowder made of other saline substances besides nitre; different modes of proving powder, examination of powder; description of workshops, mechanics, and utensils, &c. &c. with a variety of engravings. We have merely to remark, that this work of Bottée and Riffault (a large quarto volume, of 340 pages, besides the plates, which make a distinct volume) ought to be in the possession of every gunpowder manufacturer, as it contains all the information known on that subject. Of this fact there can be no difference of opinion, that in consequence of the great attention paid to the subject of gunpowder in France, not only by the government, but by scientific associations and individuals, their knowledge generally must be more minute and accurate, and their works, as authentic records of facts, more to be depended on.
Besides many interesting works, and memoirs in French,[20] there have appeared some valuable dissertations in the English language. Mr. Coleman, in his paper in the Phil. Mag. ix, p. 355, may be considered the first, who, as superintendant of one of the Royal powder mills, was enabled to present a body of facts on this subject.
As the mode of manufacturing gunpowder at the Royal Powder Mills of Waltham Abbey, in England, may be interesting and useful, in connection with the different processes already given; we will introduce in this place the account of Mr. Coleman, having extracted it from the Artist's Manual, &c. of the author, and having taken it from the original memoir of that gentleman.
The ingredients of gunpowder are taken in the following proportion, namely, 75 of saltpetre, 15 of charcoal, and 10 of sulphur. The saltpetre used is almost entirely that which is imported from the Indies, which comes over in the rough state mixed with earthy and other salts, and is refined by solution, evaporation, and crystallization. After this it is fused in a moderate heat, so as to expel all the pure water, but none of the acid, and is then fit for use. The great use of refining the nitre is to get rid of the deliquescent salts, which by rendering the powder made of it liable to become damp by keeping, would most materially impair its goodness. The sulphur used is imported from Italy and Sicily, where it is collected in its native state in abundance. It is refined by melting and skimming, and when very impure, by sublimation. It should seem that the English sulphur, extracted in abundance from some of the copper and other mines, is too impure to be economically used for gunpowder, requiring expensive processes of refining.
The charcoal formerly used in this manufacture was prepared in the usual way of charring wood, piles being formed of it and covered with sods or fern, and suffered to burn with a slow smothering flame. This method however cannot with any certainty be depended on to produce charcoal of a uniformly good quality, and therefore a most essential improvement has been adopted in this country, to which the present superior excellence of American powder may be in a good measure attributed, which is, that of enclosing the wood, cut into billets about nine inches long, in iron cylinders placed horizontally, and burning them gradually to a red heat, continuing the fire till every thing volatile is driven off, and the wood is completely charred. But as the pyroligneous acid, the volatile product of the wood heated per se, is of use in manufacture, it is collected by pipes passing out of the iron cylinder, and dipping into casks where the acid liquor condenses. This acid is used in some parts of calico-printing, chiefly as the basis of some of the iron liquors and mordants for dark-coloured patterns. The wood before charring is barked. It is generally either alder or willow, or dog-wood, but there does not appear to be any certain ground for preferring one wood to another provided it be fully charred.
The above three ingredients being prepared, they are first separately ground to fine powder, then mixed in the proper proportions, after which the mixture is fit for the important operation of thoroughly incorporating the component parts in the mill. A powder mill is a slight wooden building, with a boarded roof, so that in the event of any moderate explosion, the roof will fly off without difficulty, and the sudden expansion will thus be made in the least mischievous direction. Stamping mills were formerly used here, which consisted simply of a large wooden mortar, in which a very ponderous wooden pestle was made to work, by the power of men, or horses, or water, as convenience directed. These performed the business with very great accuracy, but the danger from over-heating was found to be so great, and the accidents attributable to this cause were so numerous, that stamping mills have been mostly disused in large manufactures, and the business is now generally performed by two stones placed vertically, and running on a bed-stone or trough.