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the Government establishment or by some of the private makers in this country.
Saltpetre, constituting as it does, three-fourths of gunpowder, is its mainspring. It comes to us from the East Indies, principally from Bengal and Oude. In these countries it generally occurs as a white incrustation on the soil, being also mixed with it to a considerable depth. The earth is scraped up and boiled with water; and the solution after being concentrated by the heat of the sun and evaporated by artificial heat, yields impure crystals of the salt, which are largely imported into this country, packed in bags of coarse sacking. In this state the salt is known as 'grough saltpetre.' The bags when emptied of their contents are turned to account by the saltpetre refiner, who after soaking and boiling them to extract all the salt with which they are impregnated, disposes of them to the makers of coarse wrapping paper, for the manufacture of which they are well adapted. East Indian saltpetre has hitherto been a very expensive article. During the Indian Mutiny the price rose to 597. per ton; within the last few years it has sold at from 387. to 407. That furnished by Bengal is much preferred, the Madras and Bombay imports fetching a lower price in consequence of the much larger quantity of impurity contained in them. When delivered in the form of 'grough saltpetre' it contains from 1 to 10 per cent. of foreign matter which must be removed before the salt is fit for use. The refining process now followed was borrowed from the French, and is a very simple and pretty one, depending entirely on the fact that the saltpetre is greatly more soluble in hot than in cold water, while the impurities present, principally common salt and various salts of lime, do not present the same disparity in their solubility at different temperatures. This causes the saltpetre to crystallize out of a solution when cooling, and the impurities to remain behind.
Sulphur as an ingredient of powder requires little notice. Though we are supplied principally by Sicily and the volcanic districts of the Mediterranean, we are by no means dependent on them, as the element can be easily extracted from the iron and copper pyrites which are found abundantly in this country. The foreign sulphur contains from three to four per cent. earthy impurities, and the finest, known as 'Lercara firsts,' fetches at present about 77. per ton in the London market. The private powder-makers generally get rid of the impurities by a simple melting; at the Royal powder factory a more expensive and tedious process of distillation is followed. As an ingredient of powder, sulphur is chiefly valuable on account of the low temperature at which it inflames, thus facilitating ignition and accelerating combustion.
Charcoal, though chemically a simple substance, is the ingredient which is least understood. This may arise from the fact that it has been little studied by chemists at home or abroad. A good paper on charcoal as an ingredient of gunpowder has yet to be written. There is little doubt that the greater part of the unexplained anomalies in powder-making depend more or less directly on the charcoal, the fitness of which for the purpose depends mainly on the method adopted for obtaining it. Chemical analysis fails to answer the question why one kind of wood affords a better charcoal for powder than another. So does microscopical examination. All that can be said is that the lighter woods generally yield lighter and more combustible charcoals. And yet the dogwood or wild cornel-tree, which manufacturers say makes the strongest of all powders, and which is exclusively used for the fine powder employed with our breech-loading firearms, is a dense, comparatively heavy, slow-growing wood. In the elaborate researches of Bunsen, Schischkoff, and Von Karolyi, respecting the products of the combustion of gunpowder, this question of different descriptions of charcoal seems to have been entirely overlooked, and unfortunately it appears to be one not easy of solution. For even the charcoal of a single species of wood is found to vary, not only in density, but in chemical composition, with the temperature at which it has been produced, thus greatly complicating the question..
Alder, willow, and dogwood are the only woods used in the Government establishment in this country-the two former for cannon powder, the latter exclusively for that intended for small arms. Private makers use the same, generally employing the dogwood for the finest sporting powder; and using also other cheaper woods for common blasting powder. Though all three woods grow well in England, the greater part of the Government contracts are supplied by Belgium and Holland, Sussex, however, yielding large quantities of fine alder and willow. The wood is felled in the spring of the year, cut into lengths of three feet, and peeled, in which state it is delivered by the contractors. Dogwood, which is cut when small, is made up into long bundles, and is worth from 127. to 157. per ton; alder and willow costing about a third of the price. There has been a great increase in the prices of woods of late years in consequence of a greater demand on the Continent.
The art of combining the three ingredients and preparing from them powders of various sized grains of different degrees of hardness and polish, is very much as it was left to us by the fertile genius of Sir William Congreve. Various modifications in the machinery used have from time to time been proposed, and of late years several patents have been taken out for improved methods of
effecting the incorporation, but as a rule the routine of manufacture is the same every where as it has been for the last fifty years.
The ingredients are weighed out and mixed in the proportions required; the saltpetre moist, as it comes from the refinery, the sulphur and charcoal in a state of fine powder, the former having been ground under iron rollers, the latter in a species of mill resembling a large coffee-mill. To convert the mixture into powder, a long and careful grinding or incorporation under heavy runners' of iron or stone is necessary. The mere mixture, at first termed 'green charge,' differs from gunpowder in being not nearly so easily ignited, and in being much slower in burning. It is the slowness of combustion which renders accidents in the mixing-house more terrible than explosion in any of the subsequent processes. The slow and lasting flame produced burns into the bone, instead of scorching and dashing its victims to pieces, as finished gunpowder does. The difference in effects may be well illustrated by burning a little powder and an equal quantity of 'green charge' on a glass plate. The former flashes off, leaving no residue, and doing the glass no injury; while the latter will coat it with portions of half-burnt saltpetre and brimstone, and shiver it to pieces. Fortunately, accidents to the mixers are comparatively rare. The last on record took place at Messrs. Hall's Works at Faversham in 1867 and resulted in the death of four men, whose bodies when recovered were, according to a newspaper account, black, and charred, and horribly disfigured.' No cause was assigned for the accident, and it is impossible even to guess at one where such care and precaution are exercised as are usual in all large gunpowder factories. But the lesson taught by such accidents is plainly this, that the danger of powder-making begins at the beginning; and that from the very commencement too great care and precaution, even in preparing the ingredients, cannot be exercised.
A visitor to any of the great establishments will be struck with the apparently needless precautions which are observed. From the buildings in which the ingredients are refined, to the magazines where the finished product is stored, the same care and vigilance are exercised to guard against the accidental introduction of any fragment of iron, or stone, or sand, however minute. More obvious dangers, such as lucifer matches, cigar lights, &c., it is presumed are guarded against by an examination of the clothes of every person who is allowed to enter the factory. The danger attending the introduction of fragments of iron or gritty particles is the risk of their getting under a workman's foot on the floor, or amongst any part of the machinery, and so causing a spark, which would at once ignite
the clouds of powder dust with which everything, even the workmen's clothes, are saturated. Hence the floors of all powder buildings are covered with leather, fastened down with copper tacks, and kept constantly moist; and no one is permitted to enter or set foot in one till he has donned large magazine shoes made entirely of leather, which are never allowed to be taken out of the door.
The incorporation or milling of the green charge is the principal operation in powder-making; indeed, it is powdermaking. The charge goes to the mill a mere mixture, and leaves it gunpowder. Nothing that it afterwards undergoes adds to its strength or explosiveness; the succeeding operations are merely intended to make it into the most convenient form for use, storeage, and transport. The milling is done by subjecting the mixture to the action of two large iron edge-runners, weighing about four tons each, working round a perpendicular spindle on a cast-iron bed. In the older mills the edge-runners and bed are made of black Derbyshire marble. By the Act of Parliament the charge must not exceed 50 lbs. in weight for fine, and 60 lbs. for blasting powder, so great is the risk of accident in the trituration and pressure to which the powder is subjected. The millman occasionally enters the mill to moisten the charge with distilled water, and to rake it up from time to time, while two wooden ploughs' fixed to the runners keep the composition from working away from under them. The amount of water added from time to time is very slight, only enough to prevent the charge flying off as dust, but not enough to dissolve and crystallise the saltpetre, which would destroy all the incorporation that had been effected, and certainly not enough to retard in any way the explosions which so often happen. The time of incorporation varies. It is found that in a few hours, generally from three to five, according to the weight and speed of revolution of the runners, a thorough incorporation is effected, and that the resulting powder will not be benefited in strength by continuing the process, though it may be to a slight extent improved in quality, leaving less residue on combustion, a point of great importance with our marksmen and sportsmen as regards the fouling of their pieces. Hence fine sporting powder is sometimes milled for twelve hours; that used with our Enfields and Sniders not as much as half of this time. Cheap blastingpowder, and the stuff which is exported for the Africans and Chinese, in addition to having a less proportion of saltpetre, receive hardly any milling at all. That time is money, is peculiarly applicable to powder-making, for with a limited number of mills, and being by law only allowed to work a certain weight of charge in each, a manufacturer can only produce a limited quantity.
The process of incorporation is far the most dangerous of all the operations connected with gunpowder. In the mills alone may explosions be expected; in all other processes they are the exception, and fortunately few and far between. Hence the restriction as to the amount of mill charges. But the accidents are rarely attended with fatal injuries. The mills being selfacting, do not require constant watching; so the millmen only enter them occasionally, to moisten the charge or rake it up from the bed; consequently, the chances of their being caught by an explosion are small.
The structure of the buildings is the lightest possible, the ends being of brick, the roof and sides of thin boarding, so that when a charge explodes under the runners, the force of the blow' is expended on the boarding instead of being confined and damaging the machinery.
On the thorough and effectual milling which it receives depends a great deal of the excellence of English powder, and no other method of incorporation has been devised which gives equally good results. The use of incorporating mills is becoming general in foreign countries, and gradually superseding the French Moulins à pilons' and Moulins à tonneaux,' in the former of which the composition is stamped under a number of large pestles, and in the latter shaken up in barrels along with a number of metal balls, and so rudely incorporated. These processes are neither so effectual nor so expeditious as the English process, and it may safely be asserted that it is impossible to produce good gunpowder without having recourse to the incorporating mill. Gunpowder being not a chemical compound, but a mere mechanical mixture, it follows that the more intimate and thorough the mixture is the better will be the powder produced; and it may be taken for granted that nothing but a mechanical operation will suffice to effect the thorough mixing required.
The powder leaves the mill in a state partly of soft cake, known technically as mill-cake,' and partly of dust. In this state, though perhaps as fiercely explosive as at any other, it is not fit for the use of the artillerist or sportsman. In addition to being in an inconvenient form for use, the presence of a large quantity of dust renders it peculiarly liable to attract moisture. The first thing to be done with it, then, is to compress it into hard cakes in the hydraulic press. To effect this, it is placed between gun-metal or copper-plates, in layers about half an inch thick, packed in a strong massive box of wood, lined with bronze, and subjected to a pressure of 70 tons on the square foot, becoming thus converted into 'press-cake,' a hard compact cake resembling slate in appearance. If small-arm powder of dogwood charcoal is being made, it receives only Vol. 125.-No. 249.