CHAPTER II EXPLOSIVE MIXTURES GUNPOWDER Gunpowder. Gunpowder may be taken as a representative explosive mixture. It is a very intimate mixture of potassium nitrate (saltpeter or niter), sulphur, and charcoal. Although these substances do not act upon each other at the ordinary temperature, when thoroughly mixed and heated they are momentarily dissociated (separated), the elements immediately rearranging themselves as new products which are largely in the form of highly heated gas. Ingredients of Gunpowder.-The ingredients of gunpowder are easily obtained and in large quantities, saltpeter and sulphur occurring naturally, the one mixed in the soil of certain countries, notably India, the other in caves and the vicinity of volcanoes, while charcoal is merely the residue of charred wood. Proportions of the Ingredients.-The three ingredients of gunpowder may be mixed in greatly varying proportions and each mixture will be explosive, but, for ordinary service gunpowder, experience has shown that a powder containing— 75 parts Saltpeter 10 parts 15 parts is the best, and until recently the majority of military nations adopted it. The large proportion of sulphur used in the earlier powders 1 Gunpowder and its manufacture are described because certain of these powders are still retained in the service. is no longer necessary since the introduction1 of percussion caps, friction primers, etc.; and since when present in large quantities it introduces in the powder certain disadvantages, the percentage of sulphur in modern gunpowders is reduced as much as possible, being only from 2 to 3 per cent. in the cocoa powders, which are decidedly the best for guns of large caliber. In order to secure uniform results and safety during the process of manufacture, the ingredients, before being mixed, are separately pulverized. The saltpeter, if used immediately after being purified, is sufficiently fine and requires no further reduction; but if it has been stored and become caked, it, like the sulphur, is ground to a very fine powder in a machine similar to an ordinary "mortar-mill." Because it is very porous and quickly absorbs moisture, the charcoal is not stored in large quantities or for any length of time, but is prepared about two weeks before it is required for use, when it is ground to powder in a machine resembling a large "coffee-mill," and then stored in air-tight metal boxes. Manufacture of Gunpowder. The ingredients are now ready for the manufacture of the powder, which consists of the following processes: 1. Mixing the ingredients.-A 50-pound charge is carefully weighed in the proper proportions and placed in a gun-metal or copper barrel (or drum), through the center of which passes an axle to which are attached several fork-shaped arms, also made of gun metal. When in operation the barrel and axle carrying the arms revolve in opposite directions, and at the end of five minutes the charge is thoroughly mixed. 2. Incorporating or "milling."-The charge is next uniformly spread in the "incorporating mill" and slightly moistened with 1 In gunpowder, saltpeter acts as the oxidizer and charcoal as the combustible, while the sulphur was originally added to lower the point of ignition, although it also served to increase the amount of heat produced and to further expand the gases. 1 water, and subjected to continued grinding under heavy rollers. The product is known as "mill-cake." 3. Breaking down the "mill-cake."-After removal from the incorporating mill, the mill-cake is "broken down," or reduced to "powder meal," by being passed through two pairs of gunmetal toothed rollers. 4. Pressing. The powder meal is next placed in the "press box," where it is compressed into hard slabs or sheets. Next to incorporation, pressing is the most important step in making gunpowder. The principal advantages obtained by pressing are: first, the slabs or sheets when made into grains of the required size absorb less moisture from the air; second, the lasting qualities of the powder are greatly increased; third, the powder is less liable to be reduced to powder in transportation; and, finally, it supplements the object sought in incorporating, inasmuch as by it the ingredients are brought into a closer union, thereby producing greater uniformity in the grain. The effect of pressing upon the density of the powder can not be overestimated and will be referred to again. 5. Granulating.-The slabs or sheets as they come from the press box are known as press cake, and are passed to the granulating machine, which is similar to the breaking-down machine, consisting essentially of three or four pairs of gunmetal toothed rollers, the size of the teeth of which vary according to the size of grain required. 6. Dusting. The granulated powder is next passed through revolving reels covered with canvas cloth in which the dust formed during the last step is removed. 7. Glazing. As a general rule, all modern military powders are glazed. This is done by introducing the charge of granulated powder in oaken barrels containing small quantities of graphite or plumbago (about one-half ounce of graphite to 1 This is done for the threefold purpose of preventing powder-dust from flying about, facilitating the incorporation, and reducing the effects of an explosion in case of an accident. one hundred pounds of powder), and causing the barrels to revolve rapidly. At the end of six hours the grains will have acquired a fine gloss, while all sharp angles and corners will have been rounded off. The object of glazing is to diminish the formation of dust during transportation and to render the powder less hygroscopic (that is, less liable to absorb moisture from the air). Properties of Gunpowder.-Good gunpowder should be composed of hard angular grains which do not soil the fingers when handled, and have a perfectly uniform dark-gray color. The grains when broken should present a clean fracture, homogeneous in appearance, without any visible specks of saltpeter or sulphur, and of a dark grayish or brownish color, according to the kind of charcoal used. When new it should be free from dust, and a small quantity flashed upon a porcelain or copper plate should leave no residue or foulness. It should not absorb more than from 0.5 to 1.5 per cent. of water when exposed to air of average dryness. The grains should be sufficiently hard to stand transportation without being broken. The property which exercises the greatest influence upon the general character and action of gunpowder is its density,1 which should vary between the limits of 1.60 and 1.85 according to the kind of powder. 1 Density must not be confounded with hardness, which seems to bear a direct relation to the pressure exerted in compression ("pressing"). Although a very high density can not be obtained without producing a considerable degree of hardness, still a powder may be very hard without being very dense: For example, "powder meal" containing 6 per cent. of water can be made very dense by the application of a moderate pressure, while that containing 1 per cent. of water can be brought to the same degree of density only by the exertion 1 Density is the ratio which the weight of a given volume of the substance (in this case, powder) bears to the weight of an equal volume of distilled water at 60° F. of enormous force. Of the resulting powders the latter will be the harder. No experimental proof is necessary to show that if two grains of powder of equal size, one of which is twice as dense as the other, be ignited in the open air the denser will take longer to burn completely; for the former not only has a closer and less porous texture of grain, but contains, bulk for bulk, a larger amount of matter to be burned from the same surface. It is evident, therefore, that the density of the powder, which can be varied at will, must be its most important physical quality, or property. Recapitulation.-Gunpowder may be taken as the representative of the explosive mixtures, and consists of saltpeter (niter), sulphur, and charcoal. The several steps in the manufacture of gunpowder and their objects are: (1) Mixing the ingredients. (2) Incorporating the ingredients, to bring the pulverized ingredients into such intimate contact that each particle of the powder shall contain, if possible, a particle of each ingredient. (3) Breaking down the "mill-cake," so that it can be introduced into the press box. (4) Pressing, to give strength and density1 to the powder. (5) Granulating, to regulate the surface of combustion. (6) Dusting, to prevent the absorption of moisture, and to insure uniformity of combustion. (7) Glazing, to diminish the formation of dust during transportation, and also to protect the grains from the action of the moisture of the air. Of the properties of gunpowder enumerated, the most important are its density and that of being able to resist the action of the moisture of the air. 1 The density is also affected by the kind of charcoal used and the amount of water used to moisten the ingredients before being introduced into the press box. |