Gunpowder is a well-known explosive mixture composed of saltpetre, charcoal, and sulphur mixed together in certain proportions, somewhat varying in different countries and in different descriptions of powder.
The early history of gunpowder is very obscure; but there appears to be little doubt that the explosive nature of saltpetre (the great bulk of which comes either from India or China) when mixed with charcoal or carbon was known to the Chinese for many centuries before the Christian era. It may be assumed that the discovery of this property of saltpetre was accidental: a wood-fire lighted on the earth where saltpetre was mixed with the soil would bring the two ingredients together, and the action of the heat would be sufficient to show the nature or property of the mixture so brought about when raised to a certain temperature. It is certain that fireworks were known in China from very early periods; but in a pamphlet written by Colonel Omodei (Turin, 1834), and later in an article in the Athenæum of December 26, 1868, by Captain (now Lieut.-General) Henry Brackenbury, R.A., the question as to the first invention of gunpowder was fully discussed, and the conclusion arrived at was that there is great reason to doubt whether either the Chinese or any other Asiatic people invented gunpowder in its true sense, or were the first to use it as a propelling agent. It was left for more western nations to develop the discovery of the Chinese, and our first knowledge of the use of gunpowder as a military agent dates from the 7th century, when it was used by the Byzantine emperors, under the name of Greek Fire (q.v.), in the defence of Constantinople against the Saracens, who, discovering the secret of its manufacture, used it against the Crusaders, not however as a propelling agent, but in the form of rockets or liquid fire. Its first use in Europe as a propelling agent was in Spain, where both the Moors and Christians used some description of artillery as early as the 12th century. Roger Bacon first introduced it into England. Whether he discovered it independently of foreign aid, or whether he conceived the idea from ancient manuscripts, is uncertain; but the latter is the more probable, as the name first given to it was crake, presumably a corruption of the word grece. Bacon's discovery dates from a period early in the 13th century, but, owing to the crude and uncertain means adopted for mixing the ingredients, it was of no practical value till the German monk, Berthold Schwarz, introduced, somewhere about the year 1320, a method of manufacture by which the ingredients were thoroughly incorporated; the meal powder thus made was first used in England as a propelling agent by Edward III. in his war against the Scotch in 1327, the tubes from which he propelled the shot being called crakeys of war. The same king subsequently used cannon at the battle of Crécy in 1346. From that date the use of gunpowder throughout Europe soon became general, the Russians, who in 1889 celebrated the 500th anniversary of its introduction into Russia, being the last to adopt it. Until the reign of Queen Elizabeth by far the larger quantity of powder required by the English was obtained from abroad; but in her reign its manufacture was introduced into England. The earliest English powder-mills of which there is any record were established at Long Ditton and Godstone, in Surrey, by George Evelyn (John Evelyn's grandfather) in 1590; the Faversham mills were started soon after this date, as were also those at Waltham Abbey. The mills at Faversham subsequently became the government powder-factory, and in 1787 the government also bought the mills at Waltham Abbey, which have remained in its hands down to this day. The Faversham mills were given up by the government after the peace of 1815; they were soon after bought by Messrs John Hall & Son, who still retain them.
The mode of manufacture adopted in England when these several powder-mills were all thoroughly established remained practically unchanged up to within the last thirty-five years. But before proceeding with a description of the manufacture and of the different powders now in use, it will be desirable to consider very briefly the part played by the several ingredients of which gunpowder is composed, and the chemical action which takes place on ignition. The saltpetre or nitrate of potash, , acts as a magazine of oxygen, with which it readily parts when raised to a certain temperature. When the powder is fired, the oxygen of the saltpetre converts most of the carbon of the charcoal into carbonic acid, , a portion of which combines with the potash of the nitre to form carbonate of potash, , the remainder existing in the state of gas, and the nitrogen is liberated. The sulphur, which performs the part of a second combustible in gunpowder, is for the most part converted into sulphuric acid, , and forms sulphate of potash. The reaction on firing the gunpowder may be expressed by the equation . The heat generated by the explosion evolves a large quantity of elastic gases, the expansive power of which is greatly increased by the heat. The pressure being equal in all directions, the work done on the projectile in the bore of the gun is due to this elasticity and expansive force. The method adopted for measuring the amount of pressure in the bore of the gun will be dealt with hereafter.
Sir Frederick Abel, K.C.B., F.R.S., and Sir Andrew Noble, K.C.B., carried out two series of most exhaustive and complete experiments on fired gunpowder, and the conclusions they arrived at were communicated by them to the Royal Society in two papers (1875-80) under the head of 'Researches on fired Gunpowder.' The results are summarised as follows: when fired in a confined space (1) the products of combustion are about 57 per cent. by weight of ultimately solid matter and 43 per cent. of permanent gases; (2) the permanent gases occupy about 280 times the volume of the original powder; (3) the tension of the products of combustion when the powder entirely fills the space in which it is fired is about 6400 atmospheres, or 42 tons per square inch; (4) the temperature of explosion is about .; (5) the chief gaseous products are carbonic acid, nitrogen, and carbonic oxide; (6) the solid residue is mainly composed of potassium carbonate, sulphide, and sulphate.
From the foregoing description of the part played by the nitrate of potash it might be thought that it would be highly advantageous to make gunpowder with some nitrate containing a larger percentage of oxygen than nitrate of potassium; and as a matter of fact there are a large number of nitrate mixtures other than gunpowder in which nitrate of sodium, barium, or ammonium are substituted for the nitrate of potassium. But unfortunately they are extremely hygroscopic, so that gunpowder made with them would, under ordinary circumstances, soon become useless on account of the damp it would absorb from the atmosphere. In a hot dry climate nitrate of soda powders would doubtless be valuable, besides being much cheaper to manufacture than nitrate of potash powders: indeed such powders were used to a considerable extent in the construction of the Suez Canal; but, as these powders are not in general use, it is unnecessary to refer to them further.
Process of Manufacture.—The method of manufacture of gunpowder at the Royal Gunpowder Factory at Waltham Abbey (fully explained in the official handbook) may be briefly described. As in all other explosives, it is essential that the ingredients of which the powder is composed should be as pure as possible. The selection and preparation of the charcoal is of the greatest importance; for, without any change in the proportions of the components, the properties of the gunpowder are capable of great variation from the quality of the charcoal used in its manufacture. The ingredients are first reduced to a fine powder by grinding. They are then mixed by hand in the proportion of 75 per cent. by weight of saltpetre, 15 of charcoal, and 10 of sulphur, and are next thoroughly incorporated in a wet state in a powder-mill into a cake called a mill-cake. This cake is then broken down between copper-plates into meal. From this meal-powder all granulated powders are made. The meal is compressed in a press-box, the amount of compression it undergoes being dependent on the density of powder required. After compression the press-cake is broken into pieces ready for granulating, which is done in the granulating machine, the powder passing between gun-metal rollers till it is broken into grains of the required size, different powders being made to pass through sieves whose meshes are of the size of the grain required. There is a considerable amount of dust formed by the granulating process, so that after granulating it is necessary to dust the powder previous to glazing it, which is the next operation. It is glazed in glazing drums, which, revolving rapidly, impart a glaze to the powder simply by the friction set up. The powder is now stoved or dried in copper-trays in a drying room, which is heated to a temperature of about ., and the powder is left in this room from one to two hours according to the amount of moisture that it contains. Formerly all powder was granulated, but the enormous increase in the size of the guns now used necessitated the introduction of other descriptions of powder—viz. cut and moulded powders. In the cut powders, after the process of pressing, the press-cake, instead of being granulated, is first cut into strips, and these strips are then cut into cubes, and the powder so made is called cubical or pebble powder; there are at present two sizes—viz. -inch and -inch cubes. In the moulded powders, as is implied in the name, each grain or piece of powder is moulded or pressed in a separate mould. This is done in a hydraulic machine. The exact quantity of granulated powder required to form each prism is deposited in a block containing sixty-four moulds; the powder in these moulds is then pressed by plungers exactly fitting the moulds till the required density is obtained. This powder is called prismatic powder, the grains or pellets being in the form of hexagonal prisms nearly 1 inch in height and about inch across, with a hole in the middle about th of an inch in diameter, the object of which will be explained later on. There are two descriptions of prismatic powder—black and brown. The proportions of ingredients in the black powder are the same as in all other English military powders; but in the brown powder the charcoal is made from straw, instead of from wood, and the proportion of the ingredients are as follows: viz. saltpetre 79, charcoal 18, and sulphur 3 per cent.
Gunpowder more nearly fulfils the objects required in a propelling agent than any other explosive hitherto discovered. These objects are (1) a maximum muzzle velocity with even and low pressures; (2) uniformity of action, so that the same results may always be expected; (3) freedom from fouling; (4) durability—i.e. that it is not liable to injury in transport, and that its condition does not materially alter when stored.
The advantages of gunpowder over other explosives are (1) that, the rate of combustion being gradual, the explosion is not so severe on the bore of the gun as in the case of more violent explosives; (2) the ingredients of which it is composed are easily produced and are cheap; (3) it is, with proper precautions, safe in manufacture, in store, or in transport. Experiments made by the Explosives Committee have shown that any alteration in the proportions of the ingredients has not so great an influence on the 'explosiveness' or rate of burning as the density, hardness, size, and shape of grain and amount of glaze. The density of grain has an important effect on the rate of burning. By absolute density is meant the amount of powder actually present in a certain bulk—i.e. if different quantities of meal-powder, containing an equal amount of moisture, be pressed into cakes of the same size, that which contains the most meal will be densest; then, if these cakes be ignited simultaneously, the cake which has the least density will be the first to be completely burnt. Hardness does not depend on density; increased hardness is given by pressing the meal in a moister condition. Size and shape of grain are also important characteristics to be considered in connection with the explosiveness of powder. Other things being equal, a larger-grained powder burns slower than a smaller-grained; and in grain of equal weight that which has the largest surface will burn the quickest. A highly-glazed powder, again, burns slower than an unglazed one, probably because the glaze somewhat retards its thorough ignition. The temperature at which powder ignites varies from to F., according to the nature of the powder, the finest sporting powder igniting at the higher temperature.
It is only of late years that all these points have received the attention they deserve, but they have been forced upon the authorities by the necessities of the times. As soon as ships began to be plated with armour, guns had to be made which could throw a projectile capable of piercing that armour; and as the armour increased in thickness so did the guns increase in size and power, throwing heavier projectiles, which necessitates an enormous powder charge to propel the shot. In former days there was a beautiful simplicity about powder. Practically there were but two kinds, one for muskets called fine grain or F.G., and the other for cannon called large grain or L.G., and no particular attention was paid to the quality; it was certainly not subjected to the searching proofs and tests which all powders made in these days have to undergo.
The first improvement in powder took place on the introduction of rifled arms, when a rifle fine-grain powder or R.F.G., slightly larger in the grain than F.G., and a rifle large-grain powder or R.L.G., the grains of which were about twice the size of those of L.G., were introduced for small-arms and cannon respectively. The R.F.G. powder was improved and made of a rather smaller grain, the size of grain being from th to th of an inch, on the introduction of the Martini-Henry rifle, and this powder, known as R.F.G.2, has not since been altered. But in cannon powder, or powder for large guns, the development has been great and continuous. When R.L.G. powder was introduced it was the largest grained and slowest burning powder then in existence in England; but as the guns were made larger and larger it became necessary to use a slower-burning powder, which led first to the introduction of R.L.G.2, having grains varying in size from 3 to 6 to the inch; that is, the grains must pass through a sieve of 3 meshes to the inch, and must not pass through one of 6 meshes to the inch. A short time afterwards a still larger powder called R.L.G.4, with grains of nearly half an inch in size, was introduced. This was soon followed by pebble powder, a still further development of R.L.G., for guns of large calibre for which R.L.G.4 was not suitable. The first pattern of pebble or P. powder was cut in cubes of about half an inch in size, and a second pattern or P.2 was made in cubes of about inch in size, with rounded edges. Both these powders have a density of 1.75, whereas that of the R.L.G. powders is 1.65. Next in order came the prismatic powders, of which there are two descriptions—viz. prismatic black or Prism1 and prismatic brown or Prism1 brown, a description of which has been given under the head of manufacture. There are two other powders of a special pattern which may be mentioned—viz. M.G.1 which is used only in the 1-inch Nordenfelt machine gun, the size of grain of which is considerably larger and of more even size than that of R.F.G.2; and Q.F.1, a powder which at present is used only in the 3- and 6-pounder quick-firing guns, the size of grains being about half an inch square by about of an inch thick.
The powders of other nations differ but slightly from those manufactured in England; the method of manufacture is the same in principle, but the proportions of the ingredients vary to a slight degree in every nation.
Having now described the various powders in use, it remains to say a few words about velocities and pressures. The great desideratum with all firearms is to obtain the maximum velocity with the minimum of pressure, and in the experiments and investigations carried out by the committee on explosives, and by Sir F. Abel and Sir A. Noble, this end was kept in view. It has already been pointed out that the rate at which powder burns depends greatly on the density, hardness, size, and shape of grain: the greater the density the slower it will burn; the larger the grain the slower it will burn, simply because the amount of lighting surface is reduced in proportion to the volume; and the smoother the surface of the grain the slower it will burn for the same reason. A powder therefore composed of, comparatively speaking, small grains of irregular size and shape, burns very rapidly, and generates a large volume of gas suddenly, thus setting up a very high pressure in the bore of the gun. In some of the experiments of the committee the pressure recorded in the bore of the gun was as high as 60 tons to the square inch; but, as Captain Noble had previously discovered that the maximum pressure of powder fired in a confined space did not exceed 42 tons, this extra pressure could only be due to wave-action, a sudden evolution of gas locally causing a vibratory motion of the gas. This led to the introduction of larger-grained powders, so as to retard the burning of the charge, but this did not entirely do away with the wave pressure. It was next sought to diminish the pressure by giving air-space to the charge, as it was found that the density of the charge—not the density of the grain—materially affected the pressure; this air-space was given by enlarging the chamber of the gun, and although it became necessary to increase the charge so as to maintain the same muzzle velocity, it was found that the pressure was sensibly diminished.
But whilst Great Britain was still adhering to pebble powder, other nations had adopted a prismatic powder with a hole through the centre of the pellets or prisms. The idea of this perforated powder is due to General Rodman, an American artillery officer, who thought that by this means a charge of powder would evolve the gas at a more uniform rate. In a charge composed of solid grains, the grains being lighted on the surface and burning towards the centre, the surface giving off gas rapidly decreases, and therefore the volume of gas given off by the grain will be greatest when the charge is first fired, and will rapidly fall off; hence the maximum pressure on the bore of the gun is set up almost before the projectile commences to move. To overcome this, the idea occurred to General Rodman to have a hole through each large grain or pellet of powder, so as to give the grain an interior as well as an exterior lighting surface, so that as the exterior lighting surface decreased the interior lighting surface increased, thus preserving the ignited surface more uniform during the burning of the pellet, and therefore keeping up a more constant evolution of gas. This tends to distribute the pressure more uniformly along the bore of the gun, and increases the initial velocity of the projectile. The Russians in 1860 were the first to adopt this plan, followed by the Germans, the English being the last to take it up; it is now, however, adopted for the heaviest guns in the English service, the charge being built up of the hexagonal prisms already described, so arranged that the holes through each column of prisms shall be continuous from one end of the charge to the other. The introduction of this powder has enabled the type of gun to be entirely altered; a description of the guns now in the service will be found under the head of Cannon, Rifles (q.v.). We are indebted to the Germans for the invention of brown or cocoa powder; it is claimed for this powder that it gives a higher initial velocity with less pressure than the same charge of black powder.
The same principles as have been enunciated here with regard to guns should govern the selection of powder for mining or blasting purposes. If it is desired to shatter a mass of rock, a very violent quick-burning powder, having a great disruptive force, should be used; but if it is only desired to dislodge, say, a mass of coal without shattering it, a denser, slower-burning powder should be chosen.
The instrument generally used for taking velocities is a chronograph, the invention of Major De Boulengé of the Belgian Artillery. It consists of a brass column supporting two electro-magnets. No. 1 electro-magnet supports a long cylindrical rod, called the chronometer, covered by a zinc tube; No. 2 electro-magnet supports a shorter rod. Two screens of copper wire are placed at certain fixed distances in front of the muzzle of the gun. No. 1 electro-magnet is magnetised by the current passing through the first screen, and No. 2 by the current passing through the second screen. As the shot passes through the first screen the current is broken, and the rod or chronometer suspended by No. 1 electro-magnet falls by gravity. Similarly, when the shot passes through the second screen, the shorter weight suspended by No. 2 electro-magnet falls on to a disc, which, pressing a spring, causes a knife to be released, and this, darting forward, strikes the chronometer in its fall, making an indent in the zinc tube. The distance of this indent from the zero point being measured off on a scale specially graduated for the instrument gives the velocity of the shot between the two screens, from which can be calculated the muzzle velocity. Another scale gives the time of flight.
The pressures in the bore of the gun are calculated by means of a crusher gauge. A small copper cylinder is inserted in the gauge, which is screwed into the gun at that part where it may be desired to measure the pressure. The copper cylinder is measured before and after the discharge of the gun, the amount by which it is shortened by the force of the explosion being the measure of the force, or pressure, exerted. The actual pressure in tons to the square inch is calculated by means of a mathematical table prepared for different coppers.
See Wardell, Gunpowder and Gun-cotton (1889); Cundill, Dictionary of Explosives (1895); Eissler, Modern Explosives (1889); Guttmann, Manufacture of Explosives (1895); Sanford, Nitro-Explosives (1896). For Smokeless Gunpowder, see GUN-COTTON, RIFLES; and for laws relating to gunpowder, see EXPLOSIVES.