EXPLOSIVES are substances capable of developing a sud den pressure on their surroundings, this development of pressure being caused by the rapid conversion of the explosive substance into gases having a much greater volume than the original ex plosive, and to the further expansion of these gases by the simul taneous generation of heat. Explosives may consist of gases, liquids, or solids; for example, the explosive mixtures of gas or petrol vapour with air in the internal-combustion engine, and nitroglycerin, and gunpowder are familiar cases of these three classes respectively.
The starting-point in the story of explosives is the discovery by Friar Roger Bacon of the process of purifying a salt by crystalli zation from aqueous solution. By applying this process to the purification of naturally occurring saline deposits, Bacon obtained, for the first time, pure nitre or potassium nitrate, and one may imagine that with this pure nitre he attempted to imitate one of the "fiery" or "incendiary" compositions which had long been known and used. Instead of a smouldering and fuming action, the result was the first artificial explosion. As Col. Hime states (Gunpowder and Ammunition, 1924), there is no evidence of the knowledge or existence of a real explosive before Bacon's experi ments. Fuming, fiery or flaming compositions, such as sulphur and various kinds of natural pitches or bitumens, were not diffi cult to obtain, and these along with charcoal were easily made into incendiaries. The addition of the natural nitre, patron, etc., can easily be accounted for by a happy accident. Col. Hime, after a thorough examination of all the available evidence, concludes that Roger Bacon was the first maker of the "fundamental" mixture leading to gunpowder. For a surprisingly long time gunpowder (of sorts) was practically the only known explosive.
In the last three-quarters of a century many explosives have been invented or discovered. These modern explosives vary widely in composition, chemical constitution and the kind of explosion they produce. It was not until the i 9th century was well advanced that gunpowder had a serious rival either for military or civil employment. Slight modifications in its composition were made, more by accident than by calculation, in order to render it more effective and local in action as a blasting agent for mining. This result was attained mainly by increasing the proportion of sulphur at the expense of the charcoal. Still later, military modifications, known as "brown" powders, were introduced for special uses, in which most of the charcoal was substituted by a "caked cellulose," whilst the proportion of sulphur was reduced to one-fif th and the nitre increased. Speaking generally, the era of new explosive sub stances started soon of ter the discovery of the compounds col lectively termed gun-cotton or nitrocellulose and nitroglycerine, although fulminating silver and a few similar substances may have been known at a prior date. Gun-cotton and nitroglycerine may be considered as commencing the series of explosives derived from nitric acid in a more or less direct manner. Later, other sub stances, such as picric acid (or lyddite), were discovered, more or less accidentally, to be explosive under certain special conditions, and following on this discovery it was found that analogous sub stances (nitro-compounds) of a slightly simpler type of constitu tion than picric acid could also be induced to explode, especially when submitted to violent or sharp percussion, a mode of explo sion distinguished as detonation. With more complex nitrogen compounds such as the hydrazines, and, later still, the azides, there have been valuable additions to the list of compounds showing in various degrees the detonating form of explosion.
Careful study of the heat production or absorption in chemical actions of combination or decomposition places practically all chemical compounds in one of two classes : (I) exothermic com pounds, which give out heat energy during the process of forma tion, and (2) endothermic compounds, which can be formed only by having energy, as heat or in some other form, expended on them during the process of formation. Chalk, water and sand are good examples of exothermic compounds, since much energy— mostly as heat—was dissipated during the formation of these sub stances, and therefore they are not very active in taking part in chemical actions. Consequently they cannot, with advantage, be employed in explosive compounds or mixtures. On the other hand many explosive compounds are endothermic and, on explosion, liberate a part of their stored-up energy. The essential properties of any explosive are that it should be capable of (a) complete or nearly complete conversion into gaseous products, (b) generation of heat; (c) rapid and simultaneous production of gas and heat. The formation of gases in the products of explosion is usually due to a process akin to combustion or burning, in which the combustible parts of the explosive are oxidized or burned to gaseous oxides such as carbon monoxide, carbon dioxide and steam. This process is called "internal burning" and differs from ordinary burning in that the oxygen required is not derived from external sources, but is already contained in the explosive in some readily available form. The production of heat is due either to the exothermic nature of the products of internal burning, or, in some cases, to the breakdown of an unstable endothermic com pound into its elements.
The speed at which explosive action takes place may vary con siderably with different substances, and with the conditions under which explosion is initiated, the variation extending from a few metres per second to several thousand metres per second. Explo sions at lower speeds may be utilized as sources of power in the internal-combustion engine, where the pressure of explosion is developed sufficiently slowly to move a piston against an external load. Similarly, in small arms or ordnance, the pressure of explo sion of the "propellent" charge or cartridge is utilized to drive the bullet or heave the shell along the bore of the weapon. Explosions having speeds of several thousand metres per second have a dis ruptive or shattering effect (Fr. brisance) on their surroundings, the duration of the pressure being so short that it cannot be used to produce a propulsive or heaving effect. Explosive effects of this order are called detonations and are made use of in high explosives for military purposes, also for many blasting explosives used in mines and quarries.
Classification may also be based on chemical considerations, i.e., according as the explosives are simple chemical compounds or mixtures. Within the first class are carbon compounds containing combined oxygen which, on explosion, oxidizes the combustible elements, carbon and hydrogen, to form the gases of explosion. These explosive compounds are formed by the action of nitric acid on various organic materials, and are usually nitric esters or nitro compounds. The combined nitrogen present in these forms is on explosion mainly liberated as elemental gaseous nitrogen. Included in the second class of explosives are those which, being sensitive, endothermic substances, readily break up into their elements with generation of heat, and do not depend on combined oxygen for their explosive effect. Mercury fulminate and lead azide are examples of this class. Explosive mixtures may contain substances in the above-mentioned group together with non-explosive sub stances containing oxygen, or consisting of combustible matter. Such mixtures are of two kinds—those containing explosive com pounds, e.g., cordite, amatol, and those made up of non-explosive materials, e.g., gunpowder.
Blasting explosives are used in engineering and mining opera tions for breaking up rock or minerals. For many years gunpowder was the only explosive available, until Nobel introduced dynamite in 1867, followed by blasting gelatine and gelignite in 1887. These blasting explosives may be classified in two groups : (I) High explosives. (a) Containing nitroglycerine, e.g., dynamite, gelignite, etc. (b) Not containing nitroglycerine, e.g., ammonal, etc. These are fired by a detonator containing mercury fulminate (q.v.) or other suitable initiator. (2) Gunpowders. These are of the black powder type, e.g., bobbinite, a typical black powder explosive con sisting of potassium nitrate 63.5%, charcoal 19.5%, starch 3%, sulphur 2%, paraffin wax 8% and moisture 3%. Typical composi tions of blasting explosives are given in the following tables: cumulative decomposition of the nitrocellulose, finally resulting in explosion. On the basis of these discoveries improved methods of washing the nitrocellulose were introduced and resulted in the manufacture of a safe, stable product.
The chemical action between cellulose and nitric acid may be represented by the equation The proportion of nitrogen in the form of nitrate groups is represented by the variable quantity, x, the value of x depending on the time and temperature of nitration and on the composi tion of the mixture of nitrating acids. The function of the sul phuric acid is to remove water formed in the action and thereby to prevent dilution of the nitric acid. The properties of the nitrocelluloses vary with the percentage of nitrogen they contain (i.e., according to the value of x), particularly in their behaviour towards organic solvents. Nitrocellulose containing 13.o% or more nitrogen content can be detonated by percussion or by the use of a detonator. Their chief properties and uses are summarized in the following table : Of these foregoing groups certain explosives may safely be used for getting coal in fiery mines, and are accordingly on the British list of permitted explosives. All are explosives of the mixture class, containing explosive and non-explosive compounds. Among the latter are inert substances such as chlorides and oxalates which act as coolers in reducing the flame temperature. The individual types of explosive will now be described, but gunpowder and picric acid (lyddite) are dealt with in separate articles under their own headings.
Nitroglycerine and Nitrocellulose.—Nitroglycerine, glyceryl trinitrate or trinitrin was first made by Sobrero in 1846. It is manufactured by spraying glycerol (glycerine) into a mixture of nitric and sulphuric acids; the nitroglycerine separates as an oily liquid and is purified by washing and filtration. Its use as an explosive was suggested by Nobel in 1863, but owing to its sensitive nature, the danger in its transport and use prevented its employment unless mixed with solid materials to form a plastic mass which could be handled with safety. Such mixtures are cordite, dynamites and blasting gelatines. Nitroglycerine, a liquid of density 1.6, exists in two solid forms melting at 2° C and 13° C. When soaked in absorbent paper it burns quietly with a green-edged flame, but when heated it decomposes just before boiling, and finally detonates. It is used medicinally for the treatment of asthma and angina pectoris in the form of a 1 % solution in alcohol.
Nitrocellulose is the name given to the nitric esters formed by the action of nitric acid on cellulosic materials, e.g., purified cotton or wood pulp. Pelouze in 1838 discovered that cotton could be converted into a violently inflammable substance by nitration with nitric acid, and in 1845 Schonbein discovered its value as a high explosive, also improving the method of preparation by the addition of sulphuric acid to the nitric acid used in nitration. Many attempts were made to utilize this substance as an explo sive for civil and military purposes, but owing to a series of serious accidents in manufacture, storage and use, it was aban doned temporarily. Sir Frederick Abel of the Royal Gunpowder Factory, Waltham Abbey, England, discovered subsequently that these dangers were due to the presence in the nitrocellulose of minute traces of acid-forming substances which were not removed by the processes of washing then in use and caused a The manufacture of a smokeless propellent consists in the mechanical incorporation of the nitrocellulose with the other constituents in such a manner as to give a product which is gelatinous or colloid in its nature and free from the porosity characteristic of untreated nitrocellulose. In the case of nitro cellulose propellents, gelatinization is effected by kneading the nitrocellulose with a volatile liquid such as acetone, the product being a dough-like mass which can then be rolled into sheets or pressed through dies to form strips or cords. The volatile solvent is then removed by heating, leaving the nitrocellulose in a horny non-porous condition. With nitroglycerine propellents, gelatini zation can be effected in various ways: (1) Heat treatment. In this method the mixture of wet nitrocellulose with nitroglycerine and any other constituents of the powder is passed between hot rollers which first expel moisture and finally cause absorption of the nitroglycerine by the nitrocellulose to form a translucent mass, e.g., ballistite. In some cases non-volatile substances are added to the mixture which facilitate gelatinization and act as stabilizers in lessening subsequent deterioration of the finished propellent, e.g., Ardeer cordite. The gelatinized mass can then be rolled or pressed into the required shape and is ready for use. (2) Kneading with a volatile solvent, as in the case of nitrocellulose powders, followed by pressing and drying, e.g., cordites.
The chief drawbacks of smokeless propellents are the erosion of the bore of the gun caused by their use, and the brilliance of the flash of discharge, the latter being specially undesirable in the case of propellents used for warlike purposes. Erosion is caused by the high temperature of the gases of explosion and is less with powders of the nitrocellulose type than with those containing nitroglycerine. This effect is due to the deficiency of oxygen in nitrocellulose and to the excess of oxygen in nitro glycerine. An illustration of this is found in the cordites : Cor dite Mark I. is highly erosive owing to its high percentage of nitroglycerine, but cordite M.D. is much less so. Reduction of the temperature of explosion can be effected by the admixture of substances which absorb heat or produce gaseous nitrogen in place of the exothermic oxides of carbon in the gases of ex plosion. This not only has the effect of reducing erosion but also reduces the brightness of the flash of discharge.
Propellents.—Propelient explosives are required for : (I) The propulsion of shell or other projectiles in rifled arms: (a) ord nance, e.g., guns, howitzers, mortars; (b) small arms, e.g., pistols, rifles. (2) The propulsion of shot in smooth-bore guns for sport ing purposes. For many years the only explosive available as a propellent was gunpowder which is far from being an ideal ma terial for the purpose. Its uncontrolled rate of burning and smoke-producing properties are the chief obstacles to its use. The discovery of nitrocellulose and nitroglycerine led to the development of an entirely new type of propellent, usually called smokeless powder. These propellents differ not only in being smokeless, but also in that the rate of burning is uniform owing to the non-porous texture of the powder preventing the hot gases of explosion penetrating below the surface layer.
Modern smokeless propellents may be either nitrocellulose propellents, consisting mainly of nitrocellulose, or nitroglycerine propellents, consisting mainly of nitrocellulose plus nitroglycerine. In addition to these principal constituents, small quantities of other substances are in some cases added (a) to act as "stabil izers" in preventing the deteriorating action of acid substances formed by the slow decomposition of the nitrocellulose or nitro glycerine, or (b) to act as "moderants" in regulating the rate of burning of the propellent. Typical compositions of some modern smokeless propellents are given in the following table : Trinitrotoluene (T.N.T.).—This is a derivative of toluene and in some respects similar to picric acid (q.v.). Toluene, is a liquid boiling at 110° C, obtained from the lower boiling portions of coal tar (q.v.) or from Borneo petroleum. Generally speaking, it is more easily brought into chemical re action than benzene, but not so easily as phenol, Nitric acid acts on it in several stages, the highest stage being the trinitro-compound or T.N.T., The lower stages of nitration are represented by mono- and di-nitrotoluene. They and T.N.T. itself were employed, either alone or mixed, in the dye industry for quite a long time before any explosive property was associated with them. There are six possible modifications of trinitrotoluene, these isomerides differing in form, colour, melting point, chemical reactivity, etc. (see IsoMERIsM). The ordinary variety of T.N.T. used as an explosive is represented in the fol lowing graphic formula as the most symmetrical isomeride, i.e., In the manufacture of T.N.T. a mixture of nitric and sulphuric acids is employed with the addition of oleum or sulphuric acid containing some sulphuric anhydride, thus minimizing the amount of water in the acid mixture. To this mixture the tol uene is gradually added with constant agitation, jacketed iron vessels being employed so that cooling or heating may be ar ranged as required. Nitrous fumes are conducted away and re converted into nitric acid. The oily product is separated from the dilute mixed acids, which are run off and concentrated for fur ther use. The oil is poured into water, where it solidifies, and is then washed free from adhering acids and purified by treatment with a solution of sodium sulphite, which dissolves all the unsym metrical trinitrotoluenes (present as by-products) but has no action on the symmetrical variety.
The pure substance melts at 8 2 ° C, and when heated to about 150° C it gives off a little gas which is much increased at about 18o° C; the melt darkens as the temperature rises, and intumes cence may take place at or below 300°. By rapidly heating a few grams of the melted substance in a test tube over a large flame, it is possible to get quite a marked explosive effect. When large quantities catch fire, the burning may become explosive, although this substance, when pure and alone, is one of the safest of high explosives. Some unexplained explosions have taken place, which may possibly have been due to the presence of a trinitrobenzoic acid, produced by too drastic nitration, such an acid being more sensitive than T.N.T. itself. Minute quantities of substances containing available oxygen, e.g., chlorates, nitrates, etc., in con tact with nitro-compounds, such as trinitrotoluene, greatly increase the sensitiveness of these compounds to heat or percussion. Ni trate of ammonia, however, is almost the only substance of this class capable of being safely employed in admixture with trini trotoluene or like substances on a large scale (see below).
Trinitrobenzene, C6H3 (NO2) 3, is difficult to make, otherwise it would doubtless be largely employed for both civil and military purposes. The lower nitration stages—mono- and di-nitrobenzenes —are constituents of various blasting compositions.
Amatol.—Ammonium nitrate has been in use for a considerable time for blasting purposes in mine work, especially coal mining, and was first employed with trinitrotoluene in 1914. After the safety of this mixture had been proved through a wide range of proportions and conditions, it was largely employed under the name of amatol, a composition of 8o parts of ammonium salt to 20 of trinitrotoluene being favoured. The advantage of such a mixture is that practically no carbon is left unburat on its explosion. The ammonium nitrate is a self-combustible and may, in favourable circumstances, respond to a detonator. The neces sity for using special explosives in coal mines instead of ordinary gunpowder drew attention to ammonium nitrate as not being so hot, producing much steam and nitrogen and no red-hot solid particles, and therefore less liable to ignite coal dust or inflam mable gases. Ammonium nitrate is hardly to be regarded as a possible substitute for the nitre of gunpowder owing to its del iquescent properties. Compositions of nitro-naphthalene, par affin, rosin, etc., were formerly used for blasting, being mixed as dry powders, pressed into form, and waterproofed by treatment with paraffin wax. Since then many changes have been made on this type of explosive, particularly in regard to the aromatic nitro-compounds introduced, generally with a view to greater safety or power.
Ammonal. The enormous amount of heat evolved by the oxidation of some powdered metals, such as that of aluminium in thermite compositions, led to its being introduced into ammo nium nitrate mixtures. For ordinary blasting work (out-of-doors) one uses a composition such as : ammonium nitrate, 7 2 % ; alu minium, 25%; charcoal, 3%; or ammonium nitrate, 47%; alu minium 2 2 % ; T.N.T., 3 o% ; charcoal 1 %. The velocity of ex plosion of these mixtures may be in the vicinity of 4,00o to metres per second. For coal mines the aluminium must be reduced to a few units per cent, the larger proportion of am monium nitrate keeping down the temperature ; thus : ammonium nitrate, ; aluminium, 2.5-3.5%; charcoal, 2-3%.