EXPLOSIVES.
Classification, Characteristics and Properties.

General Classification. Explosives are classified generally as follows:—

1. Explosive mixtures of the nitrate class.

2. Explosive mixtures of the chlorate class.

3. Explosive compounds of the nitro-substitution class. 4. Explosive compounds of the nitric-derivative class.

5. Explosives of the Sprengel class.

6. Fulminates and Amides.

7. Ammunition.

Of the seven classifications of explosives, we are dealing with but four in the subject of Bomb Fighting, namely, as classified above, 1, 3, 4 and the Fulminates.

Nitrate Class. Explosive mixtures of the nitrate class. The best known example of this class is gunpowder, the characteristics of which are that it consists of a mechanical mixture of nitrates with some base containing charcoal or other substance yielding carbon. The nitrates carry the oxygen which combines with the base, under favorable circumstances developing a large volume of gases at a high temperature, so that if the powder is confined at the time of explosion there will be produced an enormous disruptive effect. The standard composition of gunpowder is:—

It might be interesting to note that the charcoal employed for military and sporting powder is made from dog-wood, while for inferior grades of powder willow and alder are used.

Explosive Compounds of the Nitro-Substitution Class and Nitric-Derivative Class. The two explosives of these classes which are generally known are gun-cotton and nitro-glycerine, with special preparations made from them, such as dynamite, blasting gelatine, etc.

Gun-Cotton. Gun-cotton is made by treating suitably prepared cotton with a mixture of one part by weight of nitric acid and three parts sulphuric acid. The immersion lasts 48 hours, the temperature being maintained at 60 deg. F. The cotton is then subjected to a thorough and prolonged washing, after which it is carried through various processes to prepare it for use. The cellulose of the cotton has thus been converted into tri-nitro cellulose. By varying the strength of the acids different degrees of nitration may be obtained. Gun-cotton is extensively used for military purposes.

Gun-cotton differs but slightly in appearance from ordinary cotton. It has a harsh feel and is less flexible than common cotton. It becomes highly electrified when rubbed between the fingers and appears luminous when rubbed in the dark. It is entirely insoluble in hot or cold water, but dissolves in a mixture of ether and ammonia. It will rarely take up more than two per cent. of moisture from the atmosphere. It is insensible to pressure, percussion or friction, unless closely confined or firmly compressed. It burns with a flash, but without explosion if brought into contact with burning or incandescent bodies. Wet gun-cotton will not burn or explode. Its ignition temperature is 360 degrees F. Pure gun-cotton will undergo no spontaneous decomposition and is the safest explosive known. Although it will not explode when wet, it may be detonated in this condition by a Mercury Fulminate Detonator with a small initial charge of dry gun-cotton in contact with it.

Nitro-Glycerine. Nitro-glycerine is a nitric ether, or specifically a glyceryl tri-nitrate. Different degrees of nitration yield the mono-di- and tri-nitro glycerine, respectively; the latter being the nitro-glycerine of commerce. It is made by treating an exceedingly pure quality of glycerine with a mixture of nitric and sulphuric acids, the proportions commonly adopted being 3 parts of nitric acid, 5 parts of sulphuric acid, and from 1 to 1.15 parts of glycerine. The glycerine is added very slowly and with constant stirring. The agitation of the mixture is now usually accomplished by compressed air.

When made from the purest ingredients, nitro-glycerine is an oily looking fluid, as clear and transparent as water. When freshly made it is whitish and opaque, but on standing it clears. The specific gravity at normal temperature is about 1.6 deg. F., when frozen 1.735 deg. F. Nitro-glycerine dissolves in alcohol, ether, methyl-alcohol, benzine, etc. Freshly made, opaque nitro-glycerine freezes at from 2.2 deg. F. to 7.6 deg. F., while the transparent, or clear, product freezes at from 39.2 deg. F. to 37.4 deg. F. In its frozen state it is less sensitive to shock or concussion than when it is in liquid. It may be completely evaporated when at a temperature of 158 deg. F. Its ignition temperature or firing point is 356 deg. F. Exposed to a temperature of 365 deg. F. it boils with evolution of vapors; at 381.2 deg. F. it volatilizes slowly; at 392 deg. F. it evaporates rapidly; at 422.6 deg. F. it detonates violently. From this point its behavior changes, passing through temperatures at which it explodes with constantly lessening violence until, at a dark, cherry-red heat, it assumes a spheroidal state and fails to explode. This applies to small quantities only. When gradually heated it is certain to explode at 356 deg. F.

Picric Acid. When coal tar is subjected to a fractional distillation the portion which comes over up to a temperature of 170 deg. C. is called “light oil” and contains all the compounds of low boiling paint contained in tar, and from this several of our most valuable explosives can be obtained. When these light oils have distilled over the next fraction, or “middle oil,” yields phenol or carbolic acid, a body which nitrated gives picric acid, which is the basis of the French high explosive “melinite” and the English “lyddite.”

Picric acid consists of a very strong nitric acid and carbolic acid, and is a very high explosive. It was introduced by Turpin, who mixed it with collodium and called it “melinite,” by which name it is known in the French Service. It forms with metals a class of salts (picrates). The potassium salts were suggested as a bursting charge for shells nearly fifty years ago. Sprengel and, later, Turpin, employed the acid itself as an explosive. It was possible to get a great weight of explosive into small space, as the acid could be melted and poured into the shell in a molten condition. Picric acid is a very safe explosive, but has the drawback of acting on metals, forming “picrates,” some of which are more sensitive to disturbing influences than the acid itself.

Lyddite. Lyddite consists of melted solidified picric acid, and has the same disadvantage of forming “picrates” when in contact with metal, making it necessary to varnish the interior of shells when used in them. Experience with lyddite shells shows them to be very erratic, due to the fact that they require a very powerful detonator, the use of which is very dangerous, as they may cause a premature explosion.

T. N. T. These disadvantages in picric acid led to its being largely replaced by tri-nitro toluol, or T. N. T., which has a bursting pressure of 119,000 pounds per square inch as against 135,820 pounds for picric acid. Yet the advantages of the former more than compensate and warrant its use being preferred. T. N. T. does not act on metals to create sensitive salts and is, therefore, perfectly stable. The French name for T. N. T. is “Tolitype,” the Spanish “Trilite,” and the German “Trotyle.” It is produced by heating troulue with a mixture of nitric acid and sulphuric acid.

Troulue. Troulue is a liquid hydro-carbon obtained along with benzine.

Tetryl. Tetryl is another coal tar product containing more nitrogen than lyddite, and is employed in detonators with a little lead azide, making a less sensitive and safer preparation than fulminate of mercury.

Aunnonal. Aunnonal is a mixture of T. N. T., aluminum in fine powder and nitrate of ammonia, and a trace of charcoal. It is safe and powerful, but has the disadvantage of attracting moisture, and for that reason does not always explode.

Dynamite. Dynamite is the most generally used of any blasting material in the world. It was invented in 1866 by Alfred Nobel. Its principal consisted in using an absorbent commonly called a “dope,” which would take up the nitro-glycerine and hold it after the manner of a sponge.

A suitable dope should possess a cellular structure so that the nitro-glycerine may be subdivided into minute globules, each being held separately in its own cell, completely isolated from every other. In this condition its sensitiveness is greatly reduced, depending, of course, on the amount of nitro-glycerine absorbed. Dynamite may be classified, according to the nature of the absorbent used, as follows:—

The explosives’ bases in (b), as above, may be of the nitro-substitution class, or the nitric-derivative class.

In choosing the dopes for inert bases of dynamite where wood pulp or sawdust is employed, it should be of some porous wood such as spruce or basswood. Woods differ considerably in the amount of nitro-glycerine they will absorb, ranging from 60% to 85%. Before introducing the nitro-glycerine they should be thoroughly dried. Good dynamite should not feel greasy. There should be no trace of free nitro-glycerine inside the wrapper of the cartridge. Slowly heated dynamite explodes at a temperature of 356 deg. F. If rapidly heated, it explodes at 446 deg. F. These temperatures apply only to Kieselghur dynamite. The American dynamites containing wood pulp and nitrates will explode with somewhat lower temperatures. Like nitro-glycerine it is most sensitive to shock and friction just above the freezing point. According to the dope used, it freezes at from 42 deg. F. to 46 deg. F. It is nearly, but not quite, insensitive to shock or friction when frozen. (See page 88, Manual of Field Engineering.)

Monobel. Monobel consists of:

Fulminates. These are the most powerful and dangerous explosives in common use. They consist for the most part of metallic salts of fulminic and amic acids. The commonest fulminate, known as mercury fulminate, is formed by dissolving mercury in nitric acid, to which solution when cool is added 110 parts of alcohol. Water is then added, causing a grey fulminate to precipitate. This is carefully washed and air-dried. The operation is attended with great danger. The color of fulminate varies from a white to dirty grey. Its specific gravity is 4.42 deg. F. It has a sweetish taste and is highly poisonous. It is extremely sensitive to heat and shock of every kind. Its firing point when slowly heated is 306.5 deg. F., and when rapidly heated 368.6 deg. F. When wet it is less sensitive, but not secure against explosion. The slightest friction will provoke its explosion. It may be destroyed safely by treating it with alkaline sulphides.

Fulminates of other metals are capable of being made, such as fulminates of silver, gold, platinum, zinc and copper, but these are more violently exploded and less stable. The only one which has come into any use being a silver compound. Mercury fulminate is the explosive used in the manufacture of detonators. (See page 89, Manual of Field Engineering, 1911.)

Detonators and Fuses. (See pages 89, 90 and 91, Manual of Field Engineering, 1911.)

Theory of Explosives and Fumes. Definition of Explosive: Explosive is a substance either solid, liquid, or jelly, which, when subjected to a shock, suddenly changes from solids, etc., to gases, at a very high temperature, tearing to pieces any vessel which may contain them.

Definition of an Explosion: An explosion is a chemical reaction which is completed in an exceedingly short period of time with the evolution of a large quantity of gas at a very high temperature. If this reaction occurs in a body which is closely confined, the expansive effect of the highly heated gases produces disruptive effects. If the suddenness of the reaction is very great, disrupted action upon solid objects in contact with the body may be obtained even when it is unconfined, because the cohesion of these objects can be overcome more readily in an instant of time than the inertia of the surrounding air. This has given rise to the popular error that nitro-glycerine and other high explosives act downwards; as a matter of fact they act with equal force in all directions. It is evident, therefore, that the violence of an explosion depends upon three things, namely, the time occupied completing the reaction of the explosive body; the temperature produced by the reaction, upon which directly depends the expansive forces of the resultant gases; and the quantity of gas evolved by the reaction. A fourth consideration—whether the products of the reaction are the result of one set of chemical actions occurring simultaneously; or whether the set of new compounds react upon each other, producing a second set of compounds.