~Detonators~, or caps, are metallic capsules, usually of copper, and resemble very long percussion caps. The explosive is pure fulminate of mercury, or a mixture of that substance with nitrate or chlorate of potash, gun-powder, or sulphur. The following is a common cap mixture:— 100 parts of fulminate of mercury and 50 parts of potassium nitrate, or 100 parts of fulminate and 60 parts of meal powder. Silver fulminate is also sometimes used in caps. There are eight sizes made, which vary in dimensions and in amount of explosive contained. They are further distinguished as singles, doubles, trebles, &c., according to their number. Colonel Cundill, R.A. ("Dict. of Explosives"), gives the following list:—
No. 1 contains 300 grms. of explosive per 1000. " 2 " 400 " " " " " " 3 " 540 " " " " " " 4 " 650 " " " " " " 5 " 800 " " " " " " 6 " 1,000 " " " " " " 7 " 1,500 " " " " " " 8 " 2,000 " " " " "
Trebles are generally used for ordinary dynamite, 5, 6, or 7 for gun-cotton, blasting gelatine, roburite, &c.
In the British service percussion caps, fuses, &c., are formed of 6 parts by weight of fulminate of mercury, 6 of chlorate of potash, and 4 of sulphide of antimony; time fuses of 4 parts of fulminate, 6 of potassium chlorate, 4 of sulphide of antimony, the mixture being damped with a varnish consisting of 645 grains of shellac dissolved in a pint of methylated spirit. Abel's fuse (No. 1) consists of a mixture of sulphide of copper, phosphide of copper, chlorate of potash, and No. 2 of a mixture of gun-cotton and gun-powder. They are detonated by means of a platinum wire heated to redness by means of an electric current. Bain's fuse mixture is a mixture of subphosphide of copper, sulphide of antimony, and chlorate of potash.
In the manufacture of percussion caps and detonators the copper blanks are cut from copper strips and stamped to the required shape. The blanks are then placed in a gun-metal plate, with the concave side uppermost—a tool composed of a plate of gun-metal, in which are inserted a number of copper points, each of the same length, and so spaced apart as to exactly fit each point into a cap when inverted over a plate containing the blanks. The points are dipped into a vessel containing the cap composition, which has been previously moistened with methylated spirit. It is then removed and placed over the blanks, and a slight blow serves to deposit a small portion of the cap mixture into each cap. A similar tool is then dipped into shellac varnish, removed and placed over the caps, when a drop of varnish from each of the copper points falls into the caps, which are then allowed to dry. This is a very safe and efficacious method of working.
At the works of the Cotton-Powder Company Limited, at Faversham, the fulminate is mixed wet with a very finely ground mixture of gun-cotton and chlorate of potash, in about the proportions of 6 parts fulminate, 1 part gun-cotton, and 1 part chlorate. The water in which the fulminate is usually stored is first drained off, and replaced by displacement by methyl-alcohol. While the fulminate is moist with alcohol, the gun-cotton and chlorate mixture is added, and well mixed with it. This mixture is then distributed in the detonators standing in a frame, and each detonator is put separately into a machine for the purpose of pressing the paste into the detonator shell.
At the eleventh annual meeting of the representatives of the Bavarian chemical industries at Regensburg, attention was drawn to the unhealthy nature of the process of charging percussion caps. Numerous miniature explosions occur, and the air becomes laden with mercurial vapours, which exercise a deleterious influence upon the health of the operatives. There is equally just cause for apprehension in respect to the poisonous gases which are evolved during the solution of mercury in nitric acid, and especially during the subsequent treatment with alcohol. Many methods have been proposed for dealing with the waste products arising during the manufacture and manipulation of fulminate of mercury, but according to Kæmmerer, only one of comparatively recent introduction appears to be at all satisfactory. It is based upon the fact that mercuric fulminate, when heated with a large volume of water under high pressure, splits up into metallic mercury and non-explosive mercurial compounds of unknown composition.
In mixing the various ingredients with mercury fulminate to form cap mixtures, they should not be too dry; in fact, they are generally more or less wet, and mixed in small quantities at a time, in a special house, the floors of which are covered with carpet, and the tables with felt. Felt shoes are also worn by the workpeople employed. All the tools and apparatus used must be kept very clean; for granulating, hair sieves are used, and the granulated mixture is afterwards dried on light frames, with canvas trays the bottoms of which are covered with thin paper, and the frames fitted with indiarubber cushions, to reduce any jars they may receive. The windows of the building should be painted white to keep out the rays of the sun.
Mr H. Maxim, of New York, has lately patented a composition for detonators for use with high explosives, which can also be thrown from ordnance in considerable quantities with safety. The composition is prepared as follows:—Nitro-glycerine is thickened with pyroxyline to the consistency of raw rubber. This is done by employing about 75 to 85 per cent. of nitro-glycerine, and 15 to 25 per cent. of pyroxyline, according to the stiffness or elasticity of the compound desired. Some solvent that dissolves the nitro-cotton is also used. The product thus formed is a kind of blasting gelatine, and should be in a pasty condition, in order that it may be mixed with fulminate of mercury. The solvent used is acetone, and the quantity of fulminate is between 75 to 85 per cent. of the entire compound. If desired, the compound can be made less sensitive to shocks by giving it a spongy consistency by agitating it with air while it is still in a syrupy condition. The nitro-glycerine, especially in this latter case, may be omitted. In some cases, when it is desirable to add a deterring medium, nitro-benzene or some suitable gum is added.
[Illustration: FIG. 34. METHOD OF PREPARING THE CHARGE.]