A good guhr should absorb four times its weight of nitro-glycerine, and should then form a comparatively dry mixture. It should be pale pink, red brown, or white. The pink is generally preferred, and it should be as free as possible from grit of all kinds, quartz particles, &c., and should have a smooth feeling when rubbed between the finger and thumb, and should show a large quantity of diatoms when viewed under the microscope. The following was the analysis of a dried sample of kieselguhr:—Silica, 94.30; magnesia, 2.10; oxide of iron and alumina, 1.3; organic matter, 0.40; moisture, 1.90 per cent.
The guhr is generally dried in a reverberatory muffle furnace. It is spread out on the bottom to the thickness of 3 or 4 inches, and should every now and then be turned over and raked about with an iron rabble or hoe. The temperature should be sufficiently high to make the guhr red hot, or the organic matter will not be burnt off. The time occupied in calcining will depend of course upon the quality of the guhr being operated upon. Those containing a high percentage of water and organic matter will of course take longer than those that do not. A sample of the calcined guhr should not contain more than 0.5 per cent. of moisture and organic matter together.
After the guhr is dry it requires to be sifted and crushed. The crushing is done by passing it between iron rollers fixed at the bottom of a cone or hopper, and revolving at a moderate speed. Beneath the rollers a fine sieve should be placed, through which the guhr must be made to pass.
The kieselguhr having been dried, crushed, and sifted, should be packed away in bags, and care should be taken that it does not again absorb moisture, as if it contains anything above about five-tenths per cent. of water it will cause the dynamite made with it to exude. The guhr thus prepared is taken up to the danger area, and mixed with nitro-glycerine. The nitro-glycerine used should be quite free from water, and clear, and should have been standing for a day or two in the precipitating house. The guhr and nitro-glycerine are mixed in lead tanks (about 1-1/2 foot deep, and 2 to 3 feet long), in the proportions of 75 of the nitro-glycerine to 25 of the guhr, unless the guhr is found to be too absorbent, which will cause the dynamite to be too dry and to crumble. In this case a small quantity of barium sulphate, say about 1 per cent., should be added to the guhr. This will lessen its absorbing powers, or a highly absorptive sample of guhr may be mixed with one of less absorptive power, in the proportions found by experiment to be the best suited to make a fairly moist dynamite, but one that will not exude.
The mixing itself is generally performed in a separate house. In a series of lead-lined tanks the guhr is weighed, placed in a tank, and the nitro- glycerine poured on to it. The nitro-glycerine may be weighed out in indiarubber buckets. The whole is then mixed by hand, and well rubbed between the hands, and afterwards passed through a sieve. At this stage the dynamite should be dry and powdery, and of a uniform colour.
It is now ready to be made up into cartridges, and should be taken over to the cartridge huts. These are small buildings surrounded with mounds, and contain a single cartridge machine. Each hut requires three girls—one to work the press, and two to wrap up the cartridges. The cartridge press consists of a short cylinder of the diameter of the cartridge that it is intended to make. Into this cylinder a piston, pointed with ivory or lignum vitæ wood, works up and down from a spring worked by a lever. Round the upper edge of the cylinder is fastened a canvas bag, into which the powdery dynamite is placed by means of a wooden scoop, and the descending piston forces the dynamite down the cylinder and out of the open end, where the compressed dynamite can be broken off at convenient lengths. The whole machine should be made of gun-metal, and should be upright against the wall of the building. The two girls, who sit at tables placed on each side of the press, wrap the cartridges in parchment paper. From these huts the cartridges are collected by boys every ten minutes or a quarter of an hour, and taken to the packing room, where they are packed in 5-lb. cardboard boxes, which are then further packed in deal boxes lined with indiarubber, and fastened down air tight. The wooden lids are then nailed down with brass or zinc nails, and a label pasted on the outside giving the weight and description of the contents. The boxes should then be removed to the magazines. It is well to take a certain number of cartridges from the packing house at different times during the day, say three or four samples, and to test them by the heat test. A sample cut from a cartridge, about 1 inch long, should be placed under a glass shade, together with water (a large desiccator, in fact), and left for some days. A good dynamite should not, under these conditions, show any signs of exudation, even after weeks.[A]
[Footnote A: For analysis of dynamite, see chapter on "Analysis," and author's article in Chem. News, 23rd September 1892.]
~Properties of Kieselguhr Dynamite.~—One cubic foot of dynamite weighs 76 lbs. 4 oz. The specific gravity of 75 per cent. dynamite is, however, 1.50. It is a red or grey colour, and rather greasy to the touch. It is much less sensitive to shock than nitro-glycerine, but explodes occasionally with the shock of a rifle bullet, or when struck. The addition of a few per cent. of camphor will considerably diminish its explosive qualities to such an extent that it can be made non-explosive except to a very strong fulminate detonator. The direct contact of water disintegrates dynamite, separating the nitro-glycerine, hence great caution is necessary in using it in wet places. It freezes at about 40° Fahr. (4° C.), and remains frozen at temperatures considerably exceeding that point. When frozen, it is comparatively useless as an explosive agent, and must be thawed with care. This is best done by placing the cartridges in a warming pan, which consists of a tin can, with double sides and bottom, into which hot water (130° Fahr.) can be poured. The dynamite will require to be left in for some considerable time before it becomes soft. On no account must it be placed on a hot stove or near a fire, as many serious accidents have occurred in this way.
Frozen dynamite is a hard mass, with altered properties, and requires 1.5 grm. of fulminate instead of 0.5 grm. to explode it. Thawing may also cause exudation of the nitro-glycerine, which is much more sensitive to shock, and if accidentally struck with an iron tool, may explode. It is a dangerous thing to cut a frozen cartridge with a knife. Ramming is even more dangerous; in fact it is not only dangerous, but wasteful, to use dynamite when in a frozen state.
Dynamite explodes at a temperature of 360° Fahr., and is very sensitive to friction when hot. In hot countries it should never be exposed to the rays of the sun. It should, however, not be kept in a damp or moist place, as this is liable to cause exudation. Sunlight, if direct, can cause a slow decomposition, as with all nitro and nitric compounds. Electric sparks ignite, without exploding it, at least when operating in the open air.