Rock Blasting / A Practical Treatise on the Means Employed in Blasting Rocks for Industrial Purposes

by George G. André

The action of a detonator upon a chemical compound is different. In this case, the explosion seems to be due more to the vibration caused by the blow than by the heat of the gases from the detonator. Probably both of these causes operate in producing the effect. However this may be, the fact is certain that under the influence of the explosion of the detonator, the molecules of a chemical compound, like nitro-glycerine, are broken up simultaneously, or at least, so nearly simultaneously, that no tamping is needed to obtain the full effect of the explosion. Dynamite is always, and gun-cotton is usually, fired by means of a detonator. A much larger quantity of explosive is needed to detonate gunpowder than is required for dynamite, or gun-cotton, since, for the former explosive, a large volume of gases is requisite. Dynamite detonators usually consist of from six to nine grains of fulminate of mercury contained in a copper cap, as described in the preceding chapter. Gun-cotton detonators are similar, but have a charge of from ten to fifteen grains of the fulminate. An insufficient charge will only scatter the explosive instead of firing it, if it be unconfined, and only explode it without detonation, if it be in a confined space.

Section V.—Some Properties of the Common Explosive Agents.

Gunpowder.

—The combustion of gunpowder, as we have seen, is gradual and comparatively slow. Hence its action is rending and projecting rather than shattering. This constitutes one of its chief merits for certain purposes. In many quarrying operations, for instance, the shattering action of the chemical compounds would be very destructive to the produce. In freeing blocks of slate, or of building stone, a comparatively gentle lifting action is required, and such an action is exerted by gunpowder. Moreover, this action may be modified by using light tamping, or by using no tamping, a mode of employing gunpowder often adopted in slate quarries. The effect of the violent explosives cannot be modified in this way.

Gunpowder is injured by moisture. A high degree of moisture will destroy its explosive properties altogether, so that it cannot be used in water without some protective covering. Even a slight degree of moisture, as little as one per cent. of its weight, materially diminishes its strength. For this reason, it should be used, in damp ground, only in cartridges. This is, indeed, the most convenient and the most economical way of using gunpowder in all circumstances. It is true that there is a slight loss of force occasioned by the empty space around the cartridge, in holes that are far from circular in shape. But at least as much will be lost without the cartridge from the moisture derived from the rock, even if the hole be not wet. But in all downward holes, the empty spaces may be more or less completely filled up with dry loose sand.

The products of the explosion of gunpowder are partly gaseous, partly solid. Of the former, the most important are carbonic acid, carbonic oxide, and nitrogen. The sulphuretted and the carburetted hydrogen are formed in only small quantities. The carbonic oxide is a very noxious gas; but it is not formed in any considerable quantity, except in cases of overcharging. The solid products are compounds of potassium and sulphur, and potassium and carbon. These constitute the smoke, the dense volumes of which characterize the explosion of gunpowder. This smoke prevents the immediate return of the miner to the working face after the blast has taken place.

Gun-cotton.

—The combustion of gun-cotton takes place with extreme rapidity, in consequence of which its action is very violent. Its effect is rather to shatter the rock than to lift it out in large blocks. This quality renders it unsuitable to many quarrying operations. In certain kinds of weak rock, its disruptive effects are inferior to those produced by gunpowder. But in ordinary mining operations, where strong tough rock has to be dealt with, its superior strength and quickness of action, particularly the latter quality, produce much greater disruptive effect than can be obtained from gunpowder. Moreover, its shattering action tends to break up into small pieces the rock dislodged, whereby its removal is greatly facilitated.

Gun-cotton may be detonated when in a wet state by means of a small quantity of the dry material. This is a very important quality, inasmuch as it allows the substance to be used in a wet hole without protection, and conduces greatly to the security of those who handle it. When in the wet state, it is uninflammable, and cannot be exploded by the heaviest blows. Only a powerful detonation will bring about an explosion in it when in the wet state. It is, therefore, for safety, kept and used in that state. Since it is insensible to blows, it may be rammed tightly into the bore-hole, so as to fill up all empty spaces. The primer of dry gun-cotton, however, which is to detonate it, must be kept perfectly dry, and handled with caution, as it readily detonates from a blow. Gun-cotton, when ignited in small quantities in an unconfined space, burns fiercely, but does not explode.

The products of the combustion of gun-cotton are:—carbonic acid, carbonic oxide, water, and a little carburetted hydrogen or marsh-gas. On account of the insufficiency of oxygen, already pointed out, a considerable proportion of carbonic oxide is formed, which vitiates the atmosphere into which it is discharged. Overcharging, as in the case of gunpowder, causes an abnormal quantity of the oxide to be formed.