=171. Requirements.=—The desirable features in an explosive to be used in trenching and tunneling in rock are: (1) stability in make up so as not to deteriorate in strength or to become dangerous during storage, (2) imperviousness to ordinary variations in temperature and moisture, (3) insensibility to ordinary shocks received in transportation and handling, (4) not too difficult of detonation, (5) convenient form for transportation and loading and for making up charges of different weights, (6) the non-formation of poisonous gases when fired, (7) imperviousness to water and usefulness in wet holes, (8) power without bulk, etc.
=172. Types of Explosives.=—Explosives which fill some or all these requirements can be divided into two classes, deflagrating and detonating. A deflagration is an explosion transmitted progressively from grain to grain. A detonation is a sudden disruption caused by synchronous vibrations of a wave-like character. The deflagrating explosives are represented by gun-powders and contractors’ powders. They must be carefully tamped in the hole to develop their full power and they must be ignited by a fuse or flame. They are valueless in water or moist holes. These powders are used mainly for loosening frozen earth, soft sandstone, cemented gravels and similar materials where a thrusting action rather than a disruption is desired. The detonating explosives are most commonly represented by the dynamites. These are exploded by a shock usually caused by another explosive which has been ignited by a fuse or electric spark, and which is known as the “detonator.” Detonating explosives are more powerful than deflagrating explosives and are used in all but the softest materials.
_Gunpowder._—This is a mechanical mixture of sulphur, charcoal, and saltpeter generally in the proportions of 10 parts sulphur, 15 parts charcoal, and 75 parts saltpeter (sodium nitrate). It weighs about 62½ pounds per cubic foot and produces about 280 times its own volume in gas at a pressure of 4.68 tons per square inch at a temperature of 32 degrees F., which amounts to a pressure of approximately 38 tons per square inch at the temperature of explosion of 4,000 degrees F.
_Blasting Powder._—This is a mixture of 19 parts sulphur, 15 parts charcoal, and 66 parts saltpeter. These powders are made in different size angular polished grains, from the size of a pin head to sizes just passing a ⅜ to ½ inch hole. The larger the grains the slower the action of the powder.