BLASTING, the technical term for split ting and breaking up any object by means of gunpowder or some other powerful explosive. The operation, which is of extensive use in quarrying, mining and other branches of en gineering, is performed by boring a hole or tunnel in the substance to be exploded, by means of a drill, sometimes called a jumper, loading it with gunpowder, and igniting this by means of a fuse burning so slowly as to allow the parties employed to retire. to a safe distance before the explosion takes place. At one time it was sup posed that the force of the explosion depended on the firm packing of the gunpowder in the hole by means of small chips of stone, sand, etc. One of the most important modern im provements in blasting is the firing of the charge by electricity. This mode is especially applicable to submarine blasting, and was first practised for that purpose by General Pasley, in 1839. The only thing necessary is to make an interruption in the conducting wire at the point where the explosive is placed. In passing the electric current, a spark produced at the inter ruption fires the charge. The effect being in stantaneous the operator can fire any number of charges on the same wire simultaneously.
Two classes of explosives are used in blast ing: low explosives, fired by direct ignition; and high explosives, fired by an intermediate detonator. To the first class belong the ordi nary black blasting powders. These differ from common gunpowder in being required to pro duce a more rapid explosion. To this end a larger proportion of saltpetre is used. The quickest formula is 76 parts saltpetre, nine parts sulphur and 15 parts charcoal, in use in England and America. At the other end of the scale is the blasting powder of Austria — 61 parts saltpetre, 18 parts sulphur and 21 parts charcoal. Chief among the high explosives in use for blasting is dynamite, or some one of its many variations. Blasting gelatine and gela tine dynamite are also in favor for certain kinds of work. The first consists of 92 parts of nitro-glycerine and eight parts of collodion cot-. ton; the second of 80 parts of blasting gelatine and 20 parts of woodpulp or other similar ab sorbent. Fatalities resulting from blasting in coal mines have led to the invention of a series of ex plosives known as °safety powders." There are three classes of these; the first containing large quantities of volatile salts — the water of crys tallization of these salts lessening the tempera ture of the explosion, as with the °Oliver Flameless Powder." The second class are the nitrate of ammonia powders, which give very little flame. The third class are nitro-glycerine
powders in which the explosive is so combined with other ingredients as to give a very short flame and a low temperature, as with °Car bonite.° As compared with black powder, car bonite may be used with the same degree of safety in loads of 200 pounds as with one pound of common black blasting powder.
In mining work single-hand boring is the common practice — that is, each miner holds his own drill as well as striking with the ham mer. In quarries, however, and large excava tion work sets of two or three men work to gether, one handling the drill and the others striking. Where the holes are very deep the weight of a long drill rod is sufficient without the hammer, the rod being lifted two or three feet and allowed to fall. After each blow the drill is lifted a little and turned in the hole. The dust is frequently lifted out to prevent choking. A better plan is to introduce a small running stream of water. For removing small volumes of rock in mines, quarries and other engineering enterprises at a single blast, small shot blasting is the most common method ern ployed. This consists of drilling a small num ber of holes in the rock from one and one quarter to three inches in diameter and from 18 inches to several feet in depth which are then filled with dynamite or blasting-powder or some other safe and easily handled explosive, and properly connected by fuse, or with a magneto-machine or electric battery by electric wires. The space above the explosive is then plugged up with sand, dirt, clay or other matter, and the charge exploded. For breaking the rock into small pieces so as to be more easily removed (as in excavating for a foundation) the holes are drilled close together and heavily charged, but where it is unnecessary to break into small pieces (as in quarrying), and large shapely masses are more desirable, the holes are drilled in rows with greater distance between and filled with a smaller amount of explosive. This will split the rock practically along one line and will not shatter it as in the first case. In excavating tunnels, it is in many cases desirable to remove a mass of rock the size of the tunnel cross-section, an object which is generally ac complished by drilling and firing a small num ber of converging holes, thus forming and re moving a cone-shaped or wedge-shaped centre core. This central opening thus formed is en larged by drilling and blasting successive rings of holes around it.