96. The charge may be gunpowder or high explosive. If the former, it must be thoroughly tamped. If the latter, tamping will greatly increase its effect; but it is in some cases preferable to obtain the desired effect by increasing the charge and saving the time taken in tamping.
97. Blasts are fired by electric fuses, Bickford fuse, firing-tubes, needles, etc.
The fuses have been already described.
The firing-tube is a very small iron pipe, which is inserted in the powder charge and the tamping rammed around it.
After the tamping is finished the tube may be filled with fine powder poured in it if the hole is vertical or inclined downward, or straws filled with powder may be inserted if it inclines upward or is horizontal. A “squib” of wet powder is also sometimes placed in the tube and ignited, when it passes down the tube like a rocket and fires the charge.
The needle is a smooth copper wire, longer than the depth of the hole. It has a ring handle, by which it may be turned around and withdrawn. It is inserted in the charge, the tamping is well rammed around it, and it is withdrawn, leaving a pipe in the tamping, through which fire may be communicated, as described for the firing-tube.
98. Tamping.—The best and safest tamping is perfectly dry silicious sand, poured in the hole so as to fill it completely, but not rammed. It cannot be used in holes which incline upward nor when the needle is used.
In such cases moist clay, brick-dust, etc., are used. The first layers are pressed in upon the charge, and the subsequent ones thoroughly rammed down with a copper tamping-bar. A hammer is used with the bar, when necessary, in deep holes and hard rock.
With high explosives no tamping should be used except dry sand or water. Holes which incline upwards should receive an extra charge and be untamped.
99. Determining the Charge.—The charge of gunpowder or high explosive required for any particular hole in ordinary blasting can be best estimated by an experienced blaster. If one is not to be obtained, an approximate estimate for the first experiment may be made from the formulas (counting the high explosives about four or five times as strong as gunpowder for ordinary use), and the charges for subsequent blasts may be estimated from the effects of those first fired.