174. Strength.—The relative weights for equal strength of various explosives are given in Table 62.
| TABLE 62 | |
|---|---|
| Relative Weights of Explosives with the Same Strength as a Unit Weight of 40 Per Cent Dynamite | |
| Explosive | Relative Weight |
| Picric acid | 0.86 |
| Gun powder (well tamped) | 3.10 |
| Straight dynamite, 15% | 1.45 |
| Straight dynamite, 20 | 1.33 |
| Straight dynamite, 25 | 1.28 |
| Straight dynamite, 30 | 1.18 |
| Straight dynamite, 35 | 1.07 |
| Straight dynamite, 40 | 1.00 |
| Straight dynamite, 45 | 0.93 |
| Straight dynamite, 50 | 0.86 |
| Straight dynamite, 55 | 0.83 |
| Straight dynamite, 60 | 0.78 |
| Low-freezing dynamites are the same as straight dynamites | |
| Smokeless powder, well tamped | 0.74 |
| Triton | 0.86 |
| Blasting gelatine | 0.43 |
| Gelatine dynamite, 30% | 1.28 |
| Gelatine dynamite, 35 | 1.21 |
| Gelatine dynamite, 40 | 1.14 |
| Gelatine dynamite, 50 | 1.04 |
| Gelatine dynamite, 55 | 0.97 |
| Gelatine dynamite, 60 | 0.90 |
| Gelatine dynamite, 70 | 0.83 |
| Ammonia dynamites are the same as gelatine dynamites. | |
| Chlorates (sprengle) Rack-a-rock | 1.33 |
| Guncotton | 0.72 |
175. Fuses and Detonators.—The explosion of gunpowder and other deflagrating explosives is caused by the direct application of a flame led to the charge by a powder fuse, or they may be fired by a blasting cap which is itself exploded by the heat from a fuse or an electric spark. The powder fuse is a cord made up of a train of powder securely wrapped in a number of thicknesses of woven cotton or linen threads and usually made waterproof. Ordinary fuse burns at about 2 feet per minute but there may be wide variations from this rate due to the quality of the fuse, moisture, temperature, or pressure. Moisture tends to retard the rate, pressure to increase it. Instantaneous fuse will burn at about 120 feet per second. It is distinguished from the ordinary safety fuse both by eye and touch due to the rough red braid with which it is covered. It is used in firing a number of charges simultaneously. Powder fuses are lighted by the application of a flame or smoldering torch to the freshly cut or opened end exposing the powder grains. Cordeau Bickford is lead tubing filled with triton, in which the flame travels at about 17,000 feet per second. This is also used for igniting charges simultaneously.
The detonation of an explosive is caused by the shock or heat of the explosion of a more sensitive substance which has been exploded by a powder fuse or electric spark. The common method of detonating explosive charges is by the firing of a blasting cap. These caps are copper cylinders, closed at one end, about 1½ inches long and ¼ to ⅜ of an inch in diameter, or larger. They contain a mixture of about 85 per cent fulminate of mercury and 15 per cent potassium chlorate held in place by a wad of shellac, collodion, or paper. The strength of detonators is based on the weight of fulminate of mercury and is designated as shown in Table 63.
| TABLE 63 | |
|---|---|
| Strength of Blasting Caps | |
| Blasting Cap, Commercial Grade | Grains Fulminate of Mercury |
| 3X or Triple | 8.3 |
| 4X or Quadruple | 10.0 |
| 5X or Quintuple | 12.3 |
| 6X or Sextuple | 15.4 |
| 7X or Number 20 | 23.1 |
| 8X or Number 30 | 30.9 |
| Single strength | 12.3 |
| Double strength | 15.4 |
| Triple strength | 23.1 |
| Quadruple strength | 30.9 |
The force of the explosion is markedly affected by the strength of the caps, the effect being greater for low-grade powders. For 40 per cent dynamite the explosion caused by a 5X cap is 15 per cent stronger than that caused by a 3X cap. For 60 per cent dynamite the difference is only 6 per cent. The deterioration of the caps will reduce the strength of an explosion noticeably. With straight dynamite, 3X caps are generally used, but with gelatine dynamite 6X or heavier caps must be used. Caps may be tested by exploding them in a confined space and noting the report and the effect on the shell. A full strength cap will tear the shell into minute pieces, while a deteriorated cap will merely tear it into three or four large pieces. An ordinary blasting cap is shown in Fig. 120 together with other equipment for blasting.
Firing by electricity is generally safer and more satisfactory than by the use of ordinary caps and powder fuses. The explosion is more certain and its exact time is under the control of the operator. Fig. 121 shows a section through an electric blasting cap or detonator, commonly called an electric fuse. Delayed action electric detonators are made by inserting a slow-burning substance between the platinum bridge and the detonating substance. The time of delay is controlled by the depth of the slow-burning substance. Delayed action detonators are useful in tunnel work where it is desired to explode the charge in three or four stages in order that the debris from one charge may be out of the way of the following, and that the forces of the explosions may not serve to nullify each other.
Fig. 120.—Blasting Supplies.
Courtesy, Aetna Powder Co.
176. Care in Handling.—Some of the don’ts in the handling of explosives recommended by the U. S. Army Engineer Field Manual are: in the use of nitro-glycerine explosives of all kinds—