primer cartridge be placed first in the hole; this method is sometimes termed "back initiation." A more important point in priming is that the electric detonator should be inserted in the primer so that the "business" end of the detonator points toward the main part of the explosives charge, because the electric detonator produces a powerful shock, which exerts its greatest force on the charge of explosive away from the charged end of the cap and in line with its longer axis. Lack of care in preparing and placing the primer may cause misfires or hangfires or may burn the charge in the hole. Failure of the charge to explode also may be due to several causes, such as damp explosive at the cut end of a stick of explosive or foreign material between the sticks. PREPARING THE PRIMER The primer may be prepared in a number of ways to fulfill these requirements or to meet special or unusual conditions. In making primers it is also important that the electric detonator be secured in the primer so that it cannot be pulled out during the preparation and firing of the charge. Kinking of the leg wires or the use of half hitches to fasten them should be avoided wherever possible, as these may cause broken wires or short circuits. The leg wires should be protected as much as possible against injury. Figure 46 shows approved and objectionable methods of preparing primers. Method A.—The cartridge is pierced near its center with a wooden skewer, making a transverse hole as shown in figure 45. Another hole is made in one end about coincident with the long axis of the cartridge and deep enough to admit an electric detonator. A loop of the leg wires is inserted in the first hole and drawn through it and over the lower end of the cartridge. After the detonator is inserted in the end hole the loop is drawn tight. In the final position of the leg wires illustrated the primer would be used for back initiation. For front initiation the outby end of the leg wires would be up instead of down. Method B.-The cartridge is pierced by transverse and longitudinal holes as in method A. The third diagram of method B shows the detonator inserted and the leg wires in position for back initiation. The fourth diagram is similar to the third except that the leg wires are in position for front initiation. Method C.-The knot shown may result in kinking and breaking of the leg wires, and the detonator may be dislodged during the charging of a borehole. This method is inferior to either methods A or B and is not recommended. CHARGING THE HOLE After the primer and other cartridges are placed in the blasthole, in an amount not exceeding the permissible limit, incombustible stemming is inserted with a wooden tamping bar. In some instances, it has been found advantageous to leave an air space in front of or back of the explosive charge or around the charge by using cartridges of smaller diameter than that of the borehole. Such blasting is termed "cushion" blasting and is used principally to give larger "lump" production. In some States (Pennsylvania, for example) it cannot be practiced without permission from the State mining department. FIGURE 46.—Methods of Priming Cartridges. Methods A and B Are Satisfactory; Method C Is Not Recommended. Charges of permissible explosives exceeding 11⁄2 pounds, but not exceeding 3 pounds, may be used only when— 1. Boreholes are 6 feet or more in length. 2. Explosives are charged in a continuous train without deliberately deforming or crushing the cartridges. All cartridges should be in contact with each other, and the end cartridges should touch the rear of the hole and the stemming, respectively. 3. Class A or class B explosives are used. These classes of explosives produce volumes of toxic gases that are not objectionable when the explosives are charged and fired in a permissible manner. The 3 pounds does not apply to solid rock work. The detonator leg wires are unshunted and connected to the shotfiring cable, which has been shunted at the battery end, and the shotfiring cable is strung out as the shot firer retreats to a place of safety before firing. Figure 47 shows a shot firer tamping a hole with clay stemming, attaching a shot-firing cable to the charge, and unshunting the cable before connecting to a permissible shot-firing unit. FIGURE 47.-A, Tamping Charge With Clay Without Using Paper (the Preferred Way); B, Attaching Cable to Charge After It Has FIRING PROCEDURE Before firing a shot the shot firer should warn all persons in the vicinity of the blast of his intentions, and if necessary he should post guards to keep persons away from the shot. He should give ample warning not only when using explosives but also when using Cardox and Airdox as the blasting devices. A good procedure is for the shot firer to go to a safe position, connect the shot-firing cable leads to the blasting unit, give warning by shouting "Fire" 3 times at 2-second intervals, and then apply the electrical energy (fig. 48). After the smoke and fumes from a shot have been removed sufficiently by ventilation, the shot firer should return to the face, examine the roof, and in a gassy mine test for methane. MISFIRES A waiting period of at least 15 minutes should elapse before anyone returns to the face when a misfire occurs in electric blasting. After such failure the blasting cable should be disconnected from the source of power and the battery ends short-circuited before electrical connections are examined. These precautions also should be taken when blasting with Cardox. If it develops that the circuit outside the borehole is not the source of failure, the misfire, both with explosives and Cardox, should be handled under the supervision of a certified foreman or other competent person. Explosives should be removed by firing a separate charge at least 2 feet away from, and parallel with, the misfired charge or by carefully washing any stemming from the borehole with water and inserting a new primer. If a separate shot is fired, careful search should be directed toward the recovery of the undetonated charge. A misfired shot should be handled under the direct supervision of a foreman or a competent person designated by him. EXAMINATION AFTER BLASTING A working place should be examined carefully after blasting has been completed. The roof should be tested and, if necessary, the shot firer should direct that loose roof be taken down and the place timbered adequately before any coal is loaded. The air should be tested for methane in a gassy mine; if it can be detected with a flame safety lamp or if more than 1 percent can be detected with an electric methane detector, it should be removed by properly directed ventilation before the workmen proceed to load coal. The shot firer should examine the place carefully to determine that no fire is burning. 301286°55——6 |