moisture. Oils such as gasoline or kerosine, grease containing light oils, turpentine, or linseed oil should not be allowed to come in contact with the fuse as these oils may penetrate to the powder and cause misfires.

The average burning rate of fuse is about 40 seconds to the foot.19 Short fusing should not be tolerated, and no fuse under 30 inches in length should be used in blasting. A minimum length of fuse should be adopted for each class of work, and fuse should always project at least 1 foot out of the hole. A standard practice should be adopted by each operation for trimming fuse to insure proper rotation of shots and to avoid cutting any fuse too short for safety. The use of master fuse lighters or the practice of "bunch spitting" helps to decrease the time consumed in lighting rounds. Some timing device should be provided to insure against overstaying the safe limit of time while spitting; hot wire spitters are being widely used for this purpose, but the advantage of their use is lost if men do not leave when the spitter is burned out. When all the holes cannot be safely lit with one spitter, two men can divide the work if their spitters are lighted at the same time. There should be the fewest number of men possible at the face when spitting, as unnecessary men only increase the exposure and may confuse or distract the miner doing the spitting; where one man spits all the holes, another should be present to give any needed assistance in spitting or supplying a light.

Tools, excess explosives, or other materials should be taken away from the face to be blasted, and all necessary work should be completed before spitting. Air or spray valves should be arranged so that no time is lost in opening them on the way out.

POSITION OF DETONATOR IN PRIMER

REQUIREMENTS FOR PRIMERS

A primer is a cartridge of explosive containing the detonating device for setting off an explosive charge. It is placed in the borehole so that the remainder of the charge is detonated when the primer is fired. Primers should be made carefully to meet the following requirements for safety and reliability of results:

The primer should be assembled so it can be loaded safely, easily, and in the preferred position in the charge.

The igniter or detonator should be secured so it cannot be pulled out of the primer cartridge without use of some effort. The fuse should not be subjected to sharp bends or the wires of electric detonators to tight knots.

The cartridge should not be slit, as the whole cartridge is necessary to hold the detonator properly and to protect it from abrasion and blows during loading.

In preparing primers maximum protection is afforded the detonator when it is put in the exact center, parallel with the long axis of the cartridge; however, this arrangement may present difficulties in loading, and modifications may be made accordingly. Holes for inserting detonators in cartridges may be punched with a pointed punen of wood (as shown in fig. 24) or nonsparking material, such as the handle of a cap crimper. The hole should be longer than the detonator and big enough so the detonator can be inserted without 19 Harrington, D., and Warncke, R. G., The Burning Rate of Fuse: Bureau of Mines Inf. Circ. 7281, 1944, 10 pp.

use of pressure. In the method generally recommended the hole for the detonators is made in the end of the cartridge so that when the detonator is inserted it will be approximately in the center, entirely surrounded by the explosive and protected from impact. Another method is to punch a slanting hole in the side of the cartridge and near the middle so that when the detonator is placed it will be approximately in the center of the cartridge; this method has some hazards and disadvantages. When capped fuse is used it should be tied to the cartridge with string to prevent the detonator from being pulled out in handling.

Any hole not in the center of the cartridge is unsatisfactory, as the detonator will not be in proper position for efficient work and may

FIGURE 24.-Preparing primers and loading holes.

result in accidental detonation upon impact when handling or loading. Fuse should not be laced through a cartridge because the bending may result in cracking the protective covering of the fuse, making it possible for moisture to enter and cause a misfire; besides, there is an added chance for side spits to ignite the charge.

In a method of making primers that has been generally used in some sections of the Southwest, the capped end of the fuse is inserted centrally in one end of the cartridge; the primer is placed near the bottom of the hole, with the end containing the detonator in that direction and the fuse turned back along the cartridge. This method has the advantages that the primers can be easily made and quickly loaded, the detonator and fuse are not easily pulled out in loading or by the firing of other holes, the charge can be tamped, and the

detonator is pointed toward the bulk of the charge. The disadvantage of the sharp turn of the fuse around the end of the cartridge is that the fuse may be pinched, cutting off the train of fire, or moisture may enter through cracks at this point. This type of primer has been

B000

FIGURE 25.-Methods of preparing primers in electric blasting. successful only in dry holes, with a good grade of fuse at warm temperatures, and is not recommended by explosives manufacturers. Primers with electric blasting caps can be made safely with either of the methods described for detonators and fuse. Manufacturers of explosives recommend making primers with electric blasting caps by punching a hole from the center of one end of the cartridge in a slanting direction to come out at the side 2 or 3 inches from the end, doubling over the leg wires of the electric blasting cap and pushing

them through the hole and looping them around the cartridge, punching another hole in the same end straight down, and inserting the cap so that it is well-covered by explosive and takes up the slack on the wires (fig. 25, method 2). In this method the wires do not cross each other, and the detonator is correctly placed in the cartridge.

Another method of priming with an electric detonator that can be used with less difficulty for small-diameter cartridges and will enable the primer to be used either near the top or the bottom of the borehole, consists in punching a hole transversely through the cartridge, running the cap through this, and then punching another hole in the top of the cartridge for the cap. This avoids crossing the wires and places the cap in the proper position pointing toward the bulk of the charge (fig. 25, method 1). Such a primer does not hang vertically from the wires and hence it is not so convenient for loading in vertical as in horizontal or slanting holes.

The wires of electric blasting caps should never be fastened around explosive cartridges by half-hitching them, as a strong pull is likely either to break the wires or to cut the insulation (fig. 25).

In priming with delay electric detonators it may be difficult to punch a hole long and large enough in a cartridge to take the longer delays, and forcing the detonator into hard or tightly packed cartridges is likely to ruin it. In such cases it is better to cut the cartridge lengthwise down one side, place the detonator inside, then close the halves tightly about the detonator and tie them securely.

SAFETY PRIMERS

Safety primers of wood or fiber have been tried at a number of mines and tunnel projects. These are plugs with holes for the detonators and wires in which the live ends of the detonators are flush with the ends of the plugs. The primer is placed in the bottom of the hole with the detonator pointing back toward the collar. The advantages are that explosives are not left in the bottom of the hole, the detonator is held parallel to the charge, explosives at the bottom of the hole can be strongly tamped without danger of accidental discharge, and live primers thrown out in the muck pile are less dan

gerous.

The use of safety primers has not become general, probably because they must be placed in the bottom of the hole (which may not be favorable to full efficiency of the blast), the volume of carbon monoxide produced is increased, and there is a small extra cost. It is also probable that one reason for slow acceptance of such a device is the inertia of established practices. Tests conducted by the Bureau of Mines with wooden and cardboard safety primers at the Mount Weather Testing Adit indicated that about twice the volume of carbon monoxide is produced when these safety primers are used in firing an average 4-foot round; the proportion would be decreased in firing rounds of greater length. In well-ventilated places the disadvantage of the increase in carbon monoxide should be weighed against the benefits derived from the greater safety in handling and loading wooden safety primers.

POSITION OF PRIMER IN BOREHOLE

The position of the primer in the borehole has an effect on the efficiency of the explosives charge and the possibilities of misfires.

The detonation of the primer has greater force in the direction in which the base of the detonator is pointing, so if no other factors were involved the primer should be at one end of a charge of explosives and the detonator should point toward the bulk of the charge. Demonstrations reported by the Bureau indicate that the force of the explosion increases as it progresses through the charge from the detonator.20 On this basis, without consideration of any modifying influences, the best position for the primer would be at the outer end of the charge in order to have the greatest force applied at the back of the hole. Advantage can be taken of this method where single shots are fired or where a number of shots are fired at one time, since there will be no danger of misfires from cutting of the fuse or pulling out of the primer by preceding shots. Where stemming is used, one cartridge should be placed between the primer and the stemming to protect the primer from damage and prevent accidental detonation during tamping.

In no case should the primer itself be tamped. In raises or wherever holes inclined upward are loaded the primer and the following cartridge should be put into the hole at the same time. When this is done, enough pressure may be applied with the tamping stick to insure that the primer will stay in place.

In blasting rounds where all the holes do not fire at the same time the fuse of a number of holes may be cut off at the collars by the blast of the first holes to fire, primers and other cartridges may be pulled from holes, and portions of some holes may be broken off, carrying the primers with them if the primers are near the collars. To avoid misfires from these causes, the primer should be placed near the back of the hole, usually with one unprimed cartridge at the back to protect the detonator. The unprimed cartridge may be tamped well before the primer is loaded.

With electric detonators the primer can be placed safely so that the detonator points toward the charge, but where fuse is used it must be bent around the end of the primer or left pointing toward the bottom of the hole. In wet ground the concussion of blasting may force moisture into the fuse at such bends, causing misfires; to avoid this risk the primers of lifters may be placed near the outer end of the charge with the detonator pointing inward, as there is less danger of the fuse of such holes being cut off.

In hard rock the explosive must be compacted in the drill hole to have as great a loading density as may be obtained by ordinary methods to secure the maximum effect of the explosive and to make sure that the detonation will not be halted by any gas in the charge. With the exception of the primer, the charges should be tamped to fill completely the section of the drill hole, and to do this it has been general practice to slit the paper of the cartridges, which allows them to spread when pressure is put on them with the tamping stick. A type of cartridge has been developed with a perforated-paper wrapping so that slitting is unnecessary. The space in the drill hole is usually incompletely filled unless the explosive used is of a plastic nature; unless the hole is in firm rock there may be gaps caused by loose material between the cartridges or irregularities in the sides of the hole. If the loading stick does not have a square-cut end nearly 20 Gardner, E. D., Drilling and Blasting in Open-Cut Copper Mines: Bureau of Mines Bull. 273, 1927, pp. 52-53.