BURNING EXPLOSIVES.

Most high explosives, especially if in quantity, become heated to their exploding temperature after burning some time and they then explode violently from the slightest cause. A fall of a few inches, a blow from some object, or a kick with the foot is apt to cause them to explode. For this reason, leave burning explosives alone. Do not, under any circumstances, attempt to move them. Allow them to burn until consumed.

INCOMPLETE DETONATION.

In some instances, when permissible explosives are not completely detonated or exploded, some of the explosive burns, and the duration and length of the flame are so increased that the explosive becomes very unsafe. Incomplete detonation and setting fire to the charge occur when the detonators used are not strong enough, when the explosive is partly or entirely frozen, and when the detonator is displaced.

FIGURE 7.-Arrangement of a double charge in the same hole, electric detonators

being used.

SIZE OF DRILL HOLE.

Since permissible explosives are made in cartridges of uniform size for certain grades and of uniform length for all brands; if the miner knows what size he is to use he can so drill the hole that the cartridge will fit it snugly. In this respect these explosives have a distinct advantage over black blasting powder, which is generally furnished loose and must usually be made into cartridges after reaching the working place.

FIRING A DOUBLE CHARGE OF PERMISSIBLE EXPLOSIVE IN THE SAME HOLE.

Conditions frequently suggest that a charge of explosive be distributed in different parts of the same drill hole. Part of the drill hole may pass through coal that is harder than the rest of the bed; balls and streaks of sulphur may occur in one part of the bed or a part of the face may project so that an unequal distribution of the explosive is required, as when the mining machines have cut the coal so as to leave one part of the face in advance of the other. Under such conditions some mines using permissible explosives arrange the cartridge of explosive as indicated in figure 7. The entire length of the drill hole is 5 feet and the diameter is 2 inches. Two sticks

measuring 8 by 12 inches are placed at the point of the hole and onehalf of a stick, or a cartridge 4 inches long, is placed near where the extra work is to be done. An electric detonator is placed in each of the charges and they are wired as shown in figure 8, in series, as all shots must be when more than one shot is to be fired and a battery is to be used to explode them. This practice although it is followed in some coal fields is not to be commended because the electric detonator in the half cartridge, or the outside one, may be defective, whereas that in the inside charge may prove to be good and the explosion of the latter will blow the first charge unexploded into the mine air where it may burn and cause an explosion, or may fail to explode and later may be exploded by the miner striking it with his pick while loading the coal.

MAKING CHARGES.

As permissible explosives are put up in cartridges of standard size they are easier to carry and are much more readily made up into charges and prepared for firing than black blasting powder is.

8 in.

FIGURE 8.-Arrangement of detonator wires for double charge.

They have all the convenience of dynamite and are less dangerous. A charge of permissible explosives can be prepared and fired much more quickly than a charge of black blasting powder.

SIZE OF CARTRIDGES.

The different kinds of permissible explosives are put up in cartridges that are from inch to 2 inches in diameter, but are all 8 inches long. When the drill hole is of less diameter than the cartridge of permissible explosive that is to be put into it the explosive must be removed from the wrapper and a new cartridge made that will fit the drill hole. If the cartridges are much smaller than the drill hole they are often slit lengthwise and the parts tamped into the drill hole. This should not be done if the hole is a wet one, because a permissible explosive will lose strength and fail to explode if left long in contact with water.

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PLACING THE DETONATOR.

Caps" or detonators should be securely clamped to the fuse with a crimper when fuse is used, and they should not be wholly

buried in the explosive, but at least one-eighth of an inch of the "cap" or detonator should stick out.

A common cause of failure in attempting to fire shots by aid of fuse is due to the loosening of the detonator from the fuse because of insecure crimping. When fuse is used the detonator should be tightly crimped to the fuse by a proper "cap " crimper.

DETONATOR SHOULD FIT TIGHTLY IN CARTRIDGE.

If the detonator does not fit snugly into the cartridge, the space left about the detonator prevents the full force of the detonator being exerted on the explosive, for part of the force will be wasted on the air that fills the space. The weaker the initial detonation the weaker will be the explosion, so that it is important to watch all these details.

DETONATION.

When high explosives are exploded or detonated, which is the proper method of firing high explosives, the stronger and quicker the action of the detonator (exploder or cap) or electric detonator the greater will be the shock given the explosive, and the more effective will be its explosion. In order to insure the charge exerting its greatest force it should be properly tamped and should be detonated quickly and completely. If the explosion of the detonator or electric detonator is slow or incomplete; then the detonation of the charge will be slow and incomplete; more of the explosive will be needed to do the required work and larger quantities of poisonous gases will be given off than there would have been had the detonation been complete. When the detonation is quickest and most complete the least flame is produced by the explosion. A weak detonator or electric detonator that can not explode the explosive may produce enough heat to set fire to it, with the result that the burning explosive gives off poisonous gases. Again, a weak detonator or electric detonator may be strong enough to detonate an explosive, but the detonation may be too slow; in such cases the quantity and quality of the gases given off will be entirely different from what they would have been had the detonation been rapid and complete. Therefore the detonators and electric detonators used must be strong enough to give quick and complete detonation of the charge.

The cost of the detonators is so small in comparison with the cost of the explosive in the charge and the drilling and the preparation of the drill hole, that it seems foolish to buy weak detonators simply because they are a little cheaper than strong detonators. Furthermore, weak and unsuitable detonators cause high explosives to give off more dangerous gases. All permissible explosives lose a certain

amount of their strength with age and improper handling, yet by using strong detonators with them they may be made to explode completely.

IMPERFECT DETONATION BY USING LACED FUSE.

If fuse is laced through a cartridge of permissible explosive side spitting of the fuse may ignite the explosive and cause it to burn and explode before the detonator is exploded. In this way, the force of the explosive is reduced and noxious gases are produced.

LIMIT CHARGE.

There is for all the permissible explosives a limit charge of 11⁄2 pounds, which has been established by experiment. If more than this limit charge is used there is danger of gas or dust being ignited by the explosion, and it is good practice to use, if possible, only 1 pound in each shot, as this gives a factor of safety.

DANGER FROM POWDER AND PERMISSIBLE EXPLOSIVES IN THE SAME HOLE.

It is bad practice to use a permissible explosive and black blasting powder in the same drill hole. There is danger of the black blasting powder setting fire to the permissible explosive, causing a much greater amount of poisonous gases to be given off than if the explosive were detonated properly. It is not wise to use black blasting powder and permissible explosives at the same working face, for the reason that they may accidentally be placed in the same hole thereby causing fires and producing fumes.

LOOSE CHARGES SHOULD NOT BE USED.

If it becomes necessary to remove explosives from their wrappers in order to fit them to the drill hole, they should be inade into new cartridges that will fit the hole snugly. The cold surface of the drill hole and the moisture in the hole will chill the explosives and cause a loss of strength unless they are protected by the wrappers. Indeed, some of these explosives are almost incapable of proper detonation if left unprotected by the paper covering in a wet drill hole for a few

hours.

TAMPERS AND TAMPING.

Hardwood rods that fill the drill holes snugly are best suited for tamping permissible explosives. Wet slack or coal spalls must not be used for stemming. Moist clay gives the best results, and this clay should be made into dummy cartridges with thin paper covers, which can be broken easily by the tamper so that the clay is made to fill the drill hole tightly.

SHOTS ON THE SOLID.

When a drill hole that extends into the solid a few inches beyond the undercutting is charged and fired with a permissible explosive, the force does not extend greatly into the solid, but is expended mainly at the point of the drill hole. In some coal fields permissible explosives are used with more or less success in shooting coal off the solid wholly without undercutting, but like all other explosives, they do not do as good work on the solid as they do when the coal has been undercut.

BLASTING WITH PERMISSIBLE EXPLOSIVES.

FIRING WITH DETONATING SQUIBS.

In some mines in which permissible explosives are used they are exploded by what are known as detonating squibs, which are common safety squibs fitted with a detonator at the end opposite the match. These detonating squibs are placed in blasting barrels that extend from the mouth of the drill hole to the charge, and when the squib is ignited the detonator is projected violently against the charge and thus explodes it. The advantage of using these squibs is that they reduce the danger of withdrawing a missed charge, because there is no detonator in it.

FIRING WITH FUSE.

In some mines permissible explosives are fired with fuse, but this practice introduces dangers. Common fuse, when laced through the cartridge, as it often is, breaks at the bends, so that sparks reach the explosive and the latter ignites and burns. A burning permissible explosive gives off dangerous gases and loses its safety properties. There is also a tendency for miners to tamp holes with less care when they employ fuse. The detonator may be pulled out of the cartridge or come off the fuse, thus causing a misfire or delayed fire.

Weak detonators are often used with fuse, and thus detonate the explosives inefficiently. Moreover, burning fuse not only gives off a dangerous gas, carbon monoxide, but is also likely to burn through at such points as have been injured and thus to set fire to the charge or to dust in the drill hole.

BLASTING WITH ELECTRICITY.

It is generally conceded that electricity is safer, quicker, more efficient, more certain, and cheaper than any other means of setting off blasts. Often adjacent blasts in the same working face yield the best results in the quantity and quality of the coal produced when they are fired at the same time. This is possible only when the shots