EXPLOSIVE FORCE OF BLACK BLASTING POWDER.

When black blasting powder explodes in a drill hole, the solid particles of the powder are suddenly transformed into highly heated gases, which, by reason of their tendency to expand, exert pressure on the sides of the drill hole and do useful work in rending the coal or rock. The rate of burning of black blasting powder, as determined by the average results of a large number of tests at the Pittsburgh experiment station of the Bureau of Mines, is about 1,500 feet per second. When black blasting powder is exploded in the laboratory apparatus as nearly as possible in the same way as in the drill hole in the mine, 1 pound of powder at normal pressure (barometer 30 inches) and temperature (freezing point) produces 5.30 cubic feet of gas; that is, 90,000 cubic inches of gas is confined in a space of 23 cubic inches. The pressure on walls in contact with the powder exerted by the expanding gases, as determined by experiments at the Pittsburgh experiment station, is 68,500 pounds, or over 34 tons per square inch. A consideration of these facts will give some idea of the tremendous pressure exerted on the mine air by the suddenly freed and highly heated gases when a shot blows out the stemming.

BLACK POWDER AND DYNAMITE.

In blasting in headings or entries in some mines of certain districts a small quantity of dynamite was formerly used with black blasting powder. Usually a stick of medium-strength dynamite was placed at the point of the hole, and the remainder of the hole was filled with black powder. The purpose was to have the dynamite "cut ahead" of the point of the drill hole into the solid and thus give a soft place in the coal seam at which the entryman could begin his next cutting. Shooting these "combination" charges is still done in some coal fields, both in rooms and in entries, but the practice is dangerous. It causes fires and gas and dust explosions in highly volatile gaseous coals, as the flame of dynamite, though shorter, is much hotter than that of black blasting powder.

There is a great difference between the rate of detonation of dynamite and the rate of burning of black blasting powder. Dynamite usually explodes or detonates about 10 times as fast as black blasting powder burns. As a result the dynamite in a "combination " charge expends its force before the black blasting powder is well ignited. If the dynamite is put in the point of the hole, burning powder will be expelled from the hole by the quicker dynamite and ignite any dust or gas present, burn any person in its path, and, perhaps, cause an extensive, disastrous dust explosion.

When a detonator is placed in the dynamite, many charges are fired too soon because the needle that is used to push the dynamite back to

the point of the hole strikes the detonator. Or the needle may be pushed against the detonator in making a hole in the powder for the squib. The usual reason for "combination" charges is that they are thought to cost less than the "straight" charges and to do better work. However, any possible advantages in cheapness and efficiency are more than offset by the attending dangers.

CARRYING AND STORING POWDER IN MINES.

A miner should not carry into the mine more than one day's supply of powder, which is seldom over 5 pounds. This should be carried in a rustproof can, preferably of copper.

It is extremely dangerous to take large cans of black powder into the mine. The small powder cans should be carried either by the miner or in a car with a dry, tight wooden bottom, the upper surface of which is absolutely free from bolts or other metal. Because electricity explodes powder, the car should be hauled by a mule or horse after the electric power has been shut off the wires. All powder cans in the mine, when not in use, should be locked in wooden boxes, which, if possible, should be used for no other purpose than the storing of powder. If possible, each powder box should be kept in a safe place at least 100 feet from the working face, and everybody carrying an open light or smoking should always keep at least 5 feet away from this box. Squibs, fuse, and detonators should never be kept in the same box with powder.

PLACING AND CHARGING HOLES.

DEFINITIONS.

Shooting off the solid.-By shooting off the solid is meant mining coal by blasting it without undercutting or shearing. Usually one blast opens up the coal face so as to present opportunities for succeeding blasts, and accordingly the success of the second shot depends on the success of the first.

Shot. The term shot is used to denote either the loaded drill hole or the part of the coal bed that is dislodged by any particular blast. Dead hole.-A dead hole (fig. 1) is a hole that extends into the solid coal beyond the part that can be blasted off by a charge of explosives not greater than the limit charge allowed by good mining practice or by State law. Or a dead hole may be defined as a hole placed in a shot whose width at the point, measured at right angles to the line of the hole, is so broad that the heel is not strong enough to at least balance the resistance at the point.

Heel. The heel (ABEF, fig. 2) is that part of the shot, or that part of the coal bed to be moved, that is outside the powder, and it

is usually occupied by the tamped portion of the drill hole. The distance AF, figure 2, is also often called the heel of the shot.

Toe. The toe (BCDE, fig. 2) is that part of the shot, or coal bed to be moved, that extends from the outer end of the powder to the point of the hole. The distance CD is also often called the toe of the shot.

FIGURE 1.-A hole (A-G) with a dead end, C-G; a dangerous shot, H-I; and a properly placed shot, 2.

Line of least resistance.-The line of least resistance is a straight line drawn from the charge to the nearest open face of the rock being blasted. Experience has shown that the best results are obtained when this line is perpendicular to the line of the drill hole, and that the length of this line of least resistance must be shorter than the length of the drill hole. Unless such a condition exists, the powder will expend its force in the direction of the drill hole and fail to do any work. This principle is known to all experienced miners, especially those familiar with blasting coal off the solid, and they often

follow this rule when they undermine a heel that is too large for a toe, in order that the resistance of the heel may equal that of the toe, and the shot may be better balanced. The foregoing rule in regard to the length of the line of least resistance is only another way of saying that a shot should be well balanced.

In figure 2, a line from C to D would represent the line of least resistance, but it would be the same length as the drill hole. It should be at least 3 feet shorter in order to balance the shot and the Idrill hole shown.

SIZE OF DRILL HOLE.

Powder

9 feet

6 feet

C

D

TOE

Straight drill holes of uniform diameter give better results than crooked holes, and as a rule, in most kinds of coal, short holes give better results than long holes. This is especially true if the coal is shot off the solid. If coal augers are kept at a standard gage, and are kept sharp, the miners' work will be done more efficiently and safely than if the gage is lessened and the augers are allowed to grow dull. A certain gage should be determined by experiment as suitable for a given coal field, and every miner should adhere to this. Holes of large diameter increase the dangers that always attend solid shooting. After it has been demonstrated by experiment that a certain. size of hole is best adapted

6 feet

Stemming

Alfoot+++

3 feet

A

3 feet

HEEL

E

FIGURE 2.-Plan view of a poorly balanced shot.

to produce coal safely and efficiently in any particular coal field, this size should be required by law.

REGULATING LENGTH OF DRILL HOLES.

In the endeavor to prevent holes being drilled into the solid in mines where all the coal is undercut, some mine foremen insist that the miners have their longest coal augers at least 1 foot shorter than the thickness of the coal bed.

Since it is good practice to undercut the coal bed only to a distance equal to the thickness of the bed except in very thin coal seams, the practice just mentioned will prevent holes from being drilled into the solid if they are drilled parallel to the ribs.

POORLY BALANCED SHOTS.

Figure 2 shows a dangerous shot. It may blow the coal and yet produce a windy shot, as the toe is too heavy for the heel. The toe

and heel should be of the same width, and the length should be in proportion to the width of the shot. Neither the toe nor the heel of any shot should be wider than the height of the seam. If the toe is light and the heel is strong the shot will blow out at the toe, and gas or dust in the mine air may be ignited.

In shooting off the solid, the length and the width of a shot should bear a certain relation to each other. To use a miner's term, the shot should be "well balanced." If the toe is much heavier than the heel, the blast may cause an explosion by reason of the heel being blown off, even when the blast does bring down the coal. On the other

7 feet

4 feet

2

9 feet

FIGURE 3.-Plan view illustrating a safe method of shooting the shot shown in figure 2.

hand, if the toe is much lighter than the heel, a blown-out shot may result, and often does, even when the coal has been undercut. Experienced miners often undercut a heavy heel somewhat in order to balance a light toe.

In solid shooting, the width of the shot at the point (as CD in fig. 2) in beds of coal 6 feet or less in height should not be greater than the thick

ness of the coal, nor more than 5 feet at the most, and should not be more than 5 feet in beds over 6 feet thick.

A shot measuring more across the point than the height of the coal to be removed is generally dangerous as well as unprofitable to the miner. Holes perfectly horizontal and parallel with the partings in the coal, as well as those in soft streaks of coal or shale in the coal bed, are unsafe holes.

When coal is undercut no shot should be "drilled on the solid," or beyond the undercutting into the solid coal. No hole should be drilled beyond the end of the cutting or "loose end" or "chance," when coal is shot off the solid.

The shot shown in figure 2 could be made safe by drilling and firing such holes in the coal as are shown in figure 3. The holes could be fired in the order indicated in figure 3, with the result that the coal would be broken safely and in such a way as to yield the largest amount of lump coal. The quantity of powder used would also be less than in the shot shown in figure 2. Drilling the holes as shown in