Instead of an ex

a rigid tube resembling a manifold connection. pansible tube transmitting pressure to break the coal, the several plungers move outward when hydraulic pressure is applied with a pump, and the coal is broken by the combined pressure of the series of plungers acting on the interior of a borehole. Similar advantageous claims are made for this device as for the expansible hydraulic tube. A hydraulic blasting device, known as the "Hydrabuster," is shown in figure 7.

HOW EXPLOSIVES ARE FIRED

IGNITION BY FLAME

Black blasting powder in all its forms is ignited by flame, which causes it to explode. The sources of flame for igniting black blasting powder are squibs, fuse, electric squibs, delay electric squibs, and delay electric igniters.

A

Squib.-A squib, sometimes called a miner's squib, is used for igniting black blasting powder which has been charged in a borehole. miner's needle is inserted in the stemming material and when withdrawn leaves a small-diameter hole extending from the outside end of the borehole to the explosive charge. A squib is about 611⁄2 inches long and consists of a thin paper tube of powder sealed at one end by a wax plug and twisted at the other end into a paper taper, which acts as a wick or fuse. The wax plug protects the squib from moisture and is pinched off immediately before the squib is inserted in the small hole left by the miner's needle with the taper nearest the blaster. The taper is ignited by a match, open light, or touch paper and burns slowly enough to permit the blaster to retire to a safe place. When

the flame from the ignited taper reaches the powder train it produces a rocketlike effect because the squib is forced back through the small hole and its flame ignites the charge of black blasting powder. Figure 8, 4, shows the use of a needle in firing a charge of black blasting powder with a miner's squib.

The burning rate of the taper is an important safety consideration because enough time must elapse for the blaster to retreat to a place of safety. It may range from 36 to 82 seconds and may be affected materially by the mine atmosphere if squibs are stored underground.

If squibs absorb much moisture, their rocket action may be reduced greatly, or they may become worthless. The burning time of the taper depends upon whether the taper has been shortened or untwisted. It also depends upon the quality of the atmosphere in

FIGURE 8.-A, Needle used in firing black blasting powder with a miner's squib; B, hole charged with black blasting powder and primed with a fuse.

which it burns and the direction of burning, whether horizontally, downward, or upward.

Considerable smoke is produced by an ignited squib, and the needle hole that gives access to the powder charge sometimes reduces the effectiveness of the stemming material. Slight obstructions in a needle hole may prevent a squib from reaching the charge of black blasting powder, resulting in misfired or delayed blasts. To overcome this difficulty, a blasting barrel of - or 1/4-inch steel pipe sometimes is inserted before tamping in wet places or in dipping holes instead of a miner's needle. The blasting barrel is not withdrawn before the hole is fired but is recovered after the blast.

A deficiency of oxygen in mine atmosphere will reduce the burning time of a squib taper or may prevent its burning. The taper may

burn rapidly if its point is below the squib proper, it may burn at its normal rate when it is horizontal, but usually it burns more slowly if its point is above the main portion of the squib. It is essential that squibs be kept dry by storing them in metal containers used exclusively for this purpose.

Fuse.-Fuse, commonly called "safety fuse," consists of a center thread or threads surrounded by a train of fuse powder and enclosed in textile wrappings and waterproof material. It is used for carrying a train of fire to a charge of black blasting powder (fig. 8, B) or to an ordinary blasting cap. Figure 9 shows a charge of black pellet powder primed with fuse. Fuse is designed to burn at a uniform rate.

FIGURE 9.-Pellet powder primed with electric squib and with fuse.

FIGURE 10.-Preparing fuse for lighting.

Accidents due to short-fusing or to mistakes as to the rate of burning of fuse generally result in fatalities or serious injuries; they can be eliminated by educating the users of fuse to remember that the average burning rate of fuse is 30 to 40 seconds a foot. A good rule-of-thumb is to allow 2 feet a minute and not 1 foot a minute and to remember that mishandling, improper storage, and the use of certain procedures in stemming and tamping more or less affect the burning rate. To be on the safe side, therefore, assume that any fuse may burn at a rate of 2 feet a minute; even then the factor of safety is by no means large.13 The method of preparing a fuse for lighting is illustrated in figure 10. Fuse should be stored in a cool, dry, well-ventilated place; it should not be exposed to very low temperatures because it becomes brittle, and if it is handled when cold the powder train may become damaged. The better brands of fuse are waterproofed, but when they are used in very wet places the waterproofing does not overcome the difficulty

13 Harrington, D., and Warncke, R. G., The Burning Rate of Fuse: Bureau of Mines Inf. Circ. 7281, May 1944, 10 pp.

of igniting the fuse. The common methods of igniting fuse and the "spit" of the fuse itself will also ignite flammable gas mixtures. A pull-wire-fuse igniter will ignite fuse with certainty when properly adjusted to a fresh-cut fuse; it is especially advantageous in wet places because it insures ignition. The danger of ingniting gas with a fuse igniter probably is reduced, but the hazard of using fuse remains; the difference in hazard is in degree rather than kind.

Fuse is commonly packed and sold in packages, each containing two 50-foot rolls, one nested within the other. A coil of fuse should be taken to a working face and cut off after the explosive charge has been stemmed to avoid the danger of "short-fusing."

Electric squib.-An electric squib "consists of a thin metal-tube shell 11/2 inches long, closed at one end, and a deflagrating charge in the closed end. An electrical firing element attached to leg wires is sealed into the other end with a waterproofing compound and sulfur. Electric squibs are waterproof and will resist water in wet boreholes much better than the black blasting powder which they are intended to ignite. When current is applied to the leg wires the firing element flashes, ruptures the shell, and ignites the deflagrating compound. Intense flame issues from the ruptured shell into the black-powder charge and ignites it.

The leg wires of an electric squib should be shunted with a metal seal or by twisting the exposed ends together. The shunt should be maintained until immediately before the legs are connected to a shotfiring cable or circuit as a precaution against premature explosion from stray or other electric currents. Electric squibs are superior to cither miner's squibs or fuse for exploding charges of black blasting powder. Figure 9 shows a charge of black pellet powder primed with an electric squib.

Delay electric squib.-This squib is similar to an electric squib except that after electric current is applied a short time elapses before the powder charge is exploded. Delay electric squibs are made with several delay periods; the thin metal shells vary in length, the first delay having the shortest shell and each subsequent delay increasing in length. In manufacturing these squibs the several delay periods are obtained by inserting delay elements between the firing element and the deflagrating charge; delay periods in increments of 1 to 2 seconds each may be obtained.

Delay electric igniter.—A delay electric igniter is provided with a means of igniting fuse by an electric current. It consists of a copper tube with an electrical firing element and leg wires sealed into one end and a piece of fuse crimped into the other. The lengths of fuse range from 3 inches upward, increasing in increments of 2 inches to provide first, second, third, and higher delay periods. The free end of the fuse is protected by a waterproofing dip; about 1 inch must be cut from the end of the fuse to remove this dip so that the powder core of the fuse will be exposed before use. Delay electric igniters sometimes are used for rotation firing of black blasting-powder charges; this results in dependent blasting which involves a rapid succession of shots fired with one application of an electric current. The Bureau of Mines is opposed not only to the use of black blasting powder in bituminouscoal mines but to the firing of shots in rapid succession, except when

14 du Pont, E. I., de Nemours Co., Inc., Blasters' Handbook: Wilmington, Del., 11th ed., 1942, 339 pp.

driving passageways in rock, because of the hazards of dust explosions and of igniting gas released by previous shots.

In all instances where firing is done by applying an electric current the leg wires of the electric squib or igniter should be shunted until immediately before they are connected to a firing circuit to prevent accidental blasting from stray currents or electrical conductors contacted inadvertently.

DETONATION OF EXPLOSIVES

Blasting cap.-A blasting cap is a detonator for firing permissible explosives or dynamites and is fired with fuse. It is a small metal Cap shell

Powder core

FIGURE 11.-Properly and improperly cut fuse placed in a blasting cap.