During 1912 some 2,360 workers perished in the coal mines of this country. Great as are the normal dangers of coal mining, it may be stated with approximate certainty that more than half of these men would be alive to-day if all the miners and mine operators of the country had acquainted themselves with and had used with proper care the mine-safety precautions now recognized as possible.

Both directly and indirectly, one of the important causes of death in coal mines is the misuse of explosives. Falls of coal and roof, dust and gas explosions, blown-out and windy shots, premature blasts, hangfires, and misfires are all, to a considerable extent, the result of the misuse of explosives or the use of explosives that ought not to be used.

The Bureau of Mines has therefore prepared this circular on the use of explosives in the hope that it will be of service to the miner in avoiding those accidents that result from either the use or misuse of explosives. Every miner should remember that the risks he needlessly takes not only endanger his own life but also endanger the lives of his fellow miners. The carelessness of any one man, at any time, may cause the loss of the lives of all the men in the mine.

This circular aims to present in simple words some of the greater dangers and the more important precautions to be observed in the use of explosives. It is divided into two parts: The first part treats of the use of black blasting powder; the second part treats of the use of permissible explosives, dynamite, and other "high" explosives. Because black blasting powder is still used in coal mining some precautions to be observed in its use are given in this circular.

The Bureau of Mines has at Pittsburgh a large experiment station where experiments with explosives have been made continuously since the beginning of this work in 1908. The results of these investigations and the hearty cooperation of explosives manufacturers with the bureau have brought about the development of explosives far less dangerous than black blasting powder and dynamite. These explosives are known as "permissible explosives," and their names are published by the bureau from time to time. Each day sees an increase in the use of these permissible explosives and a proportionally smaller use of black blasting powder. Dynamite as well as powder is used less as its dangers become more clearly recognized.

The primary object in the development of these permissible explosives was to reduce the frequency of disastrous explosions in dusty coal mines by the use of an explosive that would be least likely to ignite either inflammable dust or gas in these mines. The flame from the explosion of black powder lasts from 2,500 to 3,500 times as long as does the flame from these permissible explosives, and is therefore more likely to ignite inflammable gas or dust.

In addition to the lessened risk of fire and explosion, there are other advantages in the use of permissible explosives. Among them are less damage to roof, cleaner ribs, fewer blown-out shots, less scattering of coal, fewer air blasts and windy shots, and the absence of smoke. The investigations of the Bureau of Mines show that in many mines where miners have stopped using black blasting powder and dynamite and have used permissible explosives in the proper manner they have found their work more agreeable as well as safer. Several times within the past year mine disasters have awakened the operator and the miner to the need of earnest cooperation in an effort for greater safety. The miner can do little alone-nor by himself can the operator accomplish much-but by working hand in hand the operator and the miner in other countries have greatly reduced accidents. The accomplishment of this has required wise laws and regulations, based on fact and experience, and strict discipline in the mine.

To reduce the accidents due to the misuse of explosives these things are important: (1) The use of the proper permissible explosives; (2) the entire handling and firing of these explosives by shot firers who have been carefully selected and trained; and (3) strict rules for these men and careful supervision of their work.

J. A. HOLMES.

THE USE AND MISUSE OF EXPLOSIVES IN COAL MINING.

By J. J. RUTLEDGE.

BLACK BLASTING POWDER.

MANUFACTURE.

The black blasting powder used in coal mines is usually made of 73 parts of Chile saltpeter (sodium nitrate), 16 parts of charcoal, and 11 parts of sulphur. Each of these substances is ground to a fine powder; then they are mixed in the above proportion and again ground, being moistened with water during the grinding to prevent explosion. Cakes are pressed out and then broken into grains, which, after a thorough drying, are rounded and glazed by rubbing with graphite in a revolving barrel. The grains are sorted with sieves into the various sizes in which the powder is sold. The object of rounding and glazing the grains is to render them free running. The glaze also retards their taking up moisture from the air.

No one should attempt to manufacture powder in any other place than a well-equipped powder mill, and only experienced powder makers should do this work. The attempt to make and use homemade blasting powders has caused many serious accidents.

GRADING.

The sizes of black powder usually offered for sale are CCC, CC, C, F, FF, FFF, and FFFF. Of these, CCC represents grains about five-eighths of an inch in diameter, and FFFF represents grains about one-sixteenth of an inch in diameter. The table following, taken from Bureau of Mines Bulletin 17, "A primer on explosives for coal miners," shows the relation between the letters designating the sizes of the powder grains and the mesh of the screens by which the different sizes are separated.

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Relation between sizes of black blasting powder and separating sieve.

In order that the best results in the use of black powder may be obtained the grains should be of uniform size, so that the miner, having determined the best size of grain for his particular work, can easily use it again. It is unfortunate that not enough care is always taken in separating the grains into the different sizes. Formerly much care was exercised in sorting and grading black powder, particularly the larger sizes. Recently there has been a tendency among some black-powder manufacturers to mix the smaller and larger sizes of powder together and, in a few instances at least, to mark the cans containing such mixed sizes with the letter or letters of the larger size.

When the miner obtains such a keg of powder marked with the same letter as the powder that he has been using he gages his shots as he has previously done, taking it for granted that the powder is identical in character and behavior with the powder with the same mark that he has used before. He may or may not discover that the new powder contains much smaller grains mixed with the larger grains. If he does discover the smaller grains, he does not consider their presence important, and uses the same quantity of the mixed grains that he has of the larger grains. But the smaller grains which fill in the spaces between the larger grains, are more easily ignited, and burn faster than the latter, so that when the blast is fired they are the first to ignite and explode, driving the larger, partly ignited grains out of the drill hole to expend their explosive energy on the atmosphere of the mine, thus causing a windy shot. All black powder should be graded to standard sizes, and a certain number of cubic inches of it should weigh a pound.

OPENING KEGS OF POWDER.

When a miner drives the point of a coal pick into a metal powder keg, as some miners have done, there is a likely chance of a fatal ex

plosion being caused by a spark struck from the impact of the pick on the keg. Such carelessness can not be excused, for only half a minute is needed to loosen the metal cap placed over the proper opening by the manufacturer. If the keg must be opened by punching a hole in it, a sharpened stick of hard wood should be used.

PAPER POWDER KEGS.

Because some miners will drive picks into metal powder kegs and because metal kegs are sometimes exploded by contact with bare electric conductors, especially trolley wires, cardboard kegs are now used. In some mines they are the only kegs used. They are of the same size as the metal kegs and are provided with metal caps for opening. If, however, the miner uses a pick or metal object to open them, no sparks can be made to explode the powder.

MAKING CARTRIDGES.

To be safe while making cartridges the miner must set his lamp at least 5 feet away, where sparks can not be carried by the air current to the powder. Many a miner has met death because sparks, or even the burning lamp itself, have fallen from his head into the powder.

The black blasting powder for a charge should always be measured, whether it is to be used loose or in cartridges. The miner should know absolutely how much powder he is using in a shot and not guess at it. Many practical miners of long experience have rules as to the quantity of powder required in drill holes of different sizes and depths, and always follow these rules in charging the holes. The result is always increased earnings and greater safety in producing coal. Formulas have been proposed for computing the quantity of powder required, but they are not satisfactory. The best guide is practice.

DANGER FROM USE OF LOOSE POWDER.

Loading loose powder into dry holes is a practice that has come in during recent years. In some instances a pocket-making or expanding drill is employed to enlarge the point of the hole, and loose powder is put into this cavity with a tin scoop fastened on the end of a broom handle. This powder is apt to run into any cracks or crevices in the coal, and all of it is not confined to the drill hole, as is powder incased in a properly made paper cartridge. Often these cracks or crevices open when the powder begins to explode and allow it to break out of the drill hole and expend its force on the mine air. If the explosion breaks the coal at all, instead of wedging the coal, it rends the coal with great force. In addition to being dangerous, the practice of loading powder loosely wastes much powder.

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