always use a fuse and detonator, but the results are better when electric detonators and electric firing apparatus are employed. When these explosives are being tested at the Pittsburgh experiment station they are fired with electric detonators and batteries, and they are considered to have passed the tests only when they have been fired by these means; therefore they should be fired by these means when used in the mines, for, as pointed out in Miners' Circular 6, of the Bureau of Mines, permissible explosives are permissible only when used as prescribed. Other reasons why permissible explosives should be fired by shot firers are as follows: If a shot firer is employed the shots will generally be fired by means of the electric battery instead of by fuse and detonator. It is the shot firer's duty to see that the explosive is fresh, in proper condition, and properly fitted to the detonator or electric detonator; that the detonator or electric detonator is of the right strength; that all connections are properly and safely made; and that the shot is fired only when all persons are away from the face. He must see that the hole is not run into the solid, for he must measure the charge and hole and fire the shot. He must see that the hole is not overcharged. He is not interested in the loading of the coal and hence does not desire to have it broken up. He must wish to have the shot do the work it is planned to do but no more. He should fire no shot unless the working place is well timbered and take no risks in going into dangerous working places. He will not risk himself in places where a miner often goes in a hurry to load a car. When the shot firer fires the shots they are fired systematically and not intermittently as when the miner fires them. No gas is ignited as there often is when fuse is used and the miners fire their own shots. As the shot firer is a picked, experienced miner, he does not handle the detonators or electric detonators and explosives carelessly, but uses all possible care in taking the explosives into the working places and in preparing the charges, and he does not endanger the lives of others or his own, as miners often do when making up their own charges or when carrying detonators or explosives. The yield of coal per pound of permissible explosive used will be greater and the coal will be of better quality if permissible explosives are properly used. The work, if properly done, will tend to improve mine discipline, because every shot will be fired in turn and every miner will know when to expect the shot firer and will accordingly be prepared for him, so that there will be no waiting for holes to be drilled or for stemming to be prepared, but all supplies needed will be ready at the face when the shot firer arrives. What has been said above refers entirely to such mines as those in which the shots are fired by the shot firers while the men are in the mine. In those mines using permissible explosives where the shots are fired by electric means, after everyone has left the mine, the practice is, of course, somewhat different. As a rule all the shots are carefully inspected by shot firers before they are stemmed and tamped by the miner. The latter makes the connections to the electric wires at the face of the working place and the entrance to the same, but the shot firer attends to all other details, such as switches, line insulators, etc. He throws in all switches and fires the shots from the surface, and, after the shots have been fired, he makes an inspection of the entire firing system and examines all working places for shots that have failed and removes such defects as have caused shots to misfire. The general experience is that shot firers are necessary to a successful use of permissible explosives, whether the shots are fired while the men are in the mine or after they have left it. DELAY IN RETURNING AFTER FIRING. The same precautions should be exercised when blasting with permissible explosives as with black blasting powder. At least one-half hour should be allowed to elapse before returning to the face after a misfire when fuse or squibs have been used and five minutes when electric firing has been used. CLASSES OF PERMISSIBLE EXPLOSIVES. The four classes of permissible explosives described in Miners' Circular 6 of the Bureau of Mines are: CLASS 1, AMMONIUM NITRATE EXPLOSIVES. Nitrate of ammonia (ammonium nitrate) is the characteristic chemical used in this class of explosives; these explosives develop less heat when exploded than some others, but they lose sensitiveness and strength on exposure, as they absorb moisture from the air. They can not be kept ready for use long; they generally require the strongest detonators for their explosion. CLASS 2, HYDRATED EXPLOSIVES. The hydrated explosives contain substances, such as alum, Glauber's salt, and sulphate of magnesia, from which large quantities of water are obtained. On explosion this water appears as steam, which lowers the temperature of the flame and lessens the chance of its setting fire to dust and gas. However, as the strength of an explosive depends on the temperature as well as the kind and quantity of the gases set free at the time of its explosion, this lowering of the temperature of explosion weakens the explosive. CLASS 3, ORGANIC-NITRATE EXPLOSIVES. Nitrostarch is now the chief base of the organic-nitrate class of explosives. They produce only small quantities of poisonous gases on detonation. CLASS 4, NITROGLYCERIN EXPLOSIVES. Permissible explosives of the nitroglycerin class are similar to ordinary dynamite in that their chief explosive ingredient is nitroglycerin. They differ from dynamite in that chemicals are added that cause the explosives to give a much shorter and cooler flame than dynamite. Nitroglycerin explosives freeze at a temperature of 8° to 13° C. (40° to 45° F.), which is above the freezing point of water. This is a great disadvantage for this class of explosives. Some manufacturers use ingredients that lower the freezing point of the explosive. Such an explosive is often styled an "L. F. explosive." BLACK BLASTING POWDER. Although the use of permissible explosives is rapidly increasing, black blasting powder is still much used in coal mining. Realizing this, the bureau feels called upon to outline the less dangerous methods of handling this powder, and to suggest a few general precautions that have, in the light of experience, been found to be practical and wise. The first and general "don't" which the bureau wishes to urge is, Don't use black blasting powder. If it falls to your lot to use black blasting powder, the following precautions will be found useful: Never open a metal keg of powder with a pick or metal object. Use the opening provided by the manufacturer of the keg. Never make up charges or handle cartridges or powder with an open "light" on your head. Place the "light" at least 5 feet away on the return air side so that sparks from it will not fall into the powder. Never allow powder or other explosive to remain exposed. Keep it in a well-locked box at least 100 feet from the working face and in an unfrequented place. Never go nearer than 5 feet to a powder box or powder when wearing an open "light" or when smoking. Never use coal slack or coal spalls for stemming; it is dangerous. Even wet coal slack may, under some circumstances, cause an explosion. Use moistened clay, wet wood pulp, or other noninflammable material. Never withdraw a shot that has missed fire. Drill a fresh hole at least 2 feet from it but parallel to the old hole and fire this new hole. After the shot a careful search should be made for the unexploded charge to prevent its being struck by a pick and perhaps causing an explosion. Never fire a hole the second time. If the first charge proves useless, powder and labor are wasted in loading a hole a second time, Moreover, the first shot often cracks the coal so much that the second shot has a chance to blow out of the cracks, and thus a blown-out shot may result. Never use iron or steel tampers or needles. Have at least 6 inches of hard-drawn copper on the tamping end of the bar, or better still, use a hardwood tamping stick. The needle should be made entirely of hard-drawn copper. Never tamp shots with an iron or steel scraper, and do not push a cartridge into the drill hole with the scraper. The scraper rod should be tipped with at least 6 inches of brass or copper on the scraping end. Never allow the point of the coal auger to become dull or to become of less than the standard gage, for the drill hole made with it will give you trouble and you may get a "sticker." Never drill a hole past the "loose end," "chance," or "cutting" in solid shooting. If the coal has been undercut, do not drill beyond the undercutting. It is better to stop at least 6 inches short of the solid coal. Never bore "gripping" holes. Keep the holes parallel to the ribs, or as nearly so as possible. Use the side gear on the machine if you can when boring a hole. Never guess at the quantity of powder to be used. Always measure it. This course is cheaper and better than guessing. Use cartridges rather than loose powder and make them of cartridge paper. Don't use newspaper for cartridge making. Never place black blasting powder in the same drill hole with dynamite or a permissible explosive. Never use short fuse. Always have the fuse long enough to stick out at least 2 inches from the mouth of the drill hole. When the short fuse is lit any gas in the hole may be ignited, and this may result in a premature blast. Never bite a piece of the match off the squib, nor oil it to make it burn faster. Never use sulphur and gas squibs at the same working face. Never fire shots in adjoining working faces at the same time. Never return to a shot that has failed to explode until at least 10 minutes after lighting it, if squibs were used, or 12 hours after lighting if fuse was used. When shots are fired electrically be sure that all wires are disconnected from the battery, and wait at least 5 minutes before returning to the face. Never fire a rib or butt shot before a center or "busting" shot is fired. The opening shots should be fired first, in order to give the succeeding shots a chance to do their work. Never drill a hole near the remaining portion of a former shot, nor near cracks and fissures made by previous shots, because there is great danger of the powder gases on explosion flying out of the loose coal or the cracks and igniting gas or dust in the mine air. Never use squibs or any kind of fuse, except electrical fuse, in mines that make inflammable gases. Never fire a shot without making sure that the coal dust near by is well wet down. Never light a dependent shot at the same time as another shot, and never fire a dependent shot until the first shot has broken properly. Never fire a "split shot"; that is, never fire a hole that has been drilled into a mass of coal, cracked and shattered by a previous shot that failed to dislodge the coal. OPENING KEGS OF POWDER. When a miner drives the point of a coal pick into a metal powder keg, as some have done, there is a likely chance of a fatal explosion being caused from a spark struck from the contact of the pick and keg. Such carelessness is criminal, for only half a minute is needed to loosen the metal cap placed over the proper opening by the manufacturer. PAPER POWDER KEGS. Because some miners have driven picks into metal powder kegs and because metal kegs are sometimes exploded by contact with bare electric conductors, such as 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. MAKING CARTRIDGES. While making cartridges the miner, to be safe, must set his lamp at least 5 feet away, and in a place where sparks from it can not be carried by the air current to the powder. Many a miner has met death because a spark from his lamp, or even the burning lamp itself, has fallen from his head into the powder. Black blasting powder for a charge should always be measured. The miner should not guess but should know exactly how much powder he is using in a shot. 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 com |