TABLE 2.-Classes and rate of detonation of permissible explosives - Continued.

The general characteristics of the permissible explosives embraced in each of the classes shown in Table 2 are given below:

Class 1, ammonium-nitrate explosives.-To class 1 belong all the explosives in which the characteristic material is ammonium nitrate. The class is divided into two subclasses. Subclass a includes every ammonium-nitrate explosive that contains a sensitizer that is itself an explosive. Subclass b includes every ammonium-nitrate explosive that contains a sensitizer that is not in itself an explosive. The ammonium-nitrate explosives of subclass a consist principally of ammonium nitrate with small percentages of nitroglycerin, nitrocellulose, or nitrosubstitution compounds which are used as sensitizers. The ammonium-nitrate explosives of subclass b consist principally of

ammonium nitrate with small percentages of resinous matter or other nonexplosive substances used as sensitizers.

All of the ammonium-nitrate explosives readily absorb moisture from the atmosphere, and great care should be taken in storing them or in using them in damp places. They are not suitable for use in wet mines. If in such mines a cartridge of an ammonium-nitrate explosive is opened and its contents exposed for only a few hours to the damp atmosphere the explosive may deteriorate and later fail to detonate completely. The ammonium-nitrate explosives when stored in well-ventilated magazines for only a few months have shown signs of deterioration, and nearly all explosives of this class after six months' storage at the Pittsburgh testing station have either failed to detonate or have detonated incompletely when retested. For this reason the ammonium-nitrate explosives should be obtained in a fresh condition and should be used as soon as possible after their receipt. When fresh, these explosives, if properly detonated, have the advantage of producing only small quantities of poisonous and inflammable gases, and are adapted for mines that are not unusually wet, and also for mines and working places that are not well ventilated.

Class 2, hydrated explosives.—To class 2 belong all explosives in which salts containing water of crystallization are the characteristic materials. The explosives of this class are somewhat similar in composition to the ordinary low-grade dynamites, except that one or more salts containing water of crystallization are added to reduce the flame temperature. They are easily detonated, produce only small quantities of poisonous gases, and can be used successfully in damp working places.

Class 3, organic nitrate explosives.—To class 3 belong all the explosives in which the characteristic material is an organic nitrate other than nitroglycerin. The permissible explosives listed under class 3 are nitrostarch explosives. They produce small quantities of poisonous gases on detonation.

Class 4, nitroglycerin explosives.—To class 4 belong all the explosives in which the characteristic material is nitroglycerin. These explosives contain free water or an excess of carbon, which is added to reduce the flame temperature. A few explosives of this class contain salts that reduce the strength and shattering effect of the explosives on detonation. The nitroglycerin explosives have the advantages of detonating easily and of not being readily affected by moisture. On detonation some of them produce poisonous and inflammable gases equal in quantity to those produced by black blasting powder, and for this reason they should not be used in mines or working places that are not well ventilated.

RATE OF DETONATION.

The energy developed by the detonation of permissible explosives, like that of other high explosives, depends on the change of the small solid and liquid particles of the explosive into large volumes of highly heated gases and on the rate of detonation or the rapidity with which these gases are formed. The force exerted by these gases is the means of producing useful effects. The rate of detonation is the governing factor in judging the efficiency of an explosive as well as offering the best means for selecting explosives suitable to meet the varying conditions of coal mining.

During the conversion of an explosive into solid, liquid, and gaseous products the cooling produced by the walls of the drill hole tends to lower the temperature of the gases so that the maximum theoretical temperature, or pressure, is never reached. The more nearly instantaneous the occurrence of the explosive reaction, all other conditions being equal, the greater the volume of highly heated gases that will be produced, and the more violent the effect.

To meet the varying conditions of coal mining in this country the explosives manufacturers have devised explosives with rates of detonation that range from 4,750 to 14,560 feet (1,447 to 4,439 meters) per second. It is evident that for certain work in which a shattering effect is desired, as in driving through or "brushing" rock, or in producing coal for coking purposes, the explosive reaction should be rapid. Hence, permissible explosives having a high rate of detonation should be selected. Similarly, for use in soft friable coal to produce lump or steam coal, selection should be made of a permissible explosive that develops its gases at a slow rate, thus insuring the development of a more prolonged pressure. In medium hard coal an explosive having an intermediate rate of detonation would be expected to be most suitable, but is not always Coals vary in hardness and coal beds vary in the number and position of the joints, partings, shale bands, etc. These facts have to be considered in mining coal.

So.

An explosive having a very low rate of detonation is not always best suited for mining soft friable coal, because its energy may be lost by its gases escaping through cracks and fractures in the coal. Under such conditions an explosive having an intermediate rate produces the most economical results.

Another factor to be considered in connection with an explosive having a high rate of detonation is its possible effect on the roof or overlying strata of the coal bed. When large charges of explosives having a very high rate of detonation are used the local effect is

marked. Such charges produce small fissures that may later necessitate extra timbering to prevent falls in rooms or entries, and thus make the operation of a mine more costly.

It is well known that the pressure developed by the detonation of explosives in a closed space is directly proportional to the charging density; in other words, a 13-inch drill hole loaded with 11-inch cartridges will produce on the walls of the drill hole about one-half as much pressure per square inch as it would if loaded with cartridges of 13-inch diameter. Accordingly, explosives having a rapid rate of detonation will yield a larger proportion of lump coal if used in a hole of larger diameter than the cartridge. Such air spacing to reduce the shattering effect of an explosive is recommended by the Bureau of Mines, provided moist clay stemming is used to confine the charge and is tamped to the mouth of the drill hole.

Other means of reducing the shattering effect of an explosive are the use of an improper detonator, reducing the amount of stemming used in a drill hole, using an explosive that is frozen or partly frozen, using an explosive in cartridges of less diameter than those originally tested, and introducing foreign substances between the cartridges of an explosive; but these methods are all dangerous. They not only eliminate the safety qualities of the explosives but also increase the chance of a resultant dust or gas explosion.

It is hoped that with few exceptions the classification given will serve as a useful guide for comparing the practical value of permissible explosives. However, in order to select the most suitable, users of these explosives should conduct a series of experiments in the mine. The information given herein should eliminate the necessity of testing many different classes and brands of explosives.

PRECAUTIONS.

The following precautions should be taken to reduce the risks in storing, thawing, handling, and using explosives at coal mines:

PRECAUTIONS TO BE TAKEN IN THE STORAGE OF EXPLOSIVES.

Don't store detonators with explosives. Detonators should be kept by themselves.

Don't open packages of explosives in a magazine.

Don't open packages of explosives with a nail puller, pick, or chisel. Packages should be opened with a hardwood wedge and mallet, outside of the magazine, and at a distance from it.

Don't store explosives in a hot or damp place. All explosives spoil rapidly if so stored.

Don't store explosives containing nitroglycerin so that the cartridges stand on end. The nitroglycerin is more likely to leak from the

cartridges when they stand on end than it is when they lie on their sides.

Don't repair a magazine until all explosives are removed from it.

PRECAUTIONS TO BE TAKEN IN THAWING EXPLOSIVES.

Don't use permissible explosives or other explosives that are frozen or partly frozen. The charge may not explode completely, and serious accidents may result. If the explosion is not complete, the full strength of the charge is not exerted and larger quantities of harmful gases are given off.

Don't thaw frozen explosives before an open fire, nor in a stove, nor over a lamp, nor near a boiler, nor near steam pipes, nor by placing cartridges in hot water. Use thawers, such as are furnished by the manufacturers of explosives.

Don't put hot water or steam pipes in a magazine for thawing purposes. Where large quantities of explosives are used, a special thaw house should be built large enough to hold the quantity of explosives needed for a day's work.

PRECAUTIONS TO BE TAKEN IN HANDLING DETONATORS AND

EXPLOSIVES.

Don't carry detonators and explosives in the same package. Detonators are extremely sensitive to heat, friction, or blows of any kind. Don't handle detonators or explosives near an open flame.

Don't expose detonators or explosives to direct sunlight for any length of time. Such exposure may increase the danger in their use. Don't open a package of explosive until ready to use the explosive, then use it promptly. All explosives are injured by exposure to the action of the air.

Don't handle explosives carelessly. They are all sensitive to blows, friction, and fire.

PRECAUTIONS TO BE TAKEN IN USING DETONATORS AND EXPLOSIVES.

Don't use more than 1 pounds of any permissible explosive for one shot in a coal mine. No explosive is safe if used in large quantities under ordinary coal-mine conditions.

Don't use a detonator (blasting cap) or electric detonator of less efficiency than that prescribed by this bureau. The explosive may not explode completely, thus reducing the work done by the shot and causing the same dangers that come from the use of frozen explosives.

Don't crimp a detonator (blasting cap) around a fuse with the teeth. Use a cap crimper, which is supplied by any manufacturer of explosives.