High explosives (HE) are those that decompose by detonation. This is a very rapid (nearly instantaneous), and hence violent, process. An explosion may be initiated by sudden shock, high temperatures, or a combination of the two. The conditions under which many explosives will explode are well known.

An explosion may be initiated by elevated temperature alone, as in the following cases.

(1) Mercury fulminate by 15-second exposure to 200°C or 1-second exposure to 340°C will be set off.

(2) Trinitrotoluene will be set off by exposure to 500°C for 1 second.

(3) Tetryl will detonate in 1000 seconds at 160°C or in 0.1 second at 500°C.

(4) Picric acid will detonate in 9 seconds at 300°C or 1 second at 355°C.

An explosion of HE may also be initiated by severe shock. Sensitivity of explosives to shock may be measured in several ways, such as the impact pendulum method and the drop test. In the impact pendulum test, a heavy pendulum swings down over a sample of explosive in a dished, inclined container so arranged that there is very little clearance between the pendulum and the sample. Thus, the effect of contact between the sample and the pendulum bob is one of a combination of shock and rubbing. The height from which the pendulum is allowed to swing to explode the sample is a measure of the sensitivity of the sample to this test. The drop test consists of placing a sample upon an anvil and allowing a 5-pound weight to drop on it. The height from which the weight must drop to explode the sample is a measure of the sample's sensitivity to shock.

The table below shows the results of a drop test upon several samples. These results must be considered as relative and not by any means absolute. A solid explosive in a tightly fitting container is much more sensitive to shock.

(1) mercury fulminate = 2 in. at 5 lbs.

(2) nitroglycerin = 4 in. at 5 lbs.

(3) tetryl = 8 in. at 5 lbs.

(4) picric acid = 14 in. at 5 lbs.

(5) trinitrotoluene = 20 in. at 5 lbs.

(6) black powder (a low explosive) = 30 in. at 5 lbs.*

*From Explosions, Their Anatomy and Destructiveness, by C. S. Robinson (McGraw-Hill).

Another test for explosives is the speed at which a detonation travels. This speed is usually in the range of thousands of m/sec. Speed of detonation is found to be a function of the kind of explosive and state of compaction. There is an optimum state of compaction beyond which the explosive tends to become “deadpressed,” in which state it is difficult to make the whole sample explode. Below the point of optimum compaction the rate of detonation is found to be directly proportional to the density of the sample. Some maximum detonation rates are listed below in m/sec for some common explosives:

(1) nitroglycerin, 8500

(2) PETN, 8100

(3) tetryl, 7700

(4) picric acid, 7400

(5) trinitrotoluene, 7400

(6) lead azide, 4900

(7) mercury fulminate, 4800

(8) ammonium nitrate, 1100

(9) low explosives, 1000

It has been found that upon detonation, an explosive can cause a nearby sample of explosive to detonate “sympathetically.” The distance over which one charge can detonate another is a function of the amount of energy produced by the first explosion and the medium through which the shock wave is propagated to the second charge of explosive. For instance, the relationship for air (very approximately) would be expected to be: weight of explosive in lbs/(distance in ft)³ = 4. Thus, to calculate the maximum distance for a possible sympathetic detonation of 40,000 lbs of explosive, the calculation is:

D³ = (40,000)/4

D³ = 10,000

D = 22 ft (approximately).

According to C. S. Robinson, the formula is more nearly:

weight of explosive = 4 × (distance)^2.25

The power of the shock wave is much more rapidly attenuated in water, wood, etc., than in air, which means that if a shield of water or wood is interposed between piles of explosive the distance between them may be lessened.

Liquid Oxygen: Though not itself explosive, liquid oxygen can be dangerous when blended with highly flammable or carbonaceous materials. In this combination it is used in coal mining, quarrying, strip mining, open-cut ore mining, and in rocket fuels. Its use underground or in confined places is not recommended by the U.S. Bureau of Mines because it evolves a great deal of carbon monoxide. This type of explosive has many safety advantages. For instance, it is not itself an explosive until mixed with a flammable absorbent, a process that can be done at the last moment before firing. However, once the explosive has been made up, it is very flammable and when it catches fire it will usually detonate. Liquid oxygen explosives are not stored, as they deteriorate rapidly and lose a great deal of their explosive power in a short time.

A very dangerous fire hazard when exposed to heat or by chemical reaction with powerful oxidizing or reducing agents.

A moderate to dangerous explosion hazard when severely shocked or heated, depending upon the kind of explosive, state of compaction, degree of confinement, etc. Practically all high explosives used commercially require a detonator or cap to set them off, as compared to an igniter needed to set off black blasting powder.

Detonating Devices: To develop the desired disruptive effect of an explosive, some means must be adopted to “set off,” “fire,” or “detonate” it without killing or maiming the persons doing the blasting. Several devices or methods are being utilized, all with a view to having this work done as safely and efficiently as possible. There are two general types of devices or methods of getting explosives into action, namely, igniters and detonators. The former merely conveys a flame to the explosive mass and ignites it, while the latter transmits (originally through ignition of a small quantity of highly explosive substance by an arc, a flame, or a spark) a sharp blow that causes the explosive to disassociate, or detonate, or burn with very great rapidity. Igniters include squibs (plain and electric), fuse, and delay igniters; detonators include blasting caps (plain and electric), delay electric blasting caps, delay electric igniters with caps, and Cordeau-Bickford detonating fuse.

The squib is a small-diameter tube of straw or paper filled with quick-burning powder and having a relatively slow-burning “match head” attached to one end; the latter is ignited or lighted by an ordinary match or other flame, and its relatively slow burning allows the person handling the ignition to retire before the fire is communicated to the quick-burning material in the tube. Squibs are by no means either safe or efficient, even though still used to a considerable extent, especially in coal mining. Electric squibs are somewhat similar to ordinary squibs, except that the ignition is accomplished by means of an electric arc; electric squibs are much more satisfactory from a safety viewpoint than ordinary squibs.

A fuse (or, as it is sometimes called, “safety fuse”) consists of a fine-grained black powder core covered with cotton hemp or jute to form a ropelike material about 3/16 inch in diameter; one end of the fuse is brought in contact with the powder charge or with a detonating “cap,” and the other end (usually several feet away from the explosive) is lighted by a flame from a match or open light. The fine-grained black powder burns gradually and somewhat slowly (about 30 to 40 seconds to the linear foot of fuse) until it reaches the explosive (black powder) or the detonating cap (if some form of dynamite is used), giving the blaster time to get in the clear before the main explosion takes place. Fuses are much safer than squibs, but have their own hazards and must be used with care.

Delay electric igniters usually are a combination of electric igniters and fuses, the latter being ignited by the igniters within the blasting hole, the fuse transmitting the ignition to the explosive. Delay igniters usually are much safer than fuses, particularly for coal-mine use; but they, too, have their hazards. Delay blasting is by no means a safe procedure in coal mining, though it is a standard and relatively safe practice in metal mining and tunneling, if sensible precautions are taken.

Blasting caps or detonators are metallic cylinders (usually copper) closed at one end, about 3/16 inch in diameter, and usually less than 2 inches in length, partly filled with a small amount of relatively easily fired or “detonated” compound, the resultant shock or blow when fired being sufficient, when embedded in dynamite, to fire or detonate the dynamite mass. Ordinary blasting caps usually are fired or detonated by the flame of the fuse, the end of the latter being inserted into the open end of the detonator or cap and placed in contact with the highly explosive material in the interior of the metallic capsule or cap. Caps are extremely hazardous to handle, as they are likely to be detonated by heat, friction, or a relatively moderate blow; however, they are relatively safe if handled carefully. Partial proof of this is the fact that they are manufactured and shipped by the thousands daily and accidents are decidedly rare, primarily because the caps are at all times handled with utmost care.

Electric blasting caps are somewhat similar to ordinary caps or detonators, but the cap is fired by electricity. The electric wires are so placed in the capsule or cap that when attached to an electric current an arc is formed within the cap, which detonates the sensitive explosive material in the cap. A hazard in the use of electric blasting caps is unexpected explosions due to radio or radar-induced electric currents that may activate the cap.

Delay electric blasting caps or detonators are somewhat similar to ordinary electric blasting caps, except that several time intervals in blasting are obtained by having the electric arc ignite a short piece of fuse or some slow-burning substance before it reaches the highly sensitive detonating material in the capsule or cap. Numerous time-interval delays are obtained; in general, delay electric blasting caps are relatively safe and effective even in wet holes, though they ought not be used in coal mining if explosions of gas or dust are to be avoided. Delay electric igniters with caps or detonators are a combination of electric igniter and blasting cap, usually with suitable lengths of fuse between to give the desired delay; they have some advantages but are relatively unsafe and should not be used in coal mining.

The Cordeau-Bickford denotating fuse is a combined fuse and detonator in the form of a lead tube about 1/4 inch in diameter filled throughout its length with a high explosive, trinitrotoluene (TNT). It is fired by a fuse and an ordinary detonator or cap or by an electric cap; when fired, it detonates throughout its length (which may be up to or over 100 feet) almost instantaneously, the explosion wave traveling at a rate of about 17,500 ft/sec. Although somewhat expensive, it is relatively safe to handle and is particularly effective in deep-well drill holes in quarry and similar work, as it detonates simultaneously throughout its length, adding effectiveness to the main body of explosive that it detonates. It fires black powder as well as high explosives (dynamite, etc.), and is obtainable in lengths of approximately 500 feet wound on spools.

See also EXPLOSIVES, PERMITTED; DYNAMITE, NITROGLYCERIN, AMMONIUM NITRITE, and NITRATES.