explosive


explosive
explosively, adv.explosiveness, n.
/ik sploh"siv/, adj.
1. tending or serving to explode: an explosive temper; Nitroglycerin is an explosive substance.
2. pertaining to or of the nature of an explosion: explosive violence.
3. likely to lead to violence or hostility: an explosive issue.
4. Phonet. plosive.
n.
5. an explosive agent or substance, as dynamite.
6. Phonet. plosive.
[1660-70; EXPLOS(ION) + -IVE]

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Any substance or device that can produce a volume of rapidly expanding gas in an extremely brief period.

Mechanical explosives, which depend on a physical reaction (e.g., overloading a container with compressed air until it bursts), are little used except in mining. Nuclear explosives (see nuclear weapon) use either nuclear fission or nuclear fusion. Chemical explosives are of two types: detonating (high) explosives (e.g., TNT, dynamite) have extremely rapid decomposition and development of high pressure; deflagrating (low) explosives (e.g., black powder, smokeless powder; see gunpowder) merely burn quickly and produce relatively low pressure. Primary detonating explosives are ignited by a flame, a spark, or an impact; secondary ones require a detonator and sometimes a booster. Modern high explosives use either mixtures of ammonium nitrate and fuel oil or ammonium nitrate-based water gels.

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▪ chemical product
Introduction

      any substance or device that can be made to produce a volume of rapidly expanding gas in an extremely brief period. There are three fundamental types: mechanical, nuclear, and chemical. A mechanical explosive is one that depends on a physical reaction, such as overloading a container with compressed air. Such a device has some application in mining, where the release of gas from chemical explosives may be undesirable, but otherwise is very little used. A nuclear explosive is one in which a sustained nuclear reaction can be made to take place with almost instant rapidity, releasing large amounts of energy. Experimentation has been carried on with nuclear explosives for possible petroleum extraction purposes. This article is concerned with chemical explosives, which account for virtually all explosive applications in engineering.

Types of chemical explosives
      Basically, chemical explosives are of two types: (1) detonating, or high, explosives and (2) deflagrating, or low, explosives. Detonating explosives, such as TNT and dynamite, are characterized by extremely rapid decomposition and development of high pressure, whereas deflagrating explosives, such as black and smokeless powders, involve merely fast burning and produce relatively low pressures. Under certain conditions, such as the use of large quantities and a high degree of confinement, some normally deflagrating explosives can be caused to detonate.

      Detonating explosives are usually subdivided into two categories, primary and secondary. Primary explosives detonate by ignition from some source such as flame, spark, impact, or other means that will produce heat of sufficient magnitude. Secondary explosives require a detonator and, in some cases, a supplementary booster. A few explosives can be both primary and secondary depending on the conditions of use.

History of black powder
      It may never be known with certainty who invented the first explosive, black powder, which is a mixture of saltpetre (potassium nitrate), sulfur, and charcoal (carbon). The consensus is that it originated in China in the 10th century, but that its use there was almost exclusively in fireworks and signals. It is possible that the Chinese also used black powder in bombs for military purposes, and there is written record that in the mid-13th century they put it in bamboo tubes to propel stone projectiles.

      There is, however, some evidence that the Arabs (Arab) invented black powder. By about 1300, certainly, they had developed the first real gun, a bamboo tube reinforced with iron, which used a charge of black powder to fire an arrow.

      A strong case can also be made that black powder was discovered by the English medieval scholar Roger Bacon (Bacon, Roger), who wrote explicit instructions for its preparation in 1242, in the strange form of a Latin anagram, difficult to decipher. But Bacon read Arabic, and it is possible that he got his knowledge from Arabic sources.

      Some scholars attribute the invention of firearms (gun) to an early 14th-century German monk named Berthold Schwarz (Berthold Der Schwarze). In any case they are frequently mentioned in 14th-century manuscripts from many countries, and there is a record of the shipment of guns and powder from Ghent to England in 1314.

      Not until the 17th century was black powder used for peaceful purposes. There is a doubtful claim that it was used in mining operations in Germany in 1613 and fairly authentic evidence that it was employed in the mines of Schemnitz, Hungary (modern Banská Štiavnica, Czechoslovakia), in 1627. For various reasons, such as high cost, lack of suitable boring implements, and fear of roof collapse, the use of black powder in mining did not spread rapidly, though it was widely accepted by 1700. The first application in civil engineering was in the Malpas Tunnel of the Canal du Midi in France in 1679.

      For 300 years the unvarying composition of black powder has been approximately 75 percent saltpetre (potassium nitrate), 15 percent charcoal, and 10 percent sulfur. The saltpetre was originally extracted from compost piles and animal wastes. Deposits found in India provided a source for many years. During the 1850s tremendous quantities of sodium nitrate were discovered in Chile, and saltpetre was formed by reaction with potassium chloride, of which there was a plentiful supply.

      Chilean nitrate was not at first considered satisfactory for the manufacture of black powder because it too readily absorbed moisture. Lammot du Pont, an American industrialist, solved this problem and started making sodium nitrate powder in 1858. It became popular in a short time because, although it did not produce as high a quality explosive as potassium nitrate, it was suitable for most mining and construction applications and was much less expensive. To distinguish between them, the potassium nitrate and sodium nitrate versions came to be known as A and B blasting powder respectively. The A powder continued in use for special purposes that required its higher quality, principally for firearms, military devices, and safety fuses.

Manufacture of black powder
      Manufacture of black powder was accomplished originally by hand methods. Ingredients were ground together with a mortar and pestle. The next step was to use crushing devices of wood (wooden stamps), also operated by hand, in wooden or stone bowls. The stamping process was gradually mechanized and, about 1435, the first powder mill driven by water power was erected near Nuremberg, Germany.

      Metallic crushing devices, introduced in the early 1800s, slowly and steadily replaced the wooden stamp mills.

      In the modern process, charcoal and sulfur are placed in a hollow drum along with heavy steel balls. As the drum rotates, the steel balls pulverize the contents; this device is called a ball mill. The saltpetre is crushed separately by heavy steel rollers. Next, a mixture of several hundred pounds of saltpetre, charcoal, and sulfur is placed in a heavy iron device shaped like a cooking pan. There it is continuously turned over by devices called plows, then ground and mixed by two rotating iron wheels, which weigh from 10 to 12 tons each. The process takes several hours; water is added periodically to keep the mixture moist.

      The product of the mills is next put through wooden rolls to break up the larger lumps and is then formed into cakes under high pressure—namely, from about 210 to 280 kilograms per square centimetre (3,000 to 4,000 pounds per square inch) of pressure. Coarse-toothed rolls crack the cakes into manageable pieces and the corning mill, which contains rolls of several different dimensions, reduces them to the sizes desired.

      Glazing (the next operation) consists of tumbling the grains for several hours in large wooden cylinders, during which friction rounds off the corners, and, aided by forced air circulation, brings the powder to a specified moisture content. The term glazing derives from the fact that graphite is added during this process, forming a thin film over the individual powder grains. Glazed powder flows more readily than unglazed powder and is more moisture resistant.

      After glazing, the powder is graded by sieves into different sizes and packaged, usually in kegs.

      Because the burning of black powder is a surface phenomenon, a fine granulation burns faster than a coarse one. Grain sizes are designated as F, 2F, etc., up to 7F, which is the finest, and from C up as the grains become larger. For the A powder the letter indicating the fineness becomes 3FA, etc., and if the powder is glazed, this is followed by the letter g—e.g., 3FAg. For many years the B blasting material was offered in pellet as well as granular form. Four pellets, each 5 centimetres (2 inches) in length and from 2.75 to 6.25 centimetres (1.1 to 2.5 inches) in diameter, were packed in waxed paper cartridges. Each pellet had a hole through its centre to accommodate a safety fuse or an electric device used to ignite the powder. Pelleted powder was used almost entirely in underground coal mines, but now regulations generally prohibit both it and the granular type.

Ignition of black powder
      Black powder is relatively insensitive to shock and friction and must be ignited by flame or heat. In the early days such devices as torches, glowing tinder, and heated iron rods were used to ignite the powder and, in most cases, a train of the powder was led to the main charge in order to give the firer time to get to a safe place.

      In cannons (cannon) a small touchhole was drilled into the breech and filled with fine powder. Ignition of the charge was usually by means of a slow-burning punk. The same principle was employed in flintlock muskets (musket) and rifles (rifle) except that ignition resulted from sparks produced by contact between flint and steel.

      Percussion methods of firing guns have long been in universal use. In the most common procedure, pulling the trigger releases a hammer, which strikes an impact-sensitive explosive mixture. This explosion then ignites the black powder or other powder charge.

      Some black powder is still used as the propellant in guns in spite of the superiority of smokeless powder. Besides antique gun experts, who employ it mostly with hand-loaded shells and cartridges, hunters in South and Central America still use guns that require black powder.

      In mining, a succession of crude means for ignition (fuses (fuse)) included straws filled with pulverized black powder, reeds in which the pith was scooped out and replaced with a paste of powder and water (later bound with string and dried), or powder paste spread on wool threads. All of these fuses were ignited either by a piece of wool yarn impregnated with sulfur, called a sulfur mannikin, or some equivalent slow-burning device. A later, and extremely popular, type of fuse was formed of goose quills (quill). The quills were cut so that they could be inserted one into the other and then filled with powder. Quill fuses could be ignited directly, that is, without any delaying element such as the sulfur mannikin. Unfortunately, their reliability was not high, and they often burned erratically.

Safety fuse
      A major contributor to progress in the use of explosives was William Bickford, a leather merchant who lived in the tin-mining district of Cornwall, England. Familiar with the frequency of accidents in the mines and the fact that many of them were caused by deficiencies inherent in the quill fuse, Bickford sought to devise an improvement. In 1831 he conceived the safety fuse: a core of black powder tightly wrapped in textiles, one of the most important of which was jute yarn. The present-day version is not very different from the original model. The cord is coated with a waterproofing agent, such as asphalt, and is covered with either textile or plastic.

      The safety fuse provided a dependable means for conveying flame to the charge. Its timing (the time required for a given length to burn) was amazingly accurate and consistent, compared to that of its predecessors, and it was much better from the standpoints of resistance to water and abuse.

      Underground coal mining was formerly by far the largest consumer of black powder. From a performance standpoint, it is probably the best explosive for that purpose. Its relatively gentle, heaving action gives a high yield of lump and leaves the coal in good position for rapid loading. Before the advent of oil, gas, and electric heating and cooking, coal was produced in tremendous quantities for household use and lump demanded a premium price. But black powder has a dangerous tendency to ignite coal gas (mostly methane) and coal dust, and many mine explosions occurred. About 1880 several European governments, seeking to develop safer substitutes for black powder, set up testing stations. Similar action was taken in the United States a few years later. The result was a series of special dynamites approved for use in gassy and dusty coal mines when used in the specified manner. Their blasting action was not as good as that of black powder, but they were much safer. These dynamites are discussed below.

      The use of black powder in underground coal mines is no longer allowed in most countries. As a result, black powder production has decreased tremendously. Further, black powder is now more expensive than dynamite and is used only for special purposes. There is, for example, no substitute for black powder in certain military applications, and nothing equal to it has yet been found for the manufacture of the safety fuse. The fact that black powder is relatively nonshattering is of value in blasting certain types of stone.

      Nitroglycerin, another chemical explosive, was discovered by an Italian chemist, Ascanio Sobrero, in 1846. Although he first called it pyroglycerin, it soon came to be known generally as nitroglycerin, or blasting oil. Because of the risks inherent in its manufacture and the lack of dependable means for its detonation, nitroglycerin was largely a laboratory curiosity until Immanuel Nobel and his son Alfred (Nobel, Alfred Bernhard) made extensive studies of its commercial potential in the years 1859–61. In 1862 they built a crude plant at Heleneborg, Sweden; Alfred, a chemist, was basically responsible for the design of this factory that was efficient and relatively safe considering the state of knowledge of the times. Nevertheless, it exploded in 1864 and killed, among others, Alfred's youngest brother Emil Oskar. Although deeply affected by the accident, Alfred continued work, at first on a barge that he moored in the middle of a lake. In 1865 he erected a plant at Krümmel, Germany, and another in Sweden at Vinterviken near Stockholm. A third plant was built a year later in Norway. Nobel was granted a patent for the manufacture and use of nitroglycerin in the United States, in 1866, and since importation on a large scale was impractical, he visited the United States in an effort to interest local capital. The victim of a number of unscrupulous businessmen, he finally sold his American holdings in 1885 for only $20,000.

      Even today most experts regard Nobel's invention of the blasting cap, a device for detonating explosives, in 1865, as the greatest advance in the science of explosives since the discovery of black powder. Combined with Bickford's safety fuse, the blasting cap provided a dependable means for detonating nitroglycerin and the many other high explosives that followed it. After a number of attempts that were only partially successful, Nobel settled on a charge of mercury fulminate, which had been known for many years, in a copper capsule. With one or two minor changes, this blasting cap remained in general use until the 1920s.

      The second most important of Nobel's inventions was dynamite, in 1867. He coined the name from the Greek dynamis, “power.” The basis for the invention was his discovery that kieselguhr (diatomaceous earth), a porous siliceous earth, would absorb large quantities of nitroglycerin, giving a product that was much safer to handle and easier to use than nitroglycerin alone.

      Dynamite No. 1, as Nobel called it, was 75 percent nitroglycerin and 25 percent guhr. Shortly after its invention, Nobel realized that guhr, an inert substance, not only contributed nothing to the power of the explosive but actually detracted from it because it absorbed heat that otherwise would have improved the blasting action. He turned, therefore, to active ingredients such as wood pulp for an absorbent and sodium nitrate for an oxidizing agent. By varying the ratio of nitroglycerin to these “dopes,” as they came to be called, Nobel not only improved the efficiency of dynamite but also was able to prepare it in varying strengths, termed straight dynamites. Thus 40 percent straight dynamite contained 40 percent nitroglycerin and 60 percent dope.

      Nobel patented the use of active ingredients in dynamite in 1869. Several others obtained similar patents at about the same time, however, and the result was that no one could establish a clear-cut claim to the invention.

      Nobel's next outstanding contribution was his invention of gelatinous dynamites in 1875. There is a legend that he hurt a finger and used collodion, a solution of relatively low nitrogen content nitrocellulose in a mixture of ether and alcohol, to cover the wound. Later, unable to sleep because of the pain, Nobel went to the laboratory to find out what effect collodion would have on nitroglycerin. To his great satisfaction, he found that after evaporation of the solvents, there remained a tough, plastic material. He discovered that he could duplicate this by the direct addition of 7 to 8 percent of collodion-type nitrocotton to nitroglycerin and that lesser quantities of nitrocotton decreased the viscosity and enabled him to add other active ingredients. He called the original material blasting gelatin and the dope mixtures gelatin dynamites. The principal advantages of these products were their high water resistance and greater blasting action power than the comparable dynamites. This added power resulted from a combination of higher density and a degree of plasticity that allowed complete filling of the borehole (the hole that was bored in the coal seam or elsewhere for implantation of the explosive).

      The first large-scale manufacture of nitroglycerin in the United States is attributed to George Mowbray, a chemist of considerable ability who had followed the work of Sobrero and others in Europe with great interest. Mowbray published an advertisement offering to supply nitroglycerin. This led to an invitation to manufacture it for completion of the Hoosac Tunnel at North Adams, Massachusetts. Mowbray's plant was built near North Adams in the latter part of 1867. Most of its product went to the tunnel, but a substantial amount was shipped, frozen, throughout the eastern United States and Canada. Pure nitroglycerin, relatively insensitive in frozen form, freezes at about 11° C (52° F) and is, therefore, easy to keep frozen by packing it in ice. Before closing his plant down because of patent difficulties, Mowbray made about 450,000 kilograms (1,000,000 pounds) of nitroglycerin without accidents in either manufacture or shipment.

      One of the earliest major uses of nitroglycerin in the United States was in blasting oil wells to increase the flow of oil. E.A.L. Roberts in that country obtained a patent covering this procedure and later acquired the right to manufacture and use nitroglycerin under the Nobel patents. Theoretically, this gave him a monopoly on shooting oil wells, and his company dominated the field, but many of his competitors ignored his patent rights.

      After 1883 the use of nitroglycerin was, with a few unimportant exceptions, restricted to oil-well shooting. In recent years more efficient means have been developed for increasing oil flow. Nitroglycerin is still used occasionally because it is more economical in small wells.

      Three tunnels (tunnels and underground excavations) stand out as benchmarks in the history of the use of explosives: first is Mont Cenis (Mount Cenis Tunnel), a 13-kilometre (8-mile) railway tunnel driven through the Alps between France and Italy in 1857–71, much the largest construction job with black powder up to that time; second was the 6.4-kilometre (4-mile) Hoosac (Hoosac Tunnel), also a railway project, during the construction of which (1855–66) nitroglycerin first replaced black powder in large-scale construction; third was the Sutro mine development tunnel in Nevada (1864–74) where the switch from nitroglycerin to dynamite for this type of work started.

      After the straight dynamites and gelatins, the next important advance in dynamite was the substitution of ammonium nitrate for part of the nitroglycerin to give a safer and less expensive product. The use of ammonium nitrate in explosives had been patented by others in Sweden in 1867, but it was Nobel who made the new “extra dynamites” successful by devising gelatins that contained from 20 to 60 percent ammonium nitrate.

      During the period 1867–84, many people worked to develop nongelatinous ammonium nitrate mixtures, but nothing of value resulted, largely because ammonium nitrate is too hygroscopic; that is, it picks up moisture too readily. In 1885 R.S. Penniman, an American, found a solution to the problem by coating the ammonium nitrate with a small percentage of paraffin, or some similar substance, prior to use. With this development a series of ammonia dynamites soon became popular. Coating was discontinued when other, safer means were developed to handle the moisture problem.

      All major underground-coal-mining (coal mining) countries have similar explosives and regulations. In the United States explosives that have been approved by the U.S. Bureau of Mines for use in underground coal mines are called permissibles. Besides passing the Bureau's safety tests, these explosives must be used in a manner specified by the Bureau. In England the explosives are known as permitted; in France, explosifs antigrisouteux; in Belgium, explosifs S. G. P. (sécurité, grisou, poussière); and in Germany, schlagwettersichere Sprengstoffe. Almost without exception the major ingredient in these explosives is ammonium nitrate, chosen because of its low explosion temperature, and nearly all of them contain a cooling agent such as sodium chloride (common salt) or ammonium chloride to prevent the heat of their explosion in a mine from igniting underground gases such as methane, or a combination of them and coal dust, and causing a fire or disastrous secondary explosion. The sensitizer is usually a small amount of nitroglycerin, but in some cases it is TNT, trinitrotoluene (discussed later); for example, it is said that a typical Russian permissible would be 68 percent ammonium nitrate, 10 TNT, 20 sodium chloride, and 2 powdered bark.

      As synthetic ammonia became less expensive because of improvements in manufacture and a raw material change from coal to natural gas, the explosives industry concentrated its efforts on substituting ammonium nitrate for nitroglycerin. Two important products were (1) low-density ammonia dynamites and (2) semigelatins. Prior to their development, the density of most dynamites was about the same and was quite high. Strength was changed in the different grades by varying the amount of explosives used. The new concept was to employ the strongest formula possible, with a minimum of nitroglycerin and a maximum of ammonium nitrate, and to dilute it systematically with suitable low-density ingredients such as bagasse (the pulp remaining after extraction of sugar from the cane) so that one stick of the new product would give the same blasting action as one of the old. This provided a substantial saving to the user because the cost per stick of the new product was much lower.

      The only difference between the low-density ammonia dynamites and the semigelatins is that the latter are partially gelatinized through the use of nitrocellulose and a higher nitroglycerin content. This gelatinization provides good water resistance and a degree of plasticity that is desirable in loading holes prior to blasting.

      Means are available to obtain a moderate amount of water resistance in the ammonia dynamites without resorting to gelatinization of the nitroglycerin. The most common involve the use of water repellents, such as calcium stearate, and ingredients that form a water gel on the surface of the dynamite that slows down the further penetration of water. Examples of the latter are pregelatinized starch products and rye flour.

Low-freezing dynamite
      Attempts to reduce the freezing point of nitroglycerin began shortly after the Nobels introduced it commercially. Frozen dynamite is very insensitive, sometimes so much so that it will not give dependable performance, and it is difficult to use, since it cannot be punched for the insertion of a blasting cap or slit and tamped into a borehole. Consequently, almost all of it had to be thawed for use, and careless thawing methods caused many accidents. Not until 1907 was a reasonably successful procedure for producing low-freezing dynamite developed. This involved adding 20 to 25 percent of the liquid isomers (molecules with identical formulas but different structure) of TNT to the nitroglycerin. This was replaced for a short time by a nitrated solution of sugar in glycerin. In 1911 a practical way to manufacture diglycerin (a glycerin polymer) was discovered. Its nitration product, tetranitrodiglycerin, when mixed with nitroglycerin, reduced its freezing point materially.

      The ultimate solution to the freezing problem was found in 1925, when synthetic ethylene glycol became available. The explosive properties of ethylene glycol dinitrate are practically identical with those of nitroglycerin, and its low-freezing qualities are extremely good. Dynamite containing a mixture of it and nitroglycerin was stored in the open at Point Barrow, Alaska, for four years without freezing.

Other explosives

Chlorates and perchlorates
      Interest in the chlorates and perchlorates (salts of chloric or perchloric acid) as a base for explosives dates back to 1788. They were mixed with various solid and liquid fuels. Many plants were built in Europe and the United States for the manufacture of this type of explosive, mostly using potassium chlorate, but so far as can be determined, all of them either blew up or burned up, and no chlorate explosives have been manufactured for many years.

Sprengel explosives
      In England in 1871, Hermann Sprengel patented combinations of oxidizing agents such as chlorates, nitrates, and nitric acid with combustible substances such as nitronaphthalene, benzene, and nitrobenzene. These differed from previous explosives in that one of the ingredients was liquid and the mixture was made just prior to use. Sprengel explosives were quite popular in Europe, but consumption in the United States was relatively small except for the spectacular Hell Gate blast in New York harbour in 1885, in which a combination of 34,000 kilograms (75,000 pounds) of No. 1 dynamite and 110,000 kilograms (240,000 pounds) of potassium chlorate–nitrobenzene were used to remove “Flood Rock,” a menace to navigation. Cloth bags of the chlorate were soaked in the nitrobenzene and loaded directly from the soaking tank into the boreholes.

Liquid oxygen explosives
      In 1895 the German Carl von Linde (Linde, Carl von) introduced carbon black packed in porous bags and dipped in liquid oxygen. This, which was a Sprengel-type explosive, came to be known as LOX. Because of the shortage of nitrates, LOX was widely used in Germany during World War I. Little if any was used in World War II, however, because ample supplies of nitrates could be obtained from synthetic ammonia.

      Because the manufacture of liquid oxygen requires complicated and expensive equipment, the use of LOX was limited to areas that could consume very large quantities. In the United States several of the tremendous strip coal mines in the Midwest met this requirement. Maximum consumption of LOX explosive was about 10,190,000 kilograms (22,465,000 pounds) in 1953, but it fell to zero in 1968. Inexpensive as LOX is, it cannot compete with ammonium nitrate–fuel oil mixtures.

Nitrostarch explosives
      Nitrostarch, which is closely related to nitrocellulose, attracted early attention, but it was not until about 1905 that it proved possible to produce it in a stable form. In general nitrostarch explosives are similar to the straight and ammonia dynamites except that nitrostarch is used in place of nitroglycerin. Disadvantages are its relatively low strength, mediocre water resistance, and the fact that it cannot be transformed into gelatinous products. Nitrostarch explosives, however, do not produce the headaches from skin contact that are characteristic of mixtures containing nitroglycerin. For that reason they are still marketed.

Nitramon and Nitramex explosives
      An important advance in explosives technology was the development by du Pont in 1934 of Nitramon, a canned product with a typical formula of 92 percent ammonium nitrate, 4 percent dinitrotoluene, and 4 percent paraffin wax. Some grades contain metallic ingredients such as aluminum and ferrosilicon. Nitramon is insensitive to the action of a line of detonating cord, a commercial blasting cap, shock and friction, or the impact of small-calibre ammunition. A large primer is required for its detonation, and the one normally used is known as a Nitramon primer. This is also a canned product with Nitramon at each end but a centre section of amatol that can be detonated by either detonating cord or a blasting cap. The cans are provided in varying sizes. A minimum diameter of 10 centimetres (4 inches) for regular Nitramon is necessary to ensure proper explosive effect if individual cans in a column become separated by some material such as a rock. Special grades are made for use in seismic exploration for gas and oil in 5- and 6.4-centimetre (2- and 21/2-inch) diameters. In this case, however, the cans are threaded and intimate contact is assured because the column is screwed together.

      Nitramex is similar to Nitramon but is much stronger because it contains TNT and a metallic ingredient such as aluminum. Both it and Nitramon have been largely replaced by the water gels, which are described later.

      So far as is known, the largest commercial, nonnuclear blast in North America was made on April 5, 1958, in Seymour Narrows, which lies between Vancouver Island and the mainland of British Columbia. The object of the blast was to remove the top of a submerged twin-peak mountain known as Ripple Rock, which was only 2.7 metres (9 feet) below the surface at low tide. More than 120 vessels had been lost because of this obstacle. In preparing for the blast, a shaft was sunk on shore to the proper depth. From it a tunnel was driven to a point directly under the twin peaks, from which a vertical shaft finally was driven to the desired depth below the peaks. A series of small horizontal drifts and pockets was prepared for placement of the explosives, consisting of 1,253,000 kilograms (2,756,000 pounds) of Nitramex 2H and a special primer, fired by means of detonating cord.

      After the blast the top of the rock was a minimum of 15 metres (50 feet) below the surface and no longer a menace to navigation.

Modern high explosives
      The year 1955, marking the beginning of the most revolutionary change in the explosives industry since the invention of dynamite, saw the development of ammonium nitrate–fuel oil mixtures (ANFO) and ammonium nitrate-base water gels, which together now account for at least 70 percent of the high explosives consumption in the United States. The technology of these products is far more advanced in the U.S. than it is in other countries; so, at the present time, they have not replaced nearly as much of the older explosives in the rest of the world. In addition to a variety of packages, both ANFO and water gels are delivered in bulk by special trucks and loaded directly into boreholes.

Ammonium nitrate–fuel oil mixtures
      In 1955 it was discovered that mixtures of ammonium nitrate and fine coal dust would give very satisfactory blasting results in the large (about 22.5-centimetre, 9-inch) holes used in open-pit coal mines to remove the rock and soil covering the coal. Polyethylene bags for this material both stretched to fill the holes and provided a moderate amount of water resistance.

      Shortly thereafter ANFO was evaluated in the open-pit iron mines of Canada and the United States, with a high degree of success. From there ANFO spread to other open pits, such as copper, and to construction work such as road building. It was then found that the mixture could be air blown into holes 5 centimetres in diameter, or even smaller, with excellent results. This led to its adoption in many underground mines.

      ANFO applications were based on prilled rather than crystallized ammonium nitrate. Prills, or free-flowing pellets, were developed for the fertilizer market, which requires a coarse product that has little tendency to set and can be spread easily and smoothly. A small amount of kieselguhr is generally added to improve the flowing properties. Prills are made by allowing droplets of ammonium nitrate that is almost molten to fall freely from a high tower. When they reach the bottom, they are dry and solidified, and slightly porous, which allows them to absorb and hold a greater amount of oil and gives a more sensitive product. ANFO is almost universally prepared by mixing 94 percent of prills with 6 percent of No. 2 fuel oil. The latter imparts some water resistance and, if that is not enough, polyethylene bags can often be used to give the necessary protection.

Water gels
 Water gels, or slurries, were introduced in 1958. These were, at first, mixtures of ammonium nitrate, TNT, water, and gelatinizing agents, usually guar gum and a cross-linking agent such as borax. (Cross-linking is a form of chemical bonding.) Later, aluminum and other metallic fuels were sometimes used and vastly better gelatinizers were discovered. In addition nonexplosive sensitizers were developed that could replace the TNT if desired. When the highest possible concentration of strength is needed, however, large quantities of TNT are still used.

      Water gels have many advantages. Among them are a high concentration of strength, a high degree of water resistance, plasticity that permits them to displace air or water and completely fill the borehole, economy, ease of handling and loading, and good safety characteristics.

Nitrocellulosic (nitrocellulose) explosives
      When Christian Schoenbein (Schönbein, Christian Friedrich) invented nitrocotton (guncotton) in 1845 by dipping cotton in a mixture of nitric and sulfuric acids and then removing the acids by washing with water, he hoped to obtain a propellant for military weapons. It proved, however, to be too fast and violent. About 1860 Major E. Schultze of the Prussian army produced a useful nitrocellulosic propellant. He nitrated small pieces of wood by placing them in nitric acid and then, after removing the acid, impregnated the pieces with barium and potassium nitrates. The purpose of the latter was to provide oxygen to burn the incompletely nitrated wood. Schultze's powder was highly successful in shotguns but was too fast for cannon or even most rifles.

      In 1884 a French chemist, Paul Vieille (Vieille, Paul), made the first smokeless powder as it is now known. He partially dissolved nitrocellulose in a mixture of ether and alcohol until it became a gelatinous mass, which he rolled into sheets and then cut into flakes. When the solvent evaporated, it left a hard, dense material resembling horn. This product gave satisfactory results in all types of guns.

      In 1887 Nobel introduced another of his revolutionary inventions, which he called Ballistite. He mixed 40 percent of a lower nitrogen content, more soluble nitrocellulose, and 60 percent of nitroglycerin. Cut into flakes, this made an excellent propellant, and it continued in use for over 75 years. The British refused to recognize Nobel's patent and developed a number of similar products under the generic name cordite.

      The progress of smokeless powder in the United States was much slower than it was in Europe. Long-continued work, principally by E.I. du Pont de Nemours & Company (DuPont Company), finally resulted in a material that was excellent for guns of all types and sizes. It was first marketed about 1909 and was the most important type of smokeless powder used by the Allies in World War I. It was made from a nitrocotton of relatively low nitrogen content, called pyrocellulose, because that type is quite soluble in ether–alcohol. A small amount of diphenylamine was used as a stabilizer and, after forming the grains and removing the liquid, a coating of graphite was added. The smokeless powder most widely used in the United States at the present time is much the same. Other popular types are mostly double-base and may contain from about 20 to 35 percent nitroglycerin. Cotton linters for nitration have been almost, if not entirely, replaced by purified wood cellulose.

Blasting caps (blasting cap)
      Nobel's (Nobel, Alfred Bernhard) original fuse-type blasting cap remained virtually unchanged for many years, except for the substitution of 90–10 and 80–20 mixtures of mercury fulminate and potassium chlorate for the pure fulminate. This did not affect the performance materially and provided a substantial economy. Mercury fulminate is an example of an explosive that can be both primary and secondary. In its more compressed form it is a high density base charge; less compressed, a low density primer charge. Hexanitromannitol (nitromannite) functions in the same manner and is used that way in a very successful blasting cap.

      Extensive work was carried out on replacements for the costly mercury fulminate; by 1930 little of it remained in use, and by the 1970s it had disappeared from commercial use. Experience has shown that the cheaper replacements are actually superior.

      The dominant base-charge materials are now pentaerythritol tetranitrate ( PETN) and cyclotrimethylenetrinitramine ( RDX). These are as strong as nitroglycerin, quite safe to manufacture and handle, and relatively inexpensive. In addition to low density nitromannite, diazodinitrophenol, lead styphnate, and lead azide are widely used as ignition-primer charges. One other departure from Nobel's blasting cap is the fact that aluminum has now almost entirely replaced copper as the material used for the shell.

Electrical firing
      The principal advantages of electric over fuse firing are exact control of the time when the blast is initiated, the simultaneous firing of a number of shots, if that is desired, and the ability to obtain a very high degree of water resistance. Attempts to make electric blasting caps date back to the 1700s, but nothing of a really practical nature was developed until late in the 19th century. There were two separate problems, the cap and the means to fire it.

Blasting machines
      The first satisfactory electrical blasting machine was invented by H. Julius Smith, an American, in 1878. It comprised a gear-type arrangement of rack bar and pinion that operated an armature to generate electricity. When the rack bar was pushed down rapidly, it revolved the pinion and armature with sufficient speed to obtain the desired current. This current was released into the external, or cap, circuit when the rack bar struck a brass spring in the bottom of the machine. Smith's blasting machine was improved and made in a range of capacities; also, a small twist-type machine that employed basically the same principles was introduced. These machines are still in widespread use, although they have been replaced to a considerable extent by power firing and capacitor-discharge blasting machines. The latter have a battery power source for energizing one or more capacitors and a safe, dependable means for discharging the stored energy. They have high capacity for their weight and size and are rapidly displacing the other firing systems.

Ignition systems
      Except for the means of firing, there is little difference between electric and fuse-type blasting caps. With minor variations, the explosives used are the same.

      It was in the 1880s that the forerunner of the modern electric blasting cap was first assembled. In contrast to the spark-type ignitions previously used, it employed a fine, high-resistance wire soldered between two insulated leg wires and embedded in, or coated with, an ignition mixture. The resistance wire was either platinum or one of its alloys, and the ignition mixture was based on mercury fulminate. The leg wires were insulated with two layers of cotton thread, wound in opposite directions. Except for coal-mine caps, the wire was then run through a bath of molten asphalt. Paraffin wax was used for the coal-mine caps because its white colour provided good contrast with the black coal. sulfur, or a mixture of sulfur and mica or graphite, was used to hold the leg wires in place and seal the cap. Sulfur was well suited for this purpose because its melting point is very low and it is compatible with the explosive ingredients. Later, to obtain better water resistance, part of the sulfur was replaced by asphalt.

      In 1939 the du Pont company introduced a revolutionary new type of ignition system. nylon plastic was substituted for the cotton insulation, a rubber plug to hold the leg wires replaced the sulfur plug, and the bridge wire was welded to the leg wires instead of soldered. By that time alloys such as nichrome had largely replaced the platinum bridge wires. The shell was crimped tightly to the rubber plug, with the result that the cap could withstand a substantial amount of water pressure. All electric blasting caps are now made substantially in this way. Polyvinyl chloride is widely used for the leg wire insulation, and plastic is sometimes substituted for rubber in the plug.

      Match-head ignition, very popular in Europe, is used less widely in the United States. The ignition device consists of a piece of cardboard with a thin sheet of metal glued to each side. A bridge wire is soldered to these sheets, around the end of the cardboard, and this part of the assembly is dipped in a slurry of ignition mixture, usually based on copper acetylide. After drying, the match head is given a protective coating and is then soldered to the leg wires.

      Most countries require explosives in underground coal mines to be fired electrically but prohibit the use of aluminum-shell electric blasting caps. This is because aluminum burns with a very hot flame and is much more likely than copper to ignite coal gas. Otherwise, almost all electric blasting-cap shells are made of aluminum.

Delay systems
      Delay, or rotational, shooting has many advantages over instantaneous firing in almost all types of blasting. It generally gives better fragmentation, more efficient use of the explosive, reduced vibration and concussion, and better control of the rock. For these, and sometimes other reasons, most blasting operations are now conducted with a delay system.

      It is probable that the first use of delay firing was in tunnels. The centre was shot out first and then successive rings around it until the desired tunnel dimensions were reached. The procedure was to cut all the fuses to the same length and then trim them toward the centre; for example, the outside ring of fuses would be full length, the next ring a few centimetres shorter, and so on. In addition, the fuses were lit from the centre out, causing a little more delay in the desired direction. This method of shooting could not be used until Bickford's safety fuse, which had a uniform burning speed, became available.

      Delay electric blasting caps are the most commonly used means for obtaining rotational firing. They are of two types: (1) the so-called regular delay, which has been in use since the early 1900s, and (2) the short-interval, or millisecond, delay, which was introduced about 1943. Except for a delay element placed between the ignition and primer charges, they are the same as instantaneous electric caps.

      A typical series of regular delays would comprise 14 periods ranging from a few milliseconds to about 12 seconds. To avoid overlapping and because there is some variation in the burning speed of the delay element, the intervals are made longer in the higher periods; for example, the delay between periods 1 and 2 might be 0.8 second, whereas for 13 and 14 it might be 1.5 seconds. Ordinary delays have been largely replaced by short-interval delays but are still used to a considerable extent for such purposes as driving tunnels and sinking shafts.

      The periods in short-interval delays are usually separated by 25 milliseconds up to 200 milliseconds, by 50 up to 500, and by 100 up to 1,000 (one second). This close spacing gives improved fragmentation, the ability to fire many holes with hardly any more vibration or concussion than would be obtained with one hole, less chance that the detonation of one hole will cut off an adjacent hole, and a reduction in the quantity and cost of explosives. Short-interval delays are used above ground, in such work as excavating and quarrying, and for almost all types of underground mining. Their development is one of the major advances in explosives.

      Delay elements for electric blasting caps function in about the same way as black powder in safety fuse, except that the chemical mixtures used are much faster. At times the delay mixture is simply pressed on top of the primer mix. Usually, however, it is put in the centre of a metallic tube in lengths that will give the desired delay interval.

Detonating cord
      Detonating cord (detonating fuse) resembles safety fuse but contains a high explosive instead of black powder. The first successful one, patented in France in 1908, consisted of a lead tube, about the same diameter as safety fuse, filled with a core of TNT. It was made by filling a large tube with molten TNT that was allowed to solidify. The tube was then passed through successively smaller rolls until it reached the specified diameter. In France the product was called cordeau détonant, elsewhere shortened to cordeau. Its velocity was about 4,900 metres (16,000 feet) per second.

      In 1936 the Ensign-Bickford Company, Simsbury, Connecticut, the American manufacturers of cordeau, developed Primacord, based on French patents and constituting a core of PETN covered with various combinations of textiles, waterproofing materials, and plastics. The velocity is approximately 6,400 metres (21,000 feet) per second. Many types of Primacord are available for both military and commercial use, but the industrial varieties generally contain from 25 to 60 grains of PETN per 0.3 metre. RDX is sometimes used in place of PETN for high temperatures, because the melting points are, respectively, 203.5° and 140° C (398.3° and 284° F).

      Detonating cord has many applications in blasting. Any number of holes can be connected with it in just about any desired pattern. Attached to the blasting charge and knotted to a trunk line, it is fired by means of either a fuse-type (fuse) or electric blasting cap. Sequential shooting may be obtained by cutting the trunk lines and inserting delay connectors, which have delay periods ranging from about 5 to 25 milliseconds.

Military explosives
      Military requirements for high explosives differ in many respects from those for commercial users. Military explosives must have insensitivity to shock and friction and must be unlikely to detonate from small-arms fire and yet have excellent shattering power. They must have the ability to withstand long periods of adverse storage without deterioration and must be able to be fired in projectiles or dropped in aerial time bombs without premature explosion. Some types are required to possess almost unlimited water resistance. Many types must have complex fuses for detonation.

TNT
       trinitrotoluene (TNT) is the most useful military high explosive. Although it had been known for many years and was used extensively in the dye industry, it was not employed as an explosive until 1904. It is an excellent military explosive in itself, but its most valuable property is that it can be safely melted and cast either alone or as a slurry with other explosives. This is because there is a wide spread between its melting point and its decomposition temperature.

      It has two shortcomings: first, it is extremely insensitive in the cast form, and second, it is difficult to cast without air holes. The first problem can be overcome by drilling a hole, about 2.5 centimetres (1 inch) in diameter, the length of the charge in the shell and filling it with trinitrophenylmethylnitramine (tetryl); the second, by using a mixture of 40 percent trinitroxylene (TNX) and 60 percent TNT. This mixture not only casts perfectly but can be detonated with a smaller tetryl booster. There is no indication that any TNX was used in World War II; it is believed to have been replaced by PETN and RDX.

picric acid and ammonium picrate
      Picric acid was used as a shell explosive in Europe during the 1880s and carried through World War I on a large scale. Quantities of it were made in the United States, but the army and navy used mainly TNT.

      Ammonium picrate (Explosive D) has exceptional value as a charge for armour-piercing projectiles. Loaded in a shell with a suitably insensitive primer, it can be fired through 30 centimetres (12 inches) of armour plate and made to detonate on the far side. These armour-piercing shells were used in both World Wars.

      Early in World War I it was found that mixtures of molten TNT and ammonium nitrate were almost as effective for shell loadings as pure TNT. The mixtures most commonly used were 80–20 and 50–50 AN and TNT, known as amatol. Their principal advantages were that they made the supply of TNT go further and were considerably cheaper. In World War II the amatols were used in aerial bombs as well as artillery shells.

      To conserve TNT in World War I, a nitrostarch-base composition was also developed for loading hand grenades and trench-mortar shells.

      Several explosives, although previously known, only came into use during World War II. The most important of these were RDX, PETN, and ethylenediaminedinitrate (EDNA), all of which were cast with varying amounts of TNT, usually 40 to 50 percent, and used where the highest possible shattering power was desired. For example, cast 60–40 RDX-TNT, called cyclotol, develops a detonation pressure of about 270,000 atmospheres (4,000,000 pounds per square inch). Corresponding mixtures of PETN and TNT have almost as much shattering effect. The EDNA mixtures, or ednatol, were used only to a limited extent and for special purposes. Probably the most powerful of all nonatomic military explosives are the cast mixtures containing aluminum. The torpedo warhead Torpex, for example, is a cast mixture of RDX, TNT, and aluminum.

      A series of plastic demolition explosives with great shattering power, designated Composition C-1 to Composition C-4, has had considerable publicity. These contain about 80 percent RDX combined with a mixture of various oils, waxes, and plasticizers. The only significant difference is in the temperature range through which they remain useful. C-3 stays plastic to −29° C (−20° F) and does not exude oil below 49° C (120° F). In contrast, C-4 remains plastic to −57° C (−70° F) and does not leak below 77° C (170° F).

Shaped charges
      The shaped charge, principally the hand-fired rocket, is another highly publicized product introduced during World War II. A shaped charge normally consists of a cone made of metal or glass surrounded by a high-strength, high-density explosive and means to obtain the proper standoff, or distance to the target.

      When the explosive is detonated, the cone is collapsed and vaporized, forming a small, high-temperature jet containing particles of liner material moving at 3,050 to 9,100 metres (10,000 to 30,000 feet) per second. This strikes the target with such heat and force that the target simply flows radially from the point of impact leaving a deep, nearly round hole. As spectacular as the results are, only about 15 percent of the explosive energy is focused.

Other industrial applications

Explosive rivets
      Blind rivets are needed when space limitations make conventional rivets impractical. One type of these is explosive; it has a hollow space in the shank containing a small charge of heat-sensitive chemicals. When a suitable amount of heat is applied to the head, an explosion takes place and expands the rivet shank tightly into the hole. The shank is normally open but can be sealed to eliminate noise and the ejection of metal fragments. Most explosive rivets are aluminum, but they can be obtained in stainless steel and certain other metals. Their use is mainly in aircraft.

Explosive bonding
      Explosives are sometimes used to bond various metals (metal) to each other. For example, when silver was removed from United States coinage, much of the so-called sandwich metal that replaced it was obtained by the explosive bonding of large slabs, which were then rolled down to the required thickness. These slabs are placed parallel to each other and approximately 6.4 millimetres (0.25 inch) apart. An explosive developed especially for the purpose is placed on the top slab, and its detonation slams the slabs together with such force that they become welded. One especially valuable feature of explosion cladding is that it can frequently be applied to metallurgically incompatible metals, such as aluminum and steel or titanium and steel.

      Finally, the very fine industrial-type diamonds used for grinding and polishing are produced by the carefully controlled action of explosives on carbon.

Norman Gardner Johnson

Additional Reading
Encyclopaedic coverage of every aspect of the chemical industry is provided by Herman F. Mark et al. (eds.), Encyclopedia of Chemical Technology, 3rd ed., 31 vol. (1978–84), formerly known as Kirk-Othmer Encyclopedia of Chemical Technology, with a 4th edition begun in 1991; Ullmann's Encyclopedia of Industrial Chemistry, 5th, completely rev. ed., edited by Wolfgang Gerhartz et al. (1985– ); and Thorpe's Dictionary of Applied Chemistry, 4th ed., 12 vol. (1937–56). Works specifically on explosives include A.P. Van Gelder and H. Schlatter, History of the Explosives Industry in America (1927), a highly detailed book covering the origin and development of explosives throughout the world up to the date of its publication; M.A. Cook, The Science of High Explosives (1958), an advanced mathematical work devoted almost exclusively to theory, with a brief, interesting section on the history of explosives; C.H. Johansson and P.A. Persson, Detonics of High Explosives (1970), an outstanding book describing the behaviour of high explosives, with emphasis on experimental data; E.I. du Pont de Nemours and Company, Inc., Blasters' Handbook, 15th ed. (1969), a practical discussion of commercial blasting; Institute of Makers of Explosives, “Safety in the Transportation, Storage, Handling, and Use of Explosives” (1970), a pamphlet primarily designed for the guidance of the consumer; T.L. Davis, The Chemistry of Powder and Explosives, 2 vol. (1941–43), a general treatment of explosives with excellent coverage of pyrotechnics; T. Urbanski, Chemistry and Technology of Explosives, 3 vol. (1967), an excellent treatment of these subjects, highly recommended; N.B. Wilkinson, Explosives in History: The Story of Black Powder (1966), a popular account, written primarily as a science supplement for high school students; and U.S. Bureau of Mines, Apparent Consumption of Industrial Explosives and Blasting Agents in the United States (annual).Norman Gardner Johnson Ed.

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Universalium. 2010.

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