A detonator or blasting cap is a small explosive device or small quantity of explosive, which can be combined with a fuse, used for detonating a larger charge of high explosive. Detonators come in a variety of types, depending on how they are initiated (chemically, mechanically, or electrically) and details of their inner working, which often involve several stages. Types of detonators include non-electric and electric. Non-electric detonators are typically stab or pyrotechnic while electric are typically "hot wire" (low voltage), exploding bridge wire (high voltage) or explosive foil (very high voltage).
The original electric detonators invented in 1875 independently by Julius Smith and Perry Gardiner used mercury fulminate as the primary explosive. Around the turn of the century performance was enhanced in the Smith-Gardiner blasting cap by the addition of 10–20% potassium chlorate. This compound was superseded by others: lead azide, lead styphnate, some aluminium, or other materials such as DDNP to reduce the amount of lead emitted into the atmosphere by mining and quarrying operations. They also often use a small amount of TNT or tetryl in military detonators and PETN in commercial detonators.
History The first blasting cap or detonator was demonstrated in 1745 when British physician and apothecary William Watson showed that the electric spark of a friction machine could ignite black powder, by way of igniting a flammable substance mixed in with the black powder. From p. 500: "But I can, at pleasure, fire gunpowder, and even discharge a musket, by the power of electricity, when the gunpowder has been ground with a little camphor, or with a few drops of some inflammable chemical oil."
In 1750, Benjamin Franklin in Philadelphia made a commercial blasting cap consisting of a paper tube full of black powder, with wires leading in both sides and wadding sealing up the ends. The two wires came close but did not touch, so a large electric spark discharge between the two wires would fire the cap.Franklin, Benjamin, Experiments and Observations on Electricity at Philadelphia in America (London, England: Francis Newberg, 1769), p. 92. From p. 92: "A small cartridge is filled with dry powder, hard rammed, so as to bruise some of the grains; two pointed wires are then thrust in, one at each end, the points approaching each other in the middle of the cartridge till within the distance of half an inch [1.27 cm]; then, the cartridge being placed in the circle [i.e., circuit], when the four [Leyden] jars are discharged, the electric flame leaping from the point of one wire to the point of the other, within the cartridge amongst the powder, fires it, and the explosion of the powder is at the same instant with the crack of the discharge."
In 1832, a hot wire detonator was produced by American chemist Robert Hare, although attempts along similar lines had earlier been attempted by the Italians Volta and Cavallo. Hare constructed his blasting cap by passing a multistrand wire through a charge of gunpowder inside a tin tube; he had cut all but one fine strand of the multistrand wire so that the fine strand would serve as the hot bridgewire. When a strong current from a large battery (which he called a "deflagrator" or "calorimotor") was passed through the fine strand, it became incandescent and ignited the charge of gunpowder.Hare, Robert (1832) "Application of galvanism to the blasting of rocks," The Mechanics' Magazine, 17: 266–267.Note: Robert Hare had constructed his large battery (or "deflagrator" or "calorimotor", as he called it) in 1821. See: Hare, R. (1821) "A memoir on some new modifications of galvanic apparatus, with observations in support of his new theory of galvanism," The American Journal of Science and Arts, 3: 105–117.
In 1863, Alfred Nobel realized that although nitroglycerin could not be detonated by a fuse, it could be detonated by the explosion of a small charge of gunpowder, which in turn was ignited by a fuse.Patent for nitroglycerin: Nobel, A., British patent no. 1,813 (July 20, 1864). Within a year, he was adding mercury fulminate to the gunpowder charges of his detonators, and by 1867 he was using small copper capsules of mercury fulminate, triggered by a fuse, to detonate the nitroglycerin.See: Patent for dynamite: Nobel, Alfred, English patent no. 1,345 (May 7, 1867). Nobel, Alfred, "Improved explosive compound" , U.S. patent no. 78,317 (May 26, 1868). (See p. 2 for the description of the "percussion-cap".) de Mosenthal, Henry (1899) "The life-work of Alfred Nobel," Journal of the Society of Chemical Industry, 18: 443–451; see p. 444.
In 1868, Henry Julius Smith of Boston introduced a cap that combined a spark gap ignitor and mercury fulminate, the first electric cap able to detonate dynamite.Smith, Henry Julius, "Improvement in electric fuses," U.S. Patent no. 79,268 (June 23, 1868).
In 1875, Smith—and then in 1887, Perry G. Gardner of North Adams, Massachusetts—developed electric detonators that combined a hot wire detonator with mercury fulminate explosive.Cooper, Paul W., Explosives Engineering (New York, New York: Wiley-VHC, 1996), p. 339.See: Smith, Henry Julius, "Improvement in electric fuses," U.S. Patent no. 173,681 (February 15, 1876). Smith, Henry Julius, "Electric fuse," U.S. Patent no. 225,173 (March 2, 1880).Gardner, Perry G., "Electric fuse," U.S. Patent no. 377,851 (February 14, 1888). These were the first generally modern type blasting caps. Modern caps use different explosives and separate primary and secondary explosive charges, but are generally very similar to the Gardner and Smith caps.
Smith also invented the first satisfactory portable power supply for igniting blasting caps: a high-voltage magneto that was driven by a rack and pinion, which in turn was driven by a T-handle that was pushed downwards.See: Smith, Henry Julius, "Improvement in magneto-electric machines," U.S. Patent no. 201,296 (January 17, 1878). Smith, Henry Julius, "Dynamo-electric igniting machine," U.S. Patent no. 353,827 (December 7, 1886). Smith, Henry Julius, "Art of blasting," U.S. Patent no. 534,289 (February 19, 1895). Krehl, Peter O. K., History of Shock Waves, Explosions and Impact: A Chronological and Biographical Reference (Berlin, Germany: Springer, 2009), p. 365.
Electric match caps were developed in the early 1900s in Germany, and spread to the US in the 1950s when ICI International purchased the Atlas Powder Company, these match caps have become the predominant world standard cap type.
Purpose
The need for detonators such as blasting caps came from the development of safer secondary and tertiary explosives . Secondary and tertiary explosives are typically initiated by an explosives train starting with the detonator. For safety, detonators and the main explosive device are typically only joined just before use.
Design
A detonator is usually a multi stage device, with three parts: at first, the initiation method (fire, electricity, etc.) must provide enough energy (as heat or mechanical shock) to activate an easy-to-ignite primary explosive, which in turn detonates a small amount of a more powerful secondary explosive, directly in contact with the primary, called the "base" or "output" explosive, able to carry out the detonation through the casing of the detonator to the main explosive device to activate it.
Explosives commonly used as the primary in detonators include lead azide, lead styphnate, tetryl, and DDNP. Early blasting caps also used silver fulminate, but it has been replaced with cheaper and safer primary explosives. Silver azide is still used sometimes, but very rarely due to its high price.
It is possible to construct a Non Primary Explosive Detonator (NPED) in which the primary explosive is replaced by a flammable but non-explosive mixture that propagates a shock wave along a tube into the secondary explosive. NPEDs are harder to accidentally trigger by shock and can avoid the use of lead.Safety Data Sheet: Exel Neo (1.4S). Orica UK Limited. 11/09/2023 rev. 1.0.
As secondary "base" or "output" explosive, TNT or tetryl are typically found in military detonators and PETN in commercial detonators.
While detonators make explosive handling safer, they are hazardous to handle since, despite their small size, they contain enough explosive to injure people; untrained personnel might not recognize them as explosives or wrongly deem them not dangerous due to their appearance and handle them without the required care.
Types
Ordinary detonators usually take the form of ignition-based explosives. While they are mainly used in commercial operations, ordinary detonators are still used in military operations. This form of detonator is most commonly initiated using a safety fuse, and used in non time-critical detonations e.g. conventional munitions disposal. Well known detonators are lead azide [Pb(N3)2], silver azide [AgN3] and mercury fulminate [Hg(ONC)2].
There are three categories of electrical detonators: instantaneous electrical detonators (IED), short period delay detonators (SPD) and long period delay detonators (LPD). SPDs are measured in milliseconds and LPDs are measured in seconds. In situations where nanosecond accuracy is required, specifically in the implosion charges in nuclear weapons, exploding-bridgewire detonators are employed. The initial shock wave is created by vaporizing a length of a thin wire by an electric discharge. A new development is a slapper detonator, which uses thin plates accelerated by an electrically exploded wire or foil to deliver the initial shock. It is in use in some modern weapons systems. A variant of this concept is used in mining operations, when the foil is exploded by a laser pulse delivered to the foil by optical fiber.
A non-electric detonator is a shock tube detonator designed to initiate explosions, generally for the purpose of demolition of buildings and for use in the blasting of rock in mines and quarries. Instead of electric wires, a hollow plastic tube delivers the firing impulse to the detonator, making it immune to most of the hazards associated with stray electric current. It consists of a small diameter, three-layer plastic tube coated on the innermost wall with a reactive explosive compound, which, when ignited, propagates a low energy signal, similar to a dust explosion. The reaction travels at approximately 6,500 ft/s (2,000 m/s) along the length of the tubing with minimal disturbance outside of the tube. Non-electric detonators were invented by the Swedish company Nitro Nobel in the 1960s and 1970s, and launched to the demolitions market in 1973.
In civil mining, electronic detonators have a better precision for delays. Electronic detonators are designed to provide the precise control necessary to produce accurate and consistent blasting results in a variety of blasting applications in the mining, quarrying, and construction industries. Electronic detonators may be programmed in millisecond or sub-millisecond increments using a dedicated programming device.
Wireless electronic detonators are beginning to be available in the civil mining market. Encrypted radio signals are used to communicate the blast signal to each detonator at the correct time. While currently expensive, wireless detonators can enable new mining techniques as multiple blasts can be loaded at once and fired in sequence without putting humans in harm's way.
A number 8 test blasting cap is one containing 2 grams of a mixture of 80 percent mercury fulminate and 20 percent potassium chlorate, or a blasting cap of equivalent strength. An equivalent strength cap comprises 0.40-0.45 grams of PETN base charge pressed in an aluminum shell with bottom thickness not to exceed to 0.03 of an inch (0.762 mm), to a specific gravity of not less than 1.4 g/cc, and primed with standard weights of primer depending on the manufacturer.
Blasting caps