Explosives

An explosive is defined as a substance or mixture of substances that is capable of producing an explosion by itself, without the need for an outside source of oxygen. An explosion is a rapid oxidation reaction that liberates a large quantity of energy and is accompanied by the evolution of a large volume of hot gases and a loud noise. Explosives are often used in criminal and, more particularly, terrorist activities. Crime scene investigators and forensic scientists are called to the scene of an explosion to determine if an explosive was used, and thus, what kind of explosive was used and how it was used. Therefore, knowledge of explosives and their characteristics is paramount for a forensic scientist involved in such criminal investigations.

In general, explosives are classified into two categories: low explosives and high explosives. Low explosives are characterized by a slow rate of reaction, also said to be rapid combustion, resulting in a deflagration. Deflagration is defined as an explosion whose resulting pressure wave travels at subsonic speed (less than 340 meters, or 1,115 feet, per second). These explosives are usually designed to produce a push or to heave a mass. These explosives are also referred to as propellants. Examples of such explosives are black powder, and single-, double-, and triple-base gun powder.

High explosives are characterized by a high rate of reaction resulting in a detonation. Detonation is defined as an explosion whose resulting pressure wave travels at supersonic speed (more than 340 meters, or 1,115 feet, per second). High explosives create a powerful blasting or shattering effect.

High explosives are further classified into primary and secondary explosives depending on their susceptibility to be initiated. Primary explosives are very sensitive to heat or shock and undergo a rapid transition to detonation. They are used to provide the minimum necessary energy to initiate the secondary explosives. Examples of primary explosives include lead azide, lead styphnate, mercury fulminate, and diazodinitrophenol (DDNP). Secondary explosives are much more stable and require a higher initiation energy to be detonated. They will only be detonated by an explosion, such as the one created by a primary explosive. Nevertheless, they are generally more powerful than primary explosives and are thus, mostly used as main charges. Examples of secondary high explosives are 2,4,6-trinitrotoluene (TNT), ammonium nitrate fuel oil (ANFO), cyclotrimethylenetrinitramine (RDX), cyclotetramethylenetetranitramine (HMX), ethyleneglycoldinitrate (EGDN), and pentaerythritoltetranitrate (PETN). Secondary high explosives are designed to destruct by a shattering effect.

Most explosives are used in a combination of two or more explosives, adding the effects of the different types of explosives to the mixture. Dynamite was originally nitroglycerine absorbed into dry silica. Modern formulations of dynamite present some variations, but usually include a mixture of nitroglycerine, nitrocellulose, a fuel/oxidizer mixture, and sometimes EGDN. Semtex is a mixture of RDX and PETN. Amatol is composed of TNT and ammonium nitrate. C-4 is a plasticized composition of RDX. Pentolite is a mixture of TNT and PETN.

Detonating cords are plastic tubes filled with a powder form of explosive. These tubes are often wrapped with fibers to make them more solid. They are mostly used to link different charges by transmitting the shock wave of the detonation. They may also be used as an explosive charge by themselves. In such instances, they are typically used to perform small and accurate destruction. Detonating cords typically use PETN as their explosive content. In some instances, when the cord needs to be used in a medium with a high temperature where PETN would not be suitable, other explosives such as RDX or HMX may be used.

Boosters are defined as the components in the explosion train that propagate and amplify the shock-wave from the detonator to the main charge. They are necessary with some secondary explosives that are insensitive and the shockwave created by the detonator would not be enough to initiate the main charge. Thus, the booster is placed after the detonator and amplifies this detonation, initiating the main charge. Examples of boosters are pentolite or tetryl (2,4,6-trinitrophenyl-methylnitramine).

The forensic analysis of explosives has two interests. On intact material, it is to determine if the material is an explosive. On explosive residues, it is to determine the nature of the explosive and to profile its origin. Identifying the nature of the explosive involved may lead to the author of the crime. Explosives are usually regulated and controlled by the government. Thus, they are not easily obtained by regular citizens. Explosives are often stolen or smuggled from one country to another. Once the nature of the explosives used is identified, it is possible to relate it to recorded thefts or smuggled activities. Some terrorist groups have been known to use one particular explosive consistently, thus allowing the investigation to head one direction or another. Also, when several explosions occur over a certain time span, it is possible to establish links between them if the same explosives have been used.