READ: Arson, Firearms and Toolmarks
Arson and Explosives Evidence
Arson and Explosives Evidence
In this module, you will learn about arson, explosives, firearms and tool marks. Of those, arson, explosives and firearms are under the jurisdiction of the ATF, or Bureau of Alcohol, Tobacco, Firearms and Explosives. The mission statement of the ATF is as follows:
"ATF is a unique law enforcement agency in the United State Department of Justice that protects our communities from violent criminals, criminal organizations, the illegal use and trafficking of firearms, the illegal use and storage of explosives, acts of arson and bombings, acts of terrorism, and the illegal diversion of alcohol and tobacco products. We partner with communities, industries, law enforcement, and public safety agencies to safeguard the public we serve through information sharing, training, research and use of technology."
Chemistry of Fire Interactivity
Spontaneous Combustion
Spontaneous Combustion is a fire caused by natural heat produced process in the presence of oxygen and fuel. Often it is a combination of factors including objects that have a relatively low Auto-Ignition Temperature, insufficient ventilation causing excess heat to build-up, and the presence of oxygen. Some materials that are more prone to spontaneous combustion include hay, peat, compost piles, coal, pistachio nuts, large manure piles, and in some cases cotton and linen! What about humans? Spontaneous Human Combustion (SHC) was once billed as a scientific mystery, but is now regarded with skepticism by most in the scientific community. In the majority of the historical cases of SHC, a source of ignition such as a candle or lit cigarette was located in the area of the body. It is thought that the source of ignition was moved or cropped from the documenting evidence photos by journalists to create more sensational headlines. To explain how the bodies burned, there are several widely accepted explanations including:
- The victim was disabled or incapacitated due to illness, obesity or intoxication and was too close to a heat source such as a lit cigarette, candle or heater unit. Due to their incapacitation, the victim was unable to put out the fire on their clothing as it burned.
- "The Wick Effect" hypothesis which says that a heat source, such as a lit cigarette, burns a victim causing the skin to split. The split skin allows subcutaneous fat beneath the skin to heat and melt. The melted fat then soaks into the clothing or chair/bed that that victim is seated or laying upon. The fabric acts like a wick in a candle to keep the body burning until the fat is no longer soaking it and is used up. This hypothesis seems to be the most consistent explanation and is backed up by evidence from both animal trials and from actual cases of burned victims. It also explains why the fire is relatively confined and does not spread to other areas such as the carpeting or other objects in the room.
The Fire Triangle
Let's discuss what a fire must have in order to burn. There are three required elements needed to produce a fire. These three elements are commonly referred to as the fire triangle.
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Oxygen - Enough oxygen must be present to sustain combustion.
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Fuel - Some sort of flammable fuel source must be present. The minimum temperature need to ignite fuel is called the Flash Point.
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Heat - Enough heat must be produced to make the fuel source ignite.
The flash point is the temperature that is needed to make a flammable liquid vaporize and burn. In order for a solid object to burn, the temperature must be high enough that it will decompose into a gas which is called pyrolysis.
Explosives
Whether it is a bomb in a crowded place, a gas leak within a house that ignites or a clandestine drug lab explosion, the experts in an explosives unit risk their lives daily to keep others safe from the dangers of explosive devices. Like fire, explosives 
are also the result of an oxidation reaction except the explosive reaction is much more rapid. An explosion, or detonation, is the sudden and violent release of mechanical, chemical or nuclear energy from a confined space which creates a shock wave that travels at supersonic speeds. The rate or speeds of these reactions are what classify explosives as either high or low explosives. High explosives are substances that react faster than the speed of sound in the substance which makes them have a characteristic and very loud detonation. High explosives include dynamite, C4 and TNT and are used in commercial and military blasting. Because of the extensive potential for damage, these types of explosives are very tightly regulated. 
Low explosives are more readily available and used more often than high explosives. Black powder, fireworks and smokeless powder are classified as low explosives. These explosives react much slower than high explosives and thus react at a speed slower than the speed of sound in the substance. Because of this slower reaction speed, a low explosive does not have a technical "detonation", but a "deflagration". Deflagration is the extremely rapid burning of a material. This is much much faster than normal combustion, but slower than detonation. Without travelling faster than the speed of light, a reaction is not considered a detonation!
Explosives discharge an enormous amount of energy at the time of detonation or deflagration. This energy is sent out in a shock wave which expels not only pieces of the container it was held in, but in the case of bombs, any shrapnel, nails or other materials that may have been in the container with the explosive material. If the explosion occurs next to a building, some of the building materials such as wood or glass may be fragmented and propelled with the force of the blast. In addition to the risk of injury from flying objects during the blast, there is also significant risk of fire damage and injury to surrounding structures and victims. Incendiary devices are devices specifically created to start fires. An example of an incendiary device is a Molotov Cocktail.
Georgia Virtual, Firearms, Ballistics, Toolmarks, and Arson, CC BY-NC-SA 3.0