The crime of arson thrives on the availability and tenacity of a fire; devoid of a fire or if a fire is not tenacious enough, arson will fail. To solve arson cases, the investigators must understand fundamental and essential facts about fire. Having background knowledge on fire helps crime busters to not only understand how arson occurred but also what the perpetrators used in committing the crime. This section of the paper addresses what a fire is from its physical properties, the fire triangle, the oxidation process, accelerants, and how detectives can locate and retrieve trace elements of accelerants from fire debris.
According to the US National Fire Protection Association (2019), fire is not a thing but is instead a chemical reaction known as oxidation. As an oxidation process, fire (or combustion) happens rapidly, releasing light, heat, and sound. The heat energy released by this process (exothermic) makes temperatures to soar, causing the temperature of the fire’s immediate surrounding to rise as well. The fire also emits smoke, which is a toxic waste of fire debris. Van Wagtendonk (2006) defines fire as a process that combines hydrocarbons with oxygen to give out carbon dioxide, water, and heat energy as a chain reaction occurring fast at elevated temperatures.
The fire triangle is a framework that depicts the components that constitute a fire. The fire triangle has three elements; these are fuel, heat, and air. The fuel is the matter that the combustion process consumes; it is the source of the hydrocarbons to combine with oxygen to produce fire. For combustion to occur, there needs to be a sufficient supply of oxygen (or oxygen-containing-air). The third element is heat, which commences and sustains combustion. It is the heat that ignites the fuel into a fire; heat facilitates the combination of hydrocarbons (in the fuel) with oxygen without which combustion would not occur. The heat to sustain the reaction comes from the reaction itself. Consequently, there has been a new proposition to add a fourth element to the triangle to make a tetrahedron. The fourth element is the uninhibited chain reaction that is feedback between the heat and the fuel that produces a gaseous fuel utilized in the flame. It is out of the chain reaction that heat necessary to maintain fire(combustion) arises.
Fire as a reaction is an oxidation process. Oxidation is the process or the reaction that combines an oxygen molecule with another substance, compound, or elemental molecule forming an oxide. Oxidation can be either rapid or gradual; as an oxidation process, combustion is similar to rust formation on iron nails, the only difference being the speed of the reaction. In the case of the rapid oxidation process, the process is highly exothermic, leading to an uninhibited chain reaction that sustains the oxidation reaction henceforth, making it challenging to suppress provided the necessary reagents and conditions necessary for oxidation are available.
In combustion, an accelerant either starts or hastens the burning (fire). Primary accelerants are the ignitable liquids used to initiate or hasten a fire (Ferreiro-González et al., 2016). According to Interfire Online website (2019b), the ignitable liquids include acetone, carbon disulfide, Coleman fuel, ethyl alcohol, ethyl ether, Fuel Oil no. 1 ((kerosene, aviation fuel, coal oil, range oil), Fuel Oil no. 2 (diesel, home heating fuel), Gasoline (Motor fuel, gas), isopropyl alcohol, kerosene, lacquer, lacquer thinner, methyl alcohol, methyl ethyl ketone (MEK), mineral spirits, naphtha, paint thinner, toluene, turpentine, and xylenes. The most commonly used accelerants are petroleum-based commodities such as gasoline, diesel, and kerosene (Ferreiro-Gonzalez et al., 2016). These products are cheap and easy to access because they are not controlled commodities. Other than their ready availability, these products are easy to ignite. For instance, gasoline is highly flammable and composed of over 300 volatile hydrocarbons produced from the fractional distillation of petroleum. As Interfire Online website (2019b) reports, American forensic research facilities report gasoline as the leading commonly used and identified ignitable liquid accelerant.
Once they burn, ignitable liquids leave behind two things; ignitable liquid residues (ILRs) (Rankin & Petraco, 2014; Ferreiro-Gonzalez et al., 2016), and ignitable liquid pour patterns (Rankin & Petraco, 2014). The ILRs are the burnt remains of the ignitable liquid, while the pour pattern is a depiction of how the arsonist applied the accelerant. Investigators look for, analyze, and categorize ILRs from the fire debris following the American Society for Testing and Materials International (ASTM International) standard by way of Gas Chromatography-Mass Spectrometry (GC-MS) (Ferreiro-Gonzalez et al., 2016). Investigators obtain debris samples from a point they suspect the application of an ignitable liquid and take these samples to a laboratory. At the research facility, specialists isolate ILRs from the fire debris before vaporizing the ILR for chromatographic tests. “Passive Headspace Concentration with Activated Charcoal, ASTM E1412, is the standard practice separation of ILRs from fire debris most commonly used in the United States” (Ferreiro-Gonzalez et al., 2016, p.696). According to Interfire Online (2019a), when there is a combustion of volatile vapors atop an ignitable liquid accelerant pool, there is a distinct burn pattern left on the structure different from the patterns of other combustible material. These are the patterns detectives look for. Material lying on the floor before the fire more often presents the best potential sites to collect debris within the pour pattern on floors that impervious to moisture.
Ferreiro-González, M., Barbero, G., Palma, M., Ayuso, J., Álvarez, J., & Barroso, C. (2016). Determination of ignitable liquids in fire debris: Direct analysis by electronic nose. Sensors, 16(5), 695 – 706. Web.
Interfire Online. (2019a). Accelerant evidence collection. Web.
Interfire Online. (2019b). Fire and arson accelerants. Web.
National Fire Protection Association. (2019). NFPA – Reporter’s guide: All about fire. Web.
Rankin, J., & Petraco, N. (2014). Interpretation of ignitable liquid residues in fire debris analysis: Effects of competitive adsorption, development of an expert system, and assessment of the false positive/incorrect assignment rate. Web.
van Wagtendonk, J. (2006). Chapter 3: Fire as a physical process. In N. Sugihara, J. van Wagtendonk, J. Fites-Kaufman, K. Shaffer, & A. Thode (Eds), Fire in California’s ecosystems (pp. 38-57). University of California Press. Web.