A fire is a rapid, self-sustaining chemical reaction known as combustion, involving fuel, oxygen, and heat. Accelerants are substances intentionally introduced to drastically increase the fire’s speed, intensity, and rate of spread. They function by providing a highly volatile fuel source that lowers the energy required for ignition and rapid propagation. The presence of an accelerant at a fire scene is often a strong indicator that the event was intentionally set, leading to forensic investigation to determine the cause and origin of the blaze.
Defining and Categorizing Accelerants
An accelerant is defined in fire investigation as any fuel or oxidizer used to initiate or significantly enhance the growth or spread of a fire. Most accelerants are common ignitable liquids readily available to the public, rather than specialized chemicals. These substances primarily fall into chemical categories based on their composition and volatility.
The most common category is petroleum-based accelerants, which are complex mixtures of hydrocarbons derived from crude oil. Examples include gasoline, kerosene, diesel fuel, and lighter fluid. Gasoline, for instance, has high volatility due to its composition of alkanes, cycloalkanes, and aromatics, making it an effective accelerant. Another significant group includes oxygenated and alcohol-based solvents, such as isopropyl alcohol or paint thinners, which contain functional groups that promote efficient burning.
Forensic scientists classify these ignitable liquids using systems like the ASTM standard, grouping them based on their boiling point range, from light to heavy petroleum distillates. The volatility and chemical structure of these liquids mean they are not easily consumed by the fire, often leaving behind a chemical residue. The presence of these residues, known as Ignitable Liquid Residues (ILRs), is a primary focus for fire investigators.
The Chemistry of Fire Intensification
Accelerants intensify a fire primarily by disrupting the normal combustion process through the addition of highly energetic and easily ignitable fuel. The underlying mechanism centers on the substance’s high volatility and low flash point, which dramatically increases the rate of heat production. This chemical advantage allows the fire to grow and spread much faster than if it were consuming only solid materials.
A fire requires fuel vapor to burn, not the liquid fuel itself. Accelerants possess a high vapor pressure, causing them to vaporize easily at low temperatures. This rapid vaporization creates a rich, flammable vapor-air mixture just above the liquid surface, providing an immediate and abundant source of fuel for the flame. This ability to quickly generate a vapor cloud is why accelerant-fueled fires spread rapidly across a surface.
The flash point is the lowest temperature at which a liquid produces enough vapor to form an ignitable mixture with air, and common accelerants have extremely low flash points. For example, gasoline can produce sufficient vapor to ignite even below freezing temperatures, requiring minimal heat energy to start the reaction. This low energy requirement allows the fire to be initiated with a small heat source and sustain itself easily. Solid fuels, in contrast, must first be heated significantly to undergo pyrolysis and produce flammable gases.
This combination of properties leads to a drastically increased Heat Release Rate (HRR), which is the speed at which heat energy is generated during the fire. An elevated HRR causes surrounding materials to reach their ignition temperature much faster, creating a chain reaction of rapid fire growth. The quick, intense heat from the accelerant overwhelms the initial fuel source, resulting in a fire that burns hotter and spreads more aggressively than an accidental fire.
Physical Evidence Left by Accelerants
The intense chemical reaction and rapid spread caused by accelerants often leave behind physical indicators crucial for forensic analysis. One common sign is a deep, localized burn pattern on the floor or ground, sometimes described as a “puddle” or “low-level” pattern. This occurs because the liquid accelerant flows and pools, causing a high concentration of heat at the lowest point. This damage is inconsistent with a typical fire that burns upward.
Investigators also look for severe damage or charring in unexpected areas, such as the underside of furniture or floor coverings, that cannot be explained by the normal movement of heat. The presence of multiple, separate points of origin is another physical indicator that an accelerant was used to deliberately start the blaze in several spots. These visual patterns guide investigators to collect samples for laboratory testing.
Accelerants rarely burn off completely, especially when soaked into porous materials like carpet, wood, or soil, leaving behind unburned Ignitable Liquid Residues. These residues are collected from the fire debris and sealed in airtight containers, such as metal paint cans, to prevent volatile components from escaping. The collected samples are then analyzed using Gas Chromatography/Mass Spectrometry (GC-MS).
This laboratory technique separates the complex mixture of chemical compounds in the residue and identifies each component based on its unique mass spectrum. By comparing the resulting chemical profile to known patterns of ignitable liquids, forensic chemists can accurately confirm the presence and specific type of accelerant used. This provides definitive evidence for the investigation.