Page 12 - A Burning Question Fire Debris

of matter represents a higher amount of energy a fire accelerant, such as if a naturally occurring
starting with solids at the lowest energy state to fire ignites a can of gasoline stored in a garage. In
liquid to gas then plasma at the highest energy addition, not all accelerants are ignitable liquids, as
state. The energy that these states contain is compounds such as propane or natural gas may
stored in vibrating molecules. The faster they also be used to accelerate a fire. In the forensic
vibrate, the more energy the material contains. analysis of fire debris, the phrase “ignitable liquid”
is utilized almost exclusively, as laboratory analysis
Fires start when a combustible material and of fire debris may indicate the presence of a liquid
oxidizer is heated above the flash point of the but cannot indicate if that liquid was used as an
mixture. The oxidizer may be oxygen from accelerant.
surrounding air or other chemical compounds. For
example, in model rockets the oxidizer is contained As an extremely common fuel, the ignitable liquid
within the fuel as a powder. The flash point of a most often present in fire debris is gasoline. As
material is the lowest temperature that it can it is relatively inexpensive, readily available, and
change from a liquid into a gas, which produces an its purchase does not normally arouse suspicion,
ignitable mixture. Investigators take advantage of gasoline is also the most common accelerant used
the fact that any chemical reaction leaves behind in arson cases. Diesel fuel and kerosene are also
products that are clues. For example, when wood common igniteable liquids present in fire debris
is burned in a fire, the wood reacts with oxygen to as these liquids also possess the aforementioned
form carbon dioxide, water, and ash. The total mass characteristics of gasoline.
is conserved even though one of the products of
combusion is a gas. If we could weigh the products The presence of an ignitable liquid at the scene
they would equal the weight of the wood and of a fire is usually first detected by accelerant
oxygen that was used to start the fire. The fire’s detection canines (dogs specifically trained to
heat is generated when the molecules are broken detect the presence of ignitable liquids by smelling
apart and rearranged into new molecules, but the fire debris) or by observing the debris itself.
total number of atoms does not change. Generally, most fires do not completely burn the
substrate. Therefore, in the remaining substrate,
A number of details from a fire scene may be present a pour pattern is frequently visible when an
which indicate to investigators that the fire occurred ignitable liquid burns. A pour pattern is the pattern
under suspicious circumstances. The most common produced in a burned substrate by the presence of
example of this is the presence of an ignitable liquid. an ignitable liquid, often through the act of pouring
An ignitable liquid is one that will readily ignite when the liquid onto the substrate. Pour patterns often
exposed to an ignition source. The term “ignitable are characterized by intermixed light, medium, and
liquid” is commonly, and, scientifically speaking, heavy burning in a puddle shape that corresponds
incorrectly, used interchangeably by laymen with to the shape of the original pool of the ignitable
the term “fire accelerant”. An accelerant, however, liquid. A few examples of other indicators of the
is a compound that is used to increase the rate of presence of ignitable liquids include unnatural
combustion for materials that do not normally burn flame movement (such as in a downward direction
easily or quickly. Therefore, while ignitable liquids or with unnatural speed), gapping of wood or floor
are the most common fire accelerant used, not all seams (caused by the ignitable liquid settling into
ignitable liquids are fire accelerants. For example, an the seams or joints and then burning), damage with
ignitable liquid may be present at a fire scene under no identifiable point of origin, burned out flooring
normal circumstances and not have been used as beneath heavy appliances or furniture that would

12 THE MYSTERY OF LYLE AND LOUISE