FORENSIC ASPECTS OF FIRE INVESTIGATION Chapter 12 FORENSIC ASPECTS OF FIRE INVESTIGATION

Introduction Arson- the criminal act of deliberately setting fire to property Arson investigations often present complex and difficult circumstances to investigate: the perpetrator has thoroughly planned the act He/she is not present during the act and the destruction is so extensive. The criminalist’s function: detecting and identifying relevant chemical materials collected at the scene reconstructing and identifying igniter mechanisms Establishing a modus operandi, an offender’s pattern of operation

The Chemistry of Fire Methane or gasoline does not simply start burning when exposed to oxygen… but why? Chemically, fire is a type of oxidation, which is the combination of oxygen with other substances to produce new substances. To start fire, the minimum temperature needed to spontaneously ignite fuel, known as ignition temperature, must be reached. Combustion: Rapid combination of oxygen with another substance, accompanied by production of noticeable heat and light. Fire is matter, a transformation process during which oxygen is united with some other substance to produce noticeable quantities of heat and light (a flame). Rust is a form of oxidation.. But not one we are concerned about in this chapter

The Chemistry of Fire The heat evolved when a substance burns is known as heat of combustion. Exothermic Reaction: a chemical transformation in which heat energy is liberated Energy: the ability or potential of a system or material to do work EX- When methane is burned, the stored chemical energy in methane is converted to energy in the form of heat and light. An additional factor, besides the liberation of energy, needed to explain fire is the rate or speed at which the oxidation reaction takes place. *Once combustion starts, enough heat is liberated to keep the reaction going by itself. The fire becomes a chain reaction- absorbing a portion of its own liberated heat to generate more heat until either oxygen or the fuel is exhausted * Flammable range SO ignition (lit match) + methane/gas= FIRE Fire is matter, a transformation process during which oxygen is united with some other substance to produce noticeable quantities of heat and light (a flame). Rust is a form of oxidation.. But not one we are concerned about in this chapter Different energies: heat, electrical, mechanical, nuclear, light, chemical Flammable range: the entire range of possible gas or vapor fuel concentrations in air that are capable of burning Exothermic v endothermic

The Chemistry of Fire A fuel will achieve a reaction rate with oxygen sufficient to produce a flame only when it is in the gaseous state. A liquid burns when the temperature is high enough to vaporize it (flash point), while a solid must be hot enough to decompose into gaseous products (pyrolysis). Ignition temperature of fuel is always considerably higher than the flash point EX: Flash point of gasoline= -50 degrees F  ignition temp: 495 degrees F Glowing combustion or smoldering is burning at the fuel-air interface, such as a cigarette, wood after the flame burns out, or coals. Spontaneous combustion, which is rare, is the result of a natural heat-producing process in poorly ventilated containers or areas. Flashpoint- the lowest temperature at which a liquid gives off sufficient vapor to for a mixture with the air that will support combustion Glowing combustion- not enough heat high enough to pyrolyze the fuel

The Chemistry of Fire A fuel must be present; To initiate and sustain combustion the following is required: A fuel must be present; Oxygen must be available in sufficient quantity to combine with the fuel; Heat must be applied to initiate the combustion, and sufficient heat must be generated to sustain the reaction. ALL REACTIONS REQUIRE AN ENERGY INPUT TO START THEM

Heat Transfer The three mechanisms of heat transfer are conduction, radiation, and convection. Conduction - the movement of heat through a solid object. EX Metal v wood Radiation - the transfer of heat energy by electromagnetic radiation. EX. All surfaces that face fire are exposed to radiant heat and burst into flames when the surface reaches their ignition temperature. Convection - the transfer of heat energy by the movement of molecules within a liquid or gas. EX- water Conduction: wood v metal

The Fire Scene The arson investigator needs to begin examining a fire scene for signs of arson as soon as the fire has been extinguished. Experience shows that most arsons are started with petroleum-based accelerants. Accelerant- any material used to start or sustain a fire (i.e. gasoline/kerosene) Accelerant residue may evaporate within a few days or even hours! What to look for: Candles/ lighters Irregularly shaped pattern on a floor or ground Breaking/entering/theft The necessity to begin an immediate investigation even takes precedence over the requirement to obtain a search warrant. The search of the fire scene must focus on finding the fire’s origin flashover – when all the combustible fuels simultaneously ignite, engulfing the entire structure in flames

The Fire Scene Some telltale signs of arson include evidence of separate and unconnected fires, the use of “streamers” to spread the fire from one area to another, and evidence of severe burning found on the floor as opposed to the ceiling of a structure, due to a flammable liquid. Normally, a fire has a tendency to move in an upward direction, and thus the probable origin will most likely be the lowest point showing the most intense characteristics of burning. Fortunately, combustible liquids are rarely entirely consumed during a fire.

Collection At the suspect point of origin of a fire, ash and soot, along with porous materials which may contain excess accelerant, should be collected and stored in airtight containers, leaving an airspace to remove samples. Traces of flammable liquid residues may be located with a vapor detector (sniffer). It is important that a sampling of similar but (reasonably assumed to be) uncontaminated control specimens be collected. A search for ignitors such as matches, an electrical sparking device, or parts of a “Molotov cocktail” must also be conducted. Suspects clothing Matches are the most common igniter Molotov cocktail- flammable liquid with a cloth rad stuffed into it and lit as a fuse.

Gas Chromatography In the laboratory, the gas chromatograph is the most sensitive and reliable instrument for detecting and characterizing flammable residues. The vast majority of arsons are initiated by petroleum distillates such as gasoline and kerosene. First need to collect vapor concentration (two methods) The gas chromatograph separates the hydrocarbon components and produces a chromatographic pattern characteristic of a particular petroleum product. By comparing select gas chromatographic peaks recovered from fire-scene debris to known flammable liquids, a forensic analyst may be able to identify the accelerant used to initiate the fire.

Conclusion When a fire occurs, oxygen combines with a fuel to produce noticeable quantities of heat and light (flames). If combustion is to be initiated and sustained, a fuel must be present, oxygen must be available, heat must be applied to initiate the combustion, and sufficient heat must be generated to sustain the reaction. A fuel will achieve a reaction rate with oxygen sufficient to sustain a fire only when it is in the gaseous state.