Fire:
Arson often presents complex and difficult circumstances to investigate due to the fact that the perpetrator has thoroughly planned the act, is not present during the act, and the destruction is so extensive.
The criminalist's function is rather limited t
Chemistry of Fire
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.
The he
Fuel + Oxygen + Heat = Fire
Fuel can be any combustible material in any state of matter - solid, liquid, or gas. Most solids and liquids become a vapor or gas before they will burn.
The air we breathe is about 21% oxygen. Fire requires an atmosphere with at least 16% oxygen.
Heat is
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
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.
The necessity to begin an immediate investigation ev
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 flamm
Fire Clues: Point of Origin -
Burn patterns and other damage can help determine the point of origin, or the location where the fire started.
Fire Clues: Char Patterns -
Created by very hot fires that burn very quickly and move fast along its path, so that there can be sharp lines between what is burned and what isn't.
A char pattern on a door would help an investigator determine which side of the door the fire was on.
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Fire Clues: V-Patterns -
Fire burns up, in a V-shaped pattern, so a fire that starts at an outlet against a wall leaves a char pattern that points to the origin.
A very narrow V-shape might indicate a fire that was hotter than normal, such as one helped along by an accelerant.
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Fire Clues: Heat Shadows -
Occur when heavy furniture shields part of a wall; can help determine the origin point.
Fire Clues: Glass -
Glass fragments, windows, and light bulbs can provide clues to a fire.
Light bulbs tend to melt toward the heat source, so the "direction of melt" can indicate the direction of the fire.
The shattered or cracked glass of the windows can provide indication
Fire Clues: Chimney Effect -
Since fire burns upwards, there can be a "chimney effect" where the fire ignites at a point, the superheated gases rise upward and form a fireball, which continues straight up to burn a hole in the ceiling. If the roof is not entirely burnt, and the fire
Fire Clues: Color of smoke
Determine what type material was burning
Fire Clues: Color of flames
Indicates at what temperature the fire was burning.
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
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.
The gas chromatog