Trial Graphics: Arson: Fire Dynamics

Arson investigation starts with the fire itself. To create and sustain a fire three factors must be present. The three factors are known as the fire triangle. The fire triangle consists of oxygen, a fuel source, and heat. In most cases the percentage of oxygen concentration must be above 16%. The fuel may be any flammable substance. The heat source needs only to match the ignition temperature of the fuel.

In a fire involving arson the arsonist will have tampered with one or more of the factors in the fire triangle. The arsonist may increase the fuel load by introducing flammable material or by adding accelerants such as kerosene, gasoline or alcohol.

The arsonist may increase the oxygen content of a structure by opening windows or punching holes in ceilings and walls. Fire will follow the highest concentration of oxygen to its source. By ventilating a structure at the top and starting a fire at the bottom of the structure an arsonist can cause the fire to race upward through the structure. The fire will rapidly involve the whole structure rather than be confined to one room.

An arson fire involves the introduction of a heat source that can be as simple as a match or as complex as chemicals with very low ignition temperatures. By definition a fire is considered an arson fire when all other accidental caused have been ruled out. To say that the cause of a fire was arson and therefore deliberate, the investigator must have sufficient evidence the one of the factors in the fire triangle was tampered with.

The Fire Fighters

Arson investigations start with the observations of the fire fighters who respond to the scene. The fire fighters may not see any obvious signs of arson but may get a feeling that the fire is not behaving normally. There may be obvious signs of arson such as multiple points of origin or the presence of accelerants. What ever it is that raises the suspicions of the fire fighters at the scene it is their observations that initiate an arson investigation. (Resources:,

Next Page: London Underground