Module 3 - Arson and Explosives

An interpretation of burn patterns may help investigators determine the cause and origin of a fire. Three burn patterns that are often identified during an arson fire are the classic V, the doughnut, and the ignitable liquid pour. Examination of burn patterns may reveal important information regarding the cause of the fire.

- Image Source: Robert A. Corry Director, Fire Investigation Specialist

Classic V pattern: As a fire moves upwards on a vertical surface, it creates a distinct V pattern. The most severe physical damage is usually found at the bottom of the V pattern. Because this is likely the point of origin, investigators focus their investigation on this area for evidence of accelerants or other possible causes of the fire.

Doughnut pattern: When a liquid accelerant is poured on carpet and lit, it tends to create a circular ‘doughnut’ type pattern. After the fire has been extinguished, evidence of accelerant is often found inside the ‘doughnut’ because the melted carpet material in the doughnut interior protects the carpet padding (which is saturated with fuel) from the effects of the fire.

- Image Source: Robert A. Corry Director, Fire Investigation Specialist

Ignitable liquid pour pattern: Intense burn patterns are caused by ignitable liquid hydrocarbon accelerants such as gasoline, kerosene, or diesel that have high boiling points. When hydrocarbons burn, they tend to cause physical damage and distinct dark-coloured patterns (see photograph above). Accelerants with high vapour pressures, such as alcohol, acetone, and paint thinner, tend to ‘flash and scorch’ surfaces. Therefore, they cause less physical damage and more superficial scorching.

- Image Source: Robert A. Corry Director, Fire Investigation Specialist

When an ignitable liquid fuel source has been used to start a fire, intense burn patterns often appear on the areas where the liquid has been poured directly. Dark-coloured burn patterns tend to occur at the lower locations of uneven surfaces where fuel may have pooled before it ignited. These burn patterns are especially common on nonporous surfaces such as linoleum floors, tiles, and laminates. When an ignitable liquid fuel is poured onto porous surfaces such as carpets and wood floors, it may be absorbed into the materials as well as wood joists below the floor creating dark, distinct rundown burn patterns as shown in the photograph above.

Painting of the Great Chicago Fire, 1871

- Image Source: Wikipedia.org