Seven Myths About Arson

Fire investigators have long used certain rules of thumb to identify arson. Many have been proved incorrect.

By Douglas Starr|Monday, October 24, 2011
RELATED TAGS: MATERIALS SCIENCE
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A typical flashover sequence: A fire starts on a sofa; the fire generates a hot gas layer; the hot gas layer combusts; and finally, everything ignites floor to ceiling (flashover), enveloping the room in a blaze.
Courtesy of NIST

1 Crazing of windows, in which hundreds of cracks appear in the glass, indicates rapid heating and means an accelerant was used to start the fire.
REALITY:
Crazing is caused by the rapid cooling of window glass, as when water from a fire hose strikes a hot window.

2 Burn marks on the floor indicate that a fire was purposely set, because heat rises and fire only burns upward. It must have been set by pouring a liquid on the ground and lighting it.
REALITY: When a fire reaches flashover—the point at which an entire room ignites—extreme radiant heat will produce burn marks or even burn holes in the floor..

3 Melted metals, such as doorway thresholds, indicate that a liquid fire starter must have been used in order to reach temperatures that exceed their melting points.
REALITY: Wood fires, especially those that reach flashover, frequently exceed the melting point of metals..

4 Burn marks under doorway thresholds or under furniture indicate that a liquid accelerant must have been used to start the fire, since the liquid must have been poured and then seeped.
REALITY: Post-flashover fires commonly cause burning under thresholds and furniture..

5 Spalling, or surface chipping of concrete, indicates that a liquid accelerant must have been poured on the concrete surface and lit.
REALITY: Many factors can cause this effect, including differential expansion between the heated surface and the interior. Accelerant poured on the concrete actually protects it by providing a cool, evaporative surface..

6 Alligatoring, the appearance of blisters on the surface of burned wood, points to a fire’s origin. Small, flat blisters result from a slow burn; large, shiny blisters indicate rapid heating and hence the use of an accelerant.
REALITY: There is no scientific evidence for any such correlation. Both types of blisters can appear on the same burned wall..

7 Sharply angled V-pattern burn marks on a wall denote a fast-burning fire that must have been started with a liquid accelerant.
REALITY: Patterns can result from a number of factors, including ventilation, air currents, location of fuel, and the materials burning objects are made of.

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