Debunking Arson Indicators

Sidebar: 'Flashover defense' gets two arson convictions reversed in the 1980s.
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Sidebar: 'Flashover defense' gets two arson convictions reversed in the 1980s.

Given the lingering false beliefs about fires, it seems remarkable that lawyer Larry Hammond successfully used information about the phenomenon of flashover to help get two convictions reversed in the 1980s when few had even heard of it. The point of flashover is when a fire's heat becomes so intense that everything flammable in a room spontaneously ignites, windows blow out and a fire becomes non-survivable. The revelation was that much seen in the aftermath of accidental fires that reach flashover is indistinguishable from effects long considered proof of arson.

One of Hammond's successes was John Henry Knapp, who was freed in 1987, shortly before a date with the executioner, after 17 years in prison for the arson murders of his daughters. He was retried, but Hammond was ready. Fire experts Marshall Smyth and Richard Custer carefully reconstructed two full-scale rooms that were then burned and videotaped to show the jury flashover in action.

Although the jury hung, Knapp's release was secured, and Hammond credits the revelations about flashover. "We proved beyond a shadow of a doubt," he said, "that without so much as a thimble of flammable liquid you could watch a room go from a simple open flame to a 2,000-degree floor-to-ceiling fire in two and a half minutes. And we did it twice."

Debunked "Arson Indicators":

  • "Crazed glass" is the name given to windows marked with a web of very fine lines after a fire, an effect long cited as proof of arson. The "crazing" is really caused when firefighters' cold water hits hot glass.
  • "Spalled concrete," concrete marked with pitting, holes and brown marks or discolorations after a fire, doesn't prove that a fire accelerant was used, only that the concrete got very hot.
  • "Charring," "char blisters" and "alligatoring" are names given to sometimes shiny blister marks seen deep in burned wood and long attributed to arson. But those marks don't prove that an accelerant was used or how long the wood burned.
  • So-called "pour patterns," irregularly shaped burn patterns once believed to result from an arsonist using accelerant, have many innocent explanations. They can be caused by fall-down from objects like plastic lamps or bookcases or from furniture melting downward. Just about anything that burns can leave such marks.
  • It's a myth that arson fires burn hotter than accidental fires — faster, perhaps, but not hotter.