Read previous posts about dubious areas of forensic science by following these links:

Part 1: Introduction
Part 2: Fingerprints
Part 3: Bite Mark Analysis

Arson investigation, unfortunately, is a field where junk science has been the rule rather than the exception. Rarely are houses intentionally set on fire to allow investigators to study the unique effects of burning houses. There are subtle yet distinct differences in how houses burn in comparison to other fires. Many arson investigators rely on their intuition and have been slow to adapt to studies that show that their assumptions are bogus.

For example, it used to be that arson investigators interpreted signs that a fire burned particularly hot as proof that an accelerant, such as lighter fluid, was used. Experiments have shown that wood and gasoline fires burn at nearly identical temperatures. A natural wood fire can reach temperatures in excess of 2,000 degrees, hot enough to melt or burn many household objects. Melted copper wiring, melted steel bedsprings, and “crazed glass” all were used as evidence of arson. But crazed glass is usually the result of rapid cooling—such as when firefighters put out a blaze—and the upper ranges of pure wood fires are enough to melt copper and steel.

Another common misconception is that floors won’t burn unless an accelerant has been used. One experiment was all it took to disprove that theory. Investigators in Duval County, Florida were trying to prove that a fire could not have happened the way a defendant claimed it did. In the process, they proved his case. They spent $20,000 to hire experts and equip a condemned house with heat and gas sensors. They lit a couch on fire, without using any accelerants, and watched what happened. The fire quickly consumed the couch and filled the room with smoke. Just four and a half minutes later the room suddenly burst into flames, including the floors, walls, and furniture. The prosecutor quickly dropped the case against the defendant.

The experimental house burned so rapidly because of “flashover.” Flashover occurs when smoke and gas in a room build, to the point where it finally explodes in flames, including the floor. The house reached over 1,000 degrees just before flashover. Before this experiment, investigators were aware of flashover, but most believed it took longer to occur, especially without a liquid accelerant. It is impossible to say how much more the field of arson investigation would advance with a few more experiments.

It is still remarkably easy to be certified as an arson investigator in many states. Some states require as little as 40 hours of training before someone can be deemed an expert in arson investigation. All over the country, there are poorly trained investigators that are still using techniques that have no basis in science.