For many years fire investigators believed that they could use certain “burn indicators” to show that a fire was purposely-set instead of accidental. As a result, many people were sent to jail for arsons they never committed. The field has been modernized in recent years: arson investigations are more scientific than ever before. Nonetheless, several myths endure “proving” cases of arson – myths that send the innocent to prison:
1) Crazing of glass: Investigators had long assumed that a “crazed” pattern in glass – in which the glass shows thousands of tiny cracks – indicated rapid heating and therefore the use of a fire accelerant. Experts now know that crazing occurs from the rapid cooling of glass, not heating. In other words, almost any time water is sprayed onto a super-hot window crazing can occur.
2) Concrete spalling: Spalling, or the heat-induced crumbling concrete, was thought to indicate that a fire-starting liquid had soaked into the concrete, indicating arson. Scientists now know that spalling occurs almost any time concrete gets very hot.
3) Collapsed furniture springs: It once was assumed that couches and other furniture with collapsed springs indicated a hot-burning fire and the use of accelerants. According to FBI laboratory tests, springs can lose their tensile strength at the high temperatures found in normal house fires or from the way the springs were originally manufactured. There is no link to the use of accelerants.
4) Alligatoring of wood surfaces: Investigators thought that the appearance of large-shiny blisters on wood (resembling alligator skin) indicated a hot, fast-burning fire typical of arson and that flat blisters indicated a cooler, slower fire. It’s now known that neither of those correlations is true.
5) Floor-level burning: Because heat rises, an extensively-burnt floor was thought to indicate an artificially-started fire. Actually once “flashover” occurs – in which the room becomes thoroughly engulfed in flames – floors will burn as commonly as walls.
6) Melted metals: For years investigators thought that melted aluminum thresholds indicated the use of a liquid accelerant. It’s now known that but wood can burn hot enough to melt metal, depending on the burn and ventilation conditions.