When a fire is deliberately set, the person often uses an accelerant — a substance, usually an ignitable liquid such as petrol, to start the fire or make it spread faster and hotter. Detecting accelerants is a key part of distinguishing arson from an accidental blaze.
Signs at the scene
Accelerant use can leave tell-tale traces:
- irregular "pour" patterns on floors, where liquid ran and burned,
- unusually rapid or intense fire spread inconsistent with the available fuel,
- multiple, separate points of origin.
None of these is proof on its own — investigators weigh them together and against innocent explanations.
How it's detected
Suspected accelerant is confirmed by a combination of tools:
- electronic vapour detectors ("sniffers") at the scene,
- trained accelerant-detection dogs, which can pinpoint tiny traces,
- and, decisively, laboratory gas chromatography–mass spectrometry (GC-MS) on debris samples, which identifies the specific ignitable liquid.
Interpreting presence and absence
Two cautions matter. Presence of an ignitable liquid isn't automatic proof of arson — many household products and plastics contain or produce hydrocarbons, so context is essential. And absence doesn't rule arson out — accelerants burn away, and many deliberate fires use none at all. As with the rest of fire investigation, the conclusion comes from the whole picture, tested against the science.