How do firemen know the cause of a fire after everything has burnt down?

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Often times you hear about the cause of a fire being a cigarette for example. However, isn’t the cigarette already long gone after the house/forrest/etc. has burnt down? How is it still possible find out what started the fire?

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Burn patterns are different, and sometimes you can smell something characteristic, like gasoline. Identifying where the fire started can help (if it’s an ash tray probably it started with a cigarette). There are also chemical tests but I don’t know a lot of details.

Each fire has their own “signature” depending on the materials, any accelerants (such as gasoline), temperature and weather conditions, etc. Knowing *where* the fire started, from tracing the spread of the fire, gives clues as to what might have ignited the fire.

For example, let’s say there’s a house that burned down. Investigators may notice that certain parts of the house burned much more rapidly than others, indicating that someone might have intentionally poured an accelerant in the basement and set fire to it there as an act of arson.

Another example, specifically referencing the Kings Cross fire, might have a blaze starting under the escalators. Investigators were able to identify the combustible material beneath the escalator, combined with buildup of grease and oil, and recognised the commuters’ habit of disposing their cigarettes between the handrail and the gap in the steps.

While the cigarette is long gone, there are only so many ways a fire can start, and examination of the remaining evidence will narrow it down to something as small as an exposed wire or a burning cigarette.

*most* structure fires the houses are total losses where it burns down to the ground. Most areas, especially with those with professional paid departments, usually get the fire under control before it causes a total loss. These houses will general have lots of “clues” on what started the fire. Most house fires the houses are able to be rehabbed and used again.

The answer is things like chemical traces left at the site and burn patterns.

On the other hand 10ish years ago it came out that a significant chunk of arson analysis techniques had no real scientific backing or proof of any sort, like bite comparison turned out to be. It was quite a big thing for a bit that I lost track of.

I was once told newspaper is an effective accelerant that burns up entirely, unlike petroleum based alternatives