Eli5: How can they tell where a fire starts and how it grew after the building has already been burned down?
In: 6
There’s probably lots of ways, but the way I know is that they try to figure out what was burning the longest. The stuff that looks the most burnt, melted, and crispy had to have been burning for longer than the stuff that just has a little layer of ash on it. And then they can assume that whatever started burning first must have been burning the longest, so the fire must have started where the most crispy stuff is.
The way the burns are within the structure and the evidence left behind can be pieced together to get an understanding of what caused it or where it started.
Typically, the area that received the most damage and where the fire burned hottest, will be where the fire started. Things like the materials that burned or how badly burned was it will give them these clues. Metals, for example, have different melting points that can help indicate temperature.
Residue left behind can also give an indication if an accelerate was used, like lighter fluid or kerosene.
TLDR: a lot of science, math and experiential history comes into play.
Items and materials behave differently, depending on the circumstances of the fire, and are different depending on how they are burned down. Take a normal camp fire. you can with a bit of experience estimate how long each log was in the fire, depending on how it looks.
If you go through the smouldering rubble, and you see that the rubble of the second floor is barely burned, but the foundation of the first floor is burned up, then you can reasonably assume that the fire was on the first floor, not on the second floor.
It is similar like reconstructing an aircraft crash: piece by piece you put together a picture and attempt to find out, if all parts suits known fire patterns.
SYL
Material which has been doused with an accelerant burns hotter than the same material would otherwise. So in cases where an accelerant was used there’s often a visible as well as a chemical trail left behind.
An investigator once told me you could literally see the splash marks on the walls in blacker carbon) where a person had walked around with a can of fuel.
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