Arson Information for Thriller Writers

 (Photo credit: Wikipedia)

Jerry Coble, the Assistant Fire Marshal for Guilford County, stood in front of our class with a burned out pot in his hand. The burn went clear through the bottom. “What do you think caused this?” he asked. My mind searched for the right answer, too much beer? the distraction of a pretty girl? insufficient kitchen instruction from your mom when you were growing up? He never answered. I’m guessing “pretty girl” was correct. Heroes and damsels go together after all.

Fires can be caused by three things:
Accidental - “Whoops, baby, you’re so smokin’ hot, I thought it
                     was you that was settin’ off my fire alarm.”
Incendiary - “Aw, honey, I was just trying to get a spark goin’ I
                     didn’t think your whole trailer would go up!”
Providential - “I think the Lord above knew I’d get to meet you
                      when he sent that bolt of lightening right into your
                      place, and you had tah run out ah the shower and
                      into my arms that ah way!”

Believe it or not, and I know this will stretch your imagination, but most arsonists are men. Men with a motive.

The most common motivators are:
* Fraud
* Crime concealment
* Vanity (they want to be a hero)
* Civil disorders
* Rash action of a juvenile - Under six, it’s an accident. If it’s
                  a child over six they are probably acting out of
                  spite or revenge.
* Spite/Revenge - now if this is a woman and she’s extracting her
                  revenge at night, she will stick close to her house or an
                  area with which she is familiar. If this were a man, he
                  will go anywhere. 
* Pyromania - and who doesn’t feel a touch of pyromania from
                  time to time? Especially when the moon is full and the
                  night air is crisp. But here is an interesting fact
                  SERIAL KILLERS often start out as fire setters.
                  Creepy huh?

Now go back to crime concealment. Are you trying to conceal a murder? A fire will get rid of a whole lot of evidence. If the victim was strangled or poisoned, even shot or stabbed in a way that a bone was not nicked, the method of death would not be determinable. No blood spray patterns. No bodily secretions. No finger or foot prints. But don’t think that the body will disappear. The trunk bones of the body will remain and the body can be identified by the teeth. Though some bodies are so far gone that it takes an expert eye to see the remains amongst the debris. HINT many bodies are found not from seeing them but smelling them. A charred body smells like BBQ and the bodies will attract flies just like unburned bodies do.

So you’ve caught your house on fire to hide your murder scene. Did you do your do diligence? HINT A house built 100 years ago will burn slower than a new house. The plastics used in today’s houses make them burn hotter and faster. If you need your scene to survive the blaze, you can write the scene into a turn of the century (1900s) house and that will give the fire fighters a better chance of getting there and putting out the blaze in time to save your crime scene or your good guys in desperate straits.

Another HINT if you are doing a crime cover up - it is almost impossible to get a fingerprint off a gas can. The plastic and oily residue means that there is a 99.9% chance that there is no fingerprint.

Also, if your villain is throwing a Molotov cocktail make sure it is in an easily broken GLASS container - not plastic, or it will only bounce. But bombs are easy to build. You just need a fuel and an oxidizer. We asked Jerry for a few simple recipes, like so many fifties’ housewives looking for directions on how to bake a prize winning chocolate cake. His hands went to his hips, and he shook his head in disbelief. Did we really think he was allowed to stand up there and teach us bomb-making techniques? Guess not. Sigh. Well there’s always the internet.

There are several stages to a fire:
* Incipient - first of beginning fire.
* Free Burning- rapid heat, spreading flame, lots of oxygen is being
                    consumed.
* Steady State - the fire burns throughout the house or structure, 
                    very high temperatures.
* Smoldering Stage - Burning is reduced to glowing embers.

If someone were to run up and kick down the door during the SMOLDERING STAGE there would be rapid oxidation and the flames will leap up with rapid flame and heat. This is how BACK DRAFT takes place. This is bad news for our heroes!

FLASH OVER - you must have an open flame. Everything reaches a critical point all at the same time and BOOM! It’s Armageddon. When this happens, vapors come off the heat and it LOOKS like an accelerant was used. However, a flash over is usually at table height. If an accelerant is used the markings are usually on the floor. HINT if your villain has done his homework then he knows about flash points and accelerant trails. This villain would make sure to disperse the accelerant at tabletop level to mimic a flash over.

Ah but not so fast. There are still some tell tale signs that would be much harder to hide. This is Pyrolysis. Pyrolysis is the chemical decomposition of a solid through the action of heat. When something burns, there is a demarcation of where it burned (char layer) and the normal wood, untouched by the fire. If this is cupped then they can tell that a liquid of some kind has gone into the cracks and carried the fire deeper at the cracks making the pyrolysis curved.

They would also be looking for fire marks. Fires burn up and out like a fan. If the fire burned down then it was probably following an accelerant trail.

They are looking for crazing on glass - that is where the glass heated and then cooled rapidly. This can show the use of an accelerant in the area - but also the use of water and even possibly a flashpoint. So, careful with the crazing detail.

Also, if you are describing a fire scene the MOST smoke will be further away from where it started and LESS smoke will be close.

In an ELECTRICAL FIRE it would have started from inside the wiring (where there is copper) if it was accidental and from an external source if it was set. Careful here - make sure there was nothing that could do the deed accidentally like a wire near a heat source.

A great way to start an on-purpose fire - take a lit cigarette and place it in a book of matches. Set this down on a bed near the pack of cigarettes. Better still, spill a glass of booze on the bed in the same area. Make sure it’s tucked in near a pillow and the mattress. It will take about three hours. The foam material will melt and drip down. Foam padding is HIGHLY TOXIC when it burns.


Not a smoker? The best place to start a fire is in the kitchen. It is very, very hard to detect if a fire were set on purpose in a kitchen.


What if someone was burning their own house? (BAHAHAHAHAHA! my computer alerted me that this was a non-standard question. Well, I hope so!) HUGE CLUE they will have taken the photos out. It’s the only thing that can’t be replaced.


I hope some of these notes prove helpful.
If you have any questions please let me know and I will try to answer them.

Related articles

• Seven Myths About Arson (discovermagazine.com)
• Spark of Truth: Can Science Bring Justice to Arson Trials? (discovermagazine.com)