Arson Science Rewritten

An anonymous reader handed us a link to an AP story about advances in the science of arson investigation. Many assumptions about fire, long held by investigators, have been overturned in recent years as scientists have come to understand concepts like 'flashover'. The repercussions of these findings is having an effect not unlike the use of DNA in crime-solving; people are being set free, and old cases are being re-examined. From the article: "Significantly, flashover can create very hot and very fast-moving fires. And it can occur within just a few minutes, dashing the concept that only arson fires fueled by accelerants can quickly rage out of control. The studies began to chip away at the old beliefs -- critics call them myths -- but it took years. Through the 1980s, texts at the National Fire Academy in Emmitsburg, Md., still taught the traditional techniques. It wasn't until 1992, when a guide to fire investigations by the National Fire Protection Association -- 'NFPA921: Guide for Fire and Explosion Investigations' -- clearly laid out, in a document relied upon by authorities nationwide, that the earlier beliefs were wrong."

Hmmm, Not in my training and experience

[dsci]

I used to be a volunteer firefighter and also served as a fire investigator. My experience began around 1981 or so; later (late 90s), I worked for a Police Department doing crime scene work and part of that was fire investigation.

From TFA:

Up until the 1990s, this is what fire investigators were taught:

Fires always burn up, not down.

I was NEVER taught that; just the opposite. Fires tend to burn up FASTER than they burn down, but geez, anyone who has ever actually WATCHED a fire burn knows this statement is nonsense.

Fires that burn very fast are fueled by accelerants; "normal" fires burn slowly.

I was NEVER taught that; just the opposite. We were taught that accelerates were ONE WAY a fire MIGHT burn faster than you would expect under similar conditions. We were also taught that is EXTREMELY difficult to gauge how fast a fire "should have" burned. I did my first chemical test on fire debris in 1986 using GC/MS via a very simple headspace analysis on a sample that the state lab sent back as negative (my test was positive for something, perhaps ambient artifacts, but was an educational run, not an 'official' test). With the negative test result, we sure did not try to use evidence of 'how fast that fired burned' to assert the presence of an accelerant.

Arsons fueled by accelerants burn hotter than "normal" fires.

Somebody is oversimplifying the concept of "fire load" here. There are a WHOLE LOT of things than can make a fire burn hotter than 'normal.' In fact, as a common theme I am trying to represent, "normal" is not a well defined term for real-world fires. Rural firefighters and investigators certainly knew this before 1992.

In fact, this statement glosses over another issue about arson - they often, quite often, don't involve 'accelerants' at all.

The clues to arson are clear. Burn holes on the floor indicate multiple points of origin. Finely cracked glass (called "crazed glass") proves a hotter-than-normal fire. So does the collapse of the springs in bedding or furniture, and the appearance of large blisters on charred wood, known as "alligatoring."

The clues to arson are clear?? Man, I clearly remember in the early 1980's being taught exactly the OPPOSITE of what this article says was the "norm" back then. Perhaps it was taught somewhere, but not in RURAL North Carolina. Absolutely NONE of these "clues" are evidence of arson - only of certain fire conditions.

What we were taught in our arson investigation classes, and what I came to learn through experience, is that arson was/is and EXTREMELY difficult crime to prove. That means it is difficult to prove that a fire was arson, much less who did it.

Truthfully, based on my experience, I don't see the point of this article. It asserts 'beliefs' about fire investigation pre-1992 that just are not true.

And finally, the article gives the tragic story of the Lee family that occured in 1989. While presenting NONE of the evidence that was used to convict him, the story creates the straw man that just because it was 1989 and fire investigation changed (around then, according to the article), he must be framed. I don't know of his guilt or innocence, but that's a might big leap of logic.

Re:Wooden houses?

[Anonymous Coward]

On the Pacific coast, very few buildings are built out of brick or masonry, for three important reasons: 1) it's expensive, 2) along much of the Pacific coast there's an abundant supply of wood, and 3) they tend to fall down for reasons entirely unrelated to fire.

Irrespective of what your house is made of, one of the keys to keeping it from burning is a non-flammable roof. Many houses succumb to fire when another fire outside the house (forest fire, nearby building fire, brush fire, etc...) drops flaming material on the roof. If you don't want your house to burn in one of those scenarios, put on a metal roof and keep the trees trimmed back.

Background: Fire Development

[Christopher_Edwardz]

Here is some interesting information from a book on my shelf on Arson:

From: Fire Investigation; DAÉID, NIAMH NIC; CRC Press, Boca Raton, FL, USA, 2004. ISBN: 0-415-24891-4

(Excerpt: Chapter 1)

A fire develops through a number of fairly predictable stages. Initially a source of ignition is required at a site suitable for flaming combustion to occur. The materials begin to burn in a sustained ignition with an open flame which remains once the initial source of ignition is removed. This ignition is localised to the first fuel ignited. The fire plume emits hot gases generating a heat flux. These gases typically containing soot, water vapour, CO2, SO2 and other toxic gases. Convection carries these products and heat to the upper parts of the compartment and draws oxygen in at the bottom to sustain combustion. The increasing gas layer at the ceiling radiates heat into the room.

Growth Period

Convection and radiation spread the flames upwards and outwards from the original fuel package until nearby fuels reach their AIT and become involved in the fire. Radiative heat may spread the fire laterally depending on factors such as the proximity of fuel packages to each other. The fire grows by progressively spreading to involve adjacent combustible items. Hot gases composed of toxic gases, partially combusted pyrolysis products, soot and smoke rise to form a fuel rich layer at the ceiling, the temperature of which steadily increases. The lower part of the room will still be rich in oxygen and the rate of burning within the area continues to increase with a consequent increasing release of heat. As the fuel rich gas layer gets lower it may eventually ignite as some of its constituents may reach their AIT or by direct flame contact. This stage is called flameover and involves a rolling flame front within the hot gas layer.

Flashover

Even without flameover occurring the hot gas layer is radiating heat into the room. This causes items in the room to progressively heat up and when the layer reaches a temperature of approximetely 600C it is generating approximately 20 kW/m [2]. In a normally proportioned room this is sufficient to raise the temperature of cellulosic fuels within the room (furniture, carpets, etc.) to their AIT and simultaneously ignite in a process called flashover. Flashover is a transition from a fire involving one fuel package after another to a fire which involves all available fuel in the compartment. At the time of flashover, ventilation in the compartment becomes a restriction on the amount of oxygen available for combustion to occur, and the minimum size fire that can go to flashover in a given room is a function of the ventilation provided through an opening (ventilation factor).

Post-Flashover

Fire is a balancing act between fuel, heat and air. If the ventilation is limited then the fire will progress at a slower rate involving slower temperature rise and greater production of smoke. Ignition of the smoke layer will take longer or may only occur outside of the compartment if the oxygen supply is limited. If the fuel does not burn fast enough or produce enough heat, flashover may not be reached. Once post-flashover or steady state is reached all involved fuels will continue to burn as long as oxygen is available until the fuel is consumed.

Smouldering

Eventually, as the fuel available becomes exhausted open flaming combustion becomes gradually less and glowing combustion becomes more prevalent. This can also occur if the oxygen levels in a developing fire drops (below c.16%). The fuel may still remain in a heated state and the reintroduction of oxygen can cause the fire to re-ignite with explosive speed. Such a scenario is sometimes called backdraft.

AIT: Automatic Ignition Temperature (as in 451 Fahrenheit)

In general, if arson investigation used to be like TFA states, then yes, I'd say that it has come a long way baby

Re:Wooden houses?

[tetromino]

IMHO, the difference in building materials can mostly be explained by the differences in the buildings.

Americans typically live in small, freestanding, single-family houses. For such small buildings, a wood+drywall construction is probably the most cost-efficient, especially considering North America's plentiful supplies of lumber. On the other hand, Europeans tend to live in apartment buildings, which require the use of stronger materials like brick, cinder block, and reinforced concrete.

Unfortunately, these rational choices of construction materials have become cultural values. Thus wealthy Americans are happy to buy enormous mansions made of toothpicks and cardboard, while some Russians I know are spending their meager resources on building small single-family homes out of concrete.

Re:If you've ever seen how fast a fire moves...

[Hektor_Troy]

Ah, yes. I always seem to forget, that evidence that completely exhonorates someone, is never "significant evidence".

Of course, the ACLU seems to think differently [aclu.org]:

Frank Lee Smith, Florida
Convicted 1985; cleared (after death) in 2000

Mr. Smith was convicted of the rape and murder of a child. After the trial and sentencing the chief witness recanted her testimony. But Smith nevertheless was scheduled for execution. He died of cancer in January 2000, while on death row before the completion of the DNA test results that proved his innocence ten months later.

Now obviously, being cleared 15 years after your first conviction doesn't count. Nor does having the chief witness recanting their testimony.

But who cares. It's not like it was important, right? After all, he was a filthy child raping murderer, who deserved what he got.

Oh, wait.

Former Volunteer Firefighter

[JetScootr]

I trained in 1979ish and then they were teaching how to fight flashover. One exercise involved a fully-involved brick "bedroom". Flames were up to the ceiling and spread across the entire room. The test: Put out the fire with a 1-2 second wide spray from a 2 1/2 inch hose. That's about 10-20 gallons of water.
The trick is don't spray the hay, spray the ceiling over the hay - that's where the heat is. This is opposed to the conventional technique of spraying "the base" of the fire.
The near-explosive boiling of water to steam takes away a coupla hundred degrees of temprature, and the sudden increase in humidity reduces the flame potential of aerable fuels like cloth, blankets, hay, etc ("aerable" as opposed to dense fuels like solid wood). The snuff-out is impressive.
I don't think the article is right about this being "new", although it's possible the info spread slowly in different regions.