Forensics: How to Investigate a Fire

Forensic Analysis is a vital part of Fire Investigation. By its very nature, fire destroys the evidence of how it started. And not only that, but the methods of putting out a fire often increase the damage to any evidence an investigation hopes to find. As a firefighter friend put it:

“When you put a pulse of water on the fire, it saturates everything—1 liter of water becomes 9000 liters of steam…” (Findley, 2021)

Fire investigation is also dangerous. Even after extinguishing the fire, structural damage can leave a building unstable and liable to collapse. But evidence of volatile accelerants needs finding, therefore the Fire Investigation must begin as soon as possible after the flames are out.

Fire!

How does a Fire get going?

Accidental

A ignition source is needed to start a fire. Some of the accidental ways fires start are from electrical faults and unattended candles.

Spontaneous Combustion

I can hear you laughing at me already. But some sources of fires are self-heating substances, which, if left unventilated, can spark into fires. A painter dumps some rags covered in Linseed oil in a corner, locks up and goes home for the weekend. The Critical Ambient Temperature increases and the pile of rags ignites. A bale of hops can ignite from the centre if it is not carefully stored (Fire-scene Patterns).

Deliberate

A fire investigator is there to discover if a fire was accidental or deliberate. Often a fire is intentionally started in a number of places at once, so finding multiple points of origin is a clue. The most obvious way of  intentionally starting a fire is with a match. Of course, if the arsonist wants to get clever they can come up with electrical timers or a Molotov cocktail (Cafe&Stern, 1995-2017).

Accelerants

After ignition, a fire needs a source of fuel. Paper, as mentioned above, works well. Petroleum products make a fire burn fiercely. Alcohol is another product that burns well.  A fire requires three things to allow it to progress: Heat, Oxygen and fuel as shown in image 1(Leavell, 2017).

Image1: Fire tetrahedron Credit (Leavall, 2017)

What happens next?

The development of a fire depends on the amount of fuel that is present. Once ignition happens, then the fire enters the growth stage. A second fuel item is ignited and a third, and the fire spreads until Flashover occurs. See image 2 (Leavell, 2017).

Flashover

The point at which every available surface has been heated to combustion point by radiation from the fire. At this point everything ignites.

Image 2: Stages of a fire Credit (Leavall, 2017)

The Fire is fully developed at this point. Finally the fire will run out of available fuel and enter the decay phase. In an enclosed fire, such as a house fire, the limiting factor is usually oxygen. Once that is used up, then the fire dies.

Once the fire is out, it is time for the investigation.

Fire Investigation

Forensic Science is particularly helpful is in determining if a fire was deliberately set or was an accident. The Fire Investigator must determine the point of origin. This is the place where the evidence as to whether this was accidental or deliberate will be found.

Where to start?

Even while the firefighters are working on the blaze, it is possible to start an investigation. As always witnesses are important in determining if a crime has occurred, but with a fire it is vital to get their information.

Witnesses

But everything is black and burned and it all looks the same. This is where witness information is so helpful. While evacuating the building, people will be aware of the location of the fire so they can avoid it. They often take note of the colour of the flames, which is another helpful piece of information for the investigating officer. Evaluating their testimony is the first step in the long process of investigating a fire (Fire Investigation).

Flames

As my firefighter friend puts it, “It’s not always orange, either. I’ve seen green flames, purple, blue. You name it.” (Findlay, 2021) You will be aware from science class that chemicals burn with different colours (Fire Investigation). This helps the investigator know what was stored in the building before the fire.

What does the point of origin look like?

The first area to examine for the start of the fire, is the area with the most damage. That makes sense, doesn’t it? In some cases, there is little fuel at the initial burning point and the largest amount of damage occurs elsewhere, but in most cases, the worst burning will be where the fire has burned for the longest time, and that will be where the fire started (Fire Investigation).

Burn Patterns

Like experts who read Blood Patterns for solving crime (see article here), there are also expert who read patterns of burning. Initially, a fire burns upwards and out, so if there is a classic ‘V’-shaped mark on a wall, that is a reliable indicator of the initial burn. This works best if the fire was slow to start, in a fast-developing fire, then the heat will increase in a uniform manner preventing the easily identifiable marking.(Fire-scene Patterns).

Smoke soot deposits on cooler surfaces, so away from the fire; another indicator of where the most intense burning occurred.

Electrical Ignition

The progression of a fire can sometimes be traced by studying the electrical arcs. Most fires act on wires rather than the wires being the cause. As the fire attacks the wiring, it causes arcs and the circuit breakers to trip, preventing any more arcing on that wire. In an arc, the wires develop characteristic blobs.  Image 3 (Carey) shows a wire displaying characteristic arcing.

Image 3: Wire showing the characteristic blobs of electrical arcing credit (Carey)

While I said that fire acts on wires, sometimes wires can arc because of an electrical fault, throwing off sparks for ignition. If the arc you find is inside a toaster with a melted casing, then you can be reasonably sure that was the ignition point (Carey).

Collecting Samples

With the point of origin located, sample must be collected. People who set fires deliberately, expect the destruction to hide evidence of their crime. Yet, it is possible to find the smallest trace of evidence at the scene.

Even a match can be found. The investigator collects the ashes for examination under a microscope. The heads of matches contain the fossilised remnants of small creatures, diatoms, as seen in image 4 (Ingram).

Image 4: Microscope picture of diatom from a burned match head credit (Ingram)

Each manufacturer has different sources for their chemicals so the diatoms are different from their competitors (McDermid, 2014). All evidence to lead the investigator to a potential suspect.

For other types of evidence, the human nose can sometimes detect the presence of accelerants, but for really sensitive detection we call on sniffer dogs. Once a potential source of evidence is identified, then samples are collected. If accelerants have been poured onto a carpet then traces might have soaked down and remain to be identified (Cafe, 1995-2017). Carpets with foam backing often show up as having petroleum products so it’s important to acquire reference samples from areas away from the burning. Samples of potentially volatile substances must be carefully packaged and tightly sealed in either glass or a clean metal paint tin. A plastic container does not work; not only is it made from petroleum products, some volatile substances degrade plastic.

Laboratory Analysis

Once the samples are correctly collected, they are transported to the laboratory for specialised testing. This is usually done via Headspace Analysis in a Gas Chromatograph. A ‘headspace’ is ‘the gas above the sample in a chromatography vial’ (Mathias, 2013). As seen in image 5 (Green, 2005).

Image 5: The headspace in a chromatography vial credit (Green 2005)

In performing ‘headspace analysis’ the scientist is testing for the presence of the analyte in the gas. This ‘headspace’ is injected into a Gas Chromatograph and analysed in the normal way. The sample can be heated to encourage the analyte to enter the headspace (Mathias, 2013).

In Image 6, below, the gas chromatograms of fire residue show that interpretation requires expert knowledge. Different types of accelerant are difficult to distinguish apart. (Fernandes, 2002).

Image 6: Different accelerants can have similar Chromatography graphs credit (Fernanades 2002)

History

In 1666 the Great Fire of London swept through the city and much of the old medieval landscape. Charles II created a committee to investigate as alarming rumours propagated. Foreigners were to blame, everyone insisted. In Forensics: The Anatomy of crime,  the author describes how a surgeon stood on a church tower and swore blind that he saw fire start in many places at once. The committee interviewed eyewitnesses, some of whom declared they had set the fire. But eventually, the committee’s members sorted through the information and ruled that the fire had started accidentally in a baker’s shop. This was an unsatisfactory conclusion to most people, but until Michael Faraday published a book on how fires work in 1861, then no one knew how to investigate a fire properly (McDermid, 2014).

Finally

In the history of fire investigation, the only evidence available was the testimony from witnesses. Now, we have investigators who understand how a fire propagates and laboratory scientists who can detect trace residue among the burned evidence of a fire.  People starting a fire deliberately must hope the evidence is destroyed, but forensic analysis supports a proper fire investigation and uncovers what the fire starter believed to be hidden. The forensic science of Fire Investigation is very complex and requires years of training. But their role in solving crime is invaluable.

image credits:

Heading image: https://commons.wikimedia.org/wiki/File:Aircraft_Rescue_Firefighting_training.jpg

References

Cafe, T., (1995-2017) Basics of Fire Investigation. TC Forensics. https://www.tcforensic.com.au/docs/article12.html

Cafe, T, & Stern, W., (1995-2017) Is It Accidental Fire or Arson? TC Forensics. https://www.tcforensic.com.au/docs/article3.html

Carey, N. Arc Mapping. Hawkins. https://www.hawkins.biz/insights/insight/arc-mapping

Fernandes, M. & Lau, C. & Wong, Wing. (2002). The effect of volatile residues in burnt household items on the detection of fire accelerants. Science & Justice – SCI JUSTICE. 42. 7-15. 10.1016/S1355-0306(02)71791-7.

Findlay, R. & Knipe, V., (2021, March 3) Private Message Facebook.

Fire Investigation. The Forensics Library. https://aboutforensics.co.uk/fire-investigation/

Fire-scene Patterns. what-when-how: in depth information and tutorials. http://what-when-how.com/forensic-sciences/fire-scene-patterns/

Green, JD. (2005) Found at: https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/headspace-analysis#:~:text=Headspace%20analysis%20is%20a%20technique,enclosed%20container%20(Figure%201)

Ingram, W. Winston Ingram Diatoms Images. https://microscope-microscope.org/microscopy-image-gallery/microscopy-diatoms/

Leavell, D., Berger, C., Fitzgereld, FA., & Parker, B. (2017). Fire Science Core Curriculum-module 3. Oregon State University. https://catalog.extension.oregonstate.edu/em9172module3/html

Mathias, J. (2013). Guide to Headspace Analysis. InnovaTech. Feb 14. https://www.innovatechlabs.com/newsroom/225/guide-headspace-analysis/

McDermid, V. (2014). Forensics: The Anatomy of Crime. Profile Books LTD.