Theo Nash
Fire investigators are tasked with examining the scene of a fire to determine whether the cause was accidental or deliberate. Burn patterns, for example, can indicate the area of origin and the course of the fire. The presence of ignitable liquid residues, tampered or damaged utilities, and unusual burn patterns are all pieces of physical evidence that can be collected and analysed to support the investigator in reaching a conclusion. One tell-tale sign of burning alcohol is orange flames, although smaller quantities of alcohol burn blue.
| Characteristics | Values |
|---|---|
| Smoke colour | Alcohol may burn blue, but large quantities burn orange |
| Burn pattern | A "V" shape identifies the point of origin, with the bottom being the point of ignition and the rest of the "V" being formed by the upward and outward progress of the fire |
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What You'll Learn
Burn patterns can indicate the fire's origin and path
Burn patterns are a crucial aspect of fire investigation, providing valuable insights into the origin and progression of a fire. By examining the geometry and characteristics of burn patterns, investigators can reconstruct the scene and determine the fire's path.
One of the key indicators of a fire's origin is the presence of V-shaped or U-shaped burn patterns. These patterns, often referred to as "plume generated patterns," are created by the upward movement of hot gases and smoke. The area directly above the fire's origin typically exhibits the most intense markings, with the patterns narrowing toward the point of origin.
Circular patterns, commonly found on ceilings or upper support beams, also indicate the presence of a fire plume. The centre of the circular pattern often aligns with the fire's origin point below. These patterns provide clues not only about the origin but also the intensity and progression of the fire.
The depth of charring and the extent of soot deposits are additional factors that investigators consider. Deeper charring suggests prolonged exposure to intense heat, while shallow marks indicate a quick-burning fire. Soot trails, indicating airflow direction, can help determine how the fire spread and whether it was influenced by ventilation.
Furthermore, patterns on door jambs, referred to as "indicators," reveal the direction of fire travel. By comparing both sides of the jamb, investigators can determine the side from which the fire originated. The area of greatest destruction, or greatest mass loss, may also pinpoint the fire's origin, as it indicates the location of the most intense fire activity.
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Alcohol burns blue, but large quantities burn orange
The colour of a flame can indicate the presence of burning alcohol. Pure alcohol burns with a blue flame, which may have yellow accents. However, when large quantities of alcohol are burned, the flame may appear orange. This colour change is due to incomplete combustion, which occurs when there is more carbon per mole of alcohol.
Isopropyl alcohol (IPA), for example, burns with an orange flame. This is because there is more incomplete combustion occurring with IPA, resulting in a smoky orange flame and the smell of soot. In contrast, ethyl alcohol burns with a blue flame because it combusts more completely.
The colour of a flame can be used as physical evidence to determine the cause of a fire. For example, burn patterns and the presence of ignitable liquid residues may indicate foul play. The presence of accelerants, such as gasoline, can also be tested for using analytical techniques.
The observation of a blue flame with yellow accents, as seen in burning ethanol, can be explained by the chemical reaction of ethanol combustion. This reaction produces excited carbon dioxide, resulting in a blue flame with a relatively low temperature.
It is important to note that the colour of a flame is not the only indicator of burning alcohol, as other factors such as the smell and temperature of the flame can also provide clues. For example, ethanol combustion does not produce any smell, while IPA combustion produces a smoky odour.
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A V pattern identifies the origin point
Fire investigators examine burn patterns to determine the cause of a fire and identify its origin. Burn patterns are the result of flame, smoke, or heat plumes intersecting a horizontal or vertical surface such as a wall, ceiling, or piece of furniture. Fire plumes are three-dimensional and conically shaped, so they usually leave a two-dimensional V-shaped pattern when they intersect a wall and a circular pattern on a ceiling. The area directly above the fire’s origin will usually have the most intense markings.
Common shapes include V- or U-shaped patterns that narrow toward the point of origin. The direction of staining, or smoke trails, indicates the upward movement of smoke and gas, often revealing the fire’s origin. Soot trails can also indicate the direction of airflow, often leading to areas of increased fire activity. If soot is more concentrated near a doorway, it may suggest that the fire spread through that entry point.
When evaluating V patterns in large, open areas or fully developed commercial fires, investigators must use caution when evaluating the fire patterns they observe. Lampshades can ignite and drop, and curtains, paintings, and wall posters can burn and fall to the floor, creating secondary V patterns.
The presence and depth of char can also help investigators identify fire intensity and duration, and pinpoint the fire's origin, as the most intense charring often occurs near the point of origin.
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Ignitable liquid residues may be present
Fire investigators are tasked with examining the scene of a fire to determine whether the cause was accidental or deliberate. The presence of ignitable liquid residues (ILRs) in fire debris is often a critical piece of physical evidence that can indicate whether a fire was started intentionally.
The identification of ILRs in fire debris is a complex process due to the presence of various compounds and products of combustion and pyrolysis. Fire debris samples are labelled based on the substrate and the elapsed time since the fire. Different ignitable liquids, such as gasoline, diesel, ethanol, and kerosene, leave distinct signatures in the fire debris, allowing investigators to identify the specific liquid used.
To determine the presence of ILRs, investigators employ analytical techniques such as gas chromatography and mass spectrometry. These techniques must be performed within a specific timeframe, as the validity of the testing depends on prompt sampling. The interpretation of data from fire debris is a challenging step, requiring specialized knowledge and techniques.
In cases of suspected arson, the presence of ignitable liquid residues can provide strong evidence of foul play. Arsonists often use accelerants, such as gasoline or ethanol, to start or accelerate a fire. These accelerants can leave behind residues that can be detected and analyzed using techniques like chromatographic analysis and headspace-mass spectroscopy.
It is important to note that not all ignitable liquids are necessarily indicative of arson. Some alcohols, such as ethanol, become flammable at 100 proof (50% alcohol by volume) and can ignite easily. High concentrations of alcohol can release more flammable vapors, increasing the risk of accidental fires. Therefore, the presence of ignitable liquid residues must be carefully analyzed and interpreted within the context of the fire scene and other available evidence.
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Utilities may be tampered with or damaged
When investigating a fire, it is crucial to determine whether utilities have been tampered with or damaged, as this can indicate foul play. Utilities may include gas, electricity, and water supplies. Signs of tampering or damage can include:
- Moving or altering the position of knobs, switches, or controls on utilities.
- Damage to the physical components of the utilities, such as broken or melted parts.
- Disruption to the utility lines or connections, indicating possible interference.
- Evidence of forced entry to access utilities, such as broken locks or windows.
- Unusual or unauthorized modifications to the utilities, suggesting intentional interference.
Investigators should also be aware of potential evidence destruction or contamination at the scene. Overhaul and salvage activities, as well as the use of certain tools, can inadvertently destroy or alter evidence related to utilities. Weather conditions, vehicle leaks, and external fuel-powered equipment can also impact the integrity of utility-related evidence.
To effectively investigate tampered or damaged utilities, the following steps should be taken:
- Secure the scene and limit access to essential personnel, including utility company representatives and investigators with expertise in fire and utility examination.
- Conduct a thorough preliminary scene assessment, documenting and photographing any visible signs of tampering or damage to utilities.
- Collect and preserve physical evidence related to the utilities, such as residue samples, tool marks, or disturbed soil/debris indicating possible excavation.
- Interview witnesses and gather information about any unusual activities, suspicious individuals, or changes in utility behaviour observed prior to the fire.
- Analyze the collected evidence using appropriate techniques, such as gas chromatography and mass spectrometry, to identify the presence of any ignitable liquids or other indicators of foul play.
- Consult with experts in utility operations and fire investigation to interpret the evidence and determine the cause of the fire accurately.
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Frequently asked questions
Burning alcohol gives off blue flames. However, when large quantities of alcohol are burned, the flame color can turn orange.
The burn pattern of alcohol is often identified as a "'V'" shape, with the bottom of the "V" being the point of ignition, and the rest of the "V" shape being formed by the upward and outward progress of the fire.
Investigators can identify the presence of burning alcohol by analyzing the burn patterns, the color of the flames, and through the use of specialized analytical techniques.
One challenge in identifying burning alcohol as evidence is the potential for the alcohol to be completely consumed by the fire, leaving no physical traces. Additionally, the presence of other flammable materials at the scene can complicate the identification process.
