Chemistry of Fire
“All fires and explosions are merely different faces of the same force — combustion.”
The combustion reaction, or burning, creates heat and light, resulting in fire. For a fire to start, three key elements are needed: a fuel source, oxygen (O2), and heat. These elements form the fire triangle. An additional factor, a self-sustaining chemical reaction, creates the fire tetrahedron. A fire cannot be ignited or extinguished by smothering (removing oxygen), cooling (removing heat), or starving (removing fuel) if any of these conditions are not fulfilled.
The fire caused indoors (i.e., inside a room) mainly depends upon the following: –
1. Fuel Sources:
- Common materials include furniture, gas cylinders, electrical appliances, clothing, and bedding.
2. Heat Retention Influences:
- Construction materials and insulation properties of walls and roofs affect how much heat is retained.
- The type and thickness of these structural components play a crucial role.
3. Ventilation Influences:
- The number, size, and placement of windows and doors determine airflow and oxygen availability.
- Gaps or spaces between floors, walls, and roofs can also affect ventilation and fire spread.
Flames can appear in different colours depending on what is burning. The colour of a flame is determined by the light it gives off, which changes with the material. For example, flames are often red, yellow, or orange when carbon is present. Inorganic materials like copper can create more noticeable colour changes, resulting in a green flame.
Identifying Elements with Flame Test: Forensic Blog
How fast a fire can spread?
Fire & Arson Investigation
It depends upon the following two factors:
Physical State of Fuel
A fuel achieves a reaction rate with oxygen sufficient to produce a flame only when it is in the gaseous state because only in this state can molecules collide frequently enough to support a flaming fire. This remains true whether the fuel is a solid such as wood, paper, cloth, or plastic or a liquid such as gasoline or kerosene.
Fuel Temperature
The temperature of a liquid fuel must be high enough to vaporize it. The vapour that forms burns when it mixes with oxygen and combusts as a flame. The flash point is the lowest temperature at which a liquid gives off sufficient vapour to form a mixture with air that will support combustion. Once the flash point is reached, the fuel can be ignited by an outside temperature source to start a fire.
Generating vapour is more complex with solid fuel such as wood. A solid fuel burns only when exposed to heat intense enough to decompose the solid into gaseous products. Thus, such combustion accompanied by solid organic matter by heat is termed pyrolysis.
Indicators to Distinguish Between Slow Burning & Fast Burning Fire
A handbook was released in 1980 named “Fire Investigation Handbook”, which meticulously dealt with various indicators of how fast a fire has spread or the extent of burning it has caused. The importance of understanding these indicators cannot be overstated, as they play a crucial role in not only determining the origin of a fire but also in improving fire prevention techniques. Unfortunately, the scientific studies recommended in the survey had not been conducted, leading to a reliance on anecdotal evidence and outdated practices. Consequently, most of the myths used to incorrectly determine that a fire burned faster or hotter than normal proliferated within the field. Some of these myths have been discussed below: –
Alligatoring of Wood – Slow fires produce relatively flat alligatoring. Fast fires produced humpback, shiny alligatoring.
Spalling of Concrete – The spalling of concrete indicates an intense, high-heat fire. It is caused by the rapid boiling of the moisture trapped in the concrete.
Fire patterns: A wide-angle or diffuse “V” pattern generally indicates a slowly developing fire, while a narrow, sharply defined “V” pattern generally indicates a fast-developing, hot fire.
Glass effects – Crazed or irregular pieces of glass with light smoke deposits indicate a rapid build-up of heat.
Floor damage— Although floors seldom receive damage similar to ceilings, even in the case of total burnout, they can develop patterns that give the appearance of flammable liquids.
Phases of fire
Every fire is unique; they differ greatly from one occurrence to the next. Thus, it is impossible to connect the two distinct instances.
A natural fire goes through four basic stages. The heating of the possible fuel source or sources is the first stage, the incipient ignition phase. The growth stage, or the second stage, is characterized by visible blazing combustion and ignition. The fire may become completely formed inside the compartment and reach the so-called “flashover” situation when it intensifies and reaches high enough temperatures—roughly 1,100 °F or 600 °C). This condition typically signals the start of the third stage, known as the burning period. The impact of the fire on the building components is greatest during this most severe stage. When roughly 70% of the combustible elements in the compartment have burned, the degradation phase usually starts.
Procedure of Investigating Fire and Arson Scene
Fire Scene Investigation
Finding the fire’s origin (seat) and its most likely cause are the main goals of a fire investigation, which also aims to establish whether the occurrence was accidental, natural, or intentional. Determining the cause is essential to prevent future occurrences of the same incident (natural or accidental) or to permit a legal investigation (intentional fires). Thus, fire can be classified as follows:
Natural fire
The fire has been used for purposes like cooking, i.e., fire is ignited, not for malicious purposes.
Accidental Fire
This unintentional fire is caused by human carelessness. For example, fire is caused by short-circuiting.
Spontaneous fires
This type of fire is accidental, in which no external factor, i.e., changes in internal factors, causes the cause of fire. For example, this type of combustion may occur when combustible matter, such as coal or hay, is stored in bulk.
Malicious fire
Also known as Arson. This type of fire is intentionally set by criminal acts to destroy property. In such fires, there is always some motive behind the cause of doing such an act. In such cases, arson investigators have to look for the following: –
- Any kind/proof of incendiary evidence/ accelerants
- Proof of opportunity
- Witness Statements
Burned Bones as Important Forensic Evidence: A Review
Looking For Incendiary Evidence/Accelerants
When sufficient evidence has been gathered to prove that the fire’s origin is found in more than one place, it is a sure sign of arson fire and has been deliberately caused. Incendiary evidence might be found.
- Having more than one point of origin.
- Absence of accidental or natural cause of fire.
- A container with a residual accelerant or ignition device is also important.
Proof of Opportunity
It deals with:
- Security and other aspects as to who had access to the building before the fire occurred.
- Insurance fraud
- demolition of old property.
- This is done to eliminate competition, i.e., mostly in cases of business-related fraud.
Witness Statement
The witness should be asked the following questions:
- What time was the fire first noticed?
- How did the fire spread?
- Any kind of smell associated with the flame?
- What colour of flame was associated?
- Any kind of explosion which was heard?
- What other sorts of activities were observed around the scene?
- Opinion as to how the fire has been started?
Determination Of Origin Of Fire
According to NPFA (National Fire Protection) guidelines, an arson investigator must form a scientific opinion, including defining the problem, collecting data on the origin, developing a hypothesis on the cause and origin of the fire, and testing and validating the hypothesis. One of the most significant theories that an investigator forms and tests during the investigation is the origin of a crime.
If the right origin is not identified, the subsequent cause identification will likely be incorrect. If the origin cannot be recognized, neither can the cause.
Thus, a systematic and scientific approach should be developed while investigating an arson crime scene.
The origin of fire can be determined from the following evidence: –
- Through eye-witness testimony or by any digital evidence/ records like CCTV surveillance.
- By studying fire patterns.
- By analyzing fire dynamics.
Other indicators determining the “point of origin” will include a V-shaped pattern, the deepest char, the greatest destruction, multiconnected fire, and the formation of an inverted cone on the walls.
The Null Hypothesis
An open mind is the most valuable tool a fire investigator can bring to a scene. Fires happen frequently, but as many false arson convictions have surfaced over the years, examining them can be exceedingly challenging. It is essential to implement some basic safeguards to avoid major mistakes because determining the cause and origin of a fire can be somewhat uncertain. One strategy to stop inaccurate arson decisions is assuming the fire is accidental until contrary evidence is proven.
Negative Corpus Methodology
Some investigators use the presumption that a fire was purposefully started instead of the null hypothesis, which holds that a fire is accidental. The fire investigator thinks he is skilled enough to recognize and identify any potential source of accidental ignition and to locate proof of such a fire cause in every instance. As a result, the investigator is at ease, claiming, “All accidental causes have been eliminated; therefore, the fire must have been intentionally set,” even in the complete absence of evidence proving an incendiary cause.
| EXTERIOR EXAMINATION OF THE FIRE SCENE | INTERIOR EXAMINATION OF THE FIRE SCENE |
|---|---|
| • During exterior examination, the damage, location of the fire, and description of heat or smoke should be noted. • The person in the surrounding area should be investigated, or the first person who reported the fire should be investigated. • Condition of doors, windows, or another point of entry. Search for suspicious items like ladders, containers, or any other items within the search perimeter. • External weather conditions, wind velocity and direction humidity. | • During exterior examination, the damage, location of the fire, and description of heat or smoke should be noted. • The person in the surrounding area should be investigated, or the first person who reported the fire should be investigated. • Check the condition of doors, windows, or other points of entry. Search for suspicious items like ladders, containers, or any other items within the search perimeter. • External weather conditions, wind velocity and direction humidity. |
In addition to other investigation members, a separate team of Ignitable liquid detection canines (IGL canines) is formed. These dogs are specifically trained to locate and respond to the presence of certain classes of ignitable liquids by their associated odour.
The recommended minimum equipment to be used during the fire scene examination includes:
- Personal protective equipment
- Flashlight (preferably lantern style)
- Writing materials (clipboard or similar)
- Assorted small tools or multi-purpose tools (screwdrivers, wire cutters, knives)
- Measuring devices (20 and 100 ft tape measures)
- Camera, film, electronic media
- Shovel or other hand tools
- Rubber gloves
- AC voltage tester
Below is a flowchart regarding the complete scientific fire investigation as per the guidelines of NFPA.
Chapter 18: Origin Determination
Initial Survey: Safety Measures First
Just like a crime scene, a fire is necessary to ensure the safety of all individuals at the crime scene. By means, fire scenes are hazardous locations. In addition to others who might be in danger while conducting their investigations at fire sites, such as other investigators, labourers, equipment operators, property owners, and attorneys, fire investigators have a duty to themselves. The investigator will receive fundamental recommendations in this chapter about various safety issues, such as personal protective equipment (PPE). Safety measures that should be looked out for include: –
- Lack of visibility, structural instability, and energized electrical circuits.
- Leaking fuel gases, but the investigator should also investigate other obvious hazards, such as fire gases (CO and HCN).
- Not to conduct any investigation while endangering someone else’s life.
- Fire scene examinations should not be undertaken alone. At least two individuals should be present to ensure assistance is available if an investigator is trapped or injured.
- Avoiding investigations for long, continuous periods, putting in all their strenuous physical labour.
- The safety of bystanders may be accomplished by merely roping off the area and posting “Keep Out” signs and barricade tape, or it may require the assistance of police officers, fire service personnel, or other persons serving as guards.
Documentation & Collection Of Evidence
The goal of documenting any fire or explosion investigation is to accurately record the investigation. Documentation of fire investigation involves: –
(i) Note-taking
(ii) Diagrams/Drawings
(iii) Photography
(iv) Report.
RAW or FINE JPG format images should be gathered at the maximum resolution the camera can produce. Although RAW files are more flexible, they can quickly fill up a memory card because of their size. JPEG files have sufficient resolution for most uses. Remember that most photography formats now record the sequence in which images were captured and, provided the camera’s clock is calibrated correctly, the exact moment the picture was taken.
Drawings are another essential piece of paperwork needed for all but the most straightforward inquiries. A floor plan should be the minimum drawing for the majority of buildings. Sometimes, the building owner/operator can provide an “as built” drawing of the structure, especially in commercial or industrial buildings (Assuming that these documents were not destroyed in the fire). Fire escape plans are frequently displayed in multiple locations on the walls of business buildings.
While note-taking consists of the following information: –
(i) Whether fire occurred inside or outside
(ii) Dimensions of floor/room
(iii) any burnt appliances or fuses.
(iv) material floor, whether made of plywood, plank, or tile.
(v) whether any fire suppression devices or mechanisms were present.
The purpose of report writing is to document an accurate reflection of the investigator’s observations, activities, analyses, and conclusions. The report should contain facts and data the investigator relied upon to reach any opinions or conclusions. The report should also explain how the investigator reached each opinion.
The following information is usually found in a report:
- (1) Date the report was submitted
- (2) Date, time, and location of incident
- (3) Date and location of examination(s)
- (4) Name of the person or entity requesting the report
- (5) The scope of the investigation (i.e., tasks assigned and tasks completed)
- (6) Nature of the report (e.g., preliminary, interim, final, summary, supplementary)
- (7) Name of person(s) preparing the report
Preservation and Collection of evidence
Every attempt should be made to protect and preserve the fire scene because evidence could be easily destroyed or lost in an improperly preserved fire scene. The responsibility for preserving the fire scene and physical evidence does not lie solely with the fire investigator but should begin with arriving fire-fighting units or police authorities. Lack of preservation may result in the destruction, contamination, loss, or unnecessary movement of physical evidence.
The most common evidence is flooring samples, furniture, and other materials suspected of containing ignitable liquid residues. Ignition sources comprise another type of evidence collected to determine their possible involvement in causing the fire.
When samples are obtained for ignitable liquid residue analysis, the samples least likely to contain residues should be collected first if possible. Thus, the chance of cross-contamination is reduced.
The evidence containers should contain the following information: –
- Unique case number or file number identifying the fire scene
- Sample’s serial number
- Description of the substrate material
- Location of the sample
- Data collected
- Initials of the investigator collecting the sample
Proper sealing of evidence should be done, too, to maintain the Chain of Custody.
- Polyester or nylon evidence bags made for the purpose are suitable fire debris containers.
- When ignitable liquid is found in a liquid state, it can be collected using any one of a variety of methods, such as a new syringe, eye dropper, pipette, siphoning device, or the evidence container itself. Sterile cotton balls or gauze pads may also absorb the liquid. Such ignitable liquid should be stored in a glass jar.
- According to NFPA guidelines, gaseous samples should be collected in air sampling cans designed explicitly for this purpose.
- Electrical switches, receptacles, thermostats, relays, junction boxes, electrical distribution panels, and similar equipment and components are often collected as physical evidence. It is recommended that these types of electrical evidence be removed intact and in their original condition.
- According to NFPA guidelines, metal containers can collect liquid and solid accelerant evidence.
- It is advised that appliances and other equipment be left in situ when their size or broken state makes it impractical to remove them completely. Analysis and experimentation. Frequently, though, only one or a small number of an appliance or piece of equipment’s parts can be gathered as physical evidence.
References: –
- [1]https://www.researchgate.net/publication/309367060_Towards_sustainable_slums_understanding_fire_engineering_in_informal_settlements
- [2] Fire Investigation: Historical Perspective and Recent Developments
- [3] NFPA Guidelines for Fire and Explosion Investigations 2021 edition.
- [4] Scientific Protocols for Fire Investigation, CRC Press
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