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Investigation Process The Investigation
Information Gathering
Interviewing witnesses and suspects in arson investigation Fire Patterns Selected examples from NFPA 921. NOT a complete set or a substitute for NFPA 921. Read the Guide Yourself!!!!
The plume of hot gases rising from a fire is like that of a cone, with it’s apex located near the source of heat. In an undisturbed plume this cone will have a half-angle of approximately 15 degrees. An object burning near a noncombustible wall will leave a V-shaped damage pattern, with half-angle of 10-15 degrees, regardless of heat release rate. Soot and smoke normally adhere to a wall. In Clean burn, the soot and smoke accumulation is burned off. Ventilation Generated Patterns. The flow of air and combustion gases can also leave distinct burn patterns. Consider a closed door. Hot combustion gases can escape a room past the top of the door, while cool air can enter past the bottom. In fires where the hot gas layer extends to the floor, combustion products may escape under the door as well. Wood Char. Because of wood’s use as a predominant building material, charred wood is likely to be found in nearly all structural fires. In a fire, or when exposed to an intense heat source, wood undergoes chemical decomposition that drives off gases, water vapor, and various pyrolysis products as smoke. The solid reside that remains is mostly carbon, more commonly known as char.
Some variables that can affect char analysis:
Factors that affect char
Most importantly, no specific time of burning can be determined from char measurements alone!!! Spalling. Spalling is the pattern left by the breakdown in surface tensile strength of concrete, masonry, or brick. The appearance of spalling is characterized by distinct striations and surface material loss. Other common features include cracks, breaks and crater-shaped chipping. These forces that lead to spalling are believed to result from one or more of the following: (a) Moisture present in uncured or “green” concrete (b) Differential expansion between reinforcing rods or steel (c) Differential expansion between the concrete rnix and the aggregate (This is most common with silicon aggregates. (d) Differential expansion between the fine grained surface finished layers and the courser grained interior layers (e) Differential expansion between the fire exposed surface and the interior of the slab
Spalling of concrete, masonry, or brick has often been linked to unusually high temperatures caused by burning accelerant. While spalling can involve high rates of heat release or a rapid change in temperature, an accelerant need not be involved. The primary mechanism of spalling is the expansion or contraction of the surface while the rest of the mass expands or contracts at a different rate. This could be cause by rapid cooling induced by water used to fight the fire. A very common misconception is that spalling of concrete at a fire scene has been, in the past, thought to be a positive indicator of a liquid accelerant-involved fire. The rapid cooling of a heated mass of concrete, brick, or masonry can also cause spalling. A common source of rapid cooling in a fire situation is extinguishment by water. The presence or absence of spalling at a fire scene cannot, in itself, be considered as an indicator of the presence or absence of liquid fuel accelerant.
Melting. Melted material found in a fire scene can lead to estimates of the minimum temperature to which that material was subjected. This can be done by using the melting point of the material as a reference. Care must be taken when examining materials such as glasses and plastics, which have a range of melting points. It is best to have a sample analyzed to determine the true melting temperature. Sometimes with metals, melting can occur at temperatures below the stated melting point. This can occur when a metal of low meting point drips onto a metal of higher melting point, and the two mix. The resulting alloy will have a melting point lower than the higher melting point…sometimes lower than both melting points. Copper (wire, pipes) alloying is often found; iron (steel) alloying is relatively uncommon. Glass. In a fire investigation, much can be determined from the types of breakage of deposits present on glass. Of course, there are several variables involved, including the glass composition, thickness, temper, heating/cooling history, and several other factors. If a pane of glass mounted in a frame that protects it’s edges is exposed to an extreme heat source, high temperature differences can develop that will lead to cracking. (Some research suggests a 70C difference between center and edge temperatures is sufficient.) Glass with no edge protection can withstand a much higher temperature difference. Crazing is glass breakage, characterized by random patterns of short straight or crescent-shaped cracks in glass. The cracks may of may not extend through the entire thickness of the glass. The pressure normally produced by fires in buildings is not sufficient to break glass windows or blow them out of their frames. However, an overpressure, such as that caused by a backdraft send window glass flying many yards. Stains on glass, or the lack thereof, can offer some insight. Glass (or fragments) that has no soot staining probably was in flame contact, or failed early in the fire. Rapid heating could also result in stain-free glass. Many have interpreted oily soot stains, especially those with hydrocarbon residues, as proof-positive of a liquid-accelerant used in the fire. One must remember, however, that plastics are made from hydrocarbons, and incomplete combustion of plastic can lead to think, oily sooting.
Light bulbs of 25 Watts or more are pressurized, and on the size facing severe heat, the glass can soften causing the glass to bubble out. This is known as a “pulled” light bulb. Bulbs less than 25 Watts, under vacuum, will be pulled inward on the side of heating for similar reasons. The investigator should use caution when examining light bulbs; they may have been turned in their sockets.
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Plume Generated Patterns.
Depth
of char analysis should be used to evaluate fire spread, rather
than fire duration. A long-standing and very common misconception
is that wood chars 1 inch every 45 minutes. (link to Myths here)
The relative depth of char from point to point can be used to deduce
the direction of fire spread. Char will be most severe in areas
of high/long exposure, ventilation, or where fuel was present.
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