Charles C. Roberts, Jr.

After a fire, thermal patterns remaining at a fire scene are an important ingredient in fire investigation. Charred wood, soot deposition, melting, spalling and structural deformation are examples of thermal patterns caused by fires. When performing fire cause/origin analysis, fire investigators often seek out thermal patterns to aid in determination of the fire origin. The following article is a mini-encyclopedia of examples of thermal patterns along with what can and cannot be gleaned from the analysis of thermal patterns.

Figure 1a Classic V-pattern

Figure 1a depicts the classic V-pattern relied upon by many investigators. When hot gases from a fire rise in a natural convection mode, (as a result of heat related gas buoyancy) the gases tend to spread out as they rise, forming a V. Consequently, the fire investigator looks to the base of the V as the origin of the fire. In Figure 1a, the soot pattern, on the walls of the home, has spread outward near the eaves, suggesting that the fire origin is at grade level (arrow). Soot patterns inside the home were less severe, suggesting that the fire started on the outside of the wall.

Figure 1b Melted gas meter and pressure regulator

A closer look, at the base of the V, shows burned gas piping as shown in Figure 1b. Severe charring of wood and melted aluminum casings are indicative of a fire origin in this vicinity. It was determined that frost heaving caused movement of the natural gas piping, resulting in gas leakage through a compression fitting. The exact ignition source was undetermined. A possible ignition source could be a water heater, just inside the wall, igniting the natural gas before an explosive volume of gas could have accumulated. Air infiltration is a typical avenue for natural gas to enter a home, especially in the winter. Figure 2 is a view of another classic V-pattern in a kitchen. The pattern is typical of an overheated appliance on the kitchen countertop. The arrow points to severely burned wallboard at the base of the V-pattern where a decomposed, plastic coffee pot housing was found, the apparent cause of the fire.

Figure 2 Kitchen fire

Figure 3 Propane gas leakage causes a fire

Propane gas is heavier than air and can cause random burn patterns throughout a building, provided there is sufficient venting of the deflagration so that an explosion does not occur. In Figure 3, the propane cylinder on the fork truck (far left arrow) leaked, resulting in pooling of propane gas on the floor. A water heater pilot light is the likely ignition source of the propane. After ignition, random pools of propane ignited, causing multiple V-patterns as shown by the arrows on the right side of the photo. The amount of propane released before ignition was small, unable to sustain an explosion, but sufficient to cause random burning throughout the building, depending on the amount trapped in areas throughout the building. Multiple V-pattern observation is often a tenet of an opinion by an arson investigator that a fire of incendiary cause has occurred. Although multiple V-patterns have occurred in this example, the nature of the patterns is unique to propane gas accumulation and is not necessarily an arson.

Figure 4 Electro discharge milling machine

Machine shops often use numerically controlled, electro discharge, milling machines. These machines use electrical arcs to erode away metal on the work piece to manufacture a finished part. The electrode and work piece are submerged in a dielectric oil for cooling and reduced oxidation of the electrode head. The machines run on a computer program and are often left running overnight, unattended. If the oil level drops below the electrical arc, ignition of the oil can occur, causing a smoky fire. A float switch is designed to shut off the machine, if the oil level drops below a predetermined level. Figure 4 is a view of an EDM machine that was left operating overnight. A fire resulted, virtually destroying the building. Burn patterns on the side of the oil reservoir reflect a low oil level (arrow). The oil in the reservoir tends to cool the side of the tank, showing a different oxidation pattern than the upper side of the tank where high temperature oxidation is noted. A failure of the float switch is a possible cause of this fire.

Figure 5 Char depth

Wood char depth is a burn-related pattern utilized by fire investigators. In Figure 5 the charring of wood joists, often called alligatoring, is a clue to the fire origin. The arrow shows the most severe char depth right above the furnace flue pipe, a typical consequence of flue pipes situated too close to combustible materials. Heating of the wood, over time, tends to lower its ignition temperature until it is ignited as a result of heat transfer from the flue pipe.

Figure 6a Before the fire

Figure 6a is a view of a propane gas fired heater in a produce truck, used to keep produce from freezing in the winter. The arrow points to pyrophoric decomposition of the wood near the heater exhaust. The discoloration of the wood is an indication that ignition of the plywood sheathing is eminent. Figure 6b is a view of a heater in an identical truck that caught fire, destroying the produce and part of the van. The arrow points to the area where the plywood sheathing had been installed. In this vehicle, pyrophoric decomposition of the wood has reduced the ignition temperature to nearly that of the exhaust pipe, resulting in the fire.

Figure 6b After the fire

Figure 7 Passenger seat auto fire

Figure 7 is a view of the passenger seat in a 4-door sedan. Burn patterns tend to focus on the seat as an origin of the fire. Melted polymers, such as polyurethane, are evident on the seat and dash. The damage to the dash is most likely a result of a fire on the seat. The vehicle was found in this condition, with the fire self- extinguished from lack of oxygen (doors closed). Arrows point to remains of emergency flares near the center of the seat. There are no vehicle related ignition sources in that area. Is arson ruled out in this case? Probably not, since spontaneous ignition of road flares on seating in automobiles is unlikely.

Figure 8 Fuel line repair

Burn patterns may not always point to the actual square inch where an ignition occurred. Fuel leakage in a vehicle, typically sprays fuel throughout the engine compartment, causing a diffuse burn pattern. Inspection of the fuel lines may show deficiencies that are causative to the fire, as shown by the improper fuel tube repair in Figure 8. (See Fuel Line Failures, Claims Magazine, May 1992)

Figure 9 Fire remains in a dishwasher

Smoke damage occurred to a home as a result of soot evolution from a dishwasher. The dishwasher was in the drying cycle at the time of the fire. A view inside the washing compartment yields thermal patterns that are clues to the fire cause. The most severely burned area in the compartment is shown near a collection of utensils. The upper arrow, in Figure 9, shows a steak knife with the wood handle burned away. The lower arrow points to the heating coil used in the drying cycle. Thermal patterns are consistent with one of the steak knife handles being ignited by the hot drying coil in the washer. Figure 10 shows a 100-foot extension cord wrapped in a neat coil and placed on a rug. The homeowner had connected a new dehumidifier to the cord in a finished basement. Thermal pattern analysis indicated melting of insulating material on the extension cord. There was smoke damage to the home, but no other areas suffered the degree of melted polymer than the electrical cord. Apparently, the extension cord was too long for the appliance, causing a voltage drop and resistive heating. The coil, itself, limited heat transfer from the wiring. The consequences were increased heating and increased electrical resistance, resulting in ignition of the insulation material and carpeting.

Figure 10 Extension cord coil

Figure 11 Typical gas fired water heater

Burn patterns on water heaters are dramatic, in that one views a gradation of thermal patterns from the bright white of the original painted surface to the severely corroded, oxidized, sheet metal. Figure 11 depicts a typical V pattern with the base of the V near the gas valve. Caution should be exercised when interpreting such a thermal pattern. Possibilities of the fire cause are gas valve failure, gas pipe leakage, combustible materials placed too close to the heater, and flame rollout. In the case of Figure 11, flame rollout ignited nearby garbage bags.

Figure 12 Burn pattern near pipe joint

One large loss involving a 2-story home occurred during the winter, while the owners were on vacation. Approximately 6 inches of water was found in the basement as well as water vapor throughout the home. Wallboard had deteriorated and fallen from wall studs exposing several burned areas, like that shown in Figure 12. Some investigators were interpreting the burned areas at the pipe joints to be a result of a malfunction of the water heater. What had actually occurred, was a freeze failure of a few wall-mounted water pipes during unusually cold weather. This caused hot water to run through the pipe fractures into the basement, generating the large amount of water vapor that damaged the drywall. The burn patterns at the pipe joints, were formed during construction of the building when plumbers soldered the pipes with torches and were unrelated to the loss.

Figure 13 The grounder

Some thermal patterns yield inconclusive results, such as the “grounder’ shown in Figure 13. The destruction is so widespread that pinpointing the fire origin, based on thermal patterns, is difficult. Other thermal patterns can be more definitive, as evidenced by previous examples. Thermal pattern recognition and interpretation will most likely remain a significant ingredient in fire cause/origin analysis.