Determination of Fire Patterns and Traces due to Liquid Accelerants on Floor Coverings

This project has been a joint venture between SINTEF NBL, the Criminal Investigation Department (KRIPOS) and the Police Station in Trondheim. The objective was to clarify which visual fire patterns and chemical traces to expect after fires started with four different liquid accelerants on four different types of floor coverings, depending on several fire parameters. Such parameters may be the extent of secondary damage due to fire intensity, amount of supplied water during fire fighting and the delay in time before the sampling. In this context a chemical trace is the residual liquid accelerant detected by use of a chemical analytical technique termed Gas Chromatography. All fire tests where carried out in SINTEF NBL's laboratories.
Additional tests with solid combustibles on the same floor coverings were also carried out. The purpose of these tests were to find if the visual fire patterns from such materials differ much from those from liquid accelerants, or if the patterns can be misinterpreted as fire patterns from liquid accelerants.

THE MAIN CONCLUSIONS

19 tests were performed with primary fire in four different liquid accelerants together with secondary fire, on four different floor coverings The secondary fire, a 60-140 kW effect gas fire, simulated the spread of fire from the liquid accelerant on the floor to the entire room.
The secondary fire was simulated by setting fire to propane gas flowing through a vessel filled with sand/gravel. This causes a plume of flames sweeping along the walls and the ceiling of the room. See Figure 2.
Five additional test with solid combustibles on the floor covering, four reference tests with secondary fire only and three tests with a closed half full and full bottle of liquid accelerant in secondary fire, were carried out also. This adds up to a total of 31 tests.

Fire tests with 1,5-3 dl petrol, diesel oil, white spirit and methylated spirit were carried out on 1,1 and 1,8 mm thick vinyl, carpet and felt (both from 100 % polypropylene) coverings, in addition to unvarnished pinewood. The objective of the experiments was to find when to expect visual patterns and chemical traces (using Gas Chromatography) of flammable liquids deliberately used to accelerate the spread of fire.

The following main conclusions could be drawn from these tests:

• Fire technical traces of liquid accelerants may leave traces on floor coverings depending of the type of covering and accelerant. The chance to detect visual fire patterns and traces of liquid accelerant are often good even if the floor covering is severely damaged as a consequence of the liquid fire (the primary fire) and secondary fire in the room.
• Visual patterns from solid combustibles in fire, such as polyurethane foam burning on a floor made of unvarnished pinewood, might be misinterpreted as fire patterns from liquid accelerants. See Figure 3. The fire damage on the floor should not alone be sufficient to determine whether or not a liquid accelerant has been used. Visual fire patterns may indicate that a liquid accelerant has been used, and can also indicate where to sample for Gas Chromatography. As a rule, one should be very careful to use visual fire patterns only as basis or in proof of arson using liquid accelerants.
• Chemical traces of liquid accelerants are often found, even in locations where it is not expected (for instance in the center of the fire on the floor).
• Even if relatively small quantities of liquid accelerants have been used and the floor covering is more or less completely burnt due to the secondary fire, it is still possible to find accelerants when using Gas Chromatography.
  Even if it is not found any indication of liquid accelerants using Gas Chromatography, the use of accelerant can not be excluded. It is important to secure samples as quickly as possible after the fire.
• It is possible to point out visual fire patterns from liquid accelerants on vinyl floor coverings if the secondary fire damages are not too comprehensive.
  On a carpet and especially on unvarnished pinewood, and also to some extent on felt coverings, visual fire patters will not show as clear fire patterns except if the liquid is applied as a trailer.
• The chance of finding chemical traces using Gas Chromatography is probably just as good as the chance of finding clear fire patterns from liquid accelerants.
• In floor covering, which absorb liquid accelerants effectively, it is most likely to find chemical traces in the center of the fluid fire.
  This does not apply to floor coverings that do not absorb the liquid effectively, for instance vinyl and pinewood. In this case liquid accelerants will be found around the edge of the liquid fire.
• Chemical traces may also be found outside the pool, especially if the liquid accelerant is white spirit. White spirit that did not burn, had a tendency to move away from the secondary fire in the room.
• In general, it is easier to find chemical traces from liquid accelerants in absorbent floor coverings (carpet and felt) than in floor coverings that do not absorb the liquid (vinyl and unvarnished pinewood).
• Water used to extinguish the fire may reduce the possibilities to find chemical traces from liquid accelerants. This apply primarily to water soluble liquid like methylated spirit and ethanol.
• A plastic bottle containing methylated spirit or white spirit will not explode when the bottle is exposed to an intensive fire. The unfilled part of the bottle will deform and make the bottle leaky long before the liquid temperature is high enough to obtain particularly high evaporation and pressure buildup in the bottle.
   

CONCLUSIONS

The Possibilities of Finding Visual Fire Patterns and Chemical Traces from Liquid Accelerants:

• The chance of finding fire technical traces of liquid accelerants on floor coverings, both visually and by means of chemical analysis (Gas Chromatography), is good. This apply even when using moderate quantities of liquid accelerants in a pool on the floor covering. The sampling for chemical analysis must however be done as soon as possible after the fire.
• The chance of finding chemical traces is relatively good even after a rather intensive fire in the room that results in flashover. However this applies only if the sampling is done relatively soon after the fire. Flashover reduces primarily the chance of finding visual patters.
• Liquid fire alone produces visual patterns on the floor covering even when the quantity of liquid accelerant is moderate. The floor covering needs not absorb the liquid for the liquid fire to produce fire patterns in the floor covering. A primary fire with petrol and diesel (with paper wick) leaves considerable more distinct fire patterns in vinyl than methylated spirit, which again produces more distinct patterns than white spirit.
• It may be problematic to find fire patterns from white spirit, especially on floor coverings that does not absorb the liquid effectively (vinyl and to some extent unvarnished pinewood). This is because the liquid has a tendency to move away from the fire. Further, that the extent of the fire in these tests was rather small and had a rather short duration because the fire extinguished by it self long before the liquid was burnt.
• The chance of finding clear visual fire patterns from fire in petrol and diesel (with paper wick) is considerable larger than when using white spirit, and to some extent methylated spirit. If paper wick had been used in the tests with white spirit, the visual fire patterns from white spirit would probably be mush alike the patterns from fire in diesel. The first mentioned liquid accelerants will however easier lead to flashover, and because of this, it may still be hard to establish visual fire patterns from the primary fire.
• If a paper wick had not been used in the fire with diesel, the liquid fire would hardly led to any patterns at all. The liquid would probably not been set on fire.
• It was especially difficult to establish visual fire patterns on unvarnished pinewood if the fire lead to a flashover. It is somewhat easier to find visual fire patterns on vinyl than on a carpet.
• The use of a wick seem to make relatively distinct patterns in a carpet. These patterns are different from the more or less fire damaged carpet. This is the case as long as the floor covering is not damage by a total flashover. See Figure 5.
• If the vinyl covering is not glued properly to the floor, it will have a tendency to bulge. This may cause particular burn patterns. These damages coincide with the bulge in the floor covering (to the right in Figure 4). This is due to the insulating ability of the air pocket underneath the bulge, which serves as an insulator and thus causes the floor covering to reach higher temperatures in these locations, compared to the parts of the covering that are glued to the floor.
• The center of a pool fire is the location where chemical traces of liquid accelerants in an absorbent floor covering are most frequently found. Because this is the location where the floor covering is generally most burnt, one has earlier supposed that it would not be possible to find chemical traces in this location. Thus one has failed to get samples in this location. In fact, it is in the center of the liquid fire where it is easiest to trace liquid accelerants in such floor coverings.
• It is also possible to find chemical traces outside the liquid pool, especially when white spirit has been used on nonabsorbent floor coverings. The white spirit that did not burn, had a tendency to move away from the secondary fire in the room. Fire in 3 dl white spirit showed that the fire died by itself after approximately 45 seconds with most of the liquid remaining unburned.
• Use of water as the fire extinguisher may reduce the chance of finding chemical traces of liquid accelerants. This apply primarily to water soluble fluids, like methylated spirit or ethanol. For fluids which are not soluble in water, like petrol, diesel and white spirit, the use of water may increase the chances of tracing the liquid accelerant because the water may prevent further vaporization of the liquid. This is because the water can seal up volatile fluids, reducing the vaporization rate.
• It is easier to find chemical traces of white spirit than of methylated spirit, especially in connection with water. Further, it is somewhat easier to find traces of petrol than of diesel.
• "Sniffer" (Gas detector) is a suitable tool for examination at the scene of fire, immediately after the fire. Sniffer is recommended especially as a tool to determine the location for sampling for GC analysis.
   

Misinterpretation of Visual Fire Patterns:

• Visual fire patterns due to fire in solid combustibles, such as acrylic fibre, cotton fabric or polyurethane foam burning on a floor, may in certain situations be misinterpreted as fire patterns from liquid accelerants (see Figure 3).
• It is especially easy to misinterpret fire patterns from solid combustibles (for instance polyurethane foam) as fire patterns from liquid accelerants on floor coverings such as unvarnished pinewood.
• Heat radiation from the fire surroundings will cause a more or less diffuse or gradual transition zone between carbonized, burnt and not fire damaged parts of the floor covering. Heat radiation will not cause sharp borders between burnt and not burnt parts of the floor covering, like liquid accelerants would do (see Figure 6).
• Acrylic fibre leave behind a distinct heap of carbonized, coky material which totally or partially protects the floor covering underneath. Polyurethane foam however, will be completely burnt, but leaves sometimes patterns which reflect the shape of the polyurethane foam. This "footprint" is easy to distinguish from other fire damages on the floor. Fire in a hanging cotton curtain will expose the floor insignificantly because nearly the entire curtain will be burnt hanging. 
   

The Fire Response of a Closed Plastic Can of Combustible Liquid:

A full, unopened plastic bottle containing methylated spirit will not explode and spurt liquid on the floor when it is exposed to fire. It is deformed rather steadily, with almost all the liquid still inside. The liquid shields/cools down the bottle. The part of the bottleneck not filled with liquid, deforms and becomes leaky, so that the liquid can flow out of the bottle, steadily and gently.

A half full, closed bottle containing methylated spirit or white spirit will also slowly melt down, and some of the liquid will flow out. Some liquid remained in the strongly deformed remains from the bottle. See Figure 7.

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