Recognizing and Identifying BURN PATTERNS
ARSON IVESTIGATION
Understanding how flammable liquid behaves when used as an incendiary device will aid in its identification.
Photos by Bob Pressler and Glenn Usdin.
We have all heard fire investigators say that they have observed flammable liquid burn patterns at a fire scene and that their conclusions as to the fire’s origin and cause were based upon that observation.
What is it about these burn patterns that makes investigators point to flammable liquid as the cause?
There are significant indicators in the burn pattern to allow for this interpretation: low burning; flooring material charred; and flooring material charred in a “puddle configuration” over a wide area. While fire investigators will be able to rattle these off as justification for their decision, often it is done as a matter of rote and without the complete understanding as to how these indicators were formed.
Especially while on the stand in a courtroom, investigators’ findings would have a more professional tone if the explanation is based on the physical and scientific principles of matter and energy as they apply to the conditions that result when a flammable liquid is involved at a fire.
All liquids have certain physical properties, and viscosity is one of them. The controlling factors in the viscosity of a liquid is the adhesion and cohesion that the liquid’s molecules have for one another. Put another way, it is the affinity the molecules have for one another that causes their restraint in flow. Hence the greater their affinity, the greater their viscosity (the converse is also true).
This molecule attraction is in force in all parts of a liquid, even at its outer edges. At the outer edges this attraction is known as surface tension, and it is this factor that keeps liquid from continuously dispersing across a flat surface. Since there is uniform molecular attraction throughout the liquid, under ideal conditions the resulting shape will be round. The viscosity of the liquid will determine the size of this round configuration. That is, when two equal amounts of liquid of differing viscosities are applied to a flat surface, the liquid with the greater viscosity will form the smaller round or puddle configuration.
Factors that would change the puddle’s ideal round configuration are gravity and the liquid being applied to an angled or uneven surface (causing running patterns). Also, the force and/or angle of the application of the liquid causes splash or splatter patterns depending on the angle of application.
When a source of ignition is applied to a flammable liquid puddle on a flat surface, there will be a simultaneous eruption of flame over the top of the puddle’s entire surface, no matter how expansive, because of the vapors being emitted from the liquid. The heat generated by these flames is transmitted upward and outward and back into the liquid itself, causing the liquid to boil.
The physical reaction of boiling in the liquid occurs at a temperature lower than the temperature necessary for ignition. The temperature of the boiling liquid is generally less than the ignition temperature for the floor’s surface (usually wood); therefore, the liquid acts as an insulator for the floor’s surface.
The only point at which the surface of the liquid makes contact with the surface of the floor is at the edge of the puddle. Here, all three methods of heat transfer (convection, conduction and radiation) act upon the floor’s surface and raise it to ignition temperature, causing pyrolysis and oxidation (burning) of the flooring material all around the puddle’s edge.
As the burning process continues, the puddle is consumed in a steadily decreasing concentric configuration. This action at the edge of the puddle is enhanced by a positive heat feedback generated by the burning floor surface and the burning edge of the puddle drawing away from one another.
Charring, therefore, is deeper at the outer edges of the puddle and deepest at the original edge of the puddle because this is where burning has occurred for the longest period of time. This prolonged burning causes the resultant burn pattern to have the same shape as the original puddle.
The charring at the puddle’s edge causes a depression in the floor’s surface, allowing the flammable liquid to flow into it and burn at a level lower than the floor’s surface. This enhances the burning and the depth of char.
The resultant burn pattern, therefore, not only takes the shape of the original puddle, but is also convex. As seen in the illustration (figure 1), the char is deepest at the outer edges and tapers upwards to the center. The center of the burn configuration may exhibit no char at all because all the fuel (liquid) has been consumed before reaching this point resulting in a negative heat feedback and a cessation of pyrolysis.
Similar char patterns
The fire investigator can be misled by similar looking char patterns, but a close examination should be able to eliminate any confusion.
When burning draperies fall to the floor by their own weight, they land in a crumpled ball. This allows for the free flow of air (oxygen) between and around the crevices, resulting in flaming combustion at the floor’s surface. Heat can then be transferred to the flooring material within the configuration of the drapery pile. This applied heat pyrolyzes the flooring, and the available air allows for combustion when ignition temperature is reached. This causes a char pattern in the shape of the drapery pile to form in the flooring material, the depth of which is dependent on the intensity of the flame produced and the duration of time that the pile of draperies is allowed to remain in place and free burn upon the floor. (Suspect char patterns near windows and doorways need an extra close examination.)
When foam rubber (such as in a sofa or chair) begins burning, it melts and flows. While molten and on fire, foam rubber will drop onto the surface of the flooring material, causing it to pyrolyze and combust. As this process continues, the puddle that the dripping foam rubber forms steadily increases in diameter. The molten foam rubber lying on the floor’s surface will exclude oxygen, begin to solidify, and thereby act as an insulator and protect the floor’s surface beneath it. The depth of the char pattern will basically remain the same as the foam rubber puddle, and be fairly even across and within the final configuration formed by the molten foam rubber because of this insulating effect.
Let’s examine an incident in which gasoline was used as the flammable liquid. By applying the facts we know about gasoline’s physical and chemical properties, we can understand why we get “low burning” when gasoline ignites and combusts.
One gallon of gasoline weighs 6.664 lbs. and gives off 19,250 Btus per pound. Therefore, one gallon of gasoline will release a total of 128,282 Btus. Gasoline generally burns at the rate of one inch in five minutes, no matter how great a volume or how great an exposed surface area is involved.
If a gallon of gasoline is spread out, creating a pool whose depth is 1/8 inch, this pool would be consumed in 5/8 of a minute or 37 1/2 seconds. Therefore, that gallon of gasoline would release those 128,282 Btus within 37 1/2 seconds.
This rapid release of such an intense amount of heat quickly pyrolyzes the exposed wooden surfaces and other combustibles within the proximity of this gasoline pool. This heat is dissipated within a short time frame (37 1/2 seconds), allowing normal fire propagation to continue after that point.
This phenomenon causes greater charring and damage at the lower levels of the fire origin area in the incipient stage.
If the fire continues beyond this point, normal propagation will result in the heat at the ceiling level to be about three times as great as the heat at the floor level. A stage will be reached during the course of the fire where the charring at the upper levels of the room will be equal in depth and dispersion as the initial charring at the floor level. Since normal fire progression is upward and outward (causing the greatest damage at the ceiling level), this evenly charred room indicates that low burning occurred.
Concrete spalling occurs mainly as a result of trapped moisture in the concrete absorbing heat and, if not able to dissipate it, turning into steam. Since steam expands to 1,700 times its original volume as water, it causes concrete to break off in large and small chunks. The rapid and intense heat necessary for this action is provided by the presence of a flammable liquid.
When a flammable liquid is poured or splashed upon a vertical surface (such as a structure’s wall) the combustion characteristics of that liquid remain the same. However, the force of gravity as it acts upon the liquid has a greater impact on the resultant char pattern.
Gravity will cause the liquid to run downward on the vertical surface, and if the bottom edge of the liquid pool meets an obstruction it will divide into separate pools. These new pools will then follow separate, downward flow paths jntil surface tension overcomes gravity at the lower tips of each flow path. The resultant pattern generally resembles a hand with the fingers pointing downward (see figure 2).
If this liquid is ignited, it will char the surface on which it rests; the palm and upper portions of the fingers of the burn pattern will be scorched or less deeply charred than the lower portion of the fingers and fingertips. The reason for this is that the greatest volume of the liquid, and therefore the greatest source of heat, will, by the force of gravity, be pooled in the fingertips of this “runny fingered” char pattern.
Flammable liquids are solvents and when applied to a surface will react, if they can, by emulsifying the material on the surface and penetrating that material to various depths.
This emulsification and penetration will usually cause a discoloration (staining) of the material via a chemical reaction even when the liquid is not ignited.
The conditions and results described above would occur under ideal situations. However, there are many variables and factors that must be taken into account when viewing a char pattern, because they will alter the “ideal results.”
Some of these factors could be a floor that is uneven, old, porous, or cracked and separated between floor boards. The composition of the floor’s material and any surface covering would also alter the ideal burn pattern.
