Crime Scene Training

Arson Investigation: The Chemistry of Fire – Part 2

Fire Scene photos courtesy of the Union Fire Co., Medford, NJ

Posted by: Don Penven, Technical Support Group

Flammable Materials

The term “Flammable Liquid” or “Flammable Solid” are misnomers. Liquids and solids do not burn. Rather, the flames we see are the result of the temperature reaching a point where rapid oxidation can occur. In the case of a liquid like gasoline, which has a flash point of -45 to -50 degrees F, when the flash point is reached the liquid is vaporized. When gasoline’s self-ignition temperature is reached, that is 495 degrees F, this vapor ignites (oxidizes), providing that sufficient oxygen is present.

Likewise, a block of white pine when exposed to heat decomposes into gaseous products that are flammable. This pyrolysis of solids, when combined with sufficient oxygen, will result in a flame as temperatures reach about 250 degrees F.

In practice, the laboratory criminalist’s function is a rather limited one. Usually he or she is expected only to detect and identify relevant chemical materials collected at the scene and to reconstruct and identify trace amounts of gasoline, kerosene or other accelerants in the debris collected.

Rule Out Accidental Causes

There is no scientific test that will determine whether an arsonist has used a pile of rubbish or paper to start a fire. Furthermore, a fire can have many accidental causes, including faulty wiring, overheated electric motors, improperly cleaned and regulated heating systems, and cigarette smoking. These usually leave no chemical traces.

Thus, the final determination of the cause of a fire must take numerous factors into consideration, and requires an extensive on-site investigation. The ultimate determination must be made by an investigator whose training and knowledge have been augmented by the practical experiences of fire investigation.

We know fire is a transformation process during which oxygen is united with some other substance to produce noticeable quantities of heat and light (a flame). Therefore, any insight into why and how a fire is initiated and sustained itself must begin with the knowledge of the fundamental chemical process of fire—oxidation.

Oxidation Defined

A simple description of oxidation is oxygen combining with other substances to produce newsubstances. However, oxidation involves other factors, especially in the circumstances of a fire, that relate to the outcome of the process. For example, we know that when methane unites with oxygen, it burns. But the mere mixing of methane and oxygen will not produce a fire. Nor will gasoline burn when it is simply exposed to air. However, light a match in the presence of any one of these fuel-air mixtures (assuming proper proportions) and you have an instant fire. What are the reasons behind these differences? The explanation lies in a fundamental but abstract concept—energy. Heat is energy.

Exothermic Reactions

Certain chemical reactions can create heat by breaking chemical bonds and forming new chemical bonds. All oxidation reactions, including the combustion of methane, are examples of reactions in which more energy is liberated than the amount that is required to break the various bonds. This excess energy is liberated as heat and often as light and is known as the heat of combustion. Such reactions are said to be exothermic.

It is apparent from these considerations that all reactions require an energy input to start them. We can perhaps think of this requirement as an invisible energy barrier that has been erected between the reactants and the products of the reaction. The higher this barrier, the more energy will be required to initiate the reaction. Where does this initial energy come from? There are many sources of energy. However, for the purpose of this discussion, it will be necessary to look at only one—heat. 

Energy Barriers

 

  For gasoline the energy barriers are quite high, and a high temperature must be applied to start the oxidation of this fuel. Hence, before any fire can result, the temperature of these fuels must be raised to a value that will allow the heat energy input to exceed the energy barrier. This temperature is known as ignition temperature. It is quite high for common fuels. Once the combustion starts, a sufficient amount of heat is liberated to keep the reaction going by itself. In essence, the fire becomes a chain reaction, absorbing a portion of its own liberated heat in order to generate even more heat. The fire will continue to burn until either the supply of oxygen or the fuel is exhausted. 

Normally, an ordinary lighted match provides a convenient igniter of fuels. However, the arson investigator must also consider other potential sources of ignition—for example, electrical discharges, sparks, and chemicals—while reconstructing the initiation of a fire. All of these sources have temperatures in excess of what is needed to meet the ignition temperature requirement of most fuels. 

The Physical State of the Fuel and the Temperature

Although the liberation of energy explains many important features of oxidation, it does not offer a complete explanation for all the characteristics of the reaction. Obviously, although all oxidations liberate energy, they are not all accompanied by the presence of a flame. There is, therefore, one other important consideration that must be taken into account before our understanding of oxidation and fire is complete. This additional factor is the rate or speed at which the reaction takes place. In our description of fire and oxidation we need be concerned with only two: the  physical state of the fuel and the temperature.

A fuel will achieve a reaction rate with oxygen sufficient to produce a flame only when it is in the  gaseous state. It is only in this state that molecules can collide frequently enough to support a flaming fire. This remains true even though the fuel that may be feeding the flame is a solid such as wood, paper, cloth, or plastic, or a liquid such as gasoline or kerosene. 

FOLLOW-UP INVESTIGATION

When the fire is of a suspicious nature, a number of courses of investigation are available. Credit information may lead one to believe that the fire was set for monetary gain.

A check with local vendors or merchants may indicate accounts in arrears. Verify if the mortgage (or mortgages) are current. Determine if recent insurance coverage was secured. This information may absolve the property owner of suspicion. In the case of industrial or commercial establishments, if it appears the owner was not involved, check employee records and interview persons recently terminated.

 TYPES OF EVIDENCE

Real Evidence

Any material substance that tends to prove the facts in question in a court case. This might include a collection of oil-soaked rags, a discarded fuel can, fingerprints, plants such as candles, electrical timers, etc. 

Direct Evidence

Testimony based on the five physical senses of a witness. An eyewitness’s positive identification of a fleeing suspect is a form of direct evidence.

 Opinion Evidence

A lay person’s opinion on certain commonly accepted facts is usually acceptable, such as density of smoke or intensity of flames several feet above the structure. As long as this testimony is very general and not specific, it will often stand. Expert opinions are based on the experience, training and education of the witness. These opinions may include the intensity of a fire that causes steel to melt, indicating the presence of an accelerant. 

Circumstantial Evidence

Circumstantial evidence is indirect in that it infers certain things such as heavy insurance coverage, a mortgage close to foreclosure or service accounts in arrears. It may also include threats from a competitor or former associate. 

Documentary Evidence

This includes insurance policies, bank statements or business records. Photographs and sketches of the scene are another form of documentary evidence as are sworn statements. 

SUMMARY

The annual cost of property consumed by fire runs into the billions of dollars and the loss of life is staggering. Therefore, it is the task of the fire scene investigator to discover the cause of a fire, whether it was intentional of unintentional. Valuable information can be supplied to legislators, building inspectors and insurance companies that could lead to new laws, safety standards and construction practices directed at eliminating potential fire hazards.

Coming Soon…

Arson Scene Investigation Procedure-Part A

Arson Scene Investigation Procedure-Part B

References:

Crime and Clues. “Arson Investigation,” http://www.crimeandclues.com/index.php/crime-scene-investigation/40-crime-scene-processing/96-arson-investigation. January 31, 2010

Inter Fire Online, “The Pocket Guide to Accelerant Evidence Collection,” <http://www.interfire.org/res_file/aec.asp> January 31, 2011

International Association of Arson Investigators. User’s Manual for NFPA 921 2nd Edition. Sudbury, MA. Jones and Bartlett Publishers, 2005.

Sirchie Finger Print Laboratories, The Arson Investigator's Manual, Youngsville, NC, Sirchie Publishing, 2011

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Arson Investigation: How Investigators Solve Arson Cases – Part 1

Posted by: Don Penven, Technical Support Group

Arson investigators face considerable problems when investigating a fire scene. Unlike the majority of crime scenes frequently encountered by criminal investigators, a fire scene offers a completely different set of problems. Physical evidence generated by the perpetrator of crimes of homicide, robbery, rape and burglary is altogether different from what may be expected at a fire scene. Most of the physical evidence generated by the arsonist is often consumed by the fire.

An arson investigation begins with an understanding of the fire itself—the physics of fire if you will. The arson investigator will begin by questioning the first firefighters on the scene as well as witnesses, When materials burn they generate particular-colored flames and smoke. For example: wood produces a yellow or red flame with gray or brown smoke. On the other hand, gasoline and other hydrocarbons produce a yellow or white flame with black smoke.

But observing black smoke is not always a sure indicator that the fire was intentionally set. The structure may have had hydrocarbons stored there, like gasoline, kerosene, oil-based paint, acetone and paint thinners.   

                                               (Accelerant Burn Pattern on Concrete Floor)

Another point to clarify is to determine if any doors or windows were left open. And did any one notice any peculiar odors that might indicate that an accelerant was used. An accelerant is any substance that speeds the burning and spreading process of a fire. Accelerants include materials like hydrocarbons (gasoline, kerosene, etc.), paper, plastics, and other materials, which can cause a fire to spread more quickly or burn more fiercely than it would otherwise.

It is also important to determine whether any windows were covered to prevent the early stages of the blaze being seen from the outside.

For a fire to begin and sustain itself, three factors known as the “Triangle of Fire” must be met. The triangle of fire consists of oxygen, a fuel source and heat.

• The normal air present in most structures is sufficient to permit combustion. An arsonist may help this situation along by opening doors and windows.
• The fuel source may be simply surrounding flammable materials such as paper, furniture, wooden structures, etc., but the fuel source could be materials made available by the arsonist.
• And finally we need a heat source, Defective wiring is often cited as the probable cause of a fire since it may serve as a heat or ignition source, or the heat source may simply be a single match.

Once the fire has been completely extinguished and it is safe to enter the structure the investigator will attempt to locate the “low point” or point of origin. This is accomplished by examining the burn patterns. The charring of wooden surfaces can give a clue as to how intense the fire was as the wood will be charred.  Intense heat causes deep charring or “alligatoring,” but this does not necessarily indicate that accelerants were used at a specific location.

                                                  (Example of "Alligatoring")

We tend to think of a wooden beam, floor joist, door frame etc., as actually burning. But the physics of fire tells us that when wood is heated to high temperatures over a period of time it undergoes pyrolysis. In other words, the wood gives off a flammable gas that does the actual burning. The intense heat causes the charring of the wood.

In simple terms, fire is the visual manifestation of rapid oxidation. The principal byproducts of this oxidation are carbon monoxide and carbon dioxide.

The investigation generally begins with a walk-through of the structure. What one would expect to be present in a dwelling are items of furniture, appliances, TVs, computers, beds and bedding, clothing, etc.

In an office setting or retail store one expects to find other items like office furniture, file cabinets, and computers, while a retail establishment will have stocked shelves. If expected items are missing, Arson may have caused the blaze.

During the initial search the investigator will look for heat sources. The source could be electrical, chemical or intentional. Multiple hot spots are a dead giveaway, since this would be a rare occurrence without the aid of accelerants.

Fire scene investigation is a very complex process. The first goal of the investigator is to rule out that fire was accidental in nature. When all explainable sources are eliminated, then it is necessary to consider the fire as “suspicious in nature,” and look for human influences and actions that may have played a role.

Fire scene investigators undergo many months of intensive training, and the purpose of this and any future posts to follow is to provide the reader with an overview of what arson investigation is all about. 

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Arson Scene Photos Courtesy of the Franklin Co. (NC) Sheriff's Dept.

References:

Inter Fire Online, “The Pocket Guide to Accelerant Evidence Collection,” <http://www.interfire.org/res_file/aec.asp> January 31, 2011

International Association of Arson Investigators. User’s Manual for NFPA 921 2nd Edition. Sudbury, MA. Jones and Bartlett Publishers, 2005.