Investigation of Spontaneous Ignition Fires
John Stewart, CFEI, CFII
Jack Ward Fire Consultants
Spontaneous ignition has become more known and understood in the fire investigation community over the past 25 years, but is still missed by many seasoned experts. Fire, by definition, is a rapid oxidation process resulting in the evolution of light and heat in varying intensities, NFPA 921; 3.3.66. Oxidation is defined as a chemical reaction that is most commonly caused by atoms gaining oxygen. Therefore, fire investigators must have some knowledge of elementary chemistry to successfully complete origin & cause investigations. Spontaneous heating is the result of the slow oxidation of a combustible material, NFPA 921; 3.3.179. So, what exactly is oxidation?
There are four types of oxidation to be aware of:
1. Decay – very slow oxidation with a release of heat.
2. Rust – slow oxidation with a release of heat.
3. Fire – rapid oxidation with the release of heat and light.
4. Explosion – very rapid oxidation with the release of heat and a disruptive effect.
Spontaneous heating is somewhere between rust and fire. It is determined by the ambient temperature, materials that are oxidizing, type of combustible material in direct contact and its ignition temperature, the amount of oxygen, and the amount of insulation. If slow oxidation continues long enough and the heat cannot escape, the ignition temperature of the material or surrounding material may be reached causing spontaneous ignition. See NFPA 921: 3.3.180. It is believed that humidity may play a role in this as well, but chemist often disagree on this.
COMMON TYPES OF SPONTANEOUS HEATING
There are two major types of spontaneous heating that fire investigators will encounter:
1. Organic agricultural products subject to fermentation, such as hay, wood chips, saw dust, and grains.
2. Easily oxidized combustible materials that produce enough exothermic (heat producing) reactions at ordinary temperatures to reach their ignition temperature, such as cotton clothing or rags soaked in linseed oil.
Most organic materials that can oxidize will do so if oxygen is introduced. The most common oxidizing agent is air. So why doesn’t all organic material self-ignite? The reason is that the rate of heat generation is slow. The rate of heat generation must be faster than the rate of heat release. Therefore, the material must be insulated, and ventilation kept to a minimum. The same goes for a rag containing linseed oil.
DIAGNOSTIC SIGNS AND PHYSICAL EVIDENCE
Determining spontaneous ignition is usually not an easy task for a fire investigator. There often are no significant post fire residues left by a self-heating process because there are no additional elements of fuel for ignition necessary other than the material itself. With no residue evidence, the investigator must look for clues that point to spontaneous ignition. The most obvious of these clues being that these types of fires will usually burn from the inside out and make a tunnel like structure in the middle. In very large masses, there may even be multiple isolated pockets of smoldering with occasional flares that reach the exterior.
Additionally, we know that spontaneous heating fires often start on the floor in closets, storage areas, and garages. The investigator must remember to always find out what was stored near the area of origin or if any painting or staining has been done and where the oily rags or brushes were left. Fuels and oxidizers follow certain physical and chemical laws. Before concluding that spontaneous heating was responsible for a fire, the investigator must be able to identify the material that was first ignited, and that the ignition source (Ignition Energy; NFPA 912; 3.3.113) was hot enough to ignite the available material.
DETERMINING SPONTANEOUS IGNITION FIRES
Spontaneous ignition can often be misread and ruled to be an incendiary fire. So, investigators should always consider these four main questions when we suspect spontaneous heating:
1. Is it an organic material that is susceptible to spontaneous heating?
2. Could it get proper ventilation?
3. Was it insulated enough to hold in the heat?
4. Could it reach the ignition temperature of the material or adjacent materials under the conditions present?
If the answer to all these questions is yes, spontaneous heating cannot be ruled out as a possible cause of the fire. But on the other hand; could an arsonist set an incendiary fire by using spontaneous heating? The answer is, of course, yes. An arsonist is only limited by his or her imagination.
Fire investigators are going to be encouraged to believe that some fires have started by spontaneous heating of oily rags. But we know that only some vegetable and animal oils are subject to spontaneous heating. Oils derived from petroleum products are not subject to spontaneous heating. They do contain carbon, but they were not derived from living organisms. It is important to note that whether spontaneous ignition is suspected or not, fire investigators always use the Scientific Method to arrive at their conclusion when investigating origin and cause, NFPA 921; 3.3.160.
This article should not be construed as legal advice on any specific facts or circumstances. The contents are intended for general information purposes only. If you would like more information regarding this subject, please contact him at 843-485-7899 or email him directly at firstname.lastname@example.org.