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Excerpts from
The Pocket Guide to Accelerant Evidence Collection

Appendix I: Glossary of Terms for the Fire/Arson Investigator

Part A. Definitions:

1. Accelerant - A substance used to initiate or promote the spread of fire. The most commonly encountered arson accelerants are ignitable (flammable or combustible) liquids. Ignitable liquids, such as gasoline and kerosene, generate heavier than air, ignitable vapors at ordinary temperatures, are immiscible (don't mix with water), and float and sheen (rainbow coloration) on surface water. Other ignitable liquids, such as ethyl alcohol or isopropyl alcohol, are miscible (mix well) and do not float or leave a sheen on water. Common structural materials and assemblies and natural or man made substances often readily absorb ignitable liquids.

2. Aliphatic - Hydrocarbons are grouped according to their molecular structure. The two major families of hydrocarbons are aliphatic and aromatic. Aliphatic hydrocarbons can be either straight chain molecules or branched chain molecules.

Hydrocarbons can be further classified as alkanes, alkenes, alkynes, and alicyclics. Alkanes contain carbon-carbon single bonds (saturated); examples of alkanes include methane, ethane, and propane. Alkenes contain carbon-carbon double bonds (unsaturated); an example of an alkene is ethylene. Alkynes are highly reactive hydrocarbons containing carbon-carbon triple bonds; an example of an alkyne is acetylene. Alicyclic hydrocarbons contain carbon-carbon single bonds and are arranged in a ring structure such as cyclohexane.

Most of the common ignitable liquid hydrocarbons used as arson accelerants are blended products and contain both aliphatic and aromatic compounds.

3. Aromatics - The second major family of hydrocarbon molecules have one or more six member (benzene) rings of carbon atoms. The simplest aromatic compound is benzene (C6H6). Aromatics have a characteristic odor.

4. Boiling Point - The temperature of a pure liquid at which point its vapor pressure is equal to or slightly greater than atmospheric pressure. Boiling occurs when a temperature is reached at which the thermal energy of the particles is great enough to overcome the cohesive forces that hold them in the liquid. Most common ignitable liquids consist of mixtures of hydrocarbon compounds; such mixtures are described as having a "boiling range". As an example, gasoline is described as having a boiling range of -45 degrees F to over 400 degrees F. This means that the lightest compound of gasoline boils at -45 degrees F and the heaviest of the more than 300 individual compounds in gasoline boils at more than 400 degrees F.

5. Carbon - A non-metallic element (Periodic Table symbol - C) found nearly pure in nature as in a diamond or graphite or as a component of coal or petroleum. Carbon, the sixth most abundant element in the universe, has the unique ability among the elements to be able to react with either metal or non-metal elements. Carbon also has the ability to form bonds with other carbon atoms to form long chain or branched molecules. About 3 million carbon compounds are known. Substances that contain carbon are called organic compounds and the school of chemistry that studies and uses these compounds is called organic chemistry.

6. Celsius - The metric and scientific method of measuring temperature was formerly known as Centigrade. Expressed as degrees (C), the freezing point of pure water at sea level is 0 degres C and the boiling point of pure water at sea level is 100 degrees C. To convert from degrees Fahrenheit to degrees Celsius subtract 32 then divide by 5/9th i.e. 68 degrees F - 32 = 36; 36 x 5/9 = 20 degrees C.

7. Combustible liquid - An ignitable liquid having a flash point at or above 100 degres F (37.8 degrees C).

8. Deflagration -A rapid (exothermic) combustion reaction proceeding through fuel at a sub-sonic speed (typically less than 3300 feet/second -1000 meters/second).

9. Explosion - An effect produced by violent, sudden expansion of gases from chemical change such as detonation of an explosive or ignition of a flammable gas, mechanical changes such as in a boiler explosion or atomic changes.

10. Explosive limits - (flammable limits) The extreme lower & upper concentrations of an air/gas mixture in which combustion or deflagration will be supported. Generally, fuels with broad flammable/explosive limits such as acetylene (2.5% to 80% by volume) are considered more hazardous.

11. Fahrenheit - The method of measuring temperature where the freezing point of pure water at sea level is 32 degrees F and the boiling point is 212 degrees F. To convert degrees Celsius to degrees Fahrenheit, multiply the Celsius degrees by 9/5 and add 32 to that number i.e. 20 degrees C x 9/5 = 36; 36 + 32 = 68 degrees F.

12. Fire - Rapid oxidation with the evolution of heat and light of varying intensities. Fire is an exothermic reaction (energy is released). Flaming fire is always a gas phase (gas - gas) reaction meaning one or more ignitable gases are combining with a gaseous oxidizing agent in the flame zone forming new compounds.

13. Flame Point - A specific temperature, at which an ignitable liquid would produce sufficient vapors to support continuous, rather than momentary, combustion. The flame point is usually is usually a few points higher than the flash point for a given ignitable liquid.

14. Flammable Liquid - An ignitable liquid having a flash point below 80 degrees F (26.6 degrees C) (USCG) or 100 degrees F (37.8 degrees C) (NFPA) and a vapor pressure not exceeding 40 psi at 100 degrees F. Flammable liquids are a class of combustible liquids.

15. Flash point - The lowest temperature at which a given ignitable liquid produces an ignitable vapor in a laboratory test. The "flame or fire point", ordinarily a few degrees higher than the flash point, is the temperature at which an ignitable liquid would produce sufficient vapors to support a sustained combustion.

16. Gas Chromatography - Flame Ionization Detector (GC - FID) - A laboratory test method which vaporizes mixtures by heating and then separates the individual components according to their boiling points and molecular weights. For each component present in a sample, the detector produces a signal proportional to the quantity of that component. The resulting visual graph is called a chromatogram.

Most of the common ignitable liquids used as fire accelerants produce characteristic chromatograms, which can be readily characterized/identified by the forensic chemist. The GC - FID is used extensively by forensic laboratories for fire debris analysis.

17. Heat Release Rate (HRR) - The amount of heat energy released by combustion expressed in Btu/sec or kilowatts (kW). The HRR of a combustible is related to its chemical makeup, physical form and the quantity of oxidant present.

18. Heavy Petroleum Distillate (HPD) (C9 - C23) - A general class of combustible liquids which includes #2 fuel oil (home heating oil) and diesel fuel. Most HPDs considered as ignitable liquids would have flash points between 100-200 degrees F. The chromatographic profile of a HPD consists of a series of normal alkanes between C9 and C23; the most abundant n-alkane above C12; and at least five consecutive n-alkane peaks must be present between C14 and C20. Branched alkanes and the compounds pristane and phytane are present between the normal alkanes. (Refer to Chart 1).

19. Hydrocarbon - Organic compounds containing only carbon and hydrogen. The three simplest hydrocarbon molecules are methane (CH4), ethane (C2H6), and propane (C3H8). Hydrocarbons have one of three molecular structures; (aromatics) contain at least one six-member benzene ring (unsaturated) such as benzene (C6H6), toluene and the xylenes; (alicyclics) non-aromatic cyclic (ring) compounds, or, "open-chain" (aliphatic) structure including such groups as the alkanes (paraffins), alkenes (olefins), and alkynes (acetylenes).

20. Hydrogen - The lightest, simplest (one proton and one electron) and most abundant element in the universe. Hydrogen is a gas without taste, odor, or color. It combines with carbon in organic matter such as plants, coal, and petroleum and with oxygen in water. Periodic Table Symbol: H.

21. Ignition temperature - The minimum temperature to which a substance must be heated in air to ignite independently of the heating source. This temperature is sometimes referred to as the auto-ignition temperature. This temperature is derived from specific laboratory testing of pure substances and serves to classify the hazard presented by the ignitable liquid. The actual ignition temperature of most substances may be somewhat higher than those reported in laboratory tests.

22. Immiscible - A term that describes substances that do not mix (e.g. oil and water).

23. Light Petroleum Distillate - (C4 - C11) A general class of flammable liquids that could include pocket lighter fluids, ethers, some rubber cement solvents and V M & P Naphtha. Chromatographically, these products contain at least 4 major peaks in the range C4 and C11. (Refer to Chart 1).

24. Medium Petroleum Distillate - (C8 - C12) A general class of flammable or combustible liquids that would include paint thinner (mineral spirits), dry cleaning solvents and some brands of charcoal starter fluids. (Refer to Chart 1)

25. Miscibility - The ability of two or more substances to mix in all proportions and to form a single, homogeneous phase (e.g. alcohol and water). A liquid that can be mixed in all proportions producing a mixture that looks like a single compound (e.g. alcohol and water).

26. Oxygen - A nonflammable (oxidizing), gaseous element that is colorless, tasteless and odorless. It is found in a free state in the atmosphere where it makes up 21% of atmospheric air. Oxygen combines with virtually all the other elements except the inert gases. Periodic Table symbol: O.

27. Oxidation - Oxidation is generally considered any reaction in which electrons are transferred. The substance that gains electrons in the reaction is considered the oxidizing agent and the substance that loses electrons is the reducing material. Oxidation and reduction always occur simultaneously. Compounds containing oxygen, chlorine, fluorine, etc. are common oxidizing agents. Compounds such as potassium nitrate, potassium perchlorate, and calcium hypochlorite are examples of oxidizers commonly encountered in explosive and incendiary compositions.

28. Pyrolysis - Transformation of a compound into one or more substances by heat alone. This process often precedes combustion.

29. Soluble - The process of dispersing one or more solid, liquid, or gaseous substances into another, usually a liquid, forming a homogenous mixture.

30. Specific gravity - The ratio of weight of a given volume of ignitable liquid compared to an equal volume of fresh water. An ignitable liquid accelerant with a specific gravity less than one will float; those with specific gravity greater than one will sink.

31. Stoichiometric Mixture - A stoichmetric mixture is one containing an optimal ratio of fuel and oxygen. Gaseous fuel - air mixtures can result in an explosion when a stoichiometric mixture exists. Fuel - air mixtures which are capable of resulting in an explosion are typically reported in MSDS as the Lower Explosive Limit (LEL) and the Upper Explosive Limit (UEL).

32. Toxicity - Calibration of physiological damage caused by a poison or toxin. Rating is included in Part B of this Appendix - Common Ignitable Liquids for those products where a rating was reported.

Toxicity Rating Dose Probable lethal dose for 150 pound person
6 - Supertoxic Less than 5mg/kg A taste (less than 7 drops)
5 - Extremely Toxic 5 - 50 Between 7 drops & 1 tsp.
4 - Very Toxic 50 - 500 Between 1 tsp & 1 ounce
3 - Moderately Toxic 0.5 - 5gm/kg Between 1 oz & 1 pint
2 - Slightly toxic 5 - 15gm/km Between 1 pt and 1 quart
1 - Practically non toxic Above 15gm/kg More than 1 quart

33. Vapor density - The ratio of the molecular weight of a given volume of a specific gas or vapor to that of an equal volume of dry air at the same temperature and pressure. A vapor density greater than one will result in a tendency of the vapor to sink. Most of the common ignitable liquids have a vapor density greater than one.

The closer the vapor density is to the value of air (one) the more rapidly that gas will tend to mix with air. Higher vapor density vapors tend to resist mixing with air. Gasoline, for example, has a vapor density of 3 to 4 times heavier than air. This means concentrated gasoline vapors will ordinarily sink rapidly in still air and tend to drop to the lowest level.

34. Vapor pressure - The pressure exerted when a solid or liquid is in equilibrium with its own vapor. The higher the vapor pressure the more rapidly a liquid will tend to evaporate.

35. Viscosity - The viscosity of a liquid is a measure of its resistance to flow resulting from the combined effects of adhesion and cohesion.

36. Volatility - The ease with which vapors are given off.

Excerpted from A Pocket Guide to Accelerant Evidence Collection, 2nd Edition, (1999).

Courtesy Massachusetts Chapter, IAAI. Reprinted with permission.

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