Forensic Science - Arson and Explosives

I. The Chemistry of Fire Oxidation – The combination of oxygen with other subtances to produce new substances. One example is the burning of methane (natural gas): CH4 + 2O2 CO2 + 2H2O

This reaction requires a spark to initiate. Combustion – the rapid combination of oxygen with another substance accompanied by the production of noticeable heat and light. Exothermic reaction – a chemical reaction in which energy is released or given off. Ignition temperature – the minimum temperature at which a fuel will spontaneously ignite.

The oxidation of a fuel differs from the oxidation of a substance like iron which oxidizes into rust. 4Fe + 3O2 2Fe2O3 Rusting and burning are both oxidation processes. The difference between them is the rate or speed at which the reaction takes place.

A fuel will achieve a reaction rate with oxygen sufficient to produce a flame only when it is the gaseous state, for it is only in this state that molecules can collide frequently enough to support a flaming fire. How then do liquids and solids burn? In a liquid fuel the temperature must be high enough to vaporize the fuel.

Flash Point – the minimum temperature at which a liquid fuel will produce enough vapor to burn. Once flash point is reached, the fuel can be ignited by some outside source to start a fire. Solids will burn only when exposed to heat that is hot enough to decompose the solid into gaseous product.

Pyrolysis – the decomposition of organic matter by heat. Flammable range - the entire range of possible gas or vapor fuel concentrations in air that are capable of burning. A fuel can burn without the presence of a flame.

Glowing combustion - burning at the fuel air interface Glowing combustion - burning at the fuel air interface. Examples are a red-hot charcoal or a burning cigarette. For most reactions, a 10ºC rise in temperature doubles or triples the reaction rate. Spontaneous combustion - a fire caused by a natural heat-producing process in the presence of sufficient air and fuel.

Explosives are substances that undergo a rapid exothermic reactions, with the production of large quantities of gases. It is this sudden buildup of gas pressure that contitutes the nature of an explosion. Detonation occurs so rapidly that oxygen in the air cannot participate in the reaction; thus many explosives must have their own source of oxygen.

Oxidizing Agent - a substance that supplies oxygen to a chemical reaction. The three requirements that must be satisfied if combustion is to be initiated and sustained are: 1. A fuel must be present. 2. Oxygen must be available in sufficient quantity to combine with the fuel. 3. Heat must be applied to initiate the combustion, and generated to sustain it.

II. Searching the Fire Scene Accelerants – any material used to start or sustain a fire. The most common accelerants are combustible liquids. Because any petroleum residues that remain after a fire may evaporate in a few hours or days, a search warrant is not required.

A search of a fire scene focuses on finding the origin. Some telltale signs of origin may include “streamers”, accelerant containers, ignition devices, or time-delay devices. Nothing should be moved until pictures, sketches and notes are taken.

III. Collection & Preservation of Arson Evidence Two to three quarts of ash and soot debris must be collected at the point of origin. Should include all porous materials and all other substances likely to contain flammable residues. (Wood flooring, rugs, upholstery, and rags.) New, clean paint cans with friction lids are good containers. Wide mouthed glass jars are also useful for packing materials.

Cans and jars should be filled one-half to two-thirds full, leaving an air space in the container above the debris. Plastic polyethylene bags are NOT suitable for packing speciments because they react with hydrocarbons and permit vapors to be depleted. Collect uncontaminated samples of materials from the scene to provide “substrate control”. If a perpetrator is arrested within a few hours, their clothing may contain residual traces of accelerant.

IV. Analysis of Flammable Residues The gas chromatograph is the most sensitive and reliable instrument for detecting and characterizing flammable residues. Most arsons are initiated by petroleum distillates such as gasoline and kerosene. Hydrocarbons - any compound consisting of only carbon and hydrogen.

V. Types of Explosives Explosion – a chemical or mechanical action resulting in the rapid expansion of gases. The speed at which explosives decompose varies greatly from one to another and permits their classification as high and low explosives.

Low explosive – explosive with a velocity of detonation less than 1,000 meters per second. For example, black powder and smokeless powder. In the low explosive the speed is called speed of deflagration (burning). This is characterized by a very rapid oxidation producing heat, light and a subsonic pressure wave.

High Explosive – Explosive with a velocity of detonation greater than 1,000 meters per second. For example, dynamite and RDX. In the high explosive, the speed is called the speed of detonation. Detonation – refers to the creation of a supersonic shock wave within the explosive charge. This shock wave causes the chemical bonds of the explosive charge to break apart, leading to the new instantaneous buildup of heat and gases.

The most widely used low explosives are black powder and smokeless powder. Black powder is a relatively stable mixture of potassium or sodium nitrate, charcoal, and sulfur. Unconfined, it merely burns. It becomes explosive and lethal when confined. Safety fuses usually consist of black powder wrapped in a fabric or plastic casing. The safest and most powerful low explosive is smokeless powder.

High explosives are classified into two groups by their sensitivity. Primary explosives – a high explosive easily detonated by heat or shock. Usually used to detonate other explosives through a chain reaction and are referred to as primers. Often found in blasting caps.

Secondary explosives – are relatively insensitive to heat, shock, or friction, and will normally burn rather than detonate if they are ignited in small quantities in the open air. Includes dynamite, TNT (trinitrotoluene), PETN (pentaerythritol tetranitrate), RDX (cyclotrimethylenetrinitramine)

Nitroglycerin-based dynamite has been replaced by ammonium nitrate – based explosives. These explosives mix oxygen-rich ammonium nitrate with a fuel to form a low-cost and very stable explosive. RDX is the most popular and powerful of the military explosives. It is often found in the form of a pliable plastic of doughlike consistency known as C-4. PETN is often used in a detonating cord to connect a series of charges so that they will detonate simultaneously.

VI. Collection & Analysis of Explosives Most important is collection of samples from the scene containing undetonated residues. EGIS® is a portable explosive residue detector. There are also chemical tests for common explosives.