<CHAP NUM="11" ID="CH.00.011">chapter 11
<FM><TTL>Forensic Aspects of Arson and Explosion Investigations
</TTL><KTSET><TTL>Key Terms</TTL>
<KT>accelerant</KT>
<KT>black powder</KT>
<KT>combustion</KT>
<KT>deflagration</KT>
<KT>detonating cord</KT>
<KT>detonation</KT>
<KT>endothermic reaction</KT>
<KT>energy</KT>
<KT>exothermic reaction</KT>
<KT>explosion</KT>
<KT>flammable range</KT>
<KT>flash point</KT>
<KT>glowing combustion</KT>
<KT>heat of combustion</KT>
<KT>high explosive</KT>
<KT>hydrocarbon</KT>
<KT>ignition temperature</KT>
<KT>low explosive</KT>
<KT>modus operandi</KT>
<KT>oxidation</KT>
<KT>oxidizing agent</KT>
<KT>primary explosive</KT>
<KT>pyrolysis</KT>
<KT>safety fuse</KT>
<KT>secondary explosive</KT>
<KT>smokeless powder (double-base)</KT>
<KT>smokeless powder (single-base)</KT>
<KT>spontaneous combustion</KT></KTSET>
<OBJSET><TTL>Learning Objectives</TTL>
<P>After studying this chapter you should be able to:
<OBJ><P><INST>< </INST>List the conditions necessary to initiate and sustain combustion</P></OBJ>
<OBJ><P><INST>< </INST>Recognize the telltale signs of an accelerant-initiated fire</P></OBJ>
<OBJ><P><INST>< </INST>Describe how to collect physical evidence at the scene of a suspected arson or explosion</P></OBJ>
<OBJ><P><INST>< </INST>Describe laboratory procedures used to detect and identify hydrocarbon and explosive residues</P></OBJ>
<OBJ><P><INST>< </INST>Understand how explosives are classified</P></OBJ>
<OBJ><P><INST>< </INST>List some common commercial, homemade, and military explosives</P></OBJ></P></OBJSET></FM>
<CASE NUM="1" TY="CS"><TTL>The Oklahoma City Bombing</TTL>
<P>It was the biggest act of mass murder in U.S. history. On a sunny spring morning in April 1995, a Ryder rental truck pulled into the parking area of the Alfred P. Murrah federal building in Oklahoma City. The driver stepped down from the truck’s cab and casually walked away. Minutes later, the truck exploded into a fireball, unleashing enough energy to destroy the building and kill 168 people, including 19 children and infants in the building’s day care center. Later that morning, an Oklahoma Highway Patrol officer pulled over a beat-up 1977 Mercury Marquis being driven without a license plate. On further investigation, the driver, Timothy McVeigh, was found to be in possession of a loaded firearm and charged with transporting a firearm.</P>
<P>At the explosion site, remnants of the Ryder truck were located and the truck was quickly traced to a renter—Robert Kling, an alias for Timothy McVeigh. Coincidentally, the rental agreement and McVeigh’s driver’s license both used the address of McVeigh’s friend, Terry Nichols.</P>
<P>Outrage at the destruction of the Branch Davidian compound at Waco had spurred McVeigh and Nichols into planning the destruction of the federal building. Investigators later recovered McVeigh’s fingerprint on a receipt for 2,000 pounds of ammonium nitrate, a basic explosive ingredient. Forensic analysts also located PETN residues on the clothing McVeigh wore on the day of his arrest. PETN is a component of detonating cord. A jury took three days to decide McVeigh’s guilt and then sentenced McVeigh to die by lethal injection.</P></CASE>
<BM><P>Arson and explosions often present complex and difficult circumstances to investigate. Normally, these incidents are committed at the convenience of a perpetrator who has thoroughly planned the criminal act and has left the crime scene long before any official investigation is launched. Furthermore, proving commission of the offense is more difficult because of the extensive destruction that frequently dominates the crime scene. The contribution of the criminalist is only one aspect of a comprehensive and difficult investigative process that must establish a motive, the <KT>modus operandi</KT>,<SIDEIND NUM="1" ID="MN2.11.001"/> and a suspect.</P>
<P>The criminalist’s function is rather limited; usually he or she is expected only to detect and identify relevant chemical materials collected at the scene and to reconstruct and identify igniters or detonating mechanisms. Although a chemist can identify trace amounts of gasoline or kerosene in debris, no scientific test can 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—which usually leave no chemical traces. Thus, the final determination of the cause of a fire or explosion must consider numerous factors 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 and explosion investigation.</P>
<H1>The Chemistry of Fire</H1>
<P>Humankind’s early search to explain the physical concepts underlying the behavior of matter always bestowed a central and fundamental role on fire. To ancient Greek philosophers, fire was one of the four basic elements from which all matter was derived. The alchemist thought of fire as an instrument of transformation, to be used for changing one element into another. One ancient recipe expresses its mystical power as follows: “Now the substance of cinnabar is such that the more it is heated, the more exquisite are its sublimations. Cinnabar will become mercury, and passing through a series of other sublimations, it is again turned into cinnabar, and thus it enables man to enjoy eternal life.”</P>
<P>Today, we know of fire not as an element of matter but as 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 must begin with the knowledge of the fundamental chemical reaction of fire—<KT>oxidation</KT>.<SIDEIND NUM="2" ID="MN2.11.002"/></P>
<P>In a simple description of oxidation, oxygen combines with other substances to produce new products. Thus, we may write the chemical equation for the burning of methane gas, a major component of natural gas, as follows:
<EQ><EQ> <DM ID="DM.11.001">CH<SUB>4</SUB> + 2O<SUB>2</SUB> ® CO<SUB>2</SUB> + 2H<SUB>2</SUB>O methane oxygen yields carbon dioxide water</DM></P>
<P>However, not all oxidation proceeds in the manner that one associates with fire. For example, oxygen combines with many metals to form oxides. Thus, iron forms a red-brown iron oxide, or rust, as follows:
<EQ><EQ><DM ID="DM.11.002">4Fe + 3O<SUB>2</SUB> 2Fe<SUB>2</SUB>O<SUB>3</SUB> iron oxygen yields iron oxide</DM></P>
<P>Yet chemical equations do not give us a complete insight into the oxidation process. We must consider other factors to understand all of the implications of oxidation or, for that matter, any other chemical reaction. When methane unites with oxygen, it burns; but the mere mixing of methane and oxygen will not produce a fire, nor, for example, 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? Why do some oxidations proceed with the outward appearances that we associate with a fire while others do not? Why do we need a match to initiate some oxidations while others proceed at room temperature? The explanation lies in a fundamental but abstract concept—<KT>energy</KT><SIDEIND NUM="3" ID="MN2.11.003"/>.</P>
<P>Energy can be defined as the capacity for doing work. Energy takes many forms, such as heat energy, electrical energy, mechanical energy, nuclear energy, light energy, and chemical energy. For example, when methane is burned, the stored chemical energy in methane is converted to energy in the form of heat and light. This heat may be used to boil water or to provide high-pressure steam to turn a turbine. This is an example of converting chemical energy to heat energy to mechanical energy. The turbine can then be used to generate electricity, transforming mechanical energy to electrical energy. Electrical energy may then be used to turn a motor. In other words, energy can enable work to be done; heat is energy.</P>
<P>The quantity of heat from a chemical reaction comes from the breaking and formation of chemical bonds. Methane is a molecule composed of one carbon atom bonded with four hydrogen atoms:
<EQ><DM ID="DM.11.300"> H
<chemdraw>ƒ
<chemdraw>H ¬ C ¬ H
H</DM></P>
<P>An oxygen molecule forms when two atoms of the element oxygen bond:</P>
<EQ><P><DM ID="DM.11.301">O <chemdraw>“ O</DM></P>
<P>In chemical changes, atoms are not lost but merely redistributed during the chemical reaction; thus, the products of methane’s oxidation will be carbon dioxide:</P>
<EQ><P><DM ID="DM.11.302">O <chemdraw>“ C “ O</DM></P>
<P>and water:</P>
<EQ> <P><DM ID="DM.11.303">H<chemdraw> ¬ O ¬ H</DM></P>
<P>This rearrangement, however, means that the bonds holding the atoms together must be broken and new bonds formed. We now have arrived at a fundamental observation in our dissection of a chemical reaction—that molecules must absorb energy to break apart their chemical bonds, and that they liberate energy when their bonds are reformed. The amount of energy needed to break a bond and the quantity of energy liberated when a bond is formed are characteristic of the type of chemical bond involved. Hence, a chemical reaction involves a change in energy content; energy is going in and energy is given off. The quantities of energies involved are different for each reaction and are determined by the participants of the chemical reaction.</P>
<P>All oxidation reactions, including the <KT>combustion</KT><SIDEIND NUM="4" ID="MN2.11.004"/> of methane, are examples of reactions in which more energy is liberated than what is required to break the various bonds. This excess energy is liberated as heat and often as light and is known as the <KT>heat of combustion</KT><SIDEIND NUM="5" ID="MN2.11.005"/>. Such reactions are said to be <KT>exothermic</KT><SIDEIND NUM="6" ID="MN2.11.006"/>. <LINK LINKEND="TB.11.001">Table <TBLIND NUM="1" ID="TB.11.001"/>11–1</LINK> summarizes the heats of combustion of some important fuels in fire investigation.</P>
<P>Although we will not be concerned with them, some reactions require more energy than they will eventually liberate. These reactions are known as <KT>endothermic reactions...
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