Chapter 4 Arson and Fire Investigation © 2012 Cengage Learning. All Rights Reserved.

Chapter 4

Arson and Fire Investigation

© 2012 Cengage Learning. All Rights Reserved

2 Forensic Science II: Arson and Fire Investigation, Chapter 4


Introduction—Objectives

1. Define combustion reactions.2. Discuss the four factors that are required to

ignite and maintain a fi re.3. Explain the conditions in which fuels will

burn.4. Examine reasons why arson is difficult to

detect.5. Identify the four categories of fire.



Introduction—Objectives

6. Evaluate the significance of burn patterns discovered at an arson investigation.

7. Discuss the proper methods for detecting, collecting, preserving, and analyzing arson evidence.

8. Describe the psychological profile of an arsonist.

9. Examine the various motives for arson.



Introduction—Vocabulary

o accelerant - in fire investigation, any material used to start or sustain a fire; the most common are combustible liquids

o arson - the intentional and illegal burning of propertyo combustion reaction – oxidation reaction that

involves oxygen and that releases heat and lighto exothermic reaction – chemical reaction that

releases heato heat of combustion - excess heat that is given off in

a combustion reaction



Introduction—Vocabulary

o hydrocarbon - any compound consisting only of hydrogen and carbon

o oxidation reaction - the complete or partial loss of electrons or gain of oxygen

o pyrolysis - decomposition of organic matter by heat in the absence of oxygen

o substrate control - a similar, but uncontaminated, sample; used for making comparisons


© 2012 Cengage Learning. All Rights Reserved6

Happy Land Fire

o Julio Gonzalez and Lydia Feliciano dated off and on for six years.

o When Lydia ended the relationship, Julio set fire to the Happy Land Club in the Bronx.

o 87 people were trapped and died.o Julio purchased the accelerant at a local

station.o Julio’s home and shoes smelled of gasoline.



Chemistry of Fire (Obj 4.1, 4.2, 4.3)

o Fire — a rapid oxidation reaction that involves a combustible material

o Oxidation — the complete or partial loss of electrons or the gain of oxygen

2NA + Cl2 2NaCl



Combustion

o Combustion reactions — oxidation reactions that involve oxygen and produce flames

o Heat of combustion — excess heat energyo Exothermic reactions — chemical reactions

that release heat



The Fire Tetrahedron

Four ingredients are required to start a fire and keep it burning• Oxygen• Fuel• Heat• Chain reaction



Flash Point & Ignition Temperature

o Most accelerants are hydrocarbons in a gas state

o Hydrocarbons are compounds that are made of only hydrogen and carbon atoms— gasoline, kerosene, lighter fluid

o In a gas state, molecular bonding is weakero Vaporization — liquid changes to gaso Flash point — lowest temperature at which

vaporization occurs



Flash Point & Ignition Temperature



Pyrolysis

o pyrolysis - decomposition of organic matter by heat in the absence of oxygen

o The vapors given off from the resins in wood are flammable and will burn.



Arson is Difficult to Prove

o The crime is usually carefully planned.o The arsonist leaves the scene before anyone

notices a fire.o The fire destroys evidence.o Extinguishing the fire also destroys evidence.



Function of a Fire Investigator

o Investigation must begin quickly, before evidence is lost

o Find the fire’s point of origino Examine possible causes

• Accidental• Arson

o Classify the fire



Function of a Fire Investigator



Determining Cause



Determining Cause



Collecting the Evidence

o Begin immediately; no warrant requiredo Collect 3-4 liters of ash from point of origin

and other suspected areaso Use portable vapor detectors, or snifferso Use trained dogs to sniff



Collecting the Control

o Place each sample in its own containero Collect a substrate control — a debris

sample that has not been contaminated by the accelerant

o Comparisons to the substrate control may help prove an accelerant was used

o Some products may look like accelerants when burned



Packaging the Debris

o Use airtight containers such as a new clean paint can and lid

o Leave head space at the top of the can



Finding the Igniter

o Matches — often burns in the fireo Cigarette lighters — often removed by

arsonisto Molotov cocktail — may leave glass

fragmentso Faulty electrical wiring — an arc causes a

predictable patterno Knowing the igniter helps form the criminal

profile



Lab Analysis

o Heat the debris container to collect vapors in the head space

o Direct headspace extraction procedure• Remove vapors with a syringe• Analyze vapors with gas chromatography

o Passive headspace extraction procedure• Suspend a charcoal-coated strip inside the can• Replace lid• Heat container 4-16 hours at 50-80oC• Charcoal absorbs the vapor



Lab Analysis

Passive headspace extraction procedure



Psychology of an Arsonist

o Sense of powero An emotional higho No typical arsonist; possible characteristics:

• Less than 25 y.o.• Father not in the home• Domineering mother• Academically challenged• Emotionally and/or psychologically disabled

• Unmarried• Living with parents• Inadequacy, insecurity• Fascination with fire• Alcoholism• Parental neglect or abuse



Motives for Arson

o Financial gain — insurance fraudo Revenge, spiteo Excitemento Vanity, hero syndromeo Crime concealment — destroy evidenceo Vandalism



Chapter Summary

o Fire is an oxidation reaction that involves a combustible material. Not all oxidation reactions produce fire.

o The fire tetrahedron represents the four requirements for sustained fire—oxygen, fuel, heat, and a chain reaction.

o Most fuels are hydrocarbons and typically have very high ignition temperatures.

o The ignition temperature is when a fuel will light and continue to burn even if the heat source is removed.



Chapter Summary

o The initial focus is to find the fire’s point of origino The burn pattern helps determine whether the fire

was natural, accidental, or deliberate.o Ash and debris are collected from the point of origin

and any other suspected area.o Each sample of debris is placed in a separate airtight

container.



Chapter Summary

o For comparison purposes, investigators also collect a substrate control.

o Two methods of collecting accelerant vapors for analysis are:• direct headspace extraction procedure and• vapor concentration.

o Gas chromatography is used to analyze the accelerant residue — comparing the lab chromatogram with one of known hydrocarbons.



Chapter Summary

o Arsonists tend to exhibit a set of characteristics that helps investigators focus their search.

o Most of the motives fit into six broad categories — financial gain, revenge, excitement, vanity, crime concealment, and vandalism.

Chapter 4 Arson and Fire Investigation © 2012 Cengage Learning. All Rights Reserved.

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