1. Forensic Aspects of Fire Investigation
Objectives:
Define oxidation.
Define energy and give examples of its different forms.
Describe the role of heat energy in chemical reactions.
Define heat of combustion and ignition temperature.
Describe the difference between an exothermic and endothermic chemical reaction.

2. Arson Investigation criminalist must establish motive
reason/purpose of fire
modus operandi
pattern of operation
find suspect

3. Fire Causes accidental arson faulty wiring
overheated electrical motors
improperly maintained heating systems
cigarette smoking
arson
chemical accelerants
gasoline, kerosene, etc
leave chemical traces
nonchemical caused
pile of combustibles
harder to prove as non-accidental

4. Chemistry of Fire Oxidation
chemical reaction with oxygen combining with a substance to form new substances
with fire (heat & light) a.k.a. combustion
CH4 + 2O2  CO2 + 2H2O
without fire a.k.a. “rusting”
4Fe + 3O2  2FeO3

5. Energy the ability or potential to do work
heat
light
electrical
mechanical
nuclear
chemical
energy can change form but cannot be created or destroyed

6. Chemical Reactions Exothermic Reactions Endothermic Reactions
more energy is liberated than is required to rearrange atoms bonded together
energy given to surroundings
surroundings get hotter (increase in temperature)
Heat of Combustion
excess heat released during combustion
Endothermic Reactions
less energy is liberated than is required to rearrange atoms bonded together
energy taken from surroundings
surroundings get colder (decrease in temperature)

7. Forensic Aspects of Fire Investigation
Objectives:
Explain why the oxidation of iron to rust is not accompanied by a flaming fire.
List the requirements necessary to initiate and sustain combustion.
Understand the three mechanisms of heat transfer
Describe how physical evidence must be collected at the scene of a suspected arson.
Describe laboratory procedures used for the detection and identification of hydrocarbon residues.

8. Ignition Temperature Energy Barrier Iron Rusting
amount of heat (energy) needed to initiate combustion
Ignition Temperature
initial temperature that must be reached for spontaneous combustion to occur
usually provided by “igniter”
match
spark
chemical
Iron Rusting
low ignition temperature
slow small energy release

9. Speed of Reaction
faster movement more collisions faster rate of reaction
Physical State
gaseous favors greatest reaction
Fuel Temperature
solids and liquids maintain a gaseous reaction when vaporized
Flash Point
lowest temperature at which a liquid fuel will vaporize sufficiently to support combustion
451 °F flash point of paper!!!!
pyrolysis takes place
each 10°C doubles or triples rate of reaction
stops only when fuel or oxygen is depleted

10. Fuel-Air Mixture combustion within definite limits Flammable Range
low fuel (lean mixture)
high fuel (rich mixture)
Flammable Range
range of gaseous concentrations in air capable of supporting combustion
gasoline 1.3%-6.0%

11. Glowing Combustion burning of fuels without flame
solids exposed to heat less than pyrolysis
red charcoals
cigarette
a.k.a. smoldering
“hot spots”

12. Spontaneous Combustion
natural heat producing process ignites a fire in a poorly ventilated area
limited situation rarely cause fires
hay with bacteria
oily (highly unsaturated fats) rags
Combustion requires:
fuel
oxygen
heat

13. Heat Transfer heat moves from high to low temperature regions
trace origin
explain how fire spreads
Transfer Mechanism
conduction
radiation
convection

14. Conduction movement through a solid
Metals are conductors
loosely held electrons better conductors
Woods/Plastics
tightly held electrons poor conductors
“insulators”
fuel contact with a conductor can spread fire

15. Radiation heat transfer by electromagnetic radiation
hot surfaces release various wavelengths
surfaces facing “fire” often ignite from radiant heat

16. Convection heat transfer by molecular movement in liquids/gases
hot gases expand and rise
in a fire these gases rise heating surfaces
surfaces can pyrolyze and ignite
flashover
simultaneous ignition of combustible fuels in a structure

17. Arson Investigation accelerants (petroleum based substances)
freeze soil to stop bacteria degradation
ignition device (candle, match, cell phone)
visual indications (stains)

18. Analysis of Flammable Residues
Headspace Technique
heat container and gather vapors with syringe
Vapor Concentration
longer slower heating
absorbent charcoal strip catches accelerant
increases sensitivity 100x
GC/MS Analysis
known readouts used to ID accelerant

19. Locating Fire Origin Fire moves up (V-shaped pattern)
Streamers – connect burnt areas
lower burning liquids flow down
Accelerant search
vapor detectors
trained canines

20. Evidence Collection and Preservation
specimens in airtight containers
“hold” vapors
Substrate Controls
sample materials NOT exposed to accelerants
burning not due to “normally” found materials
cleaning solutions or composition