Forensic Explosives

1. Forensic Science: Arson and
Explosives
CHE 113

2. Fire
• Rapid Oxidation of chemicals to release
energy (usually as heat, light, noise)
• Difference between combustion and
explosion is mostly the rate of the reaction:
– Combustion slower – explosion fast

3. Oxidation Reactions
• Is the combination of oxygen with other
substances to produce new substances
• Are reactions that exothermic (give off
heat)
• Involve both oxidation and reduction
• Not all oxidation reactions result in
flames!
– rusting
– metabolism
• Transfer of electrons
– Gain electrons – reduction
– Lose electrons - oxidation
• Rate of reaction determines the nature
of the reaction
• Chemicals that supply oxygen (lose
electrons) are known as oxidizing
agents

4. Requirements for Combustion
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 sufficient
heat must be generated to
sustain a reaction.
• Ignition temp is the minimum
temperature at which fuel vapor
will ignite
Accelerant: any material
used to start or sustain a
fire.

5. Components for Combustion
• Components needed for starting, growing and
sustaining a fire:
– Heat
• auto-ignition temperature (or kindling point)
minimum temperature where a substance will
spontaneously ignite without an external ignition.
• flash point: lowest temperature where a fuel will
vaporize sufficiently to form an ignitable mixture
with air.
– Fuel
• flammability limits: air/fuel mixtures beyond
these limits, combustion cannot be sustained
– Oxygen (oxidizer)
– Chain-reaction

6. Components for Combustion
• Components needed for starting, growing and
sustaining a fire:
– Heat
– Fuel
– Oxygen (oxidizer)
– Chain-reaction

7. Components for Combustion
• Fire Suppression:
– Heat Suppression
– Fuel Suppression
– Oxidant Suppression
– Chain-reaction Suppression

8. Spontaneous Combustion
• A natural heat-producing process that may
give rise to a fire in the presence of sufficient
air & fuel
– Ex.) hay stored in barns growing medium
for bacteria which generate heat
– Ex.) rags soaked w/ highly unsaturated oils,
such as linseed oil
– Ex.) Not people

9. Examination of evidence recovered from an arson
• Presence of gasoline, kerosene or turpentine in
debris
• Liquid gasoline may be compared for origin
• Criminalist may be able to reconstruct the ignition
mechanism used

10. Arson Investigation
Arson is the criminal act of intentionally setting fire
without lawful consent
• Fire Pattern
• Debris
• Fire Growth and features
• Accelerants
• Other

11. Arson Investigation
Arson is the criminal act of intentionally setting fire
without lawful consent
• Fire Pattern
• Debris
• Fire Growth and features
• Accelerants
• Other

12. Gasoline Residues
• Are best characterized by gas
chromatography
• Hydrocarbon: any compound
consisting
only of carbon and hydrogen
• Thin-layer chromatography is also
useful for screening debris
• Unevaporated gasoline
• 90% evaporated gasoline
• Unevaporated kerosene
• 90% evaporated gasoline

13. Explosives: Energetic Materials
• Definitions
– Compounds that react very quickly to produce heat,
light, and the rapid outward expansion of gasses
– Time is key!
• Types
– Chemical and Physical
• High explosives - explosive charge that reacts at a speed
greater than 1,000 m (1094 yards) per second
• Detonation - high explosives yielding supersonic pressure
waves (shock waves faster than the speed of sound, 343 m/s).
• Low explosives - reactions that occur at rates ofless than 1,000
m/sec.
• deflagrations - propagated mainly by a flame front rather than
a supersonic shock wave .

14. Explosives: Energetic Materials
• Requirements
– Must contain fuel AND oxidant
• Separately: ANFO (ammonium nitrate/fuel oil explosive)
• Together: TNT
• Types
– Primary: sensitive to heat, flame, shock or friction and
typically detonate quickly
– Secondary: relatively stable to heat, shock, electrical
discharge, and friction and usually require much more
energy to detonate

15. Explosives: Energetic Materials

16. Primary (Initiating) High Explosives
used in detonators
• Lead azide, lead styphnate and
diazodinitrophenol
– Major ingredient of blasting cap
– Not used as main charge b/c so sensitive; however, are
used to detonate noninitiating explosives
• PETN (pentaerythritol tetranitrate)
• TNT (trinitrotoluene)
• Dynamite
• Smokeless powder (nitrocellulose)

17. Damage from High Explosives
• Shock Wave
• Heat
• Flying Debris

18. Damage from High Explosives
• Shock Wave
• Heat
• Flying Debris

19. Low Explosives & Black Powder
• Explosive with a velocity of
detonation less than 1,000 meters
per second.
• Ex.)
• PETN (pentaerythritol
tetranitrate)
• TNT (trinitrotoluene)
• Dynamite
• Smokeless powder
(nitrocellulose)- nonmilitary
issued
• Black powder
• Black Powder ingredients:
potassium nitrate, carbon & sulfur
• Potassium chromate + Sulfuric
acid + sugar can create a low
explosive

20. Military Explosives: High Explosives
• RDX
(cyclotrimethlylenetrinitramine)
the most powerful & popular
– In the form of pliable plastic
of dough-like consistency
composition C-4
• PETN is used in TNT mixtures
for small-caliber projectiles and
granades

21. End Arson and Explosives