Chapter 14:
Arson and Explosives
Dr. J. T. Spencer
Adjunct T. L. Meeks
CHE 113
Learning Objectives
Forensic Investigations may
deal with the causes of fires and
explosions…
Learning Objectives
• What is fire and what are the
roles of the components in the
fire tetrahedron
• What type of information is
part of a fire investigation
• What is meant by arson and
what are some of the telltale
clues to intentional fires
Learning Objectives
• What are the common
explosives used today and
how are they classified
• How are the identities of
explosives determined in the
laboratory
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
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
Requirements for Combustion
1.
2.
3.
•
Accelerant: any material
used to start or sustain a
fire.
A fuel must be present
Oxygen must be available in
sufficient quantity to combine
with the fuel
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
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
Components for Combustion
• Components needed for
starting, growing and
sustaining a fire:
–
–
–
–
Heat
Fuel
Oxygen (oxidizer)
Chain-reaction
Components for Combustion
• Fire Suppression:
–
–
–
–
Heat Suppression
Fuel Suppression
Oxidant Suppression
Chain-reaction Suppression
Stages of Fire
Temperature
Types of Fire
In the US, fires are classified into
groupings depending upon the source
of fuel. The classification of the fire
is used to determine the best way to
suppress the fire. The categories of
fires are:
• Class A fire: Ordinary combustibles
such as wood, paper, cloth,
trash, cardboard, and PVC;
• Class B fire: Flammable liquid or
gaseous fuels such benzene,
gasoline, oil, butane, propane,
and natural gas;
Types of Fire
In the US, fires are classified into
groupings depending upon the source
of fuel. The classification of the fire
is used to determine the best way to
suppress the fire. The categories of
fires are:
• Class C fire: Involving live
electrical equipment, often
caused by short circuits or
overheated electrical cables;
• Class D fire: Combustible metals
and alloys, such as iron,
aluminum, sodium, and
magnesium
Types of Fire
In the US, fires are classified into
groupings depending upon the source
of fuel. The classification of the fire
is used to determine the best way to
suppress the fire. The categories of
fires are:
• Class K fire: Cooking media that
contain vegetable or animal
fat and oils.
Fire Dynamics
Heat Suppression:
The most common
firefighting agent is water.
Water largely acts to cool the
fire by absorbing large
amounts of energy.
Compounds that cool the fire
can help to bring it under
control.
Fire Dynamics
Oxidation Suppression:
Carbon dioxide and sodium
bicarbonate (solid
extinguisher) act to smother
the fire by removing oxygen
from the equation.
• Requires large amounts of CO2
• 75% of surface must be covered
• Inert gases can be used instead
due to the toxicity of CO2
Fire Dynamics
Fuel Suppression:
Removal of the fuel occurs by
either physically isolating the
fuel source or interrupting the
flow of fuel into the fire, such
as turning off a gas valve.
Fire Dynamics
Chain Reaction Suppression:
Halon acts at high
temperatures to produce
halogen atom radicals that
rapidly react with radicals
produced in the combustion
chemical chain reaction
• Removing the chain-continuing radicals
interrupts the combustion process,
terminating the fire even when
sufficient heat, fuel and oxygen are
present.
Fire Behavior
Smoldering/Growth Stage:
Small amounts of visible
smoke may be visible at the
beginning of this stage as the
heat and mixture of fuel and
oxidizer reaches a point to
support active combustion,
initially a relatively slow rate.
As this oxidation reaction
continues, the temperature
rises, increasing the rate of
the oxidation reaction.
Fire Behavior
Smoldering/Growth Stage:
The heat can be transferred by
a number of means including
radiation, convection,
diffusion and conduction
Initially, the fire’s growth is
controlled by the properties of
the fuel but, as the fire grows,
it becomes controlled by the
amount of oxygen present,
often called ventilationcontrolled fire growth.
Fire Behavior
Full Development/Active Fire
Stage:
The critical combination of
all the necessary elements
must be in place to lead to a
raging fire.
• A flashover may take place
leading into this stage.
– the rapid transition to a fully
developed fire by the nearly
simultaneous ignition of all the
flammable materials within an
enclosed space.
– occurs when the surfaces in an
enclosed space are rapidly heated
Fire Behavior
Full Development/Active Fire
Stage:
The critical combination of
all the necessary elements
must be in place to lead to a
raging fire.
• A flashover may take place
leading into this stage.
– the large amounts of unburned
flammable gases that have
accumulated from the pyrolysis of
the surfaces
– http://www.youtube.com/watch?v=Qq
MVm72FMRk
Fire Behavior
Full Development/Active Fire
Stage:
The critical combination of
all the necessary elements
must be in place to lead to a
raging fire.
• A backdraft may take place
later in this stage.
– possible when a fire burns in an
enclosed space and uses up much
of its oxygen supply, starving the
fire
– the fire itself decreases even
though there is still plenty of
gaseous fuel.
Fire Behavior
Full Development/Active Fire
Stage:
The critical combination of
all the necessary elements
must be in place to lead to a
raging fire.
• A backdraft may take place later in this
stage.
– occurs, often explosively, when oxygen is
rapidly reintroduced to the accumulated
unburned fuel such as by opening a window
or from a roof collapse, leading to a sudden
increase of oxygen leading to a flash of
combustion
– http://abcnews.go.com/GMA/video/giantbackdraft-explosion-caught-on-tape18703524
Fire Behavior
Decay Stage:
the final stage of the fire
when either the fuel is
consumed or, due to
firefighting efforts, one or
more of the elements in the
fire tetrahedron have been
removed.
• After the active flames have been
extinguished, the fire may
continue to smolder for days.
Arson Investigation
• 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
Arson Investigation
Arson is the criminal act of
intentionally setting fire
without lawful consent
• Fire Pattern
• Debris
• Fire Growth and features
• Accelerants
• Other
Arson Investigation
Arson is the criminal act of
intentionally setting fire
without lawful consent
•
•
•
•
•
Fire Pattern
Debris
Fire Growth and features
Accelerants
Other
Arson Evidence
Physical evidence
• incendiary devices (including
matches, torches, lighter,
firearms, and others
• impression evidence (e.g.,
footprints, fingerprints, tool
marks, etc.)
• the location and pattern of
debris
Arson Evidence
Physical evidence
• the presence of any trace
evidence (e.g., hair, fibers,
clothing, chemical residues,
odors, etc.)
• any unusual structural
arrangements and burn patterns
Arson Evidence
Care must be taken to prevent any
contamination of the samples and
keep them in as close to the
original condition as possible
until they reach the laboratory.
– Accelerants are most often
trapped in porous materials, such
as wood, cloth, carpeting, and
similar.
– Collecting these types of samples
in suspected points of origin are
important as often the best source
of accelerants.
GC Analysis
• 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
GC Analysis
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).
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
• Low explosives - reactions that occur at
rates of less than 1,000 m/sec.
• Deflagrations - propagated mainly by a
flame front rather than a supersonic
shock wave .
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
Explosives: Energetic Materials
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)
Damage from High Explosives
• Shock Wave
• Heat
• Flying Debris
Damage from High Explosives
• Shock Wave
• Heat
• Flying Debris
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
Military Explosives: High Explosives
• RDX
(cyclotrimethlylenetrinitra
mine) 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
Spontaneous Combustion
• A natural heat-producing
process that may give rise to a
fire in the presence of
sufficient air & fuel
Dr. John Bentley
Mrs. M H Reeser
– 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