Why are arson investigations difficult to investigate?
the perpetrator has thoroughly planned the act; perpetrator is not present during the act; destruction is so extensive
Modus operandi
an offender's pattern of operation
Criminalist's function
limited to detecting and identifying relevant chemical materials collected at the scene and reconstructing and identifying igniter mechanisms
Gasoline or kerosene
can be used as an accelerant to start a fire; found in debris
Fire can have many accidental causes that leave no chemical traces:
faulty wiring; overheated electric motors; improperly cleaned and regulated heating systems; cigarettes
Fire is a type of...
oxidation
Oxidation
combination of oxygen with other substances to produce new substances
Energy
the ability or potential of a system or material to do work
Forms of energy
heat; electrical; mechanical; nuclear; light; chemical
The quantity of heat from chemical reactions comes from...
the breaking and formation of chemical bonds
All oxidation reactions, including combustion, are examples of reactions in which...
more energy is released than is required to break chemical bonds between atoms
Combustion
rapid combination of oxygen with another substance, accompanied by production of heat and light
Exothermic reaction
chemical transformation in which heat energy is released
Heat of combustion
heat liberated during combustion
To start fire, the _____ must be reached.
ignition temperature
Ignition temperature
the minimum temperature needed to spontaneously ignite fuel
The fire will continue to burn until either the ____ or ____ is exhausted.
oxygen; fuel
An additional factor needed to explain fire is...
the rate or speed at which the oxidation reaction takes place
Two factors we consider that influence the rate of oxidation:
the physical state of the fuel;
the temperature
Flash point
minimum temperature at which liquid fuel produces enough vapor to burn
Pyrolysis
temperature hot enough to decompose solid fuel into gaseous products
Basic ingredients of a flaming fire
oxygen; sufficient heat; gas fuel-air mix
Mixture of gaseous fuel and air burns only if...
its composition lies within certain limits
If fuel is too low (___) or to great (___), ________ does not occur.
lean; rich; combustion
Flammable range
entire range of possible gas or vapor fuel concentrations in air that are capable of burning
Glowing combustion or smoldering
combustion on the surface of a solid fuel in the absence of heat high enough to pyrolyze fuel
Spontaneous combustion
the result of a natural heat producing process in poorly ventilated containers or areas; very rare
To initiate and sustain combustion, the following is required:
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;
sufficient heat must be generated to sustain the fire
Conduction
the movement of heat through a solid object
Radiation
the transfer of heat energy by electromagnetic radiation
Convection
the transfer of heat energy by the movement of molecules within a liquid or gas
Most arsons are started with ______ based accelerants.
petroleum
Accelerant
any material used to start or sustain a fire
A common telltale sign of arson...
an irregularly shaped pattern on the floor or ground resulting from pouring accelerant on the surface (pour patterns)
Telltale signs of arson
evidence of separate and unconnected fires;
the use of "streamers' to spread the fire from one area to another;
evidence of severe burning found on the floor as opposed to the ceiling of a structure, due to a flammable liquid
If a flammable liquid was used, charring is expected to be ____ _____ on the _____ of furniture, shelves and other items rather than the ____.
more intense; bottom; top
Factors that contribute to the deviation of a fire from normal behavior:
prevailing drafts and winds;
secondary fires due to collapsing floors and roofs;
physical arrangement of the burning structure;
stairways and elevator shafts;
holes in the floor, wall or roof;
the effects of firefighters in suppressing the fire
The search for traces of flammable liquid residues may be aided by the use of a...
sensitive portable vapor detector or "'sniffer
When arson is suspected, ___ to ___ quarts of ash and soot debris must be collected at the point of origin.
two; three
Samples should be packaged immediately in _____ _____ so that residues are not lost due to evaporation.
airtight containers
_____ _____ should not be used because they react to hydrocarbons.
Plastic bags
Substrate control
a sampling of similar but uncontaminated control specimens
Devices arsonists use to start a fire:
burning cigarettes; firearms; ammunition; mechanical match striker; electrical sparking devices; Molotov cocktail
One important piece of evidence is...
the clothing of the suspected perpetrator
Each item of clothing should be placed in...
a separate air tight container