Arson and Explosions

fire

The rapid oxidation of substances by combustion that includes the release of heat & light.

(Release of energy = exothermic)

arson

Criminal act of intentionally setting fire without lawful consent.

heat of combustion

the amount of heat from a chemical reaction that comes from the breaking and formation of chemical bonds

flash point

The minimum temperature a liquid fuel will produce enough vapor to burn

air-fuel ratio

Relative amounts of oxygen and fuel needed in the gaseous mixture for combustion.

class A

Ordinary Combustibles:  Wood, paper, cardboard, trash, cloth

class B

Flammable liquids/Gaseous fuels: Gasoline,oil, butane, propane, natural gas

class C

Live electrical: Short circuits, overheated electrical cables

class D

Combustible metals & alloys: Iron, Aluminum, sodium, magnesium

class K

Cooking media (vegetable & animal fats and oils)

Can’t use water on these fires

Fire Tetrahedron

the fire triangle but also including the chain reaction that continues until something has been depleted.

heat suppression

use water

oxidant suppression

Carbon dioxide, sodium bicarbonate (must be careful if chemicals are dangerous to humans)

fuel suppression

Cut-off fuel supply or flow (gas leak)

chair reaction suppression

Stop the reaction between hydrogen radicals of the combustion reaction (use chemicals that slow the H molecules down)

Incipient

Combustible gases vaporize

smoldering/growth

Low temp. that is increasing; doesn’t produce a visible flame

full development/active

Causes flashback & backdraft.

decay

One/more component of fire is no longer available.

variables that affect HRR

• Size of room

• Type of fuel

• Moisture content of fuels

• Surface area available for combustion

• Orientation of burning material (Laying flat, standing upright)

• How efficient materials burn

• Method of heat transfer

accelerants

Compounds that facilitate the initiation and /or growth of a fire, but do not necessarily serve as a fuel.

This causes fires to start faster, burn hotter, and consume fuel in a different pattern than a naturally occurring fire.

Any deviation from a naturally behaving fire suggests..

suggests arson and/or the use of an accelerant.

burn patterns

*A linear burn pattern indicates an accelerant was poured on the floor.

*An ‘inverted cone’ is the burn shape found on a wall.

-Wide cone = A slow burn (no accel.)

-Narrow cone = A fast burn (accel.)

*Burn patterns on/under the floor indicates an accelerant was poured on the floor to burn.

Portable Vapor Detector (Sniffer) & Sniffer Dogs

used to detect accelerants/impurities at a fire scene, preventing humans from inhaling toxic fumes.

• Used as a screening device

• Reaches areas that humans can’t reach.

• Safer for humans.

• Have high sensitivity to vapor detection

why is photography important?

to preserve evidence due to the changing environment of the scene (renovations, etc)

plumes…

show the path and spread of the fire

they’re most important when determining the point of origin

Why Investigate a Fire?

1) Determine the cause of the fire

2) Determine if the fire arose from ‘natural’ or accidental cause OR unlawful actions

Why investigate a fire quickly?

Fire-related evidence could be transient & short lived (accelerants tend to evaporate quickly as well as flow with water used to put the fire out)

Clues that suggest arson

Multiple points of origin, presence of accelerants, unusual burn patterns, quantity/color of smoke, changes in fire-suppression system or ventilation.

What’s the most important consideration of a fire/arson investigation?

the safety of the responders & the individuals at the location.

When are warrants needed?

not during an active fire because it is an “emergency situation. BUT they are needed when a more excessive search is needed after fire suppression.

GC/MS (gass chromatography/mass spectrometry)

Separates & identifies the components of the explosive

ex- gasoline, kerosene, etc.

How are accelerants collected from a piece of evidence?

Accelerants are volatile chemicals and usually evaporate easily.

1. Heat the sample to convert accelerants to a gas

2. Use charcoal to capture volatile vapors

3. SPME  - Silica fiber absorbs vapors

4. Rinse with a solvent to extract (usually alcohol or acetone)

explosion

A combustion reaction that produces light, heat and a rapid expansion of gases

propellant

the compound designed and used to optimize the production of the pressurized gas.  These energetic materials release more energy in a much shorter time interval than that of a flammable material

flammable materials

ignite immediately when introduced to a flame

energetic materials

a class of materials that have a high amount of stored energy capable of being released.

Ex. explosives, gun-powders, rocket fuels

physical explosion

The rapid release of gas from highly pressurized container or tank.  There isn’t a great deal of heat or light – mostly kinetic energy.

Examples – A damaged appliance that builds up pressure (a water tank, gas tank, cooking appliance)

chemical explosion

This contains a fuel source, oxidizer and initiator that provides energy to start the reaction.

low explosives

Flammable fuels where the oxidant is separate; The rate of gas release is less than 1000 m/s

Examples: Black powder, flares, smokeless powder

deflagration

damaged caused by shrapnel, fire, heat

high explosives

Rapid chemical combustion that results in gases escaping at speeds greater than 1000 m/s

ex- primary and secondary explosions

detonation

damage caused by shockwave

detonation

Results in a supersonic pressure wave;  High pressure gases that can cause damage to structures as well as humans. (shock waves faster than the speed of sound, 343 m/sec)

deflagration

A slower/weaker effect than a shockwave.  Described as a ‘pushing’ effect as opposed to a ‘shattering’ effect.

primary explosives

 Explosives that are sensitive to heat, flame, friction & shock;  The detonate easily/quickly

Lead azide, lead styphnate, tetrazene, diazodinitrophenol. (lots of lead)

secondary explosives

Explosives that are more stable to heat, shock, friction, etc.  Do not detonate as easily or quickly.

Examples:TNT, dynamite, RDX, PETN, HMX, ANFO

damage from high explosives come from…

The shockwave

-This is rapidly moving compressed gas that travels outward from the explosive.

-Shockwave produces 100 psi of pressure (Approx. 50 psi is fatal to humans)

-Injuries from the shockwave are primary injuries

damage from low explosives come from…

*Fire, heat

*Flying debris (shrapnel) - Injuries from shrapnel are secondary injuries.

What type of evidence is collected from the site of an explosion?

Soil, debris/fragments of the bomb, swab items for explosive chemical