Fire investigation Flashcards
What is the seat of the fire?
Where the fire started
What can fire dogs be used to do?
Smell the accelerants used to start the fire and therefore identify the seat of the fire
What things are needed to start a fire?
o Oxygen which must be sufficient to sustain combustion. Air allows the fire to breath. This is the most common form of fire suppression
o Fuel source (fuel is a substance that can undergo combustion. Matter can exist in 3 physical states (solid, liquid and gas), a fuel with a high surface to mass ratio will ignite more easily than a fuel or a low surface to mass ratio.
o Ignition source (combustion of a solid or liquid fuel takes place when heat causes vapours or pyrolysis product to be released). Gaseous fuels burn only when the fuel and air mixture is within flammable limits. Materials vary in flammable limits and every product has their limit.
o Chemical chain reaction heat energy is given off during combustion, which in turn raises the temperature
What are the stages of fire development and tell me about each stage?
**o Ignition **
The blue around the fire is the liquid evaporating and the vapours igniting as the fire starts to burn
o Growth and fully developed
Flashover in a room is when the fire takes over the room
This is due to the ignition of an item in a room. The fire involving the initial item can produce a layer of hot smoke which spreads across the ceiling in the room. The hot buoyant smoke layer grows in depth, as it is bounded by the walls of the room. The radiated heat from this layer hears the surfaces if the directly exposed combustible materials in the room, causing them to give off flammable gases via pyrolysis. When temperature of the evolved gases become high enough, these gases will ignite throughout their extent causing flashover
o Decay/ deflagration
When all the fuel/oxygen in the room has been consumed/used by the fire and there is nothing left to burn
What can the radius of error be used to work out?
Where the ignition started?
Whats Backdraft?
* Backdraft is when a fire uses all the oxygen in the room and then if more oxygen becomes available it will reignite the fire. Generally, the smoke may go white as it runs out of oxygen then as more becomes available can turn black
o Definition: A sudden introduction of oxygen into a closed space containing superheated products which are not yet fully burned
How does a fire spread?
*** Convection: **this is defined as the transmission of heat within a liquid or gas and is due to their difference in density. Heated liquid or gas expands and becomes lighter, thereby becoming displaced by their heavier counterpart. When this happens, oxygen is drawn in, further inciting the chemical chain reactions. The rising gases, meanwhile, go up to fuel the upper floors. In an enclosed setting, such as confined office space, the movement of the fire will most likely be forcing the gases lower in height as the heated gases spread along the ceiling and walls. Superheating then occurs in the fire, thereby causing it to rise further, but this time, carrying with its products of incomplete combustion, such as embers.
* Conduction: Conduction is the transmission of heat through materials. When there is sufficient heat present, it may be enough to ignite fuel through other objects. Combustible materials, for examples, are most susceptible to heat transmissions.
* Radiation: this is the transmission of heat by waves travelling until heat is absorbed by other objects. An example of this would be a bar heater or open fireplace radiating onto a drying rack or curtain
*** Direct burning: **This is the simplest way to spread fire: direct application. A lit.
When first attend a fire scene what is an observation to be made when thinking about when to collect evidence?
o Look for area least damaged and then use this to find where it started by where the most damage is
o Then work out the radius of error to see the damage it has caused
What can we do as forensic scientists when attending an arson scene?
o Health and safety risk assessment before even entering the scene
o PPE on
o Record scene: Photograph, video, sketch, VR etc.
o Evidence recovery plan
o Check for post fire indicators i.e., black circle on the ceiling above where the fire started and the smoke rose up
o Search for seat of fire as will give indicators if some accelerants were used
o Check for forensic evidence
o Search for accelerants
o Search for DNA
o Search for fingerprints
o Search for items of intelligence value
o Try to establish if the fire was malicious or innocent/ accidental
Tell me about fire loading and ventilation
**o Fire loading- **The fire loading of a building or compartment is a way of establishing the potential severity of a hypothetical future fire
o Amount
o Distribution
o Orientation
o Types of fuels present
**o Ventilation **
o Condition and areas where ventilation may have been enhanced for any ready
o Burns where oxygen available
Whats the point of origin?
The exact physical location within the area of origin where a heat source and the fuel interact resulting in fire or explosion
Whats fire pattern analysis?
o A fire pattern is the visible or measurable physical changes of identifiable shapes form by a fire effect or group of direct effects
o The collection of fire scene data requires the recognition and identification of fire effects and fire patterns
What factors can be used in fire pattern analysis?
- plume patterns
- wood charring
- melting temperatures
- glass-impact fractures
- plaster
Whats meant by a plume pattern?
o Fire burning up and out
o Can get an inverted plume pattern i.e., burning newspapers
o Different shapes: triangle, rectangle, cone
Tell me about the fire pattern analysis in wood charring
What layers are most important to consider in excavation techniques?
