Fire investigation Flashcards
(27 cards)
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
How can melting temperatures be used in fire pattern analysis?
o Can be used to accurately state the temperature reached in a fire at a specific location/ hight of fire
o May give indication of direction of heat travel
How can glass and impact fractures help with fire pattern analysis?
o Radial cracks form on impact
o Heat from fire creates smooth cracks
o The cracks spreads as the glass ‘bows’
o The glass shatters as the impact side is placed under compression and the non-impact side is placed under tension. Tension is also generated where the edges of the pane are ‘held’ by the frame
o As the glass shatters the compression is released and the tensioned portions try and pull the pane back into shape, scattering some glass fragments back towards the impact side
o The upper edge shows the fragmentation which occurs as the glass is ‘stretched’ under tension on the non-impact side, and the ‘hackle’ marks or striations show the characteristic curvature curving upward away from the impact side
o The edges will be polished of the broken glass
o Crazed glazing due to rapid cooling by water
**
* Glass- what to look for **
o Impact damage?
o Heat damage?
o Smoke stained
Inside
Both
Clean burn
Outside (Can get backward fragmentation if purposefully break glass)
Broken glass
Distance travelled (force)
Trajectory
*** Glass-light bulb **
o The light bulb will melt on the side the fire is
How is plaster used in fire analysis?
o Plaster removed to varying degrees
o Some cracks in the plaster as you move further away from the point of origin, where the fire started
What are some identifiable marks to look for in fire analysis?
- Pool burning
- Irregular patterns
- Protection marks- tell us what was present during the fire and may indicate being taken or melted away etc.
What layers are most important to consider in excavation techniques?
How should excavation of a fire scene be carried out?
o Archaeological style
o Teamwork required
o Potential evidence and items of interest must be identified, photographed, and recorded before removal
o Debris removed may need to be carefully examines, sieves or searched with a magnet
o Stop and re-ass frequently
Notes:
Working hypothesis may change in light of new evidence
Do not ignore inconvenient evidence or facts
Make sure all team members are aware of progress and any change
Scene reconstruction is a useful tool
What samples should I take?
- Sample of debris from seat of fire- if identified
- If carpet, cut a piece out from seat of fire
- Loose debris- about two shovels full
- If burned item, recover whole
- If fluids, recover into sterile glass or metal container and package into nylon bag
- If fragile i.e., burned paper, package into box and then nylon bag
What are some control samples?
- A sample of unburned carpet if possible
- A nylon bag control
What are nylon bags used for?
- To package evidence into
- Do not use sticky tape to seal nylon bags, swan neck and then tie with strong
