What are the different ways that we can heat things up?
- Light
- Electricity
- Friction
- Contact with something hot
- Chemical reaction
- Nuclear fission
How do you achieve ignition?
The fuel (even if its just a small part of it) needs to have its temperature raised to its ignition temperature
How does radiation/light cause fires?
- Fuel must be able to absorb more heat than it can dissipate
- Radient heat from a fire can induce another fire (flashover)
- Sunlight can be focused by lenses or mirror if positioned just right
How does electricity cause fires?
- Sparks and arcs involves the movement of current through a gas, producing plasma
- Very localised so only ignites gases or vapours
- The build up of charge is big enough to ionise the gas between into a plasma
What is the difference between sparks and arcs?
- Sparks are shorter and smaller
- Arcs are longer and maintained
How does electricity cause fires in regards to overheating?
- Electrical currents generate heat when they meet resistance
- The amount of current that a wire can carry is proportional to its diameter, material and covering
What causes overheating in wires?
- Excessive current (wrong fuse, short circuit)
- Tightly coiled wires (which cant dissipate heat)
- Poor or loose connection
- Insulation breakdown (charred wood/plastic becomes a semi-conductor)
- Aluminium fitting (old) - not as good as a conductor like copper
How does friction cause a flame in lighters?
- Friction from striking the ‘flint’ generates hot shavings (mechanical sparks)
- This process is hot enough to auto-ignite the cerium in the flint
- These sparks are formed near a fuel which comes out when your thumb touches a button
- This can also occur in poorly lubricated train bearings
What is spontaneous combustion?
When some substances have naturally occuring exothermic reactions
How is an equilibrium temperature reached?
- Heat is lost from the surface of a material
- But the interior temperature continues to rise
- This accelerates the reaction but as this happens heat gets lost more rapidly
How does the size of an object affect a chemical reaction?
- In a large quantity of material, the surface area is relatively small
- Therefore there is more material to heat up and so it takes longer to reach the auto-ignition temperature
- The equilibrium temp is higher
What naturally occuring chemical reactions can mimic arson attacks?
- Sawdust, coal dust, hay, straw and landfill waste will get broken down by microbes and will exothermically generate heat
- Unsaturated (drying) oils that are soaked up in rags and papers will oxidise in air exothermically and then become fuel
- Some reactive species oxidise in air (K, Na metal, group 1)
Why do accidental fires need evidence of origin and faults?
Arsonists may disguise an intentional fire by setting it under an oven/BBQ to make them look accidental
What are examples of that can start gas flame fires?
- Open flames
- Cooking oils
- Bunsen burners
- Portable gas stoves
- Grease
- Towels
- Paper
- Clothing
Where will you find hot and burning fragments/sparks?
- Rubbish
- Bonfires
- Lanterns
What are hot and burning fragments/sparks?
Bits of wood, cardboard, paper, which can travel whist flaming
What affects the distanced travelled by sparks?
- Material
- Size
- The wind
What is the issue with sparks?
- Can initiate secondary combustions because of burning fragments
- So cannot limit area of search to where the fire was found because the wind may have taken a spark from another fire
Why is it often unlikely that a cigarette caused a fire?
They have a low heat release rate and therefore need direct contact with a flammable material/fuel for a long time to start a fire
General aspects at a fire scene
- Anything flammable at the scene will be destroyed
- Evidence might have been destroyed
- Materials and surfaces that are far from the scene may have been affected by soot/debris or by the heat - need large cordons
- Fire fighting techniques might cause physical damage and damage to evdience
- Post fire clean up can destroy evidence
What fire-fighting techniques can cause physical damage?
- Doors and windows broken
- Ceilings collapse under weight of water
- Objects disturbed or removed
What does it mean if materials and surfaces far away from the intial scene have been affected by the fire?
- Affects ability to recover evidence
- Cordons need to be set carefully and ideally as large as possible
- DNA and fingerprints might be hard to find
Why might insurance investigators and loss adjusters be involved in an investigation?
- Fraud
- Financial fraud to profit from building insurance
- Financial fraud to profit from contents insurance
- Intentional fires to destory business records of tax fraud
What are some physical evidence that you should look for in a fire scene?
- Burn patterns - the fire will burn a certain way because it follows the rules of chemistry and physics
- Smoke records
- Temperature indication - spalling (surface flaking) of plaster
- Debris layer sequences - you can tell what fell first in a fire by looking at the order they’re lying on top of each other
- Remains of ignition devices or suspicious containers
