Forensic Fire investigation

Transcription

1 Forensic Fire investigation

2 Arson? When investigating a fire one of the questions asked is, Was this arson? This question is difficult to answer based on evidence present at a fire, which is very little. Catching Killers: The Science of Staging a Fire

3 How do fires begin? What three elements are necessary for a fire to be sustained? Fuel Heat Oxygen

4 When these 3 elements combine

5 Chemical reactions Fire is an reaction This means it releases more heat than is required to break the chemical bonds contained within it s elements Fire is also an reaction This means it combines oxygen with other substances to produce new products

6 Energy Barriers Some oxidation reactions, like oxidizing steel, require very low amounts of energy, or heat, input to start them For fire there is a much higher energy barrier that needs to be overcome for ignition to actually occur

7 Speed of reaction There are two factors that affect the speed at which a fire reacts 1. Physical State of the fuel The fuel used can only support production of a flame when it is in the gaseous state. Only in this state can molecules collide fast enough to actually support a fire. This is also true for solid fuels like wood, paper, or cloth and liquid fuels such as gasoline or kerosene.

8 How does a liquid or solid maintain a gaseous reaction 2. Fuel Temperature- the fuel must be at a high enough temperature to vaporize and mix with oxygen to combust. This is easily achieved with a liquid fuel, but is more complex in a solid fuel such as wood. A solid fuel burns only when exposed to temperatures high enough to decompose the solid into gaseous products. This process is called pyrolysis

9 Flash point video- 1:30

10 Fuel-Air mixture Any given fuel has a certain range through which the mixture of oxygen and fuel must be in order to promote combustion. If there is too little or too much fuel in the air combustion will not occur. The range through which combustion will occur is called the flammable range As an example the flammable range for gasoline is % fuel.

11 Spontaneous Combustion This is a fire caused by a natural heatproducing process in the presence of sufficient air and fuel This reasoning for a fire is often used as an excuse by arsonists, but rarely occurs in normal conditions.

12 Spontaneous Combustion

13 Heat Transfer In a fire Heat is transferred from hot areas to cooler areas by three methods: Conduction Convection Radiation

14 Conduction This is the process of the movement of heat through solid objects It is possible for heat to move through metal objects, such as nails and fasteners, to other parts of a structure.

15 Convection This is the transfer of heat energy by movement of molecules within a liquid or gas In a fire the hot gasses move upward heating other portions of the structure. This can cause other portions of the structure to pyrolyse which can cause the fuels to simultaneously ignite in a phenomenon known as flashover

16 Radiation This is the transfer of heat energy from a heated surface to a cooler surface by electromagnetic radiation A hot surface emits electromagnetic radiation of various wavelengths, and in a fire that radiation moves in a straight line from one surface to another All surfaces that face a fire are exposed to radiant heat and will burst into flames once they reach their ignition temperature

17 Investigating Fire

18 Locating the Origin Due to the tendency for heat to move upward the origin of the fire often occurs at or near the lowest point with burn damage If a flammable liquid (accelerant) was used charring is expected to be more intense on the bottom of shelves and furniture. A trail from an accelerant can sometimes be seen if an arsonist wanted the fire to move more quickly from one room to another.

19 Searching for Accelerants Most of the time an accelerant will not be entirely consumed in a fire Some of it will often seep into porous surfaces and remain behind to be detected Any questioned evidence (evidence that may contain an accelerant) is quickly packaged in an airtight container to be processed at a crime lab.

20 Other Evidence When an investigator collects evidence they believe contains an accelerant; they must also collect a control sample. This is a sample of the same substrate collected from somewhere else in the room. The investigator should also look for possible igniters of the fire which may include: Match Firearms Ammunition Electrical sparking devices Molotov cocktail

21 Analysis of Flammable Residues The Gas Chromatograph is considered to be the most sensible and reliable instrument for detecting and characterizing flammable residues This is done by changing any volatile residues to gas with heat in an airtight container then collecting and processing the gas.