Forensic Aspects of Arson and Explosion Investigation

1. Forensic Aspects of Arson and Explosion Investigation

2. The Chemistry of Fire Fire is a transformation process during which oxygen is united with some other substance to produce noticeable quantities of heat and light.

3. The fundamental chemical reaction of fire is oxidation. This is the combination of oxygen with other substances to produce new substances. CH4 + 2O2 CO2 + 2 H2O

4. In all oxidation reactions more energy is given off than is required to break the chemical bonds present. The excess energy is given off as heat and often light.

5. This excess energy is known as the heat of combustion. These are said to be exothermic reactions (heat is given off).

6. Before the reaction can take place an energy barrier must be hurdled. The higher the barrier the more energy is required to start the reaction. Usually this energy source is heat.

7. For substances like gasoline and methane the hurdle is quite high so a high temperature must be applied to begin the oxidation process.

8. Before any fire can result the temperature of these fuels must be raised to a value higher than the energy barrier. This value is known as the ignition temperature.

9. FUEL --- IGNITION TEMP. F Fuel oil ------- 495 Gasoline ----- 536 Turpentine -- 488 Kerosene ---- 410 Methane -----1076 Propane ----- 842 Wood -------- 572

10. Once combustion begins enough heat is given off to keep the reaction going by itself. The fire will continue to burn until either the supply of oxygen or the supply of fuel is exhausted.

11. Possible sources of ignition include electrical discharge, sparks, chemicals.

12. A flaming fire can only be sustained when fuel is in a gaseous state. So how does a liquid or solid maintain a fire?

13. For a liquid the temp must be high enough to vaporize it. The vapor burns when it mixes with oxygen and combusts as a flame.

14. The flashpoint is the lowest temperature at which a liquid gives off enough vapor to form a mixture with air that will support combustion.

15. Once the flashpoint is reached the fuel can be ignited by an outside source. Gasoline has a flashpoint of – 50 F but an ignition temp of 494 F.

16. With a solid it is harder to generate a vapor. The solid fuel will only burn when it is exposed to heat hot enough to decompose the solid into a gas or vapor.

17. This breakdown is known as pyrolysis. A match or other source of heat initiates the pyrolysis of the solid fuel. The gaseous products react with oxygen in the air to produce heat and light.

18. This in turn is used to pyrolyze more solid fuel into gas. • Although a flame will be supported only by a gaseous fuel, in some instances a fuel can burn without the presence of a flame.

19. Ex. Charcoal embers. This is an example of glowing combustion or smoldering.

20. Wood will burn with a flame until all of its pyrolyzable components have been expended. Then the carbonaceous residue will continue to smolder long after the flame has extinguished.

21. The chemical reaction will occur faster when the temperature is increased. For most reactions a 10 C (18 F) increase in temp will double or triple the reaction rate.

22. This increases the temp even more which increases the reaction rate which increases the temp which increases the reaction rate which increases the temp which increases the reaction rate which increases the temp …

23. Spontaneous Combustion – very rarely accounts for the cause of a fire. • Spontaneous combustion is the result of a natural heat-producing process in poorly ventilated containers or areas.

24. Examples include hay barns and rags soaked in certain chemicals such as linseed oil.

25. Oxidizing agents – explosives are substances that undergo a rapid exothermic oxidation reaction with the production of large quantities of gases.

26. It is the sudden build up of gas pressure that constitutes the nature of an explosion. Detonation occurs so rapidly that oxygen in the air cannot take part in the reaction.

27. So, many of explosives must have their own source of oxygen. Chemicals that supply oxygen are known as oxidizing agents.

28. Example – black powder • 75% potassium nitrate (KNO3) • 15% charcoal • 10% sulfur

29. The oxygen in the KNO3 acts as the oxidizing agent. • Nitroglycerin, the main ingredient in dynamite combines carbon, hydrogen, nitrogen, and oxygen.

30. When it detonates large quantities of energy are released as the molecule breaks down and oxygen interacts with the other elements.

31. Remember! 3 requirements must be met for combustion to occur.

32. 1. A fuel must be present.

33. 2. Oxygen must be present in sufficient amounts to combine with the fuel.

34. 3. Heat must be supplied to begin the combustion and sustain it.

35. SEARCHING THE SCENE • Time is of the essence. Because most arsons are started with petroleum-based accelerants such as gasoline or kerosene any remaining residue may evaporate very quickly.

36. The necessity to begin an immediate investigation of the circumstances surrounding a fire even takes precedence over the requirement to obtain a search warrant to enter and search a premises.

37. The search of the fire scene must focus on finding the fire’s origin. This is the area most likely to provide evidence of an accelerant or ignition device.

38. Investigators look for signs such as separate and unconnected fires, the use of streamers to spread the fire from one area to another, the presence of containers capable of holding an accelerant.

39. They also look for evidence such as an ignition device, signs of breaking and entering, theft, and begin to interview witnesses.

40. Once located, the point of origin should be protected. Nothing must be moved or touched before notes, sketches, and photographs are taken.

41. The most common materials used by arsonists to ensure the rapid spread and intensity of a fire are liquid petroleum products such as gasoline and kerosene.

42. Only under ideal circumstances will combustible liquids be entirely consumed during a fire. Some enters cracks in the floor, carpets, plaster, etc.

43. Water used to extinguish the fire can help slow down the evaporation of volatile liquids. Water will not interfere with lab tests used to detect accelerants.

44. Fire investigators look for traces of flammable liquid residue with the aid of highly sensitive portable vapor detectors also called “sniffers”.

45. These devices detect volatile residue by inhaling air at the scene. The air is then passed over a heated filament. If a combustible vapor is present in the air it is oxidized and increases the temperature of the filament.

46. These do not provide conclusive identification but are very helpful as screening devices. • Dogs are also trained to detect the odor of hydrocarbon accelerants.

47. COLLECTION AND PRESERVATION OF ARSON EVIDENCE. • As a general rule 2 to 3 quarts of ash and soot should be collected at the point of origin.

48. Specimens must be immediately packaged in an airtight container as no loss of residue can occur through evaporation.

49. Fluids found in open bottles or cans must also be collected and sealed. • A search must also try to locate an ignitor. These include things such as matches, cigarettes, electrical sparking devices or Molitov cocktails.

50. Another key piece of evidence is the clothing worn by a suspected arsonist if they are arrested within a few hours of the crime.