After a fire is put out and CSI technicians or fire investigators arrive to look over an arson crime scene, the first thing they want to know is where the fire started. They will look for clues to determine its origin.
Finding a fire's point of origin requires knowledge of how a fire moves through a structure. In general, fires spread sideways and up from the point of origin, but that pattern can change due to structural and decorative elements of the building. For example, stairwells may force a fire in one direction, and the chemicals in man-made carpet may cause abnormal burn patterns. Usually, the biggest amount of damage happens near the point of origin. CSI's look for evidence of igniters or accelerants as possible clues that suggest a point of origin.
Other factors than can influence or hinder CSI's' efforts in determining a point of origin are open windows, stairwells, or materials used to build or decorate the building.
After determining the point of origin, a CSI sometimes can retrace the fire's trajectory even when the building has undergone heavy structural damage. On the other hand, backtracking along the fire's path may yield the point of origin.
Searching for a V pattern in burned material is another way of finding the point of origin. It is the tendency of fire to rise and spread so that it burns a wall or other vertical surface in a V pattern, with the point of the V located at the origin of the fire.
Fuel containers and other flammable liquids also can not only hinder the search for the true point of origin but also with the locating of arson-related accelerants, simply because they are accelerants as well. Furthermore, an arsonist may have set various fires within a building or spilled a path of charcoal fluid or other accelerant throughout or around the structure, thus setting a fire with more than one point of origin.
By assessing the fire's effect on structural materials, CSI's can estimate the intensity of the fire at any particular place. Under extremely intense heat, steel beams buckle, and glass melts at around 1,500 degrees Fahrenheit. Crackling and flaking on floors and walls are indicative of places of high heat. Likewise, wooden beams, walls, and floors may scorch leaving an alligator skin-like pattern in the wake. Whenever this occurs, smaller scales tend to be closest to the hottest point of the fire.
If smoke detectors are present throughout a structure, the time at which each alarm was set off can help CSI's ascertain the path that the fire traversed throughout the building and locate the point of origin.
Liquid and volatile fuels pose special problems for arson investigators because they spread more quickly and take the shape of their containers. If the perpetrator of an arson sloshes paint thinner on a floor, the paint thinner spreads across the room, runs down the stairs, and oozes into the baseboards. Upon igniting the paint thinner, the fire follows the liquid and spreads right away, making the point of origin widely spread out.
Volatile fuels like methane gas diffuse in all directions until they fill their containers. Upon lighting up this fuel source, these containers can explode. Finding an exact point of origin in this scenario is impossible.
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