A fire investigation is an unenviable task. The devastation, charred debris, collapsed structures, water soaked ashes, together with the smoke and stench, makes the task uninviting and seemingly impossible. The basic role of an investigator at a fire scene is twofold: firstly to determine the origin of the fire (the site where the fire began), and secondly to examine closely the site of origin to try and determine what it was that caused a fire to start at or around that location. An examination would typically begin by trying to gain an overall impression of the site and the fire damage; this could be done at ground level or from an elevated position. From this, one might proceed to an examination of the materials present, the fuel load, and the state of the debris at various places.
Fire debris is a general term used to define the materials collected from a fire scene for laboratory examination. When a fire investigator suspects that a fire might have been deliberately set using accelerants such as ignitable liquids, it is possible to collect and analyze fire debris to see if such products are present.Combustion requires three elements — heat, oxygen, and fuel. Fire will be extinguished when any one of these three elements is absent. Fire does not burn solids or liquids (in general), but rather the gases formed above them. Heat acts to vaporize the liquid or solid, converting it to a gas which then combines with oxygen to “burn” above the liquid pool. Thus, when flammable liquids soak into material or run into “cracks” there will be insufficient oxygen to support combustion. In these cases residue of ignitable liquids can be collected.