79 Ignition Sources

When the centroid method has been completed, and the point or area of fire origination has been determined, that area must then be examined and inventoried for potential ignition energy sources. Such energy sources commonly include pilot lights, space heaters, electrical appliances, fluorescent light fixtures, fireplaces, chimneys, smoking materials, cooking equipment, lamps, electrical wiring and outlets, and so on.

Occasionally, the ignition of combustibles can be spontaneous, but this usually involves organic oils or materials that can readily undergo decomposition. It is commonly assumed that a pile of oily rags can catch on fire by itself. This is not true when the oil is a petroleum-based product. In fact, it is only true when the oil comes from an organic source, like linseed oil, tung oil, spike oil, and the like. These organic oils contain large amounts of organic acids that can react with air at room temperature. Motor oils and lubricants generally do not ignite spontaneously and are very stable.

Decaying fecal matter covered with straw and without ready access to air can spontaneously ignite. Also, wet hay or moist vegetative material can also spontaneously ignite when stored in bulk. However, spontaneous combustion is a multistep process initially involving bacterial decay. It typically occurs in barns, feed silos, or animal pens that have not been cleaned out in a while. The occurrence of such spontaneous combustion in one- or two-family dwellings is relatively rare compared to other causes.

The flaring up of smolders is often mistaken for spontaneous combustion. In a smolder, the combustible materials are sometimes starved for air, and thus the combustion reaction proceeds slowly. This effect usually occurs in a woven or porous material.

However, some types of smolders are simply a function of the material. Despite sufficient air, the rate of combustion for some materials proceeds very slowly. This is typical of some plastics and manmade organic materials and fibers.

In general, a smolder is any type of combustion process where the leading edge of the combustion zone moves only about 1 to 5 cm per hour. Because of this, a smolder produces modest heat and smoke, and is hard to detect when in progress. Smolders can be dangerous not only because of their obvious fire hazard, but also because while going undetected, they can release toxic gases such as carbon monoxide or even cyanide, which can slowly poison the atmosphere of an occupied room.

When an oxygen-starved smolder reaches a place where there is ample air, it may burst into flames. Similarly, when the leading edge of fire in a smoldering material reaches a more readily flammable material, it also may suddenly flare up. In both cases, the resulting fire may appear to have spontaneously combusted where the smolder finally burst into flames.

Some materials when heated to relatively low temperatures, decompose and then react to produce more heat, which eventually causes ignition of the material. This effect is also mistaken for spontaneous combustion. Polyure-

thane foam is a type of material that exhibits this behavior. When heated, perhaps by hot wiring or a hot flue pipe, the polyurethane foam undergoes chemical decomposition. The chemicals present after decomposition occurs, and react with one another and give off heat. If the heat cannot escape, it may accumulate sufficiently to ignite the rest of the foam.

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