712 Fire Spread Indicators Sequential Analysis

The point of origin of a fire can also be found by simply following in reverse order the trail of fire damage from where it ends to where it began. Where several such trails converge, that is the point of origin of the fire. In essence, the method involves determining what burned last, what burned next to last, etc. until the first thing that burned is found.

In the same way that a hunting guide interprets signs and markers to follow a trail of game, a fire investigator looks for signs and markers that may lead to the point of origin. For example, a fast, very hot burn will produce shiny wood charring with large alligatoring. A cooler, slower fire will produce alligatoring with smaller spacing and a duller-appearing char. Fire breakthrough or breaching of a one-hour rated firewall will likely take longer to occur than fire breakthrough of a stud and wood-paneled wall.

As noted, wood char and alligatoring patterns are useful indicators. The reader may recall that commercial wood is about 12% water by weight. As heat impinges on a piece of wood, the water in the surface material will evaporate and escape from the wood. The rapid loss of the water at the surface is also accompanied by a rapid loss of volume, the volume formerly occupied by the water. The wood surface then is in tension as the loss of water causes the wood to shrink. This is the reason why wood checks or cracks when exposed to high heat or simply dries out over time.

Of course, if the heat is very hot, more of the water "cooks" out, and the cracking or alligatoring is more severe. When the heat is quickly applied and then stopped, there is only sufficient time for the surface to be affected. When the heat is applied for a long time, there is sufficient time for the wood to be affected to greater depths.

Other indicators of temperature and fire spread include paint, finishes, coatings, and the condition of various materials (e.g., melted, charred, burned, warped, softened, oxidized, annealed, etc.).

For example, the paint finish on a furnace is a valuable indicator of the temperature distribution on the furnace. As the temperature increases to perhaps 250 to 400°F, the first thing to occur is discoloration of the paint. As the temperature increases over about 350 to 400°F, the paint will bubble and peel off, exposing the underlying primer. As the temperature increases again to more than perhaps 400 to 450°F, the primer will come off, and the undercoat, often zinc, will be exposed and oxidized. And as the temperature increases still more, perhaps beyond 786°F, the undercoat will melt away leaving only the bare steel, which itself oxidizes.

It is often easier to visually determine the hottest point on the furnace or appliance several days after the fire. The areas where all the paint, primer, and galvanized undercoating have been removed by high heat exposure will be bright red where the bare sheet steel has rusted.

Another example is metal ventilation ductwork. Metal ventilation ductwork is often made of galvanized steel, that is, steel with a thin exterior coating of zinc. The galvanization, which is normally shiny, will first dull and darken upon exposure to heat. When temperatures get above 500°F, the zinc will begin to oxidize significantly and will become whitish. As the zinc is heated past 500°F, it will whiten more and more. However, when the temperature approaches 786°F, the unoxidized zinc will melt and slough off, leaving bare steel exposed. (The zinc oxide itself will not actually melt until a temperature of about 3600°F, but it usually sloughs off with the unoxidized zinc underneath it.) Thus, the hottest spots on ventilation ductwork are also the red spots, where the exposed steel has oxidized to rust.

The interpretations of many such markers, "V" patterns, etc. are then combined into a logical construct of the fire path. One of the favorite tests used in the sequential analysis method is the question: "Which is burned more, the material on this side or that side?" The answer to this question then supplies a directional vector for the fire spread, and the "vector" is backtracked to another position where the question is again posed. In a sense, following a trail of indicators is like playing "Twenty Questions."

Plate 7.4 Lawnmower was stored next to furnace in garage. Furnace pilot ignited fumes from spilled gas around mower fuel tank.

In order to avoid a "false" trail due to fall down, it is common to backtrack several fire trails from finish to start. When several such trails independently converge to a common point of origin, the confidence in the answer is greatly increased.

The advantage to the sequential method is that no special assumptions need to be made concerning structural homogeneity. The disadvantages are twofold. First, it relies upon the individual skill and knowledge of the investigator to find and properly interpret the markers. Not all fire investigators have the same knowledge about materials, fire chemistry, heat transfer, etc. One fire investigator may spot an important marker that another also saw, but ignored.

Secondly, it assumes that sufficient markers are present to diagnose the fire, and can be found. This is not always true. Sometimes the severity of the fire or the fire-fighting activities themselves destroy significant markers and indicators. Also, sometimes the markers may be present but are lost in the jumble of debris. Thus, there are gaps in the evidence, and the resulting sequential analysis is discontinuous.

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