Wood Char Analysis

Wood char depths are one of many fire indicators that fire investigators utilize for determining the area of origin and cause of a fire. It was formally believed that the deeper the char the longer the fire had burned but this has since been proven to be false. Rapid fire spread is common place due to the synthetic materials used in modern furnishings found in most homes. Most firefighters can expect to encounter flashover or post-flashover conditions at the time of extinguishment because of modern furniture as compared to furnishings from forty years ago, which were natural materials such as wood, cotton, and wool. The depth of char is still a useful tool for investigators to utilize and document as it can be used to show fire patterns, although
Wood chars quickly at first and then at a much slower rate owing various factors such as the insulation effect of the new char, reduced air-fuel interactions, ventilation, heat transfer, and physical arrangement (Icove et al., 2013, p. 120). Different char rates can be caused by different species of wood, coatings, and flame retardant treatments so char depths should be compared on similar types of wood. In a controlled test, 55 pine wood 2 in. x 4 in., which are most common in residential type 5 construction, were exposed to a constant- temperature of 932 °F which produced a constant rate of charring of 0.45 mm/min (Lau, White, and Van Zeeland, 1999). This rate was determined in a controlled environment which is drastically different than real world fire events that will have numerous influences to affect char depths. These differences make utilizing char depths to determine how long a fire has burned invalid. The analogy could be made stating that holding a match to a piece of wood will result in the same damage as a blowtorch held for the same amount of time. A low intensity, long duration fire event could produce similar charring as a high intensity, short duration fire event (Herrera, 2003).
Investigators utilize a penetrating tool to measure char depth, it looks very similar to a tread depth gauge used to measure tire depths. Once on scene of a fire, investigators will create a survey grid of a room and char depth is measured and documented. The measurements are converted to lines of equal depth and each line connects depths of the same value. This diagram begins to paint the picture of the fire’s behavior and assist investigators in determining the most probably area in which the fire