Fire Safety Science Digital Archive

The degree of darkening of wood surfaces has been used to estimate the amount of heat received by a surface in circumstances where many rough measurements obtained with little effort are preferable to a few highly accurate measurements. Griffiths and Heselden have calibrated blocks of wood for this purpose. It is of interest to explore the use of the rate of darkening as a measure of the incident heat flux. Accordingly, the results obtained by Griffiths and Heselden have been re-examined. It is shown that the darkening follows a first order law and time constants are obtained for several incident flux densities. An activation energy is obtained from the variation of the time constant with the surface temperature of the wood and an equation is derived giving the intensity of irradiation as a function of the degree of darkening and the time of exposure. It is shown that the rate of darkening, as measured by the reflectivity of light from a tungsten filament lamp, has rate constants very similar to that governing the rate of pyrolysis.