Fire Safety Science Digital Archive

Vertical pieces of wood about 2.5 cm thick and 12-15 cm square have been exposed in normal ambient surroundings to the radiation from a hot source and weighed while charring. The rates of weight loss for various degrees of charring have been measured for woods of different density and permeability. The effective permeability has been altered by various devices such as sealing the edges of the wood specimens and inserting an impermeable layer between two pieces of half thickness. The rate of weight loss increases with the intensity of radiation, the density and the actual or effective permeability. Provided the charring is not too shallow the results can be interpreted in terms of a mass transfer theory in which 'Q', the amount of heat required to produce 1 gm loss of weight, is independent of the heating rate. In the absence of measurements of surface temperatures the calculations are perforce somewhat crude but the order of magnitude of Q (assuming the specific heat of the volatiles is 0.5 cal g^-1C^-1) is about 1300 cal/g for woods of low permeability and about 550 for some woods of high permeability when the weight loss is about 10 per cent (about 4 mm char). The rate of weight loss decreases and Q increases as charring proceeds. The variation of weight loss with time and least squares formulae relating the rate of weight loss at particular times to density, incident radiation and permeability are given for various experimental conditions.