The effect of varying the moisture content on bath the pilot and
spontaneous ignition times of different woods of areas 7.6 cm square and 15 cm
square has been measured over a wide range of intensities of radiation. Moisture
increases the energy required for ignition; it also increases the minimum
intensity for ignition though with pilot ignition its effect is only marked for
moisture contents above 40 per cent.
Results have been correlated on the assumption that the material is inert
and ignites at a fixed temperature. Simple heat transfer theory has been used
to calculate this temperature with values for the thermal properties appropriate
to the given moisture content and with allowance made within the term for thermal
capacity for removing the water; the effects of moisture migration have been
neglected, following the results of Williams (7).
For pilot ignition the correlating temperature is found to be 380C,
corresponding to a critical intensity of 0.31 cal cm^-2s^-1, except for fibre
insulating board which appears to ignite at a somewhat lower temperature of
330°C. Earlier experiments, with smaller specimens, gave a similar result, of
360C, with the results for fibre insulating board included. The present
correlation extends to much longer times (up to 59 min). The results show that
the choice of 0.3 cal cm^-2s^-1 as the maximum acceptable level of radiation for
building regulation purposes, gives a larger margin of safety than was originally
thought.
For spontaneous ignition the correlating temperature is found to bo 545°C,
the same as found previously for smaller areas, corresponding to a critical
intensity of 0.74 cal cm^-2s^-1. The present correlation extends to much longer
times (up to 16 min) and the results suggest that the empirical correction
necessary for the area effects is linked with density.
