Wood-water relationships, 2: The fibre saturation point of beech wood

Abstract

In order to determine the fibre saturation point of beech wood as accurately as possible, the vapour-pressure isothermal at high relative vapour pressures is measured on small blocks at 23° C., both for the adsorption cycle and for desorptions from three moisture-contents of 83, 47 and 120 per cent, the last value representing approximately complete saturation of the cell structure. Whereas on adsorption a vapour pressure indistinguishable from saturation appears to be reached at about 45 per cent moisture-content, which is much higher than that indicated by most routine methods, it is shown that on desorption the vapour pressure drops slightly at a moisture-content of about 90 per cent and also that the shape of the subsequent desorption curve is considerably influenced by the initial saturation moisture-content from which the drying commences. The observation made by Urquhart on natural cotton, that the first desorption cycle is not reproducible once the material has been completely dried, is found to apply also in the case of beech wood. It is pointed out that there is no discontinuity in size between the botanical and colloidal capillaries and that therefore the fibre saturation point cannot be defined, except as an approximate point at which the change of moisture-content begins to produce an appreciable effect on the particular property of wood under consideration.