Fundamental aspects of timber seasoning

The aim of this investigation was to develop an economically viable method of seasoning back-sawn native Tasmanian eucalypts and to reduce the seasoning time required for these timbers. The theory of unsteady state mass transfer based upon Fick's law of diffusion was used to study the drying process. The diffusion coefficients were measured directly by the diffusion cell method or deduced from a comparison of measured and calculated drying curves. The relationship between shrinkage of wood fibre and moisture concentration was obtained by slowly drying thin strips of wood and regularly measuring deformations. The effects of falling diffusivity and shrinkage with reducing moisture concentration were found to effectively cancel one-another in the case of the timbers tested during this investigation. Drying stresses in wood were studied using the theory of elasticity in anisotropic bodies. Airy stress functions were used when solving for stresses. For simplicity, sawn boards were approximated to orthotropic bodies and only boards cut with face-planes parallel/perpendicular to the principle material property directions were studied. Some elastic properties of native Tasmanian eucalypt timbers were measured on small sample populations and found to be similar to those of various North American hardwoods (with similar densities) reported by other authors. The approximate elastic properties of Tasmanian eucalypts were deduced from this comparison. The theories of mass transfer and stress analysis were combined to form a model describing the development of drying stresses. The wood was assumed to be linearly elastic and to be free of the effects of hysteresis and creep. The governing differential equations were solved by numerical methods. The model was used to assess the effectiveness of semi-permeable surface coatings in reducing drying stresses in timber. Tests on timber coated with animal glue (cologen) showed that checking (cracking) in Tasmanian eucalypts brought about by high drying stresses was reduced in severity or completely eliminated, depending upon the thickness of the coating.