Heat absorption and transfer in softwoods and their knot surfaces.
The effects of density and fibre direction on thermal conductivity were studied in order to evaluate the use of thermography to detect knots in lumber. The thermal conductivity of densified spruce (Picea sp.) wood specimens with various densities and grain angles was measured by a laser-flash method. It was found that thermal conductivity can be expressed by a combined equation of a linear relationship with the density and tensors given together with the principal components of longitudinal and transverse conductivities. The longitudinal thermal conductivities of knots and branches of hinoki (Chamaecyparis obtusa) and sugi (Cryptomeria japonica) were separated from the relationship to clear tissue, but were similar to transverse conductivity. This behaviour resembles that of compression wood of branches. From measurements of temperature increases in lumber surfaces irradiated with incandescent lamps, the heat absorbance of knots was about 1.6 times that of the surrounding wood.