The underlying physical properties that cause in the differential thermal response of knot and clear wood are not completely understood. Evidence exists that fibril angle, extractives content and equilibrium moisture content (EMC) may differ between knots and clear wood. Each of these factors is known to influence wood thermal response. Knots are also known to differ from clear wood in specific gravity. Murata and Sadoh showed that compressed spruce wood increased thermal conductivity linearly with increased wood densification. Therefore differences between knot and clear wood in fibril angle, extractives content, EMC or density may be responsible for the differential thermal response.

One objective of this study was to test methods to rapidly heat wood surface to allow knot differentiation in a lumber surface thermograph. A second objective was to determine if differences in fibril angle, extractives content, EMC or density are correlated with the differential thermal response of knots and clear wood.

Twelve species, six softwood species and six hardwood species, were studied. The six softwood species were southern yellow pine, white spruce, eastern white pine, ponderosa, douglas-fir and eastern red cedar. The six hardwood species were red oak, yellow poplar, black walnut, basswood and white ash, black cherry. Fibril angle, extractives content, EMC and specific gravity of both knot and clear wood were measured for each species.

Rapid heating of wood surface was achieved by both radio frequency and radiant heating. The power of the radio frequency heater was 35kWatts at 100 MHz frequency. Boards were exposed for 10 seconds in the radio frequency heater. Radiant heating was with two 6000 Watt halogen quartz lamps spaced 0.75 inch above board surface. A conveyor transported boards at 44 feet per minute underneath the two halogen lamps.

Radio frequency results indicate, that for softwood species, absolute temperature difference between clear and knot wood was of 3.15°F. This temperature difference should be adequate to allow differentiation of knots in a thermograph. Hardwood species clear versus knot wood temperatures differed little with a mean absolute temperature difference of only 0.26°F. This small difference would make it difficult to differentiate knot wood from clear wood in a thermograph. Rapid heating of wood surface by the two 6000 Watt halogen quartz lamps produced only minor temperature differences between clear and knot wood for both softwood and hardwood species. Softwood mean absolute temperature difference was 0.88°F while for hardwoods the difference was 0.01°F. Examination of specific gravity, EMC, extractives content, and fibril angle showed that hardwood species had considerably less difference between the clear and knot wood values for each of these variables than did softwood species. Hardwood values were 35.9, 39.7, 1.6 and 30.7 percent, respectively, of the softwood specific gravity, EMC, extractives content, and fibril angle values. Therefore, based on the four variables known to positively influence the thermal response of wood there is much greater difference in knot versus clear wood for these variables for softwood compared to hardwood species.