A stack of timber was acquired and a 300 mm (1 foot) length was removed from all boards and these were high temperature dried to identify check-prone boards. The most susceptible boards were sawn into 600 mm lengths (2 ft). These were then randomly assigned to 5 different kiln charges. These kiln charges were dried for periods of 6, 12 and 20 hours (3 charges) according to a schedule of 120°C dry bulb and 70°C wet bulb with a heat up time of two hours. The boards were then sectioned to assess the severity of internal checking immediately after drying was terminated. Four boards from the six hour charge were fitted with acoustic emission equipment and internal checking was monitored. Three pins that were an integral part of the transducer casing were pressed into the wood to achieve good contact without the need for constant force clamps.

The acoustic emission data showed that most of the internal checking occurred during the first six hours of high temperature drying at the end of which the moisture content was 72%. The maximum rate of checking occurred 1.5 to 2 hours after the start of drying, ie. when the kiln first reached its operating temperature.

The number of checks that were counted after 6, 12 and 20 hours was 93, 73 and 39 respectively. This confirms the conclusion based on the acoustic emission patterns that within-ring internal checking is not caused by wood shrinkage below the fibre saturation point, but is a phenomenon that occurs in green wood as soon as drying starts. The decrease in the number of checks after longer drying may have been caused by variability between charges, as only 5 out of 14 boards per charge showed severe internal checking. Alternatively checks may have closed up during drying to the extent that they became invisible.

It is concluded that within-ring internal checking in radiata pine sapwood begins as soon as the wood starts to dry, and that the checking process is already finished well before the wood moisture content reaches the fibre saturation point.