Effects of high-temperature kiln drying on tensile and shrinkage properties and chemical composition of boxed-heart timbers from Japanese larch ( Larix kaempferi )

The high-temperature setting method is a kiln-drying technique that rapidly dries boxed-heart timbers under high-temperature and low-relative-humidity conditions to suppress surface checking. This study examined the effects of this drying method on the properties of Japanese larch ( Larix kaempferi ) boxed-heart timbers. Three groups of timbers were subjected to different drying conditions: high-temperature setting for 18 h (HTS-18), high-temperature setting for 48 h (HTS-48), and air seasoning. After drying, each timber sample was sectioned into corner, periphery, and centre regions. Samples from each region were subjected to tensile tests parallel to the grain, fractography on the specimens from the tensile tests using scanning electron microscopy, shrinkage measurements, and chemical analyses. In all regions, the tensile strength, tangential and radial shrinkages from water-saturated to oven-dried conditions, and contents of holocellulose, α-cellulose, and hemicellulose were highest in the air seasoning timbers, followed by the HTS-18 timbers, and then the HTS-48 timbers. The rates of decrease in tensile strength, tangential and radial shrinkages, and hemicellulose content for the HTS-48 timbers relative to the air seasoning timbers were higher in the centre region than in the other regions. Rough and uneven fracture surfaces of the cell walls were observed in almost all earlywood and latewood tracheids in all air seasoning-timber regions. Smoother cell-wall fracture surfaces, which were likely brittle, were predominantly observed in latewood tracheids in all regions of the HTS-18 and HTS-48 timbers, and in earlywood tracheids in the centre region of the HTS-48 timbers. The total lignin content, comprising acid-insoluble and acid-soluble lignin, was similar for all drying conditions. These findings may be useful for gaining a thorough understanding of the properties of high-temperature setting-dried Japanese larch boxed-heart timbers, potentially improving their usability.