Microcrack behaviors in black and red heartwoods of Cryptomeria japonica were compared in this study. Black and red heartwoods have extremely different green moisture contents but similar wood structure. Small heartwood samples were prepared from these two types of green wood. Moisture contents of black and red heartwood were 201.5% and 51.3%, respectively. The samples were dried at 50 °C in a controlled-environment chamber with a relative humidity below 5%. The propagation of microcracks was continuously observed using a confocal laser scanning microscope while the samples dried. The electrical resistivity of the surface was also measured to assess surface moisture content. Results showed that the transformation of the microcracks was similar between black and red heartwoods. However, the appearance of microcracks in the black heartwood was delayed, whereas the microcracks appeared in red heartwood immediately after drying. These suggested that in-situ observation is essential for distinguishing when microcracks emerged. It was also suggested the green moisture content of heartwood has a major effect on the occurrence of microcracks. Drying conditions must be adjusted to account for the moisture content of green heartwood, even for specimens of the same species that have the same anatomical structure.
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