In situ visualization of microcracks by a confocal laser scanning microscopy system

Invisible microcracks occur on the surface of wood in the first stage of drying. It is important to investigate the morphological changes with time from generation to propagation in connection with moisture content (MC). In this study, a confocal laser scanning microscopy (CLSM) system with a controlled environment chamber was innovated to visualize the microcracks occurring during drying. Small samples of softwood Cryptomeria japonica, ring-porous Melia azedarach and diffuse-porous acacia hybrid (A. mangium × A. auriculiformis) were used. Microcracks occurred on the transverse surface in all samples with decreasing MC. The points at which microcracks occurred were in ray parenchyma or between tracheids or wood fibres and ray parenchyma. Then, the microcracks propagated in both the bark and pith directions along ray parenchyma and stopped at the maximum shape. After that, they closed with further drying. In the cases of C. japonica and M. azedarach, the microcracks almost closed and some of them could not be detected by CLSM. However, those in acacia hybrid did not close completely in the last stage of drying. These in situ observations clarified the characteristics of microcracks in relation to MC and wood structure.