Miyuki Matsuo

Aging of wood: Analysis of color changes during natural aging and heat treatment

Abstract The color properties of aging wood samples from historical buildings have been compared with those of recent wood samples that were heat treated at temperatures ranging from 90°C to 180°C. The results of kinetic analysis obtained by the time-temperature superposition method showed that the color change during natural aging was mainly due to a slow and mild oxidation process. In other words, heat treatment could accelerate the changes in wood color that occur during aging. In one sample, the color change (ΔE* ab ) after 921 years at ambient temperature was almost equivalent to that of heating (artificial aging) approximately for 6.7 h at 180°C. The results have been interpreted that the aging and the subsequent change in wood color begin at the time of tree harvesting.

The interrelation between microfibril angle (MFA) and hygrothermal recovery (HTR) in compression wood and normal wood of Sugi and Agathis

Abstract Tree growth stress (GS) consists of an elastic component and a viscoelastic locked-in component. The elastic component is released instantaneously by cutting wood, whereas the locked-in component remains. The latter can be released by hot water treatment, which is known as hygrothermal recovery (HTR). In this paper the mechanism behind HTR is described and interpreted in terms of the microfibril angle (MFA) in the cell wall as follows: during cell-wall maturation, axial tensile stress is generated in the cellulose microfibrils (CMF), whereas isotropic compressive stress is generated in the matrix of lignin-hemicellulose (MT). Some amount of microscopic stresses remains following the removal of the wood from the living stem. Hygrothermal (HT) treatment induces recovery of remaining compressive stress in the MT, which causes its expansion. Axial tensile stress in the CMF are released by HT softening of the MT. This causes the CMF to contract along its length and to expand laterally. The combined effect of the expansions of the MT and contraction of the CMF causes the wood to deform anisotropically. This is the HTR of wood. The degree of anisotropy is determined by the MFA on the basis of reinforced-matrix theory.

Determination of the change in appearance of lumber surfaces illuminated from various directions.

Abstract Optical appearance is an important aesthetic property of wood, and this unique visual characteristic needs to be described quantitatively for many industrial applications. The objective of this study is to demonstrate the change in appearance of lumber surfaces from the viewpoint of human observation. Ten different specimens of coated and uncoated fancy veneer overlaid plywood of four species and solid and print Sugi (Japanese cedar) boards were prepared. For observation purposes, a simple goniophotometric device was constructed. A series of digital images of each specimen was taken, whereas the surface was illuminated from various lighting azimuths. The images were evaluated by subtraction, correlation, and multiresolution contrast analyses. The first two methods detect reflection anisotropy on the specimen surface through comparison of two images. Subtraction analysis also distinguishes between the coated and uncoated surfaces. However, it is difficult to determine the difference between the solid and print Sugi specimens based on these techniques. By contrast, multiresolution contrast analysis renders possible evaluation of the size and degree of the change in the surface appearances by contrast values. By comparing the contrast values for every lighting azimuth and filter size, this analysis clearly determines the change in appearance peculiar to wood, such as reflection anisotropy, improvement in reflectiveness conferred by coating, and the difference between the solid and print Sugi specimens.

Continuum contraction of tension wood fiber induced by repetitive hygrothermal treatment

Abstract Tension wood, when kiln-dried, is likely to deform hugely, which is probably caused by a gelatinous layer of the gelatinous fiber. To elucidate the mechanism behind the deformation of tension wood during kiln-drying, the strains experienced by tension wood during a hygrothermal treatment that is akin to an early stage of kiln-drying were investigated. Normal wood elongated along the longitudinal axis after the first treatment and leveled off with repetitive treatments. During the first treatment, tension wood contracted significantly along the longitudinal axis, followed by small subsequent contractions, which occurred during successive treatments. This characteristic hygrothermal behavior of tension wood correlated with the areal ratio of the gelatinous layer. One of the possible reasons why tension wood behaves differently was thought to be particular behavior of gelatinous layer. This finding will contribute to the development of appropriate seasoning method for tension wood by clarifying the mechanism behind the deformation of the gelatinous layer.

Negative gravitropism of Ginkgo biloba: growth stress and reaction wood formation

Abstract Ginkgo (Ginkgo biloba L.) forms thick, lignified secondary xylem in the cylindrical stem as in Pinales (commonly called conifers), although it has more phylogenetic affinity to Cycadales than to conifers. Ginkgo forms compression wood-like (CW-like) reaction wood (RW) in its inclined stem as it is the case in conifers. However, the distribution of growth stress is not yet investigated in the RW of ginkgo, and thus this tissue resulting from negative gravitropism is still waiting for closer consideration. The present study intended to fill this gap. It has been demonstrated that, indeed, ginkgo forms RW tissue on the lower side of the inclined stem, where the compressive growth stress (CGS) was generated. In the RW, the micorofibril angle in the S2 layer, the air-dried density, and the lignin content increased, whereas the cellulose content decreased. These data are quite similar to those of conifer CWs. The multiple linear regression analysis revealed that the CGS is significantly correlated by the changes in the aforementioned parameters. It can be safely concluded that the negative gravitropism of ginkgo is very similar to that of conifers.

Delayed recovery of growth stress in tension wood induced by drying and subsequent wetting treatment

The delayed recovery of the longitudinal length in biomechanically pre-stressed wood, specifically tension wood with a conspicuous gelatinous layer in the cell wall structure, was generally induced by hygrothermal treatment. However, hygrothermal treatment should not be the sole method to induce delayed recovery in tension wood. In order to broaden our understanding of the mechanism underlying delayed recovery, tension wood of Quercus serrata was treated by drying and subsequent rewetting over many times. Interestingly, the longitudinal deformation over repeated drying and subsequent rewetting treatments (dry–wet treatments) was identical to the response of the tension wood during repetitive hygrothermal treatments. Hence, the longitudinal delayed recovery in pre-stressed tension wood was concluded to be induced not only by hygrothermal treatment but also by the dry–wet cycles.