Surface morphology and short-term water uptake of charred and coated wood
Charring of the wood surface represents a traditional alternative surface treatment technique with the purpose of aesthetics and protection. By the treatment with flame the surface of wood becomes carbonized and a few millimetres thick charred layer is formed on the top of the wooden element. Further, the charred layer can be removed by brushing, which accentuates the structure of the surface. Additionally, for appearance and protective purposes, a different kind of oils and coatings may be applied to the charred wood surfaces. Although this type of wood surface protection has been known for centuries, there is still a lack of knowledge about the water uptake properties of charred and surface finished wood. The aim of the present study was to find out how the treatment with charring, charring and brushing, and surface finishing affect the surface morphology of Norway spruce (S) and European larch (L) wood. By immersing the samples with treated radial surfaces in deionized water, the water uptake in the samples was monitored via the mass increase measurements on the tensiometer. Confocal laser scanning microscope examination showed that charring of wood greatly increased the surface roughness (S by 10-times and L by 6.5-times). Brushing of the charred wood surface further increased surface roughness (S by 21-times and L by for 23-times), completely removing the earlywood structure, while the latewood regions remained present. Surface finishing with water-borne stain noticeably increased only the roughness of the sanded wood surfaces, while the roughness of the other surfaces was not affected. In general, the S wood absorbed more water than the L wood. The highest amount of water was absorbed by the samples with the charred surface (S 0.048 g·cm–2, L 0.031 g·cm–2) and even the surface finishing of these could not prevent water absorption. The water uptake of the other surface types was quite comparable (S about 0.026 g·cm–2, L 0.021 g·cm–2). The higher water uptake seemed to be related to the higher surface roughness or to the specific surface to which the water molecules can attach and possibly penetrate into the wood.
Keywords: charring, morphology and short-term water uptake of charred and coated wood