Surface Modification of Spruce and Fir Sawn-Timber by Charring in the Traditional Japanese Method—Yakisugi

Abstract

The traditional Japanese method of wood surface charring was studied. To perform the surface charring, three sawn Norway spruce and Silver fir wood boards of dimension 190 × 24 × 4000 mm3 were tied together to act as a chimney and charred in a short time (3–4 min) with open flame at a temperature above 500 °C. Temperature inside the chimney was recorded on the three different positions during the charring process. Surface temperature of spruce increased from 0 °C to 500 °C in approx. 120–300 s while fir increased in approx. 100–250 s. The thickness of the charred layer and the resulting cupping effect were investigated at the different heights of the chimney to evaluate its variability. Temperature achieved during the charring process was sufficient to get a significant charred layer of 2.5 and 4.5 mm on average for spruce and fir samples, respectively. The analyzed samples showed a significant cupping effect to the charred side with no difference between the annual ring orientation of sawn boards. Spruce exhibit a more significant cupping effect when compared to fir, i.e., 3.2–6 mm and 2.2–4.5 mm, respectively. Furthermore, the pH values of charred samples increased significantly, which could be an indication of improved resistance against wood-decay fungi. For better insight into the traditional charring method, further studies should be carried out to execute the charring process in a consistent quality and therefore fully exploit its potential.

Review of Wood Modification and Wood Functionalization Technologies

“…The often-seen term shou sugi ban was originally a mistranslation of the term yaki sugi (literally burned/charred sugi), but it has rooted itself in the market of surface charred cladding boards produced in the USA and Europe. Surface charring can be implemented in several ways [210][211][212]. The traditional yaki sugi is made by tying boards in a triangle, setting fire from beneath and waiting while a chimney effect draws the flame upwards, spreading evenly on the boards.…”

“…Rapid heating produces much more volatiles than slow heating [225], thus reducing relative solid yield, and also increases the number of char fissures [228] that can significantly alter the pyrolysis result in the transition zone, namely by providing pathways for flow of reactants [217]. However, higher temperatures result in char with higher fixed carbon content than chars prepared at lower temperatures [212]. This "quality" is connected to the stability of the char surface during use.…”

“…Dimensional stability of surface charred wood has not been thoroughly reported in literature but cupping towards the charred side is normal due to a strong temperature gradient [212]. Both dimensional stability and hygroscopicity correlate strongly with method of charring-namely peak temperature, but also modification time [213].…”

“…In practical use the relevant organisms are at the very least greatly hindered by removal of soluble substances. Ebner et al [212] noted an increase in pH after charring, which affects the growth of common decay fungi negatively. Given that char formation begins at above 300 • C, the degradation of wood components is already very far at this point.…”

“…There are some studies related to fire-safety of buildings with traditional materials [242], but the descriptions on the manufacturing techniques, methodology and service life are practically non-existent. Recently, Ebner et al [212] have delved into the subject of traditional manufacturing methods but utilizing Austrian wood species. The results have been very similar to those reported for sugi by Okamura et al [211], regarding temperature, holding time and final char layer thickness.…”