There is a historical and traditional famous wooden bridge, Sanjo Ohashi Bridge, in Kyoto, Japan. The bridge has a unique structure, wooden poles with bronze caps on each pole head. The structure was made in 1590, and the structure has been passed down until now even after several renovations. What made it possible to preserve the wooden bridge for such a long time? Mr. Kurisaki Hiroshi from the Forest Products Research Institute of the Toyama Prefectural Agricultural, Forestry & Fisheries Research Center, and his colleagues carried out investigation to understand the traditional technique used for the bridge and learn from it for our future. HORIBA joined the investigation by elemental analysis support. We had an interview with Mr. Kurisaki to hear his motivation and passion behind the investigation.
In 2014, Mr. Kurisaki belonged to a project team for investigation on better wood preservation method by learning from traditional architectural techniques. He visited Kyoto, the former imperial capital of Japan, for a meeting with his client and he walked on the wooden bridge by chance. At the time, he noticed that some certain parts of the wooden bridge looked less-damage despite the long history. He also found all the less-damaged parts were beside the bronze caps.
He got an idea “The bronze caps seem to play a role as a physical protection to protect the wood from severe environmental conditions such as sunlight, rain, and wind, but is that all?” “Copper ion has been reported its role in preventing wooden materials from potential decay. Does these bronze caps bring the effect to this wooden bridge?”
To validate his hypothesis, he consulted HORIBA to analyze copper contents in the bronze cap and the wood materials used in the bridge by their X-ray fluorescence (XRF) analyzer. The XRF analysis revealed that higher copper contents were detected in the areas closer to the bronze caps. The copper content in wood decreases according to the distance from the bronze cap. Along with it, the wooden part had more damage and looked decayed more. The XRF result was supportive of his hypothesis.
Unexpectedly, the XRF analysis discovered one of the bronze caps used in the bridge had a different composition from the others, and it suggested that the one was made in another timing.
Mr. Kurisaki got another opportunity to investigate further about the bridge. He heard news that the bridge was disassembled for renovation during the fall of 2022 to the spring of 2023. He came back to HORIBA for additional analysis on the remaining parts that he didn’t analyze in his previous investigation.
During the additional analysis, his team focused on the roles of copper ion dissolved in rainwater. The main interest was how it immersed into the wooded parts and what effect it brought to the wood. HORIBA carried out XRF analysis on the wooden parts underneath the bridge, which are like rain gutters. The parts were located far away from the bronze cap, but the XRF analysis result revealed that higher copper content was found in the parts. The result was consistent with the appearance having less decay or damage. Thus, the additional analysis proved that the bronze cap played an additional role in preserving wooden parts indirectly via rainwater.
The interesting thing is that people in 1590 already started using copper materials to prevent wood from potential decay and that people in 2023 have used copper ion as preservative treatments for architectural woods.
Now we are struggling to find a better way to immerse copper ion into the deeper parts of wooden materials. One of the candidate’s future techniques is to use copper for nails or wood clamps. The challenge is that copper is not as hard as the conventional iron-base materials. However, we are trying to achieve both the wood preservation performance and the hardness by using copper coating on conventional nails or clamps.
Trees absorb carbon dioxide in the atmosphere through photosynthesis throughout their life to grow, but this carbon dioxide is released back into the atmosphere when the wood rotted. In other words, we can fix carbon dioxide as long as we use wood as wooden products in our daily life. Therefore, extending the lifespan of wood materials contributes to achieving circular carbon society.
Mr. Kurisaki’s team is aiming to extend the lifespan of wood materials more. HORIBA’s analysis support was helpful to prove that copper played a key role of wood preservation through the case study of the historical and traditional bridge.
*The content as well as organizations, affiliations, roles, and other names included in this article are current as of this interview.