Development of a New Insecticidal Treatment Method for Historical Wooden Structures- On-Site Inspection of the Bell Tower of Chuzen-ji Temple
On September 10th, 2018, we visited Chuzen-ji Temple to inspect the “Humidity-controlled warm air treatment ” for its bell tower. This treatment method aims to expel noxious insects harming pillars and beams of wooden structures under a high temperature (around 60°C). Usually, as the temperature increases, wooden building materials crack or strain. However, it is possible to increase the temperature inside the wood almost without affecting its physical property, since the temperature rises while the humidity in the treated space is controlled with the wood water content maintained at a certain level. The conventional yet sole insecticidal method for historical wooden structures is fumigation treatment, where a structure sealed with covering is filled with vaporized pesticide to exterminate noxious insects inside the wood. However, vaporized gas also affects human health, thus, requiring safety measures against greater risks. Accordingly, it was hard to implement such large-scale treatment for wooden structures continually. This Humidity-controlled warm air treatment is expected as a new approach to overcome such a challenge.
So far, a research team comprising the Association for the Preservation of the Nikko World Heritage Site Shrines and Temples, Kyoto University, Kyushu National Museum, Total System Laboratory Co., the Japanese Association for Conservation of Architectural Monuments, National Museum of Ethnology, Natural History Museum and Institute, Chiba, and Tokyo National Research Institute for Cultural Properties has been proceeding with the studies from basic research for application to old wooden buildings to establishment of application technique. In the basic research, we verified the humidity distribution in the treated space during the test with a chamber, as well as the temperature distribution inside the wood, measured surface strain, and effects on wooden materials. Then, following the treatment testing with a model structure by using a pilot unit manufactured to control the temperature and humidity of actual structures, we finally realized on-site treatment testing of a historical wooden structure for the second time in Japan after Aizendo Hall of Chuzen-ji Temple. We would like to move ahead with this research toward the dissemination as one of new insecticidal methods while organizing these two treatment test results obtained from two buildings of Chuzen-ji Temple.