Kumamoto Castle is composed of traditional Japanese Shikkui lime plaster
The exterior walls and interior walls at Kumamoto Castle are coated with Shikkui lime plaster finish coat. The Shirokabe plaster is the traditional Japanese Shikkui lime plaster. It is rich in plant fiber, highly durable and allows for an easily attained, highly-polished surface. It is used for smooth exterior and interior surfaces. Shikkui surface coatings become harder as time passes, as they are continuously absorbing CO2.
The image on the left portion of the screen, here, shows that the Shikkui plaster composition at Kumamoto Castle includes Ginnan seaweed, lime, straw rope, and straw fibers. The right-hand screen shows a layer made-up of: bamboo and a straw rope structural base; a rough coating of clay, which is hand-applied; a smooth coating of clay, which is trowel-applied; a middle coating of clay, which is trowel-applied; and a Shikkui Shirokabe plaster finish coating, which is trowel-applied. The layers depicted are supplemented by other such layers, unseen, which augment the application and thicken the wall of the Castle.
