Only the highest quality kiln dried cedar wood is used from the core, or heart, of the tree. Each stave fits precisely together with the circular base, and are held tightly in position on base by three stainless steel hoops. Once the tub is filled with water, the wood swells to form a watertight seal somewhat like a boat or wooden keg for maturing wine. This practice called cooperage has been widely used for centuries and is extremely effective.