By late 1944 and early 1945, much of the industrial infrastructure of Japan had been destroyed by Allied strategic bombing, and there was a growing shortage of raw materials due to Allied naval blockades and submarine warfare. Lacking in metals to mass produce hand grenades in the vast quantities that would be needed against the projected Allied invasion of Japan, the Imperial Japanese Navy Technical Bureau developed a design for a cheap, easy-to-make grenade made of ceramic or porcelain materials. Kilns famous for the production of traditional Japanese pottery, such as Arita, Bizen and Seto were pressed into service to manufacture these relatively crude weapons. There were a tremendous number of variants on shape, size and color, because the design depended on each kiln.