During the Ch'ing-li period (1041-1048) a commoner named Pi Sheng first invented the movable type. Each type was made of moistened clay upon which was carved one Chinese character. The portion that formed the character was as thin as the edge of a small coin. The type was then hardened by fire and thus made permanent.
To proceed with the process of printing, a printer smeared an iron plate with a mixture of turpentine, resin, wax, and burned paper ash. Pieces of movable type were then placed on the plate closely together and were arranged in such a way as to reflect the text of a book to be printed. They were confined within the plate by an iron fence fastened tightly to the plate.
The iron plate was then placed on a gentle fire in order to melt the mixture type so that the heads of all pieces would appear on the same level. The plate was then ready for printing.
The fact that moistened clay, instead of wood, was used as the material to make movable type was because wood was subject to change and tended to distort the Chinese character carved on it. When exposed to moisture, the surface of a piece of wooden type became uneven, and the type itself became thus unusable. Moreover, it was extremely defficult to detach it when this particular piece of type had to be used somewhere else. The movable type made of clay, on the other hand, encountered no such difficulties. When exposed to a gentle fire which melted the mixture that glued it to the plate, it requred only a gentle push by the hand before it easily came loose from the plate.