In the Mengxi bitan, the Dream Pool Essays, Shen Kuo describes how a commoner named Bi Sheng made movable type in the Qingli era (1041 to 1048). Each character was carved in clay and fired hard. Common characters such as zhi and ye had more than twenty copies each, in case one page needed them several times.
Type not in use was wrapped in paper, sorted by rhyme and stored in wooden cases. A rare character was carved on the spot. Two iron plates were used in turn. While one printed, the other was set. After printing, the plate was heated, and the type fell out for the next page.
Shen Kuo adds a condition. For two or three copies, the method was not simpler. For dozens, hundreds or thousands, it was extremely fast.