metal first developed around 3000 BC in ancient Sumer. These metal punch types can be seen as precursors of the letter punches adapted in later millennia to printing with movable metal type. Cylinder seals were used in Mesopotamia to create an impression on a surface by rolling the seal on wet clay. Seals and stamps Seals and stamps may have been precursors to movable type. The uneven spacing of the impressions on brick stamps found in the Mesopotamian cities of Uruk and Larsa, dating from the 2nd millennium BC, has been conjectured by some archaeologists as evidence that the stamps were made using movable type. The enigmatic Minoan Phaistos Disc of c. 1800–1600 BC has been considered by one scholar as an early example of a body of text being reproduced with reusable characters: it may have been produced by pressing pre-formed hieroglyphic "seals" into the soft clay. A few authors even view the disc as technically meeting all definitional criteria to represent an early incidence of movable-type printing. Woodblock printing Following the invention of paper during the Chinese Han dynasty, writing materials became more portable and economical than the bones, shells, bamboo slips, metal or stone tablets, silk, etc. previously used. Yet copying books by hand was still labour-consuming. Not until the Xiping Era (172–178 AD), towards the end of the Eastern Han dynasty, did sealing print and monotype appear. It was soon used for printing designs on fabrics, and later for printing texts. Woodblock printing, invented by about the 8th century during the Tang dynasty, worked as follows. First, the neat hand-copied script was stuck on a relatively thick and smooth board, with the front of the paper, which was so thin that it was nearly transparent, sticking to the board, and characters showing in reverse, but distinctly, so that every stroke could be easily recognized. Then carvers cut away the parts of the board that were not part of the character, so that the characters were cut in relief, completely differently from those cut intaglio. When printing, the bulging characters would have some ink spread on them and be covered by paper. With workers' hands moving on the back of paper gently, characters would be printed on the paper. By the Song dynasty, woodblock printing came to its heyday. Although woodblock printing played an influential role in spreading culture, there were some significant drawbacks. Firstly, carving the