How did Gutenberg turn letters into reusable metal?

Gutenberg made each letter as a small block of metal cast from a mould, so that one carved design could produce as many identical letters as a printer needed. Type could be set into lines, printed, and then taken apart and used again. Around 1450 the German goldsmith Johannes Gutenberg invented the metal movable-type printing press, along with innovations in casting the type from a matrix and a hand mould. China had been first with movable type, made of porcelain around 1040 AD by Bi Sheng.

The trick was the casting. A letterpunch strikes a blank die of soft metal to make a negative letter mould, called a matrix. The matrix sits in the bottom of a hand mould, and molten type metal, mostly lead and tin with a little antimony, is poured in. Antimony expands as it cools, which gives each new letter sharp edges. Gutenberg was the first to make type from that alloy, and it stayed standard for 550 years.

Why did it matter? Before Gutenberg, scribes copied books by hand, or printers carved wooden blocks, and even a small book could take months. With a reusable metal matrix, one experienced worker could produce 4,000 to 5,000 types a day. The small number of alphabetic characters needed for European languages was an important factor: a modest set of reusable designs could print any text.

Gutenberg himself paid dearly. After a dispute with his backer Johann Fust ended in a court decision for Fust, he was effectively bankrupt, though he seems to have supplied the type for a Bible printed around 1459.