Letterpress type is cast in a metal alloy that consists of lead, tin, and antimony. There are some survival examples of type from the 15th century (moveable type found in Lyon and Basel) and also some "happy accidents" occurring in incunables when type was pulled off from the form when it was being inked and not noticed, the result being a "ghost" impression on the page. This is known as fallen type.
The casting of type is divided into 3 parts: making the letter punch; striking a matrix; casting the letter. This video from the Johannes Gutenberg University in Mainz shows how it was done.
image |Fallen type found on Liber in laudem Mariae, printed by Nicolaus Götz, Cologne 1475.
Cambridge University Library, shelf mark Inc. 3A.4.9