Gutenberg was trained as a goldsmith, so he knew all about how to work with metal—even how to make the tools. Here’s how Gutenberg made type: he got a super-hard metal (steel), a medium hard metal (copper), and a soft metal (tin/lead/antimony).
Gutenberg shaped letters on the butt-end of a short steel rod—maybe he used tempered-steel files or chisels on the carbon steel rod (steel has different hardnesses depending on how it’s treated). When the letter was shaped he got the steel rod red hot then tempered it by dunking it in cold water so it would be really hard. Now he had a steel punch. He hammered the punch into a piece of copper (softer than steel) so there was an impression in it shaped like the letter.
This copper matrix was fitted into a 2-piece mold that got clamped tightly together. Gutenberg could cast copies of the letter by pouring molten tin/lead/antimony into the mold.
Watch the videos below to get a better idea:
A demonstration of how copper can be shaped by steel (skip to the 1:00 mark): https://www.youtube.com/watch?v=tXbUDCfreqE
It’s the molds which were the true technological big deal of this process. If you want to get into the details, here’s everything there is to know about them: https://www.circuitousroot.com/artifice/letters/press/hand-casting/literature/index.html
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