"Gutenberg revolutionized hot metal typesetting by coming up with an alloy mostly made of lead that could be melted and poured into a letter mold called a matrix."

The key feature of Gutenberg's alloy was that it expands when it solidifies, rather than shrinking. That's a very unusual property; most substances shrink when they freeze, rather than expanding (water is another exception to this general rule, which is why pipes break when they freeze).

The expansion is critical to getting a high-quality casting, because it forces the metal into every nook and cranny.