Type metal is an alloy (usually lead, antimony, and tin) that is used in typesetting.
One characteristic that makes it valuable for this use is that it expands as it cools, filling in any gaps present in the serifs and thinner portions of letters. This characteristic is shared by the bronze used to cast sculptures, but copper-based alloys generally have melting points that are too high to be convenient for typesetting. (Water and bismuth are other substances with this property on freezing.) As such it was sometimes referred to as "hot metal."
