Working in the forge doesn't only involve smashing very hot things with a hammer, it may also involve the skill to melt metals down to a liquid state, making molds and then casting items in metal. Depending on technology level, some metals may not be possible to fully liquidize.
In medieval times, casting is generally done with finer metals, as melting iron down to a complete liquid is something that isn't really possible with the technology of the age. Copper and tin both melt way before iron, and are commonly mixed to form bronze, which is the most common metal to use for casting.