The Alloy forge is the mannish variant of forge. Like the others, it does not accept food items, but is mostly used to smelt ores much more efficiently than a furnace would. It can be crafted and used by good and evil players, making it a useful utility. They are primarily used by the men of Middle-Earth.
The forge's main use is to permit the alloy smelting of bronze, from tin and copper. Thus, the player does not have to travel to Dwarven or Elven realms to economically produce bronze. It can also burn stone to scorched stone and be used to crack bricks. It is also used to make Gilded Iron, an alloy of the Easterling faction. For an excellent guide to Middle-earth ores and alloys, see here.