The Great Furnace is the location of Subrosias ore smelting furnace.
Link will be able to gain access of the Great Furnace after he helps out the Subrosians at the Temple of Seasons.
They tell Link to come back anytime he needs some ore smelted. When Link comes back with the Red Ore and the Blue Ore they will smelt them together and give Link the Hard Ore.
The inside of the Great Furnace
The outside of the Great Furnace