So, in keeping with the original intention of this thread.

Hearth:
A brick, stone or concrete structure used to house a fire.

Furnace:
A vessel used to smelt ore or refine smelted ore (pig iron) that traditionally uses forced air to increase the heat.

Forge:
A structure that provides a heat source to bring metal to a working temperature for fabrication.

These are a bit generic, but do they make sense?

Pete