In terms of what items they accept:

• Charcoal Forge requires both a IFluidHandlerItem and IHeat capability be present.
• Crucible has no restrictions.

In terms of what they will fill:

• Crucible will only fill a valid MoldLike implementation. (Extension of IHeat and IFluidHanderItem).
• Charcoal Forge will fill a IFluidHandlerItem, and optionally heat a IHeat (although due to the slot requirement, it's practically assured to be there). These two actions are done independently.

Ideally, we prefer using IFluidHandlerItem and IHeat independently, only require the minimum ability, and reserve use of MoldLike when necessary, which I don't believe either of these cases are.