Iron -burns-

If you overheat during the forging process* then, depending on the specific atmosphere you're in and how much oxygen is available, the surface of your piece will begin burning as it grabs oxygen from the air and starts sparking off various iron oxides.

(I use this as a demonstration for younger kids who come visit during reenactment days, because it looks -just- like a fourth of July sparkler.)

Needless to say, it entirely ruins the surface of your workpiece and requires extensive work to get it back to where it was before you inadvertently let it heat up to a burning temperature.

You can use this extra info to inform dream Vader so that the metaphor can be extended to the price of hating too much or whatnot ;-)

* During the -shaping- part, anyway. There's also the tempering part - if you heat it more than is required for the kind of heat treatment you're using, you will literally "lose your temper" and, yes, that is where that comes from.