This is what I said in a past thread about slag.

"I'd make the smelting process make a bunch of slag, a high % of the ores weight and make the ingots weigh a lot less. Then beat a tiny amount of slag out during forging also. THEN make it so you have to dispose of ALL the slag properly, cuz it's possessed with DEMONS, and they'll spawn from the slag creating an attack event proportionate to the amount of slag, within reason... This is conveniently the also reason you can't take metal though portals, cuz of the demons."

DEMONS!!!