When melting scrap gold to recast I use borax and a graphite rod. I place the scrap gold in a crucible, heat it up until it forms a melted ball, sprinkle on some borax from a salt shaker, stir with the rod which will suck up impurities then pour the gold into my mold. If the gold is extra dirty I will do the borax and rod twice. Ove time the crucible will build up a borax coating as will the rod and that helps remove impurities. After you sprinkle some borax on the hot metal you will see it floating about sucking up the crap. Hope that helped.