I use sawdust, just from my tablesaw. I dump a half handful on top of the melt and let it set for a while. I then stir with a wooden stick (paint stir stick type) briskly, and scrape the bottom and sides of the pot along the way. Let set for a few seconds and skim the stuff off the top. Ready to cast. I tried a bunch of different fluxes (lube, candles, beeswax, paraffin, charcoal, and commercial fluxes), but like plain old sawdust best.