Stubbicat.....I confess to using Marvelux, on page 57 , Lyman Cast bullet Handbook #3, says to us it. Page 17 of the RCBS Cast Bullet Manual #1, shows a can, sitting next to the pot and states the modern commercial fluxes are the best.....these being my only source of guidance back then, I followed their instructions.
Now the trick is don't use too much, I use 1/4 to 1/2 teaspoon for a pot with 12 to 15 pounds of alloy. If you get a molten glass layer on top and/or a buildup on the pot walls....that's TOO MUCH . I find it better to do 2- fluxes with 1/4 teaspoon each than one flux with 1/2 teaspoon...just seems to clean better.
After doing my two small Marvelux fluxes, before I get to casting , I flux again with just a drop or two of melted beeswax. Touch a beeswax cake to my hot stirring spoon and get just a couple drops in it and stir into melt. Not sure why but the beeswax gets the metal really clean and all blended. Again, don't use too much, it smokes and will flash to fire so use a small amount.
All of the fluxes work, the trick is using the proper amount.
And in the middle of a casting session, if the melt needs another flux I will use the cedar shavings from our wood pencil sharpeners, cedar smells nice and shavings reduce to charcoal quickly, stir them in and resume casting.
Gary