You found the culprit!  Congratulations.

From the Brownell's catalog

Far and away the most popular - and successful - bullet casting flux, MARVELUX has been on the market since 1971 filling the needs of knowledgeable and successful professional and hobbyist bullet casters. MARVELUX is non-corrosive to iron and steel, and does not produce fumes which can cause corrosion, as does sal-ammoniac (ammonium chloride). In fact, regular use will keep your pot free from rust! Reduces dross formation dramatically while increasing fluidity of bullet alloys, making it easier to obtain well filled-out bullets.

Wonder if Marvelux in itself causes no corrosion but it’s action to the alloys can cause an adverse release.

bee's wax or parafin seems to harm my porch not at all. Been casting out of doors there for decades. Did have to paint the entire house,  but didn't blame my flux. Wife wanted a different color. 
Y'all have great Thanksgiving, celebrated as you prefer. 
We have lots of dinners for the veterans and needy, all the usual places for homeless and, of course the local casinos have food if you brave the crowds. Me, I'll just stick around home and enjoy all my girls on this special day. Again, happy Thanksgiving.

bpjack wrote on Nov 22^nd, 2022 at 1:38pm:


I still have some of that I got from Walt as well .....man does it smell good.

Old candle wax works as good as anything else I've tried.  What's left over from my wife's scented candles gets busted up into small pieces and stored in a container on my casting bench.

Wayne