Advice: is it necessary to use some soft of flux-suppose painted into mold-in order to cast muzzle loader bullets and round balls, or do you just dump the lead into the mold?

Nothing is needed in the mold. Matter of fact putting some thing in the mold could cause no end of problems. Just make sure mold is clean and free of any oil or grease and start casting. Depending on the material the mold is made of it will take a little while to come up to temperture just keep casting until bullets come out sharp (or round) and wrinkle free.

I use carburetor cleaner to insure there is nothing in the mold. Then let it dry completely of course.

Alan

hose down liberally with brake cleaner, scrub with old tooth brush, hose down again, let dry, start casting.