You can't beat wax, sawdust, fat, etc. as fluxes, given their price, especially if you cast out of doors. If you don't, the smoke and odors are a real concern (sets off my smoke detectors and my wife!). Brownell's "Marvelux" is an excellent, smoke-free product, which is reasonably priced to boot. However, it has some shortcomings: It is reputed to promote rust (It hasn't in my experience.). It does leave a [water-soluble] deposit in your pot/furnace. And, like table salt, it is hygroscopic so you must keep it tightly sealed. Where smoke/stink is a concern, Marvelux is a viable option. ...Maven

one trick my friend who taught me how to cast boolets did was use the bullet lube that always seems to seap out of the lubrisizer to flux with.......... we had used the regular blue wax flux too though......