Just read the article by Glen E. Fryxell, "The Simple Act of Fluxing" on http://www.lasc.us/FryxellFluxing.htm. I've not cast in many years but am getting back in now and actually did use sawdust - my friend is a luthier so I always had plenty on hand. Though I started out with a smaller Lee bottom pour, I pretty quickly moved to dipping with a ladle and pot. As I recall, I would periodically re-flux and remove the floating particulate over the course of the smelting or casting session, though I don't recall any systematic approach.
I see the value of leaving the flux "float" on during a bottom-pour session:
But how do you guys who dip go about with your fluxing during the session?A heaping tablespoon is just about right for a 10 lb pot, stirred in thoroughly to begin with and then left in place. Halfway through the pot, the lead-pot is stirred again, this time the activated carbon (dross) is removed. By leaving the charred sawdust on the melt for the first half or so of the casting session, a barrier is formed to slow down the oxidation of the tin, and by removing the charred dross before reaching the bottom, the sequestered impurities are removed before they can sneak through the bottom-pour spout and possibly cause inclusions.
(ps: I am intrigued by the Lee 20# Magnum melt furnace. The RCBS Pro-Melt 2 looks awesome, but I'm afraid the price point would put me in immediate jeopardy of death by wife. Your thoughts on the Lee for dipper casting?).