I see no mention of boric acid powder use? (That's surprising.)_
For use as a flux? It has no carbon, so it would not flux at all. In order to flux lead alloys, you MUST introduce carbon into the metal. That does NOT just mean you put something on top of the lead, leave it there, then remove it. It
DOES mean it should be
stirred into the alloy. Get it under the surface of the melt. What to stir with, wood sticks! Get some paint stirring sticks the next time you're at the hardware store. Make sure you scrape the sides and bottom of the pot. Dirt can be trapped between the molten lead and the sides/bottom of the pot.
Sawdust is the best flux. It does
not have to be the remains of cuts made from a board. It could be chain saw shavings, or a good way to get it is the wood shavings made for pet bedding. A bag of that from a pet shop or wally world, would last a very long time. Just put about a tablespoon full on top of the hot alloy,(lead should be at least 700 degrees), when it starts smoking lite it with a match/lighter. let it char to black, then stir it into, under the surface of the melted metal.
Remove whats left as dross. It will be dirt that you do not want in your boolits. Then put some more sawdust on the top, let it char,
and leave it there! It acts as an air barrier. It's the air around you, or more specifically the oxygen, that causes the scum to appear. The scum is oxidized metal, mostly the tin. Tin reacts more quickly to oxygen, so it's mostly tin in the floating layer on the lead.
Since tin is most important in boolit fill-out, and for maintaining hardness, it's most important to keep it fluxed into the metal. Also, the higher the temperature of the lead alloy, the faster the tin will oxidize out. Seldom is it necessary to go over 700 degrees lead temp. for good boolits.