as stated the saw dust and flux are used to bring clips dirt and unwanted chaf to the top to scoop it out and discard. But if you use strieght saw dust without some flux you might be removing antimony so, a little bullet lube works best for me.
Also when it comes time to mold your bullets if you see a dull scum on your lead (it is letting you know that you need to reflux, (the tin and other antimony is starting to sepparate), reflux with lube and match (you might not need the match the lube just may catch fire on its own) and stirr till the fire goes out your lead should be shinny again.
A couple of hints, if you are using a gas check bullet there is no need to add tin just wheel weight works fine.
For flat base bullets a mix of 9&1/2 pounds of wheel weight and 1/2 pound of tin or 9 pounds wheel weight and 1 pound 50/50 solder (solid core NOT RESIN CORE)
before you lube and size your bullets weigh them a bullet design below 200 grains should not be over +/- 0.5 grains bullets over 200 grains +/- 1.0grains
If your bullet is light (you are casting a 250 grn flat base semi wad cutter (44 cal) and they all measure out at 247 that is good your tin mix is a little high no worry, but using the same batch a few of those come out at 246 or less toss them back in the pot there are air bubble in them, if there are a few at 248 your spruce cutter might be a bit lose
shinny bullets = good casting temp. frosty bullets = too high temp turn down your furnace, wrinkled bullet = cool mold or the furnace needs to be turned up a little