I have been using candle wax and shaper chips for fluxing lead for a few years, and they seem to work OK. However, occasionally small bits of  hot material will be expelled from the pot when fluxing. I caught one on the lip a few years ago and that hurt like hell! fortunately, no harm done. 
Today, I noticed that the cotton rug I drop my bullets had a frilly fringe around the edge, and if some of that hot stuff from the pot ever hit it, it could catch fire. I am casting in my garage with about 30 gallons of gasoline and other flammables. so I was a little concerned. Sure enough , about 15 minutes later I got a little "pop" when I fluxed and the dang rug caught fire. Since I had my heavy leather welding gloves on I was able to snuff it out quickly. I have no idea what causes these minor eruptions, so if anyone has any ideas, please advise. I cannot believe any moisture is the culprit, but I suppose its possible. 

NEI used to sell a rosin flux that seemed to be effective and had a very pleasant aroma. Does anyone know if that produce is available anywhere at this time?

Thanks!
Joe S

a wood-worker friend gave me a few pieces of really "pitchy" pine wood, and I just stir the pot with one of those to flux it.  One can also use about any kind of wax or oil for a good result.  What is needed/desired is some compound that will provide a reducing environment for the melt.  This allows oxides, etc, to separate easily from the melt and be easily skimmed off, without removing much lead metal.  Even fine pine sawdust works very well.   I've never had any problem with any of the above.

CHRIS

Rodeo supply companies will sell chunk rosin.
Haven't seen a difference between the black or regular for flux.
Have to break it up into pea size pieces. Works well.
Chuck

I use a salt shaker to apply the borax.

If you can find it, Rosin is available in a shaker can, almost like talk powder, it works well, also carnuba was chips also work very well for flux. 
I use both and they seem to work equaly well and both smell good. Less smoke than other waxes.
Sam

Some impurities are embedded in the flux and some are driven back into the alloy by the flux.

I did not want the impurities driven back into my alloy and started pulling the dross off the alloy as it starts to melt.

The lead:tin alloy has a lower melting point and will melt first.
So as soon as the alloy melts the dross with a higher melting temperature will float on top.

Skim it off before it gets too warm and starts mixing back into  your alloy.

Safer to not use flux and less air pollution and no explosions from accidentally adding moisture when adding flux.

And I believe you end up with a cleaner alloy by the proper timing of skimming off the dross.