So this is something that has perplexed me for many years - It's now been over a decade since Sciguy made his post related to fluxing lead with sulfur to remove zinc, and in that time, nobody has really scientifically tested the idea with anything other than anecdotal evidence as far as I can tell. So I'm going to.
Two days ago I bought 100lbs of pure lead from a metals supplier. I have also purchased a lead hardness test kit (Lee brand, which was fully reviewed and tested here: http://www.lasc.us/Shay-BHN-Tester-Experiment.htm#Lee) In addition, I bought 4.5oz of pure zinc. My plan is to purposefully and in a controlled manner alloy the zinc and lead at a rate of 2.5% - 2oz of zinc per 5lb batch of lead. With that alloy I'm going to cast some bullets, and test the hardness. Then I'm going to remelt the alloy and flux with a generous amount of sulfur, following the previously attempted methods, and cast more bullets. The hardness of those bullets will also be tested and compared to the previous batch for any significant difference.
Following that I'm going to try the other method that is frequently mentioned: slowly bringing the alloy up to temp and attempting to skim zinc off the top as it floats out of solution. This will be done with a fresh batch of alloy at the same 2.5% zinc to lead. Again, bullets will be cast before and after the procedure to see if there is any meaningful drop in hardness.
If those two tests give reasonable results, I will melt some known hardball lead (92% lead/6% antimony/2% tin) and test the hardness there, flux with sulfur and test again. This will test to see if sulfur forms a compound with either the antimony or tin to reduce hardness as well.
Hopefully this will give the community some actual statistical evidence of what's been said for year. If anyone sees any issues with what I'm proposing to do, let me know so I can take them into account. If anyone knows of testing similar to this, please let me know.
Thanks!