Many years ago, I followed the old time practise of adding murcury to the pure lead for hardening and increasing the specific gravity all in one fell blow. While it increased the weight some small amount, it didn't seem to make them harder. I would not try it today, seeing as how the fumes are so poisonous and all.
: Adding a small amound of antimony is the best method of increasing hardness, as tin is slow in doing so. 5% antimony will harden pure lead more than 10% tin mix, yet will be heavier as well.
: My .684" balls went from 480gr.pure lead, down to 466gr. in straight WW t's. This drop of 14gr. wasn't much and they shot identically to pure lead. Blowing through both shoulders of a bull moose with hardly a mark on the ball, constitutes good performance for a 'solid'. Can you picture a 12 factory slug doing that?