While reading Shifts and Expedients of Camp Life by William Barry Lord, I've came across some very peculiar sections on hunting, and bullet casting. One that really stood out to me was when describing what to bring on a long trip. While going through things like caps, molds, of course lead, the author also stated to use some kind of lead hardener. Tin and antimony were no surprise, but he also listed mercury.
This really got me thinking. I can't imagine dumping cold liquid mercury into a very hot liquid lead pot, that just sounds like a bad idea. How else would you mix them? If you could get them to mix, would it really harden it?