Lead bullets can be alloyed with tin (to make the bullets fill out better and the lead to flow easier and a hardening agent)
, arsenic (to allow it to be heat treated to greater hardness), and antimony (which is really just a hardening agent). The method of heat treating by dropping into a pail of water is probably the most common way used and is usually effective. The most precise way is to heat the bullets in a controlled oven to just under the deformation temperature and then dump them into the water. To determine the proper temperature of the oven (assuming that your oven is capable of holding a preset temp. to within a few degrees), it is necessary to test a sample of the alloy that you are using by placing it in the oven and increasing the temp. a few degrees at a time until the alloy just deforms and then backing off a few degrees so that your bullets will not deform. Now heat the bullets to that temperature and then immediately put them into the water. The closer the bullets are to the deformation point when doused, the harder the bullets will be.
Be aware that heat treated bullets will lose their hardness over time so they should be prepared as close to the time of shooting as possible.
Bullets have been allooyed with just tin as much as 30%, but everyone has their own pet formula.
For a complete treatment of cast bullets, the Cast Bullets" from the NRA is excellent as is the Lyman Cast Bullet Handbook. I'm sure there are others. Quantrill