A mixture of thoughts relative to the various replies. Battery lead is bad because it contains cadmium, the vapours of which are very toxic. Locally, some years ago, a fellow in town was buying all the wheel weights he could, in order to cast cannon balls, so there is significant risk of getting mostly wheel weights when melting down cannon balls and dive weights.. Relative to hardness, many of the better black powder cartridge shooters are using 5% alloy for their slugs, probably because its lower melting temperature allows the mold to fill more completely while leaving a slug which will still obturate in the bore. The method that I use to identify alloyed lead from pure or near pure lead is to index my thumb on my xyphoid process and drop a large ball bearing (over 1" D ) on a flat piece of the lead and compare the dent diameter with that on a known piece of pure lead;
cheers mooncoon