Lead is a funny thing, Take that ~ 10-12 bhn WW lead, and water quench when you drop from the mold, the BHN will probably be ~18-22. There are a ton of things that can affect the outcome.
Pure lead takes a lot hotter pot to hit the melting point, than an antimony mix. A lot of the theory of add tin to better fill the mold, can also be achieved by running a hotter pot.
A lot of guys look for a surface color. usually a yellowish, or straw color for optimum temp. too hot then becomes a blueish and surface oxidation starts. I find I run my Lee 4- 20 almost wide open when using pure (and pouring big lead in bulk) but with an antimony mix, I back off to ~650-700 f.
Linotype starts to melt at about 460 deg. pure lead melts at 621.