I have a 'batch' of pure lead form the walls of an x-ray room. It's dead soft & dirty - to the point that when melting it, it turns blue pluss the other colours before being completely molten and remains those colours when cooling, skiming constantly, until the lead cannot be scooped. Because it came from an X-ray room, it has to be pure - what is causing those nasty colours and rapid oxidation, I don't know. Bring to temp, and skim it, and within 20 balls (I cast fast, too) it is developing a hard chunky on top - oxidation. This happens so quickly, I could not cast (dipping) 25 to 28 balls before I was getting inclusions in the balls.
I discovered, that if I dipped 1/4" of 50/50 bar solder into the 20 pound pot, then immediatly fluxed, I could cast 50 to 100 balls depending on size, before fluxing and skimming was necessary again. Thus, with THIS pure lead, I have to add tin- I would expect the 1/4" of 50/50 might add something like a 99.5:1 mix - or maybe thinner, but it is enough to help the casting time. The balls are still dead soft. I've never run into this with any other pure lead I've used - many hundreds of pounds of it well before 1985. I don't know exactly as I've never kept track, but it's a lot.
During the 1970's I used to pick up pure lead at the telphone company sevral times a year in 300 pound lots in the trunk of my Pol. Car. It all went out the muzzles of muzzleloaders as I was shooting straight WW in pistols and rifles at that time, not tin/lead.
Tin is valuable commodity to bullet casters, especially with bp ctg. shooters. I fond it aided my pure lead ball casting, not to fill out the moulds, but to allow casting time.