I actually use solder occasionally to sweeten up the alloy, Yvsa. It's a good source of tin and sometimes antimony. The problem is finding what I need as inexpensively as possible. I'm mainly interested in the following three additives:
Tin: slight hardening of alloy, decreased surface tension. (And thus, better casting qualities.) Expensive.
Antimony: moderate hardening of alloy.
Arsenic: allows heat treating of alloy. Makes possible a moderately tough alloy that's harder than linotype. Commonly found only in wheel weights.
Tin can be purchased and added easily, although it's expensive. I have some on hand. Antimony is difficult to add to molten lead without chemical trickery or extreme temperatures and thus, I'd rather find an alloy that already includes it and just add that. Arsenic is the toughest one. I don't know what it costs or how to add it, I just know that the only really common arsenical alloy is wheel weight alloy. The arsenic is
vital if one is striving for high pressure loads; arsenical alloys can exceed linotype in hardness
while retaining some toughness (which linotype won't do, it simply shatters) when heat treated properly, and the heat treating can be as simple as dropping bullets from the mold directly into cold water. The amount of arsenic doesn't seem to be important, as long as it's there. That's what I need the wheel weights for.
So, it's not just the hardness that I'm after, but rather hardness with some toughness to resist shattering upon impact. It should be noted though that heat treated arsenical lead is far harder than any solder than I can think of -- we're talking 30+ on the Brinnel scale. (For comparison, linotype is around 22 and pure lead is 6-8.) Such bullets will often
splat on the plates rather than shatter, but are still quite hard. It has to do with the work hardening properties of lead, which actually work softens; it's thought that the sizing/engraving process that the bullet undergoes during the course of firing contributes to this, leaving a bullet with a soft exterior and hard core. It's a complicated subject and like quantum mechanics, no one truly understands it but some know how to work in accordance with it anyway. I just follow the directions that I find on the internet.
Thus, when you hear bullet casters complaining about people using WW alloy for things other than bullets, you now know why -- most folks use it for ballast or fishing weights, where any heavy metal will do. Bullet casters use it because it's nearly the perfect alloy for such work...the low cost was just icing on the cake.
(I actually cut my WW metal with scrap lead, believe it or not -- WW is
too good for bullets in its stock form around here. I'll add a bit of tin if the resulting alloy doesn't cast right.)