There has always been discussion here about what is the hardness of a lead mixture with certain amounts of antimony and/or tin. I don't have a simple formula for that.
Some years ago, I got a bunch of almost chemically pure lead (roofing lead). I also had pure tin and a bunch of monotype with a set quantity of antimony in it. At the time, I also had three hardness testers (SAECO, Potter, and Lee). They all had different scales, but I was able to use them for cross referencing. So I did a bunch of experiments mixing small amounts and testing them for hardness. This is what I found.
1) Pure lead is not Bhn 5. I understand that chemically pure lead is Bhn 4.5. The stuff I had was Bhn 4.6 to 4.7. If it tests at Bhn 5, it already has unknown amounts of antimony and/or tin in it. That affects the final hardness.
2) Very small amounts of antimony added hardness very quickly, but its affect was reduced the more that was added. In other words, a graph of percent antimony vs hardness went up very quickly at the start, but the line bent downward and plateaued fairly quickly. At small percentages (like 1%-2% or so), it added a lot of hardness, more than one Bhn for each percent. Anything over about 15% to 18% did not add any measurable hardness. I have not really seen this trait mentioned here before.
3) Tin added hardness on almost a linear fashion. Tin seemed to add very close to 1Bhn for each 1% of tin. The graph also bent downward like the antimony line, but not nearly as quickly. Even at 15%-20% it was still adding part of a Bhn for each additional percent.
4) I was not able to tell anything about the relationship with both tin and antimony. That was too complicated for my instrumentation. However, I did decide that when I had a certain amount of antimony, I should have at least an equal percentage of tin. Less than that amount of tin, the casting became harder (more rejects) and the bullet was more brittle.
I have a bunch of experiments recorded that I turned into a rough graph for Bhn prediction purposes. It does not seem to be any better than the other formulas that I have seen posted here from time to time. However, it does not seem to be any worse, either, and since I spent a lot of time on it, I use my graph.
Anyone have any comments.