In the distant past, I would cast several bullets from unknown lead alloy and then compare the weight of the bullet to the same mold poured in pure lead and #2 alloy. It was a decent way to determine the if there was more or less tin/antimony via the big difference in tin/antimony and lead density. tin = .261 antimony = .241 lead = .409
I recently scored several hundred pounds of lead. about 40 pounds in original Linotype letters and several ingots marked 4% tin 12% antimony.
Some had 90/10 marked on the ingots and one could assume 90% tin 10% lead. There are sections of solder bar that has no ratio. Then there are perfectly rectangular ingots that are bright and shinny with no markings. some ring when hit, some thud.
Density is a function of weight and volume. While I could melt samples of each, I am thinking about buying a graduated beaker and using the change in ml level to determine the volume of the ingot. Weight divided by the volume should give me an approximate density of which could be extracted an approx alloy.
Antimony might be estimated by the hardness of the lead.
Searched the forums and did not come up with any posts that addressed this. Looking for other thoughts.