Lead ingots, unknown origin

[Worn_Holster]

A while back, I purchased some lead ingots for making diving weights. To be clear, they weren't sold as lead for diving weights; they were just sold as "25 LBS.LEAD INGOTS for BULLETS, SINKERS, MOLDS". I am just setting up for casting for the first time, and the metalurgy aspect of casting has me left me wondering how you can be sure of the lead and alloy composition of lead that you find. I am loading some medium loads for my S&W Pre-29 and I know that I'll require a higher than normal hardness to prevent leading. Should I look to buy lead alloy of a known origin or attempt to purify and measure hardness of what I have got?

[mold maker]

How are you sure you need harder lead? What is the size of the bore? What size boolit do you plan to use. Do you plan to size them? Have you even tried to scratch your lead with your fingernail?
There are too many unanswered questions, to make a qualified answer, to you question?
Knowing what you really need, and what you really have is crucial. I'd suggest reading the "cast bullet" hand book by Lyman to begin with. Casting boolits can be a science, or just a hobby. Both are attainable, but the basic knowledge is key to begin either.

[Worn_Holster]

The throat diameter is .432, I am casting to .434 and I have a custom Lee sizer die to size down to .432. No, I haven't done any scratch test of the ingots... In the Fryxell Manual here on this forum, it says that Magnum revolver loads require a target hardness something in the range of 12-18 BHN. I guess that a Lyman manual should be on my read list at this point.

[Defcon-One]

I'd ad 1-2% Tin to a small sample and cast some. Try them and see. If they are too soft, then mix it 50/50 with COWW and ad the 1-2% Tin and test again. It is really that simple.

If you want to know exactly what alloy you are using, like I do, then buy lead of known content (Linotype, Isotope Lead, Pure Lead and Solder). Then you can make any alloy that you want!