I had some of them once.  They were all over the map with regard to hardness.

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agree.
I went an alternate route and made 300 and 400 lb batches of my mystery metals.  Took samples in to work and we did the xray diffraction and I KNOW what is in each batch.

Frank,
How do you know you're pressing the ball bearing with the same pressure on each piece of test lead? I'd like to try this, but seems results could vary with pressure from the vise?

The way I understand it,you put the bearing between your two samples, hopefully one is pure lead, the other is the unknown one, and apply pressure in the vise.
By pressing the two samples together with the bearing in between them, the pressure on both samples is the same.
A method for calculating hardness is in Harrisons cast bullet book.
He says not to let the ball penetrate more that about 1/4 of its diameter.
By doing some math, the Brinnell number of the alloy can be calculated, using 5 Brinnell as the number for pure lead.
But, of course, that does not tell you what specific alloying metals are in the sample, just hardness.
If, for instance, the pure lead sample has a dent in it of .250" diameter, and the unknown sample has a dent in it of .180" diameter, divide the .250 by the .180 to get 1.388, square 1.388 to get 1.926, then multiply that by the "5" of the lead hardness to get 9.63. That should be the Brinnell hardness of the unknown alloy.

Very simply cut off a piece and send it to Roto Metals and they will test it for you and tell you exactly what it is that you have and at no cost to you. I might also add that they do not do it with ball bearings and indent sizes or any other form of the state of the art home brewed opinion technologies. 

DW totaly agree bullet hardness in no way tells one the true make up of an alloy. Having played exstensively with various alloys containing antimony it is very easy to achieve a hardness to ones liking. An example would be a bullet alloy the same hardness as 1-20 pure lead and tin yet far from it being so. The bullet being thee most important component to leave the barrel it is not the place to try and save money. If one does not start out with 99.9% certified lead and tin it is anyone guess to what an alloy or its componets might be? The hardness alone as you stated surely does not play any part in one ever knowing so and thus hardness testers and or testing are both virtually useless for that intended purpose and simply a waste of good money and or time.

Well, if you have time to sent it, wait for the test and then, wait for the test results, that's fine.

Another way is take your sample to a metal recycling yard and have them test it with a hand held XRF gun. That is most likely the way Roto test theirs.

The test that I offered will get you to 99.5% and since this thread is about how to tell pure lead from scrap lead, I think that's close enough. I make all my alloy's with lead sheet or lead wire.

There are no metals, that I know of, that when alloyed with lead, won't make it harder. If there are metals that will make it softer, I'd like to know what they are.The use of a ball for testing lead hardness, is the industry's standard. 

With the above said, if you have a known, pure lead sample and the indent is the same size on the unknown sample, it will be as close to pure lead as needed. Even with the certified lead, if that .1% above the 99.9% is zink, it may cause casting problems. Trying to cast with zink contaminated lead, will make your hair turn gray, and then, fall out (huh, Bob?)

Frank