I want to say at some point before i was born they change the ingredients of whats in batteries. I was reading about how it used to be dangerous, but not unsafe. Now during lead reclamation the lead off gass chemical warfare agents. I forgot which one but its a bad one. Thats what I have read. Has to to do with battery acid reaction with mineral (i want to say potassium) and arsenic in the lead (shooting from the hip here so dont quote me).
I would asume if someone put forth the effort to clean it should be safe enough to handle. If and thats a big if, thats what it was from.
I would just melt it down and purify it again. I have a hard time letting lead go. A couple handle fulls of sawdust and let it simmer for about 10 to 15 minutes
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