A friend of mine gave me a 4X8 sheet of lead that had been in a hospital Xray room. It had dry wall glued to it. I got the dry wall off but the glue wouldn't budge with anything I tried.
I am still using it. I cut it into strips and put them in a cast iron pot and melt them on my fish cooker outdoors so all the fumes will float away.
When all the gunk has burned off I use an old spoon to dip the stuff off the top and clean the lead. Then I ladle the lead out and make it into ingots for use in my lead pot.