I agree with possible zinc contamination, years back we used to do a lot of commercial jigging in the gulf of Alaska from Cordova to Dutch Harbor and collected all the wheelweights we could get to cast 2lb - 5lb jigging and trolling sinkers, tire shops were giving the wheelweights away for free ! When that supply would temporarily dry up my old man started collecting huge worn zinc plates that came off the keels of fishing vessels and ships at the grid in the harbor, last summer I'd come across a similar crumbling and bullets would not cast properly, I had added a jigging sinker to a batch of pure lead that came from sheets of drywall I'd salvaged during a hospital reno job, thinking it was an all wheelweight sinker got me in trouble, I scrapped that zincfected batch and cast actual wheelweights and my 555 gr .500 cal bullets came out perfect,