Could have a hollow spot inside the sinker itself. A friend of mine sent me some big sinkers that he had found around rocks at a beach. I dropped one into my pot to melt down and it exploded sending lead everywhere burning my hands, had lead hanging from my eye lashes. God took care of me that day in 2011, COULD HAVE BEEN BLINDED. I posted pics here back then, maybe in the archives here. ALWAYS wear eye protection now. That sinker had a big hollow spot in center. Don't know what caused it to explode like that. Air, water ?