So I'm using sparex no.2 to remove the black from silver after I heat it. I just quench in the sparex solution and it removes the black instantly. I'm trying to heat a quarter and use sparex to remove the black but it's not working. The quarter is cupronickel which is a mix of 75% copper, 25% nickel. Any idea what solution I could use to remove the black from the cupronickel after heating it?