I have a old Lyman 20 pot and the lead will not flow from the down spout. I had the lead at 715 degrees by my thermometer and tried to run a small drill bit on a vicegrip up the hole to unplug it and it did not clear it as it normally does. If I heat up the lead again to 700 degree, would it be safe to heat the spout with a propane torch and see if it will unplug. Or do I need to try to drill it open.