This happens from poor maintenance and abusing the banger.
Once this happens from overheating it dirty will allow the carbon to fuse to the quartz when heated. There is a reason that when quartz and boro is worked, it is worked super clean. Many times the glass is cleaned with acid to remove all impurities.
The best way to clean the banger after it is chazzed is to sandblast the glass. This will not make it clear, but it will remove the chazze. It can then be fire polished by a professional glassblower.