We have found several Pyrex's on our hunts and most of them have a copper grounding 'shield' between the insulator and the peg. Because it shows through the clear insulator, I decided to see if Oxalic acid would clean up the copper. You can see the before and after results. I also spray the copper with Rustoleum clear acrylic spray paint so that it won't oxidize and turn green again - works great!