A while back There was a question about removal of a metal thimble/ insert? from a glass threaded insulator. As it turns out I had such a problem. The Amber-y ? whatever color CD 230 in center had a brassy looking threaded cup in it that I'll never know if it was original or not, however someone had tried to start pulling it out so it was torn. I had just started collecting (4 years ago) and tried a little tearing at it with needle nose pliers - DO NOT DO THAT! So, anyway I tried baths of table salt at first thinking it might eventually corrode the metal. (...ha ha...) So I eventually moved on to Oxalic acid ( Safety is a must, you need specific materials and equipment and a secure place that is not accessible to pets or children ( or even bumbling idiots) away from windows, outside.... the list goes on none of this is worth an acid burn! ok? So at first I covered the insulator in acid. I let it soak for MONTHS.... then would carefully dump the acid and replace it. ( Fumes!! DO NOT BREATH THEM!!!) I put the insulator in a plastic bucket. One time between checking the insulator fell on its side and the acid ran to the bottom of the bucket, BUT: because the metal and acid now had oxygen from the air as it was drying it made LOTS of green powder ( once again, ...story of my life ... "Oxides"!!!) So I found that a lot of light fast drying applications of acid did more in two weeks than a submersion in acid for 8+ months. PS all kinds of fisheyes on threads from some guy who doesn't know his colors, nevermind the definition of "blackglass",... and who used a needle nose plier to try to remove a thimble ( that he still has no idea of its originality to the piece).Anyway long answer short, paint acid get away, repeat. P.P.S.: I swear it was not THAT bright out that morning, though it was pretty nice.