Great piece, put it away last year, got it back out and the chain has this on it...I tried a gentle wash, didn't budge, tried harder, nada...can this come off, or is this piece toast..'
It looks like the infamous gunk known as verdigris. You could try submerging it in ketsup for a while (several hours.) It may not work completely, but you really have nothing to lose at this point This is probaly caused by moisture. for future refeence, packages of dessicant in your storage drawers can help avoid this. They are commercially available, but can readily be found in new shoes and pocketbooks
I use ketchup but I do it in 30 minute soaks. Once I forgot about a piece and by the time I got back to it the gunk was all gone - but so was the finish 
YES! Still some discoloration, but no yucky stuff! Who would have thought? Now, is it worth listing with the discoloration ( fully disclosed of course) or is this destined for the dress up box? V76
I still see verdigris..it can take awhile to remove it all..this is where the patience comes in. I also see that yes, it has caused loss of the finish. Most likely even more will show once you remove all the verdigris.
Personally I`d have a hard time selling it alone as is..others may have a diff opinion. 
I have treated others with some success, with a gold leafing marker. These are available at Michaels, etc near the framing supplies. They have to be used very lightly, and I then immediately "feather in" the painted area. You might want to try just for the experience, or mention the possibility in your listing. And yes, if you do it yourself, it should be disiclosed.
I don't use ketsup, I use vinegar and a Q tip and rub & wet with vinegar until it's gone. Sometimes the verdigris has eaten into the metal, I have always had better luck with vinegar but some pieces may require the treatment from Michael's that was mentioned. Worth a try to save a vintge peice.
Carolyn
