I taught a sterling silver metal clay ring class last October and we had some problems with inconsistent shrinkage. Even though the rings were all fired together they all had different shrinkage rates! We made the rings three sizes larger to account for shrinkage, but still some shrunk more than others. We were able to stretch some rings but unable with others.
I tried removing the black by soaking in pickling solution, heating it up with a torch until it was dull red, and finally sanding it. Sanding removed a lot of it but I didn't want to thin the ring shank too much.
I hated the fact that some of my students rings didn't come out as expected. Especially for the guys who couldn't even get the ring on their pinky! So, I thought about it for several days. And the solution it hit me!
I completed the first phase firing with the plug and paper in place.
I then placed all three items together inside a stainless steel screen to keep the carbon from causing pits by getting between the ring shank and the plug.
I fired the piece at 1500F for 1.5 hour. I wanted to get the most shrinkage pressure I could.
The paper did stick to the ring when I removed it, but that was easily fixed by sanding. Also there was a slight reaction. The metal had a beautiful blue patina to it on one side and on the other side a orange patina! I loved the colors so I kept them.