The other ring we made in the Rio Certification class was also sized without the use of a ring mandrel or ring gauge. We used the same neat mathematical formula even though this ring was made flat and bent after firing.
The construction method of this ring was difficult as the signet containing the stone was fired separately and added after the ring band was bent. The objective of this piece was to learn ring sizing, bending metal clay, setting a stone and double firing.
Usually double firing isn't difficult. Just paste the two fired pieces together with lavender oil paste (which I mentioned in an earlier post) and re-fire.
What made this piece a problem was that the amount of contact area between the signet and the ring band was minimal. I filed and filed the open ends of the ring band and could not for the life of me get them level. There was always a curve to the end which Tim told us happens because we have elbows and tend to file in an arc. So, when I inserted the (somewhat) square signet into the opening between the ends of the band there wasn't a lot of contact area. The ring fired intact, but when it was put on a ring mandrel and hammered to get a better curve, it broke at the seam.
Now I need to repair it. When I do, I'll put some kind of design between the band and the signet that gives more contact to the pasted problem.
Originally this project had the signet sitting on top of the band, which gave more contact area to the pasted joint. Doing the ring this way, made the signet stick up considerably and that is why the design was changed. I think I would have liked the other design better, (both in looks and ease of creating.) But, it was what it was.