LarryC, some excellent points. And to the OP, I apologize for turning this more and more into a marble thread. Perhaps we should copy some of the information here to a new thread about general marble information?
A quick word about using molds and their potential effects on glass. I am adressing graphite molds or rings specifically but it could apply to all manner of marble shaping devices.
As Larry said, reducing the time the marble is in contact with the mold can help a great deal. Even after you have used your marble mold a few times to shape your marble and it is "hot" it is still relatively cool compared to the heat in the marble. This can lead to chill marks where the graphite is in contact with the glass. The graphite acts as a huge heat sink. But the marble isn't just a flat surface, it has depth and an unbroken spherical surface.
If you induce chill marks in one area, the glass has cooled and contracted rapidly. The consequences can basically be thought of as there is an equal and oppisite reaction (to a degree). So if the skin contracts on one hemisphere, all that tension goes somewhere. This may lead to distortion on the opposing hemisphere or distortion deeper in the glass. If you have two layers with divergent viscosity, this may also be where the distortion manifests. As Larry C said, this may be where clear and a backing color are next to each other or opaque and tranparent layers join. If the layer that deformed is deep enough in the marble there may not be a lot you can do to repair the distortion. Or you may be able to but you have just added a lot more time to your marble.
When working a marble, or any piece, remember the effect of rapidly cooling a portion of the piece will affect a change in other areas. It may simply be you have just induced a lot of stress or it may distort the opposing side or some other area prone to distortion for whatever factors are present. I have seen issues on the opposite side of my marbles where I attach my punty. A punty will act as a heat sink as well.
One other point Larry C made that really needs to be considered no matter if you are making marbles or sculptural pieces or Some other type of piece. "Everything gets harder and more critical as the mass grows". Part of this may be our familiarity with a specific size/mass produces habits and a skill and information set we rely on. As the mass/size increases the skill set and knowledge may not be the same and we do not always adjust for that. Part of it may simply be your torch has practical limits. Not to say you cannot make monster pieces on a small torch, I know for a fact you can. In terms of practicality though it is a different story. You may be trying to work in the same way you have for smaller pieces and moving up in size is not practical and you have to change many of the techniques you used on smaller pieces. There are a variety of reasons but I agree with Larry, mass complicates matters and certain factors become more critical.
Otter
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