Frantz Art Glass has a nice Blog of glass tips, and this is one that addresses cracking in Encased Beads. You can see it at:
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About 18 years ago I was having problems with a huge bunch of my encased beads cracking on me and I was hard pressed to figure out what was causing the problem. I did lots of test beads and asked anyone who had lampworking experience what was happening to my encased beads to cause every single one of them to crack. At that time in the lampworking movement, it was impossible to get any information from the lampworkers in Italy.
One day after puzzling over this problem for months, Mike and I received a call from an Italian lampworker. This lampworker was living in the America at the time and was calling about a totally unrelated matter. He was very friendly, so Mike popped the question on how to solve this special cracking problem. This lampworker laughed and said that this was an easy problem to fix and he gave me the answer I had been searching for!!!
So what is this solution, where and when does it work? I discovered that there are certain groups of glass rod colors that are a bit more finicky than others and they are: the group that the Italians call “special” colors like all the opaque yellows, oranges, corals, reds, browns and all the opalino colors in the Effetre / Moretti palette. There are probably a few other colors that have this problem also, but the previously mentioned colors are the main culprits.
The solution is so simple that it made me laugh and it really works! When you are having cracks that run down the length of the encased bead from hole to hole which usually cracks it in half, this is the way to solve the problem. Make a core of clear glass in the center of your bead. Clear is the one color (or non-color) in the glass palette that is the most molecularly neutral. If you make the core of your bead out of clear, you are creating a buffer to the expansion and contraction of the glass against the metal mandrel.
Once you have a clear core made, you use just a thin skin of the special and or Opalino color before you do your design work on the bead. Once you have done this, you can put an encasement over your decorated core bead.
Nine times out of ten this solves the encasement cracking problem. If you continue to have a cracking issue, even after you have done this technique, there is a larger compatibility problem with the glass colors you are using. I always tag all my different C.O.E. glass rods in my studio because loose rods can always get mixed up and if you pick up an unmarked rod thinking it is compatible and it isn’t, you have just wasted a few precious minutes of your life on a bead that will fail.