I have a feeling it's one of two things. either cold tools or inconsistent heat. Cold tools on warm glass pull the heat from the places they touch and create stress. Another thing that might be happening is that you are rushing the process of getting it into the crock pot. I had this problem from time to time too. Because you aren't putting it into a 900 degree kiln, you need to make sure you bring the entire temp of the marble down by flame annealing. You may be waiting for the glow to disappear, but this can mean that the outside layer is much colder than the inside core. So after the "entire" glow of the bead has gone, make sure to wave it through the flame a few more times - not to get it molten or even red hot, but enough to bring the surface temp back up to something similar to the core temperature. I can't promise perfect results every time - as others have said, glass compatibility may be an issue too - but this should definitely help.
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