The straight-along-the-mandrel crack is typically thermal. I hate to sound like a broken record, but if you're putting them in the kiln glowing and annealing at 960-980, and you have this problem more than once in a while, you might want to borrow a pyrometer and check your kiln's temperature calibration. The 0146 steel blue has been part of their normal line for a long time, and it's a color I and many other have encased many many times with no issues. It's also a very popular kilnforming color, and is often topped with clear, and kilnforming is in some ways more demanding of compatibility than lampworking, especially with large pieces.
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-Kalera
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