I do occasionally have to heat my marbles to the point your describing and this was true for the marble pictured since it cracked in the very center while I was working it however I can't say that that's normal. I think the main difference may be how we work it I take a good 15 minutes to get mine drippy where as I imagine you blast your clear when your working with silvers. However I could be totally off base so correct me if I'm wrong.
-Zach
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