I've used NS-Indigo Aventurine many times as a backing for 1-1/2" to 2" marbles, and never had a cracking or 'checking' problem, but then again I anneal ALL my marbles that size for 5 hours at 1050, ramp down to 960 at 60 degrees/hr, let it sit at 960 for 2 hours, and then my kiln shuts off. I do not open the door, not even for a peak, until the temperature drops to about 150 degrees.
Probably everyone here will say that 5 hours is just a waste of time and electricity, but my marbles don't crack. Even when my cat knocks them off of the kitchen counter and they fall onto my stone floor they don't crack. They bounce.
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A marble a day keeps the 'willies' away.
Gerald Kappel
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