These are made from L4005D red cone 6 stoneware (M390 casting). Both are cast and a thin-walled (half of what a thrown piece would be). They are glazed only on the inside to encourage cracking/splitting if the glaze is under compression (the thermal expansion of the glaze is significantly less than that of the body). And that is what happened here. The piece on the left cracked after a tap with a hammer. Notice how the crack has opened. The piece is "spring-loaded" (press the crack together and it reopens on release). The glaze is GA6-B. The piece on the right is glazed with G1214Z. It spontaneously exploded in half about 5 hours after exit from the kiln. On further taps with a hammer pieces fractured with equal enthusiasm! The white glaze is under too much compression. But the other is closer to acceptable. It is obviously under no danger of crazing. Is the compression too great? Another thick-walled piece exiting the same kiln was glazed inside and out. It was very strong.
In ceramics, glazes are under compression when they have a lower thermal expansion that the body they are on. A little compression is good, alot is bad.