Odd Cracking

During this semester at the UNCC studio, the stoneware has started doing some weird stuff and I haven't figured out what the cause is. Numerous pieces (about 4) have cracked in a spiral pattern in the cone 10 glaze reduction kiln. The ware is dark along the crack lines, which indicates that the crack occurred before the reduction, but yet there were no signs of cracks after bisque, or during glazing. The cracks are all very clean and very sharp, more so than even S cracks tend to be. Does anyone have any clue as to what could be causing this problem to occur? Please email if you have any idea/suggestions.