Devit on cabs is usually caused by your not grinding the edge fine enough. Many people use a 120 grit bit to grind their cabs. If you fuse those, you will get scum. You need to grind much finer (320 minimum) and then add a light coating of some devit agent (spray A, clearcoat, etc).
As to cooling, devit forms above 1200°F. thermal shock happens below 1000°F. So if you crash your kiln below1200 you will not have to worry about adding to the potential of devit. Having said that, most of the major glass manufacturers’ glass is devit resistant and will not likely have devit unless you are doing multiple firings or firing very high.
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