Posted 29 April 2013 - 10:15 AM
Any time you put deep lines into a slab like that, you run the risk of it cracking along those lines. As for the discoloration, it's possible that the black underglazes fumed out and discolored the areas around them. Cobalt and copper will do that, and the black probably has one or both. I'm not convinced that's the case, but it's possible. More likely it was over fired like others suggested. I'm not surprised the reds burned out. Amaco underglazes are not necessarily all that stable at cone 6. The clear glaze can also affect color stability, so you may want to try different one. I use Speedball underglazes, which hold color perfectly at cone 6, and are cheaper than Amaco.
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