Not much more needed from what John and glaze nerd said... except YES! you really need to pre test every change in clay or glaze before doing a full batch. Fire at as close as you can come to the conditions you will fire the batch. Firing cycle can also make a big difference in the final appearance of a glaze. many of us here have learned the hard way that even going from batch to batch of a commercial clay or glaze can produce problems, and this is especially true if you mix your own glazes and change raw materials.
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