Turns brown in one area, Fired too close to open kiln peephole. Refire to wittness cone 020, being sure to stilt piece and place it away from peephole.
Powders off Applied too heavily. Rub off with soft cloth, reapply 1 light coat of MOP, refire cone 020.
Frosted look Fired too hot. Fire off at a witness cone 06. Reapply new coat of luster. Fire no hotter than witness cone 020.
Metallic sheen rubs off easily, Too cool a firing. Apply more metallic and fire to witness cone 019.
Purple or blue shadows or smudges, Contamination in the luster. Fire luster to witness cone 06 to remove, then reapply and refire. (Be sure brushes are clean.)