Is your lid stuck on?
October 1994
To remove "stuck lid" after glaze firing - place lidded form in freezer for 20 minutes, remove and immediately immerse the lower portion in very hot water and strike it sharply near lid with a wooden implement wrapped in a teatowel (being very careful not to chip the rim). Alternatively (if pot too big or freezer too full), warm pot in kiln or sink full of very hot water and sit a bag of frozen peas on lid - then "whack" as above.
Courtesy COLLECTED WISDOM | 50 years of handy hints from the Ceramic Study Group, Australia