I have had several labs tell me that they think their zirconia crowns are under sintered and had not shrunk all the way because they are fitting loose on the die. I have ran many testes and the zirconia shrinks first then becomes translucent looking. If your zirconia looks good than it had completed its shrinkage. I ran a unit up to 1500 with no hold and took it out. It looked like a piece of beige plastic but still fit on the die perfectly. Loose fitting crowns could be bad tools.