Posted 13 November 2012 - 12:02 AM
I have been working in slip cast ceramics for over 40 years. I have found that as the mold ages, it will wear. The condition which you are describing could be caused by casting too soon. Reguardless of what you have been casting or pouring, it is known that porceline can be cast more than non-porceline casting in a regular period of time. The pieces of mold could be caused by fatigue by the plaster. Remember, you made the mold. The general rule of thumb is when the cast item is dry, the mold can be cast again.
I am a certified ceramic instructor and have been so for many years.
As for putting something on the surface of the mold to protect it, do not. The slip will not set up in that area and you will ruin the mold. I am manufacturing some molds of my own and I am keeping it simple. I make the forms from hard wood and then produce the mold with good plaster. If there is a problem with my mold, I make another one and destroy the bad one.
Good luck in your actions and career