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After the mold has been drained of all its excess slip, the mold has to sit and dry. The mold pulls the water out of the clay and dries the piece out.
If you leave a piece in the mold too long, it will dry out too much and crack.
If you don't leave it in long enough, it won't let go of the mold and will probably tear or it will collapse under its own weight.
When I pour, I run a fan across the mold to help dry things out. Six to eight hours later, I go back and take the pieces out.
Larger pieces take longer to dry -- as much as a couple of days.
You can tell if a piece is ready to come out of the mold if it has pulled away from the sides of the pour hole.
When it is time to open the mold, remove the bands from around the mold and gently pull the mold apart. If the piece is dry enough, it should release immediately.
Try to pull the mold halves straight away from the piece since the clay is still soft and can be easily scarred or marred. The piece should handled carefully since it so soft.
When the piece comes out of the mold it will be a dark gray. Set the piece up and let it finish drying until it turns a light gray.
After the piece has dried to a light gray, it is ready to clean and fire in the kiln.