Introduction
The master (working) cast is the positive copy of the prepared teeth, the edentulous ridges and the rest of the arch, poured from the final impression. The restoration is made on it, so every error in the cast becomes an error in the mouth.
Gypsum products
Casts are made of gypsum: calcium sulphate hemihydrate powder that sets when mixed with water.
Mixing rules
- Measure powder and water; for die stone about 100 g powder to 20–25 ml water. The more accurate the ratio, the better the cast.
- Put the water in the bowl first, sprinkle the powder in, then spatulate.
- Pour in small amounts from one place, with vibration, to drive out air bubbles.
Requirements
A good working cast
- Reproduces every detail captured in the impression.
- Has no air bubbles, voids or distortion.
- Is trimmed at the preparation margins so the wax pattern can be carved there.
- Can be mounted precisely on an articulator.
A good die
A die is the positive copy of one prepared tooth, on which the wax pattern is made and finished. It must reproduce the preparation exactly, be free of bubbles, return to exactly the same place on the cast, and stay stable even when the cast is turned over.