The porcelain thus produced is then ground, sieved and packed. In the dental laboratory the powder is used as a slurry. The aim of spatulation, brushing, vibrating and blotting is to get the particles of dental porcelain powder as close together as possible. Firing in a vacuum causes the particles to sinter together and the porosity to decrease. A considerable degree of shrinkage occurs on firing.
Dental ceramics are inherently brittle and cracks can propagate from surfaces placed under tension.
Such forces can be generated in bending. The F - ions provide nucleating agents for the partial crystallisation of the glass. They also enable it to be cast in an investment mould, rather like a metal. The plate-like crystals produced during the ceramming process are flexible and they interlock to provide resistance to the propagation of cracks. However, the greatest strength comes from combining a metal substrate with an aesthetic veneer of porcelain.