Dental porcelain may be high fusing, medium fusing or low fusing according to
the amount of glass modifiers as follows:
1-High-fusing: 1200-1400?C: used for making denture teeth.Designing components of the restoration to minimize stress concentrations and
tensile stresses.It is done either by self-glaze
(heating the porcelain to allow flow of the glass at the
surface of the ceramic restoration), or add-on glaze
(low fusing glass is added to the surface of the
porcelain and fired). Self-glaze porcelain is preferred to an add-on glaze
because the later contains more glass modifiers and
thus has a lower firing temperature and has lower
mechanical properties. Care should be taken to avoid
pyroplastic flow during self-glaze. 6. Cooling
Slow cooling or annealing should be done to avoid surface crazing. Cooling under
pressure can be performed to reduce porosity. Properties of dental porcelain
I- Shrinkage during firing
This could be due to:
a- Loss of residual water. c- Thermal contraction due to cooling. Shrinkage can be overcome by:
Proper condensation during building. Overbuilding of the restoration. 3-
Incremental building. The use of fine-grained powder of different grain sizes. I- Porosity
It is an inherent property of fired porcelain. It affects strength by acting as stress
concentrators and affects aesthetics by acting as scattering centers that reduces
translucency. Methods of reducing porosity of fired porcelain are:
1- Firing under vacuum to remove air. 2- Firing in the presence of a diffusible gas like helium. 3- Cooling under pressure. 1lI- Chemical properties
It is chemically stable in most environments because of the type of bonding (ionic and
covalent bonding). It contains the lowest
amount of glass modifiers as Na and K so has maximum silicate cross-links and
accordingly *strength, ?solubility and 1 stability (as the amount of glass modifiers
increases, the fusion temperature decreases).Methods of strengthening of ceramics
Porcelain ceramics are brittle materials, their strength values are much lower than the
predicted values based on the strength of their primary interatomic bond.5.2.