Locate the problem and then select the possible cause to find a probably solution.
Porosity in Porcelain Layers
Surface roughness and contaminants can cause the opaque to form porosity or bubbles during firing. Follow the procedures outlined in the technical sheets for cleaning alloy surfaces. Assure that the equipment used is clean and contaminant free.
Bubbling in the opaque layer is commonly caused by too rapid a drying cycle or by placing the restoration on too hot a surface (over 600o C) before fully drying. Often high viscosity glycerides are used as the opaque liquid and these liquids may give the appearance that the surface is dry when actually underneath the surface, residual liquid remains. Carefully follow manufacturer’s drying instructions, especially of the opaque layer, prior to firing.
Applying thick or nonuniform opaque, dentin or other porcelain components can cause bubbling or porosity during firing. Avoid porcelain puddling or pooling by applying the porcelain uniformly and follow recommended pre-heating and drying instructions to assure that excess liquids are removed prior to firing.
Cracking of Porcelain
Copings must be designed with rounded corners and even application of porcelain to reduce cracking. Uneven thicknesses of porcelain may require slower cooling to reduce thermal stresses and possible cracking.
Often if the porcelain is not fully dried prior to firing, the escaping volatile liquids can cause micro-cracking on the surface which lead to cracks upon cool down. Be sure to follow the drying instructions for the porcelain prior to firing.
The coefficients of thermal expansion of the porcelain and of the alloy must be compatible. When differences exist, changes to the cooling schedules after firing might be required to reduce the stress caused by thermal expansion coefficient mismatches.
Over or under firing of the porcelain can cause thermal stresses and possible cracking. Be sure that the proper firing programs are used and that furnace used is properly calibrated.
Porcelain Looks Dull
Under firing of the porcelain can lead to incomplete melting and homogeneity of the porcelain glass phases resulting in a dullish look. Technicians should adjust the temperature by 10-15oC to have to increase the brightness. Porcelain ovens need to be calibrated periodically to assure that the recorded temperature and heating rates are correct.
The bake cycles used require a vacuum be applied to prevent unwanted oxidation of the various build-up layers, which can lead to dullness. If vacuum is insufficient because of leakages in vacuum lines, improper reading of vacuum by the various gauges, a leak in the sealing rings, et. al. can result in unwanted oxygen in the chamber. Bake oven vacuum must be periodically checked to assure proper processing.