The goal with any restoration is to stimulate natural dentition as much as possible. Restorations that contain air bubbles can cause an odd appearance at best, depending on where they are. In the worst case, they can not only affect the appearance, they can actually compromise the strength of the case, resulting in fractures.
Materials
The first step in ensuring a bubble free vibrant stacked ceramic restoration is making sure the materials are of high quality. The metal cast needs to be clean before casting. The tips of the torch should be relatively carbon free and a crucible dedicated to each type of metal shouldn't have an excessive carbon or slag buildup. Impurities entering the metal usually do so when we fail to consider what the molten metal is contacting.
Finishing
Next consider what we are to be finishing the framework once it is cast. Are you using the alloy manufacturer's recommended finishing burs, stones, and wheels? Once you have the correct tool for finishing, are you finishing in one direction, creating a satiny finish, or are you finishing in many directions? Multiple directions can trap air in the tiny folds of metal and will usually migrate to the surface by the second main body firing. If this happens, it can mean stripping the entire restoration and refinishing the metal. A properly finished restoration should be cleaned in an ultrasonic to remove any abrasive particles, dirt and oils that are attached to the surface during finishing. These tiny particles are the number one cause of bubbles in a fired ceramic restoration. The cleaned restoration should then be run on an oxidation cycle in your porcelain furnace according to the alloy manufacturer’s recommendations to expel any gasses that may be present in the metal from the casting and finishing process.
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