Introduction to Metal Ceramics
This textbook covers only one porcelain building technique and does not discuss the entire porcelain-fused-to-metal fabrication procedures.
In fixed prosthodontics, it is essential to understand basic knowledge about crown and bridge procedures, occlusal theory, crown carving techniques, and material theory and usage. For example, the operator should understand technical and physical characteristics of metal alloys. He or she should be able to select a suitable alloy, decide the correct thickness for the metal framework and size of the connectors, select the appropriate porcelain that is compatible with the thermal expansion of the alloy, decide the firing cycle and soldering procedures (presoldering or postsoldering), in addition to many other details.
Porcelain can be especially difficult to handle if the operator does not know its physical characteristics such as thermal expansion, coloration specifications, firing method, and amount of firing shrinkage, all of which is learned through experience by handling the material, not from a manual. Operators should understand how and when to use certain materials, instruments, and techniques such as high fusing investment, porcelain furnaces, and pretreatment techniques for metal surfaces.
Other factors such as alloy distortion, creating internal cracks, porcelain condensation, the color theory of shade selection and shade matching, and recontouring porcelain should also be studied. Basically, a comprehensive knowledge of dental technology is required for metal ceramics. The steps before porcelain building are discussed briefly in this chapter.
It is very important to design the metal framework for strength since the strength of the ceramics depends on its framework.
Operators tend to overcompensate for the amount of space needed for porcelain because they stress the importance of esthetics. As a result, the metal framework is thin and weak, and its strength is compromised considerably. If the metal framework for a crown is thin, it does not cause a big problem since a crown can withstand excessive occlusal pressure. On the other hand, a bridge must withstand more occlusal pressure and requires adequate strength.
To make the metal framework, the crown should be waxed completely for full contour then the wax is cut back to create enough space for the porcelain; this procedure is referred to as cut-back. Only a cut-back method can create the ideal metal framework with the most effective design to support metal ceramic restorations. The metal framework should have 0.3–0.4 mm minimum metal thickness. The larger the framework, the better strength which is especially important for a bridge. Regardless of the type of alloy, either precious, semiprecious, or nonprecious, the metal framework should have a specific thickness as mentioned above.
As mentioned previously, the strength of the porcelain-fused-to-metal restoration depends on the strength of the metal framework. This strength depends on the condition that the porcelain adheres to the metal framework firmly. For porcelain-fused-to-metal procedures, pretreatment of the metal surface directly affects the strength of the bond between porcelain and the metal surface. Pretreatment varies depending on the type of alloy and includes surface grinding, cleaning, heating, and acid pickling.
To minimize the generation of interfacial bubbles on precious metal alloys (“soft” metal alloys), the metal surface should be prepared using a fissure or carbide bur. After grinding the metal surface, clean using a steam-cleaner or an organic solvent such as CCl4 (carbon tetrachloride) or chl/>