『Historical perspective on dental casting alloys』Published by Saunders
Written by Kenneth J. Anusavice
Historical perspective on dental casting alloys
The twentieth century generated substantially new changes to dental prosthetic materials. The major factors that are dividing new developments are
（1）economy-the new material performs the same function as the old material but at a lower cost;
（2）performance-the new material performs better than the old product in some desirable way, such as ease of processing, improved handling characteristics, or increased fracture resistance; and
（3）aesthetics-the new materials provides a more aesthetic result, such as increased translucency. A brief description of the evolution of the currently marketed alloys is appropriate to understand the rationale for the development of the wide variety of alloy formulations. A summary of the major events in the evolution of dental casting alloys is shown in Table 19-1.
1905－The Lost Wax Process
Taggart’s presentation to the New York Odontological Group in 1907 on the fabrication of cast inlay restoration developed in 1905 often has been acknowledged as the first reported application of the lost-wax technique in dentistry. The inlay technique described by Taggart was an instant success. It soon led to the casting of inlays, onlays, crowns, fixed partial dentures（FPDs）, and frameworks for removable partial dentures. Because pure gold did not have the physical properties required for producing durable restorations, existing jewelry alloys were quickly adopted. These gold alloys were further strengthened with addition of copper, silver, or platinum. Gold alloys were chosen because of their biocompatibility and ease of use.
1932－Classification of Gold-Based Casting Alloys
In 1932, the dental materials group at the National Bureau of Standards surveyed the alloys being used and classified them as TypeⅠ（Soft, Vickers hardness number[HV] between 50 and 90）, TypeⅡ（Medium, HV between 90 and 120）, TypeⅢ（Hard, HV between 120and
150）,and TypeⅣ（Extra Hard, HV≧150）. This classification became the basis for American National Standards Institute /American Dental Association (ANSI/ADA) Specification No.5 and later, ISO Standards 15592. Because of the multitude of casting alloys of diverse composition and applications, it is extremely difficult to devise a universally acceptable classification system.
During this period, the results of some tarnish tests suggest that alloys with a gold content lower than 65% to75%tarnished too readily for dental use.
In the following years, several patents were issued for alloys containing palladium as substitute for platinum. By 1948, the composition of dental noble alloys for cast metal restorations had become rather diverse. With these formulations, the tarnishing tendency of the original alloys apparently had disappeared. It is now know that, in gold alloys, palladium counteracts the tarnish potential of silver, allowing alloys with a lower gold content to be used successfully.