In the early 1900s, Taggart developed a technique for making dental restorations from metal that was melted and cast into a mold using the lost wax technique (see Chapter 18). Pure metals are seldom used in dentistry, because they lack properties that make them useful in the oral cavity. Therefore, they are usually combined with other metals in portions that achieve desirable physical and mechanical properties. This combination of two or more metals is called an alloy. Alloys used with the lost wax technique are called dental casting alloys. Unlike amalgam, restorations made from these alloys are not placed directly into the preparation but are made outside the mouth using an indirect technique, then are cemented in place (see Chapters 6 and 13). Cast-metal restorations can be classified as intracoronal (preparation is made within the crown of the tooth) or extracoronal (preparation is made primarily on the outside of the crown of the tooth). An inlay is an intracoronal restoration, whereas an onlay has both intracoronal and extracoronal components in that it has an inlay preparation and also covers the outer surface of one or more cusps. Other extracoronal cast restorations include partial coverage ( and crowns) and full coverage crowns (Figure 10-1). Cast metal alloys can also be used to make fixed partial dentures (bridges) and removable partial dentures for replacement of missing teeth.

The American Dental Association classifies dental casting alloys according to their noble metal content and divides them into three categories. First are the high-noble alloys that must contain at least 60% by weight noble alloys, of which gold must account for at least 40% by weight. Second are the noble alloys that do not require any gold but must contain at least 25% noble metals by weight. Third are the base-metal alloys that do not require gold and require less than 25% by weight of noble metals. The International Standards Organization (ISO) also sets specifications for casting alloys that can be found at its website (www.iso.org).

Porcelain bonding alloys are similar to the other casting alloys with similar physical properties. They are also classified as high-noble, noble, and base-metal alloys (Figure 10-2). However, they have minor changes in their composition that make them compatible with dental ceramic materials. The most common ceramic materials used with these metals are conventional feldspar-based porcelains. The metals in porcelain bonding alloys are selected and blended so that they have the ability to withstand the high temperatures at which porcelain is fired without distorting or melting. They also have a lower thermal expansion than gold alloys used for all-metal crowns, so they do not expand too greatly and crack the brittle porcelain that lies over top. Additionally, small amounts of metals such as indium, iron, tin, or gallium are added to form the oxides on the metal surface to which the porcelain will bond (or fuse) at high temperature. Alloys that contain silver and copper may cause green staining (called greening) of the porcelain when fired at high temperatures, so they are usually avoided for use with ceramic materials. Most of the alloys are silver (white) in color, but alloys that are gold colored (yellow) contain mostly gold with platinum and palladium added to increase the melting temperature. Titanium and titanium alloys have such high melting temperatures and difficulty in handling that they have limited use for ceramic-metal crowns.