generally quite brittle and prone to fracture. Many classifications have been used to categorise dental ceramics, with the simplest, based on the material from which they are made, i.e. silica, alumina or zirconia. Silica Silica-based ceramics are highly aesthetic due to their high glass content and excellent optical properties due to the addition of filler particles which enhance opalescence, fluorescence which can mimic the colour of natural enamel and dentine. These ceramics, however, suffer from poor mechanical strength, and therefore often used for veneering stronger substructures. Examples include aluminosilicate glass, e.g. feldspathic, synthetic porcelain, and leucite reinforced ceramics. Mechanical properties can be improved by the addition of filler particles, e.g. lithium disilicate, and are therefore termed glass ceramics. Glass-ceramics can be used alone to make all-ceramic restorations either as a single form (termed uni-layered) or can act as a substructures for subsequent veneering (or layering) with weaker feldspathic porcelain (restorations termed bi-layered). Alumina Alumina (aluminium oxide) was introduced as a dental substructure (core) in 1989 when the material was slip cast, sintered, and infiltrated with glass. More recently, glass-infiltrated alumina cores are produced by electrophoretic deposition, a rapid nanofabricating process. During this process, particles of a slip are brought to the surface of a dental die by an electric current, thereby forming a precision-fitting core greenbody in seconds. Margins are then trimmed and the greenbody is sintered and infiltrated with glass. Glass-infiltrated alumina has significantly higher porcelain bond strength over CAD/CAM produced zirconia and alumina cores without glass. Alumina cores without glass are produced by milling pre-sintered blocks of the material utilizing a CAD/CAM dentistry technique. Cores without glass must be oversized to compensate for shrinkage that occurs when the core is fully sintered. Milled cores are then sintered and shrink to the correct size. All alumina cores are layered with tooth tissue-like feldspathic porcelain to make true-to-life color and shape. Dental artists called ceramists, can customize the "look" of these crowns to individual patient and dentist requirements. Alumina cores have better translucency than zirconia, but worse than lithium disilicate. Zirconia Yttria-stabilized zirconia, also known simply as zirconia, is a very hard ceramic that is used as a strong base material in some full ceramic restorations. Zirconia is relatively new in dentistry and the published clinical data is correspondingly limited. The zirconia used in dentistry is zirconium oxide (ZrO2) which has been stabilized with the addition of yttrium oxide. Yttria-stabilized