materials; in addition, the restoration may increase wear on opposing teeth. Classification by Processing Technique Powder/liquid, glass-based systems Machinable or pressable blocks of glass-based systems CAD/CAM or slurry, die-processed, mostly crystalline systems Classification of crystalline ceramics Types of dental ceramics The range of dental ceramics determined by their respective firing temperatures are: Ultra-low Fired below 850 °C - mainly used for shoulder ceramics (aims to combat the problem of shrinkage, specifically at the margins of the prep, when the early sintered ceramic state is fired to produce the final restoration), to correct minor defects and to add colour/shading to restorations Low fusing Fired between 850 and 950 °C - to prevent the occurrence of distortion, this type of ceramic should not be subjected to multiple firings Higher fusing This type is used mainly for denture teeth Laboratory procedure The dentist will usually specify a shade or combination of shades for different parts of the restoration, which in turn corresponds to a set of samples containing the porcelain powder. There are two types of porcelain restorations: Porcelain fused to metal Complete porcelain Ceramic restorations can be built on a refractory die, which is a reproduction of a prepared tooth made of a strong material with the ability to withstand high temperatures, or it can be constructed on a metal coping or core. For ceramic fused to metal restorations, the black color of metal is first masked with an opaque layer giving it a shade of white before consecutive layers are built up. The powder corresponding to the desired shade of dentine base is mixed with water before it is fired. Further layers are built up to mimic the natural translucency of the enamel of the tooth. The porcelain is fused to a semi-precious metal or precious metal, such as gold, for extra strength. Systems which use an aluminium oxide, zirconium oxide or zirconia core instead of metal, produces complete porcelain restorations. Firing Once the mass has been built up, it is fired to allow fusion of the ceramic particles which in turn forms the completed restoration; the process by which this is done is referred to as ‘firing’. The first firing forces water out and allows the particles to coalesce. During this initial process, a large amount of shrinkage occurs until the mass reaches an almost void-free state; to overcome this the mass is built-up to a size larger than the