There is an increasing demand for materials which can be processed into dental restorative products, such as crowns, inlays and bridges, by means of computer controlled milling machines.  Such CAD/CAM methods are very attractive as they allow to provide the patient quickly with the desired restoration.  A so-called chair-side treatment is thus possible for the dentist. 
However, materials suitable for processing via computer aided design/computer aided machining (CAD/CAM) methods have to meet a very specific profile of properties. 
First of all, they need to have in the finally prepared restoration appealing optical properties, such as translucence and shade, which imitate the appearance of the natural teeth.  They further need to show high strength and chemical durability so that they can take over the function of the natural tooth material and maintain these properties over a sufficient period of time while being permanently in contact with fluids in the oral cavity which can even be aggressive, such as acidic in nature. 
Secondly and very importantly, it should be possible to machine them in an easy manner into the desired shape without undue wear of the tools and within short times.  This property requires a relatively low strength of the material and is therefore in contrast to the desired properties mentioned above for the final restoration. 
The difficulty of combining the properties of low strength in the stage of the material to be processed and a high strength of the final restoration is reflected by the known materials for a CAD/CAM processing which are in particular with respect to an easy machinability unsatisfactory. 
DE-A-197 50 794 discloses lithium disilicate glass ceramics which are primarily intended to be shaped to the desired geometry by a hot-pressing process wherein the molten material is pressed in the viscous state.  It is also possible for these materials to be shaped by computer aided milling processes.  However, it has been shown that the machining of these materials results in a very high wear of the tools and very long processing times.  These disadvantages are caused by the high strength and toughness primarily imparted to the materials by the lithium disilicate crystalline phase.  Moreover, it has been shown that the machined restorations show only a poor edge strength.  The term “edge strength” refers to the strength of parts of the restoration having only a small thickness in the range of few 1/10 mm. 
Further approaches of achieving easy machinability together with a high strength of the final restoration have also been made.  EP-B-774 993 and EP-B-817 597 describe ceramic materials on the basis of Al2O3 or ZrO2 which are machined in an unsintered state which is also referred to as “green state”. 
Subsequently, the green bodys are sintered to increase the strength.  However, these ceramic materials suffer from a drastical shrinkage of up to 50% by volume (or up to 30% as linear shrinkage) during the final sintering step.  This leads to difficulties in preparing the restorations with exactly the dimensions as desired.  The substantial shrinkage represents a particular problem if complicated restorations are manufactured, such as a multi-span bridge. 
From S. D. Stookey: “Chemical Machining of Photosensitive Glass”, Ind. Eng.  Chem.,  45, 115-118 (1993) and S. D. Stookey: “Photosensitively Opacifiable Glass” U.S. Pat.  No.  2,684,911 (1954) it is also known that in lithium silicate glass ceramics a metastable phase can be formed at first.  For example in photosensitive glass ceramics (Fotoform®, FotoCeram®) Ag-particles are formed using UV-light.  These Ag-particles serve as crystallization agent in a lithium metasilicate phase.  The areas which were exposed to light are in a subsequent step washed out by diluted HF.  This procedure is possible since the solubility of the lithium metasilicate phase in HF is much higher than the solubility of the parent glass.  The glass portion remaining after said solubilizing process (Fotoform®) can be transferred into a lithium disilicate glass ceramic (FotoCeram®) by an additional heat treatment. 
Also investigations of Borom, e.g. M.-P. Borom, A. M. Turkalo, R. H. Doremus: “Strength and Microstructure in Lithium Disilicate Glass-Ceramics”, J. Am.  Ceram.  Soc.,  58, No.  9-10, 385-391 (1975) and M.-P. Borom, A. M. Turkalo, R. H. Doremus: “Verfahren zum Herstellen von Glaskeramiken” DE-A-24 51 121 (1974), show that a lithium disilicate glass ceramic can in the first instance crystallize in varying amounts as metastable lithium metasilicate phase.  However, there also exist compositions which crystallize in the form of the disilicate phase from the beginning and the metasilicate phase is not present at all.  A systematic investigation of this effect has not become known.  From the investigations of Borom it is also known that the glass ceramic which contains lithium metasilicate as the main phase has a reduced strength compared to the one of a glass ceramic which only contains a lithium disilicate phase. 
It has further been found out that the presence of ZnO in lithium silicate glass ceramics of the prior art is undesirable especially when highly translucent dental restorations are to be produced.  Under such circumstances, the strong opalescent effect caused by ZnO is apparent and results in unacceptable optical properties for a restoration which is to imitate the natural tooth material.