Lithium silicate glass ceramic

0001The invention primarily relates to lithium silicate glass ceramic materials which can be easily shaped by machining and subsequently converted into shaped products with high strength.

0002There 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.

0003However, materials suitable for processing via computer aided design / computer aided machining (CAD/CAM) methods have to meet a very specific profile of properties.

0004First 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.

0005Secondly 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.

0006The 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.

0007<patcit id="pcit0001" dnum="DE19750794A"><text>DE-A-197 50 794</text></patcit> 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.

0008Further approaches of achieving easy machinability together with a high strength of the final restoration have also been made. <patcit id="pcit0002" dnum="EP774993B"><text>EP-B-774 993</text></patcit> and <patcit id="pcit0003" dnum="EP817597B"><text>EP-B-817 597</text></patcit> 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.

0009From <nplcit id="ncit0001" npl-type="s"><text>S.D. Stookey: "Chemical Machining of Photosensitive Glass", Ind. Eng. Chem., 45, 115-118 (1993</text></nplcit>) and S.D. Stookey: "Photosensitively Opacifiable Glass" <patcit id="pcit0004" dnum="US2684911A"><text>US-A-2 684 911 (1954</text></patcit>) 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.

0010Also investigations of Borom, e.g. <nplcit id="ncit0002" npl-type="s"><text>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</text></nplcit>) and M.-P. Borom, A.M. Turkalo, R.H. Doremus: "Verfahren zum Herstellen von Glaskeramiken" <patcit id="pcit0005" dnum="DE2451121A"><text>DE-A-24 51 121 (1974</text></patcit>), 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.

0011It 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.

0012It is, therefore, an object of the present invention to eliminate these disadvantages and in particular to provide a material which can be easily shaped by computer-aided milling and trimming processes and can subsequently be converted into a high-strength dental product which also displays a high chemical durability and excellent optical properties and exhibits a drastically reduced shrinkage during said final conversion, and achieves all these properties without the need for ZnO as a component.

0013This object is achieved by the lithium silicate glass ceramic according to numbered paragraphs 1 to 17.

0014It has surprisingly been shown that by using a starting glass of a very specific composition and a specific process it is possible to provide in particular a glass ceramic which has metastable lithium metasilicate (Li₂SiO₃) as main crystalline phase rather than lithium disilicate (Li₂Si₂O₅). This lithium metasilicate glass ceramic has a low strength and toughness and hence can be easily machined into the shape of even complicated dental restorations, but can after such machining be converted by a heat treatment into a lithium disilicate glass ceramic product with outstanding mechanical properties, excellent optical properties, in particular a strongly reduced opalescence, and very good chemical stability thereby undergoing only a very limited shrinkage.

0015The lithium silicate glass ceramic according to the invention comprises the following components: <tables id="tabl0001" num="0001"><table frame="none"><tgroup cols="3" colsep="0"><colspec colnum="1" colname="col1" colwidth="29mm" /><colspec colnum="2" colname="col2" colwidth="20mm" /><colspec colnum="3" colname="col3" colwidth="38mm" /><thead><row><entry valign="top">Component</entry><entry align="center" valign="top">wt. -%</entry><entry valign="top" /></row></thead><tbody><row rowsep="0"><entry>SiO₂</entry><entry align="right">64.0 - 75.0</entry><entry>, in particular 64.0 - 73.0</entry></row><row rowsep="0"><entry>Li₂O</entry><entry align="right">13.0 - 17.0</entry><entry /></row><row rowsep="0"><entry>K₂O</entry><entry align="right">2.0 - 5.0</entry><entry /></row><row rowsep="0"><entry>Al₂O₃</entry><entry align="right">0.5 - 5.0</entry><entry /></row><row rowsep="0"><entry>Nucleating agent</entry><entry align="right">2.0 - 5.0</entry><entry /></row><row rowsep="0"><entry>Me(II)O</entry><entry align="right">0 - 3.0</entry><entry /></row></tbody></tgroup></table></tables> with Me(II)O being selected from at least one of CaO, BaO, MgO and SrO, and comprises less than 0.1 wt.% of ZnO.

0016It is preferred that the glass ceramic is essentially free of ZnO.

0017It is surprising that even without presence of ZnO the glass ceramic according to the invention fulfils the mentioned multiple requirements. This was possible by the selection of the other components and their amounts and preferably by the ratios of some of these components to each other.

0018Preferred glass ceramics are those which comprise lithium metasilicate as main crystalline phase. Such glass ceramics are also referred to in the following as lithium metasilicate glass ceramics.

0019It has also been found out to be beneficial if the glass ceramic comprises 0 to 2.0 and preferably 0 to 1.5 wt.% of Me(II)O. Me(II)O is in particular selected from at least one of CaO and MgO. Particularly preferred glass ceramics comprise 0.1 to 1.0 wt.% of MgO.

0020The nucleating agent is preferably at least one of P₂O₅ and compounds of the elements Pt, Ag, Cu and W. It serves to induce formation of lithium metasilicate crystalls and is preferably P₂O₅.

0021Further, it was shown that a specific molar ratio of SiO₂ to Li₂O serves to ensure that upon the necessary heat treatment of a corresponding starting glass mainly lithium metasilicate and lithium disilicate, respectively, is produced. This is of particular importance. While a lithium metasilicate glass ceramic essentially free of lithium disilicate results in particular in an excellent machinability, a lithium disilicate restoration essentially free of the easily dissolvable lithium metasilicate has a very good chemical stability.

0022Thus, it was found preferable that the molar ratio of SiO₂ : Li₂O is at least 2.2 : 1, preferably at least 2.3 : 1, and most preferred in the range of 2.3 : 1 to 2.5 : 1.

0023Moreover, investigations revealed that the molar ratio of Al₂O₃ : K₂O is of significance for obtaining the desired tranclucence and the predominant crystallization of lithium metasilicate.

0024It is prefered that the molar ratio of Al₂O3 : K₂O is in the range of 1 : 0.5 to 1 : 2.0 and preferably is from 1 : 1 to 1 : 2.0.

0025There also exist preferred ranges for the amounts of components of the glass ceramic according to the invention. These can be used independently from each other.

0026It is preferred that the glass ceramic comprises 2.5 to 5.0 wt.% of Al₂O₃.

0027It is also preferred that the glass ceramic comprises 70.0 to 73.0 wt.% of SiO₂.

0028It is also preferred that the glass ceramic comprises 0 to 4.0, preferably 0.1 to 4.0, more preferably 1.0 to 4.0 and most preferred 1.5 to 3.0 wt.% of ZrO₂. If the emphasis is on the achieving of a high strength of the final lithium disilicate ceramic, then 0 to 2.0 wt.% to of ZrO₂ are advantageous.

0029It is further preferred that the glass ceramic comprises at least one of the following components in an amount of: <tables id="tabl0002" num="0002"><table frame="none"><tgroup cols="3" colsep="0"><colspec colnum="1" colname="col1" colwidth="55mm" /><colspec colnum="2" colname="col2" colwidth="20mm" /><colspec colnum="3" colname="col3" colwidth="34mm" /><thead><row><entry valign="top">Component</entry><entry valign="top">wt. -%</entry><entry valign="top" /></row></thead><tbody><row rowsep="0"><entry>Li₂O</entry><entry align="right">14.0 - 16.0</entry><entry /></row><row rowsep="0"><entry>K₂O</entry><entry align="right">3.0 - 4.5</entry><entry /></row><row rowsep="0"><entry valign="bottom">coloring and fluorescent metal oxides</entry><entry align="right" valign="bottom">0 - 7.5</entry><entry valign="bottom">, preferably 0.5 - 3.5.</entry></row></tbody></tgroup></table></tables>

0030The metal of the coloring and fluorescent metal oxides is preferably selected from group f-elements and in particular from the group of Ta, Tb, Y, La, Er, Pr, Ce, Ti, V, Fe and Mn. The colouring or fluorescent oxides ensure that the colour of the final dental product matches that of the natural tooth material of the patient in question.

0031Further, the glass ceramic may comprise as additional component Na₂O in an amount of 0 to 2.0 wt.-%.

0032Additional components to enhance the technical processability of the glass may also be present. Such additional components may therefore be in particular compounds such as B₂O₃ and F which in general amount to 0 to 5.0 % by weight.

0033Generally the amount of lithium metasilicate is 20 to 80 vol.-%. It has surprisingly been shown that a specific volume portion of lithium metasilicate should be present to achieve excellent processing properties. Thus, it is further preferred that the lithium metasilicate crystalline phase forms 20 to 50 vol% and in particular 30 to 40 vol% of the lithium silicate glass ceramic. Such a part of the volume leads to the crystals being present rather remote from each other and hence avoids a too high strength of the glass ceramic.

0034If the emphasis is on the achieving of a high strength of the lithium disilicate ceramic, then the lithium metasilicate phase preferably forms more than 50 and up to 80 vol.% of the lithium silicate glass ceramic.

0035The lithium metasilicate crystals are preferably of lamellar or platelet form. This leads to a very good machinability of the lithium metasilicate glass ceramic without use of high energy and without uncontrolled breaking. The latter aspect of uncontrolled breaking is for example known from glasses which are generally unsuitable for machining. It is assumed that the preferred morphology of the lithium metasilicate crystals is also responsible for the surprisingly high edge strength of products, e.g. complicated dental restorations, can be made from the lithium metasilicate glass ceramic according to the invention.

0036The lithium silicate glass ceramic according to the invention preferably is in the form of a blank. The blank usually takes the form of a small cylinder or a rectangular block. The exact form depends on the specific apparatus used for the desired computer-aided machining of the blank.

0037After the machining, the lithium silicate glass ceramic has preferably the shape of a dental restoration, such as an inlay, an onlay, a bridge, an abutment, a facing, a veneer, a facet, a crown, a partial crown, a framework or a coping.

0038A lithium silicate glass ceramic according to the invention which comprises lithium disilicate as main crystalline phase is a further preferred embodiment of the invention. It is preferred that this lithium disilicate glass ceramic is formed in a process wherein the lithium metasilicate of a glass ceramic according to the invention is converted to lithium disilicate crystals.

0039A dental product made from lithium disilicate glass ceramic according to the invention is a further preferred embodiment of the invention. It is preferred that such product is formed in a process wherein the lithium metasilicate of a glass ceramic according to the invention is converted to lithium disilicate crystals.

0040The lithium metasilicate glass ceramic according to the invention is preferably prepared by a process which comprises <ol id="ol0001"><li>(a) producing a starting glass containing the components of the glass ceramic,</li><li>(b) subjecting the starting glass to a first heat treatment at a first temperature to give a glass product which contains nuclei suitable for forming lithium metasilicate crystals,</li><li>(c) subjecting the glass product to a second heat treatment at a second temperature which is higher than the first temperature to obtain the lithium silicate glass ceramic with lithium metasilicate as the main crystalline phase.</li></ol>

0041In step (a), usually a melt of a starting glass is produced which contains the components of the glass ceramic. For this purpose a corresponding mixture of suitable starting materials, such as carbonates, oxides, and phosphates, is prepared and heated to temperatures of, in particular 1300 to 1600°C, for 2 to 10 hours. In order to obtain a particularly high degree of homogeneity, the glass melt obtained may be poured into water to form glass granules and the glass granules obtained are melted again.

0042It further preferred that the melt of the starting glass is cooled, such as to room temperature, before subjecting it to step (b). The melt of the starting glass is also usually poured into a mould to form a starting glass blank.

0043In some cases it is convenient to control a cooling procedure such that it not only relaxes the glass, but also effects the first heat treatment of step (b).

0044In step (b) the starting glass is subjected to a first heat treatment at a first temperature to cause formation of nuclei for lithium metasilicate crystals. Preferably, this first heat treatment comprises heating the starting glass to a temperature of 500 to 600°C for a period of about 10 minutes to 3 hours. This results in formation of a great number of nuclei that ensure a very satifactory crystal growth. It also ensures that in the further processing after step (c) to give a lithium disilicate glass ceramic a very homogeneous lithium disilicate structure can be obtained.

0045It is also advantageous that the second heat treatment in step (c) comprises heating the glass product to a second temperature of 570° to 750°C, preferably 570 to 670 °C, and more preferably to about 650 °C.

0046It has further surprisingly been shown that relatively high temperatures lead to high amounts of lithium metasilicate which in turn lead to a high amount of lithium disilicate in the third heat treatment. Such high amounts of lithium disilicate impart a high strength to the ceramic. Thus, if the emphasis is on the achieving a high strength final product, then it is advantageous to carry out the second heat treatment at 680° to 720°C, and preferably 690° to 710°C and more preferably about 700°C.

0047Depending on the specific composition of a selected starting glass, it is possible for the skilled person by means of differential scanning calorimetry (DSC) and x-ray diffraction analyses to determine suitable conditions in steps (b) and (c) to result in glass ceramics having the desired morphology and size of the crystals of lithium metasilicate. Moreover, these analyses allow also the identification of conditions avoiding or limiting the formation of undesirable other crystalline phases, such as of the high-strength lithium disilicate, or of cristobalite and lithium phosphate.

0048Usually, the starting glass of step (a), the glass product of step (b), or preferably the lithium metasilicate glass ceramic of step (c) is shaped to a desired geometry by machining or by hot pressing. The machining is in particular performed by grinding, trimming or milling and preferably controlled by a computer using CAD/CAM-based milling devices. This allows a so-called chair-side treatment of the patient by the dentist.

0049It is a particular advantage of the lithium metasilicate glass ceramic according to the invention that it can be shaped by machining without the undue wear of the tools observed with the tough and high-strength prior art materials. This is in particular shown by the easy possibility to polish and trim the glass ceramics according to the invention. Such polishing and trimming processes therefore require less energy and less time to prepare an acceptable product having the form of even very complicated dental restorations.

0050Further, the lithium metasilicate glass ceramic according to the invention can advantageously be processed to a lithium disilicate glass ceramic of high strength, which usually has a content of 50 to 85 vol.% and preferably 65 to 80 vol.% of crystalline lithium disilicate phase.

0051This is preferably effected by a process wherein the prepared lithium metasilicate glass ceramic of step (c) is subjected to a third heat treatment at a third temperature of 830 to 880 °C for a period of 10 to 60 minutes. This heat treatment can also be effected when hot-pressing the lithium metasilicate glass ceramic to achieve a shaping.

0052Thus, the lithium metasilicate glass ceramic can be further processed to the lithium dilsilicate glass ceramic of desired shape e.g. by both (i) CAD/CAM and a heat treatment or (ii) a hot-pressing. This is very advantageous for the user.

0053It is also possible to use for these purposes a corresponding lithium silicate glass which comprises nuclei suitable for formation of lithium metasilicate crystals. This glass is a precursor of the lithium metasilicate glass ceramic and the lithium disilicate glass ceramic of the invention. The invention is also directed to such a glass. It is obtainable by the above process in step (b). This lithium silicate glass according to the invention comprises the following components: <tables id="tabl0003" num="0003"><table frame="none"><tgroup cols="3" colsep="0"><colspec colnum="1" colname="col1" colwidth="29mm" /><colspec colnum="2" colname="col2" colwidth="22mm" /><colspec colnum="3" colname="col3" colwidth="38mm" /><thead><row><entry valign="top">Component</entry><entry align="center" valign="top">wt. -%</entry><entry valign="top" /></row></thead><tbody><row rowsep="0"><entry>SiO₂</entry><entry align="right">64.0 to 75.0</entry><entry>, in particular 64.0 - 73.0</entry></row><row rowsep="0"><entry>Li₂O</entry><entry align="right">13.0 - 17.0</entry><entry /></row><row rowsep="0"><entry>K₂O</entry><entry align="right">2.0 - 5.0</entry><entry /></row><row rowsep="0"><entry>Al₂O₃</entry><entry align="right">0.5 - 5.0</entry><entry /></row><row rowsep="0"><entry>Nucleating agent</entry><entry align="right">2.0 - 5.0</entry><entry align="right" /></row><row rowsep="0"><entry>Me(II)O</entry><entry align="right">0 - 3.0</entry><entry align="right" /></row></tbody></tgroup></table></tables> with Me(II)O being selected from at least one of CaO, BaO, MgO and SrO, and which comprises less than 0.1 wt.% of ZnO, and comprises nuclei suitable for formation of lithium metasilicate crystals.

0054For manufacturing a dental restoration by the hot pressing technique, it is preferred to use a lithium silicate glass ingot according to the invention having nuclei for lithium metasilicate. This ingot is heated to about 700 to 1200°C to convert it into a viscous state. The heat treatment can be conducted in a special furnace (EP 500^®, EP 600^®, Ivoclar Vivadent AG). The ingot is embedded in a special investment material. During the heat treatment the ingot will be crystallized. The main crystal phase is then lithium disilicate. The viscous glass ceramic flows under a pressure of 1 to 4 MPa into the cavity of the investment material to obtain the desired shape of the dental restoration. After cooling the investment mould to room temperature the lithium disilicate restoration can be divested by sand blasting. The restoration can be further coated with a glass or a glass ceramic by sintering or a hot pressing technique to obtain the finalized dental restoration with natural aesthetics.

0055The same hot-pressing technique can be applied to the lithium metasilicate glass ceramic according to the invention which will be converted to lithium disilicate glass ceramic.

0056A preferred method for converting the lithium metasilicate glass ceramic according to the invention to a lithium disilicate glass ceramic dental restoration by the CAD/CAM technique uses lithium metasilicate glass ceramic blanks, e.g. blocks, having a strength of about 80 to 150 Mpa. These can be easily machined in a CAM unit like Cerec 2^® or Cerec 3^® (Sirona, Germany). Larger milling machines such as DCS precimill^® (DCS, Switzerland) are also suitable. The block is therefore positioned in the grinding chamber by a fixed or integrated holder. The CAD construction of the dental restoration is done by a scanning process or an optical camera in combination with a software tool. The milling process needs for one unit about 10 to 15 minutes. Copy milling units such as Celay® (Celay, Switzerland) are also suitable for machining the blocks. First, a 1:1 copy of the desired restoration is fabricated in hard wax. The wax model is then mechanically scanned and 1:1 mechanically transmitted to the grinding tool. The grinding process is therefore not controlled by a computer. The milled dental restoration has to be subjected to the third heat treatment to obtain the desired lithium disilicate glass ceramic with high strength and tooth-like color. The product can be further coated with a glass or a glass ceramic by sintering or hot pressing technique to obtain the final dental restoration with natural aesthetics.

0057The lithium metasilicate glass ceramic according to the invention can also be used for coating a dental restoration. The coating is preferably effected by hot-pressing the lithium metasilicate glass ceramic onto the restoration.

0058It was surprisingly found that the easily machinable lithium metasilicate glass ceramic according to the invention can be converted by a further heat treatment into a lithium disilicate glass ceramic product having also excellent optical properties. The conversion to a lithium disilicate glass ceramic is associated with a very small linear shrinkage of only about 0.2 to 0.3 %, which is almost negligible in comparison to a linear shrinkage of up to 30 % when sintering ceramics. The obtained lithium disilicate glass ceramic has not only excellent mechanical properties, such as high strength, but also displays other properties required for a material for dental restorations. It is emphasized that these properties are achieved without the need for ZnO as a component which may be detrimental for specific restorations in view of its strong opalescent effect.

0059Thus, a product is finally obtained which has all the beneficial mechanical, optical and stability properties making lithium disilicate ceramics attractive for use as dental restorative materials. However, these properties are achieved without the disadvantages of the conventional materials when shaped by using a CAD/CAM based process, in particular the undue wear of the milling and trimming tools.

0060In the following, further embodiments of the invention are described by numbered paragraphs. 1. A lithium silicate glass ceramic which comprises the following components: <tables id="tabl0004" num="0004"><table frame="none"><tgroup cols="3" colsep="0"><colspec colnum="1" colname="col1" colwidth="29mm" /><colspec colnum="2" colname="col2" colwidth="20mm" /><colspec colnum="3" colname="col3" colwidth="35mm" /><thead><row><entry valign="top">Component</entry><entry align="center" valign="top">wt. -%</entry><entry valign="top" /></row></thead><tbody><row rowsep="0"><entry>SiO₂</entry><entry align="right">64.0 - 75.0</entry><entry>, preferably 64.0 - 73.0</entry></row><row rowsep="0"><entry>Li₂O</entry><entry align="right">13.0 - 17.0</entry><entry /></row><row rowsep="0"><entry>K₂O</entry><entry align="right">2.0 - 5.0</entry><entry /></row><row rowsep="0"><entry>Al₂O₃</entry><entry align="right">0.5 - 5.0</entry><entry /></row><row rowsep="0"><entry>Nucleating agent</entry><entry align="right">2.0 - 5.0</entry><entry /></row><row rowsep="0"><entry>Me(II)O</entry><entry align="right">0 - 3.0</entry><entry /></row></tbody></tgroup></table></tables> and which comprises less than 0.1 wt.% of ZnO, with Me(II)O being selected from at least one of CaO, BaO, MgO and SrO. 2. Glass ceramic according to paragraph 1 which is essentially free of ZnO. 3. Glass ceramic according to paragraph 1 or 2, which comprises lithium metasilicate as main crystalline phase. 4. Glass ceramic according to any one of paragraphs 1 to 3, which comprises 0 to 2.0 and preferably 0 to 1.5 wt.% of Me(II)O. 5. Glass ceramic according to any one of paragraphs 1 to 4, wherein Me(II)O is selected from at least one of CaO and MgO. 6. Glass ceramic according to any one of paragraphs 1 to 5, which comprises 0.1 to 1.0 wt.% of MgO. 7. Glass ceramic according to any one of paragraphs 1 to 6 wherein the molar ratio of SiO₂ : Li₂O is at least 2.2 : 1, preferably at least 2.3 : 1, and most preferred in the range of 2.3 : 1 to 2.5 : 1. 8. Glass ceramic according to any one of paragraphs 1 to 7, wherein the molar ratio of Al₂O₃ : K₂O is in the range of 1 : 0.5 to 1 : 2.0 and preferably 1 : 1 to 1 : 2.0. 9. Glass ceramic according to any one of paragraphs 1 to 8, which comprises 2.5 to 5.0 wt.% of Al₂O₃. 10. Glass ceramic according to any one of paragraphs 1 to 9, which comprises 70.0 to 73.0 wt.% of SiO₂. 11. Glass ceramic according to any one of paragraphs 1 to 10 which comprises 0 to 4.0, preferably 0.1 to 4.0, more preferably 1.0 to 4.0 and most preferred 1.5 to 3.0 of ZrO₂. 12. Glass ceramic according to any one of paragraphs 1 to 11 which comprises at least one of the following components in an amount of: <tables id="tabl0005" num="0005"><table frame="none"><tgroup cols="3" colsep="0"><colspec colnum="1" colname="col1" colwidth="55mm" /><colspec colnum="2" colname="col2" colwidth="20mm" /><colspec colnum="3" colname="col3" colwidth="34mm" /><thead><row><entry valign="top">Component</entry><entry align="center" valign="top">wt. -%</entry><entry valign="top" /></row></thead><tbody><row rowsep="0"><entry>Li₂O</entry><entry align="right">14.0 - 16.0</entry><entry /></row><row rowsep="0"><entry>K₂O</entry><entry align="right">3.0 - 4.5</entry><entry /></row><row rowsep="0"><entry>coloring and fluorescent metal oxides</entry><entry align="right">0.5 - 7.5</entry><entry>, preferably 0.5 - 3.5.</entry></row></tbody></tgroup></table></tables> 13. Glass ceramic according to any one of paragraphs 1 to 12 which further comprises at least one of the following additional components <tables id="tabl0006" num="0006"><table frame="none"><tgroup cols="2" colsep="0" rowsep="0"><colspec colnum="1" colname="col1" colwidth="22mm" /><colspec colnum="2" colname="col2" colwidth="14mm" /><thead><row><entry rowsep="1" valign="top">Component</entry><entry rowsep="1" valign="top">wt. -%</entry></row></thead><tbody><row><entry>Na₂O</entry><entry>0 - 2.0</entry></row><row><entry>B₂O₃</entry><entry>0 - 5.0</entry></row><row><entry>F</entry><entry>0 - 5.0.</entry></row></tbody></tgroup></table></tables> 14. Glass ceramic according to any one of paragraphs 1 to 13, wherein the nucleating agent is at least one of P₂O₅ and compounds of the elements Pt, Ag, Cu and W. 15. Glass ceramic according to any one of paragraphs 2 to 14, wherein the lithium metasilicate forms more than 50 and up to 80 vol.-% of the lithium silicate glass ceramic. 16. Glass ceramic according to any one of paragraphs 1 to 15, which is in form of a blank or a dental restoration. 17. Glass ceramic according to paragraph 16, wherein the dental restoration is an inlay, an onlay, a bridge, an abutment, a facing, a veneer, a facet, a crown, a partial crown, a framework or a coping. 18. Glass ceramic according to paragraph 1 or any one of paragraphs 3 to 17 which comprises lithium disilicate as main crystalline phase. 19. Process for the preparation of the lithium metasilicate glass ceramic according to any one of paragraphs 2 to 17, which comprises <ol id="ol0002"><li>(a) producing a starting glass containing the components of the glass ceramic,</li><li>(b) subjecting the starting glass to a first heat treatment at a first temperature to give a glass product which contains nuclei suitable for forming lithium metasilicate crystals,</li><li>(c) subjecting the glass product to a second heat treatment at a second temperature which is higher than the first temperature to obtain the lithium silicate glass ceramic with lithium metasilicate as the main crystalline phase.</li></ol> 20. Process according to paragraph 19, wherein the first heat treatment in step (b) comprises heating the starting glass to a temperature of 500 to 600°C for a period of about 10 minutes to 3 hours. 21. Process according to paragraph 19 or 20, wherein the second heat treatment in step (c) comprises heating the glass product to a second temperature of 680° to 720°C, preferably 690 to 710°C, and more preferably to about 700°C. 22. Process according to any one of paragraphs 19 to 21, wherein the starting glass of step (a), the glass product of step (b), or the lithium metasilicate glass ceramic of step (c) is shaped to a desired geometry by machining or by hot pressing. 23. Process according to paragraph 22, wherein the machining is performed by grinding, trimming or milling. 24. Process for the preparation of the lithium disilicate glass ceramic of paragraph 18 which comprises conducting the process according to any one of paragraphs 19 to 23 and further subjecting the lithium silicate glass ceramic of step (c) to a third heat treatment at a third temperature of 830 to 880 °C for a period of 10 to 60 minutes. 25. Process according to paragraph 24, wherein the lithium disilicate glass ceramic is in form of a dental restoration. 26. Lithium silicate glass which comprises the following components: <tables id="tabl0007" num="0007"><table frame="none"><tgroup cols="2" colsep="0"><colspec colnum="1" colname="col1" colwidth="29mm" /><colspec colnum="2" colname="col2" colwidth="20mm" /><thead><row><entry valign="top">Component</entry><entry align="center" valign="top">wt. -%</entry></row></thead><tbody><row rowsep="0"><entry>SiO₂</entry><entry align="right">64.0 - 73.0</entry></row><row rowsep="0"><entry>Li₂O</entry><entry align="right">13.0 - 17.0</entry></row><row rowsep="0"><entry>K₂O</entry><entry align="right">2.0 - 5.0</entry></row><row rowsep="0"><entry>Al₂O₃</entry><entry align="right">0.5 - 5.0</entry></row><row rowsep="0"><entry>Nucleating agent</entry><entry align="right">2.0 - 5.0</entry></row><row rowsep="0"><entry>Me(II)O</entry><entry align="right">0 - 3.0</entry></row></tbody></tgroup></table></tables> with Me(II)O being selected from at least one of CaO, BaO, MgO and SrO, and which comprises less than 0.1 wt.% of ZnO, and comprises nuclei suitable for formation of lithium metasilicate crystals. 27. Glass according to paragraph 26, wherein the molar ratio of SiO₂ : Li₂O is at least 2.2 : 1, preferably at least 2.3 : 1, and most preferred in the range of 2.3 : 1 to 2.5 : 1. 28. Glass according to paragraph 26 and 27, wherein the molar ratio of Al₂O₃ : K₂O is in the range of 1 : 0.5 to 1 : 2.0 and preferably 1 : 1 to 1 : 2.0.

0061The invention is explained in more detail below on the basis of Examples.

<u>Examples</u>

Examples 1 to 8

0062A total of 8 different lithium metasilicate glass ceramics according to the invention with the chemical compositions given in Table I were prepared using the indicated second heat treatment. The obtained glass ceramics were then converted to the corresponding lithium disilicate glass ceramics using the indicated third heat treatment.

0063Firstly, samples of the corresponding starting glasses were melted in a platinum-rhodium crucible at a temperature of 1450 °C and for a period of 40 minutes. The glass melt was poured into water and the obtained granules were, after drying, again melted at 1500°C. The glass melts obtained were then poured into graphite moulds to give blocks. After relaxation of the glass blocks at 500 to 600°C for 10 minutes to 3 hours, they were subjected to the given second heat treatment. Before effecting the third heat treatment, the blocks were checked for their machinability by milling in a CAD-CAM milling machine (i.e. CEREC 3^®). Finally, the indicated third heat treatment was conducted. The crystal phases present after the second and third heat treatment were identified by XRD techniques and are given in table I.

0064Further, the opalescence of the products was visually assessed and the contrast value CR was determined according to BS 5612 (British Standard) using a spectral colorimeter (Minolta CM-3700d). The chemical stability in acetic acid was determined as well as the stability in artificial saliva. The corresponding data are to be found in the following Table II and show in particular the surprising combination of a lack of opalescence together with a high translucence and stability. The composition of the artificial saliva is given in table III.

0065The data obtained show that the lithium metasilicate glass ceramics according to the invention combine a very good machinability and high edge strength with the easy possibility to convert them by a simple heat treatment into lithium disilicate glass ceramics which have a very high bending strength as well as an excellent chemical durability and good translucence, all of which being properties which make them very attractive as materials useful for the manufacture of dental restorations.

Examples 9 to 12

0066Four glass ceramics according to the invention were prepared in analogous manner as examples 1 to 8. However, the heat treatment scheme was different. In addition each material was subjected to the schemes referred to as "Cycle A" and "Cycle B" which differ in the temperature used for the crystallization of lithium metasilicate, namely 650° and 700°C, respectively.

0067Details as to the materials prepared and tested as well as their properties are given in the table IV. It is apparent that the "Cycle B" treatment using a temperature of 700°C for the crystallization of lithium metasilicate leads to lithium disilicate glass ceramics having excellent strengths. <tables id="tabl0008" num="0008"><table frame="all"><title><b><u>Table I</u></b></title><tgroup cols="9"><colspec colnum="1" colname="col1" colwidth="27mm" /><colspec colnum="2" colname="col2" colwidth="27mm" /><colspec colnum="3" colname="col3" colwidth="27mm" /><colspec colnum="4" colname="col4" colwidth="27mm" /><colspec colnum="5" colname="col5" colwidth="27mm" /><colspec colnum="6" colname="col6" colwidth="27mm" /><colspec colnum="7" colname="col7" colwidth="27mm" /><colspec colnum="8" colname="col8" colwidth="27mm" /><colspec colnum="9" colname="col9" colwidth="27mm" /><thead><row><entry align="center" valign="middle"><b>Example</b></entry><entry align="center" valign="middle"><b>1</b></entry><entry align="center" valign="middle"><b>2</b></entry><entry align="center" valign="middle"><b>3</b></entry><entry align="center" valign="middle"><b>4</b></entry><entry align="center" valign="middle"><b>5</b></entry><entry align="center" valign="middle"><b>6</b></entry><entry align="center" valign="middle"><b>7</b></entry><entry align="center" valign="middle"><b>8</b></entry></row><row><entry valign="middle"><b>Molar ratio</b></entry><entry valign="middle" /><entry valign="middle" /><entry valign="middle" /><entry valign="middle" /><entry valign="middle" /><entry valign="middle" /><entry valign="middle" /><entry valign="middle" /></row></thead><tbody><row><entry valign="middle"><b>SiO2:Li2O</b></entry><entry align="center" valign="middle"><b>2.39:1</b></entry><entry align="center" valign="middle"><b>2.39 :1</b></entry><entry align="center" valign="middle"><b>2.4:1</b></entry><entry align="center" valign="middle"><b>2.39:1</b></entry><entry align="center" valign="middle"><b>2.39:1</b></entry><entry align="center" valign="middle"><b>2.39:1</b></entry><entry align="center" valign="middle"><b>2.39:1</b></entry><entry align="center" valign="middle"><b>2.39:1</b></entry></row><row><entry valign="middle"><b>AI2O3:K20</b></entry><entry align="center" valign="middle"><b>1:1.0</b></entry><entry align="center" valign="middle"><b>1:1.0</b></entry><entry align="center" valign="middle"><b>1:1.2</b></entry><entry align="center" valign="middle"><b>1:1.20</b></entry><entry align="center" valign="middle"><b>1:1.35</b></entry><entry align="center" valign="middle"><b>1:1.50</b></entry><entry align="center" valign="middle"><b>1:1.70</b></entry><entry align="center" valign="middle"><b>1:1.30</b></entry></row><row><entry valign="middle" /><entry align="center" valign="middle" /><entry align="center" valign="middle" /><entry align="center" valign="middle" /><entry align="center" valign="middle" /><entry align="center" valign="middle" /><entry align="center" valign="middle" /><entry align="center" valign="middle" /><entry align="center" valign="middle" /></row><row><entry valign="middle" /><entry align="center" valign="middle"><b>wt.-% (Mol%)</b></entry><entry align="center" valign="middle"><b>wt.-% (Mol%)</b></entry><entry align="center" valign="middle">w<b>t.-% (Mol%)</b></entry><entry align="center" valign="middle"><b>wt.-% (Mol%)</b></entry><entry align="center" valign="middle"><b>wt.-% (Mol%)</b></entry><entry align="center" valign="middle"><b>wt.-% (Mol%)</b></entry><entry align="center" valign="middle"><b>wt.-% (Mol%)</b></entry><entry align="center" valign="middle"><b>wt.-% (Mol%)</b></entry></row><row><entry valign="middle"><b>SiO2</b></entry><entry align="center" valign="middle">72.21 (66.12)</entry><entry align="center" valign="middle">70.64 (65.62)</entry><entry align="center" valign="middle">70.52 (65.52)</entry><entry align="center" valign="middle">70.78 (65.57)</entry><entry align="center" valign="middle">70.78 (65.56)</entry><entry align="center" valign="middle">70.78 (65.56)</entry><entry align="center" valign="middle">70.78 (65.55)</entry><entry align="center" valign="middle">70.78 (65.29)</entry></row><row><entry valign="middle"><b>K2O</b></entry><entry align="center" valign="middle">3.16 (1.85)</entry><entry align="center" valign="middle">3.09 (1.83)</entry><entry align="center" valign="middle">3.81 (2.26)</entry><entry align="center" valign="middle">3.76 (2.22)</entry><entry align="center" valign="middle">3.96 (2.34)</entry><entry align="center" valign="middle">4.16 (2.46)</entry><entry align="center" valign="middle">4.36 (2.58)</entry><entry align="center" valign="middle">3.36 (1.98)</entry></row><row><entry valign="middle"><b>Li2O</b></entry><entry align="center" valign="middle">14.99 (27.60)</entry><entry align="center" valign="middle">14.68 (27.43)</entry><entry align="center" valign="middle">14.64 (27.35)</entry><entry align="center" valign="middle">14.7 (27.38)</entry><entry align="center" valign="middle">14.7 (27.38)</entry><entry align="center" valign="middle">14.7 (27.37)</entry><entry align="center" valign="middle">14.7 (27.37)</entry><entry align="center" valign="middle">14.7 (27.26)</entry></row><row><entry valign="middle"><b>Al2O3</b></entry><entry align="center" valign="middle">3.45 (1.86)</entry><entry align="center" valign="middle">3.38 (1.85)</entry><entry align="center" valign="middle">3.35 (1.83)</entry><entry align="center" valign="middle">3.38 (1.85)</entry><entry align="center" valign="middle">3.18 (1.74)</entry><entry align="center" valign="middle">2.98 (1.63)</entry><entry align="center" valign="middle">2.78 (1.52)</entry><entry align="center" valign="middle">2.78 (1.51)</entry></row><row><entry valign="middle"><b>P2O5</b></entry><entry align="center" valign="middle">3.28 (1.27)</entry><entry align="center" valign="middle">3.21 (1.26)</entry><entry align="center" valign="middle">3.2 (1.26)</entry><entry align="center" valign="middle">3.21 (1.26)</entry><entry align="center" valign="middle">3.21 (1.26)</entry><entry align="center" valign="middle">3.21 (1.26)</entry><entry align="center" valign="middle">3.21 (1.26)</entry><entry align="center" valign="middle">3.21 (1.25)</entry></row><row><entry valign="middle"><b>ZrO2</b></entry><entry align="center" valign="middle">2.91 (1.30)</entry><entry align="center" valign="middle">3.00 (1.36)</entry><entry align="center" valign="middle">2.5 (1.13)</entry><entry align="center" valign="middle">1.8 (0.81)</entry><entry align="center" valign="middle">1.8 (0.81)</entry><entry align="center" valign="middle">1.80 (0.81)</entry><entry align="center" valign="middle">1.8 (0.81)</entry><entry align="center" valign="middle">1.8 (0.81)</entry></row><row><entry valign="middle"><b>CeO2</b></entry><entry align="center" valign="middle" /><entry align="center" valign="middle">1.88 (0.61)</entry><entry align="center" valign="middle">1.86 (0.60)</entry><entry align="center" valign="middle">2.00 (0.65)</entry><entry align="center" valign="middle">2.00 (0.65)</entry><entry align="center" valign="middle">2.00 (0.65)</entry><entry align="center" valign="middle">2.00 (0.65)</entry><entry align="center" valign="middle">2.00 (0.65)</entry></row><row><entry valign="middle"><b>V2O5</b></entry><entry align="center" valign="middle" /><entry align="center" valign="middle">0.12 (0.04)</entry><entry align="center" valign="middle">0.12 (0.04)</entry><entry align="center" valign="middle">0.07 ((0.02)</entry><entry align="center" valign="middle">0.07 ((0.02)</entry><entry align="center" valign="middle">0.07 ((0.02)</entry><entry align="center" valign="middle">0.07 (0.02)</entry><entry align="center" valign="middle">0.07 (0.02)</entry></row><row><entry valign="middle"><b>MnO2</b></entry><entry align="center" valign="middle" /><entry align="center" valign="middle" /><entry align="center" valign="middle" /><entry align="center" valign="middle">0.03 (0.02)</entry><entry align="center" valign="middle">0.03 (0.02)</entry><entry align="center" valign="middle">0.03 (0.02)</entry><entry align="center" valign="middle">0.03 (0.02)</entry><entry align="center" valign="middle">0.03 (0.02)</entry></row><row><entry valign="middle"><b>Er2O3</b></entry><entry align="center" valign="middle" /><entry align="center" valign="middle" /><entry align="center" valign="middle" /><entry align="center" valign="middle">0.12 (0.017)</entry><entry align="center" valign="middle">0.12 (0.017)</entry><entry align="center" valign="middle">0.12 (0.017)</entry><entry align="center" valign="middle">0.12 (0.017)</entry><entry align="center" valign="middle">0.12 (0.017)</entry></row><row><entry valign="middle"><b>MgO</b></entry><entry align="center" valign="middle" /><entry align="center" valign="middle" /><entry align="center" valign="middle" /><entry align="center" valign="middle">0.15 (0.21)</entry><entry align="center" valign="middle">0.15 (0.21)</entry><entry align="center" valign="middle">0.15 (0.21)</entry><entry align="center" valign="middle">0.15 (0.21)</entry><entry align="center" valign="middle">0.15 (0.21)</entry></row><row><entry valign="middle"><b>CaO</b></entry><entry align="center" valign="middle" /><entry align="center" valign="middle" /><entry align="center" valign="middle" /><entry align="center" valign="middle" /><entry align="center" valign="middle" /><entry align="center" valign="middle" /><entry align="center" valign="middle" /><entry align="center" valign="middle">1.00 (0.99)</entry></row></tbody></tgroup><tgroup cols="9"><colspec colnum="1" colname="col1" colwidth="27mm" /><colspec colnum="2" colname="col2" colwidth="27mm" /><colspec colnum="3" colname="col3" colwidth="27mm" /><colspec colnum="4" colname="col4" colwidth="27mm" /><colspec colnum="5" colname="col5" colwidth="27mm" /><colspec colnum="6" colname="col6" colwidth="27mm" /><colspec colnum="7" colname="col7" colwidth="27mm" /><colspec colnum="8" colname="col8" colwidth="27mm" /><colspec colnum="9" colname="col9" colwidth="27mm" /><thead><row><entry valign="middle"><b>Crystalline phases after:</b></entry><entry align="center" valign="middle" /><entry align="center" valign="middle" /><entry align="center" valign="middle" /><entry align="center" valign="middle" /><entry align="center" valign="middle" /><entry align="center" valign="middle" /><entry align="center" valign="middle" /><entry align="center" valign="middle" /></row></thead><tbody><row><entry valign="middle"><b>Second heat treatment: 20'/650 °C</b></entry><entry align="center" valign="middle">Li2Si2O3 Li2Si2O5 *</entry><entry align="center" valign="middle">Li2Si2O3 Li2Si2O5 *</entry><entry align="center" valign="middle">Li2SiO3</entry><entry align="center" valign="middle">Li2SiO3</entry><entry align="center" valign="middle" /><entry align="center" valign="middle">Li2SiO3</entry><entry align="center" valign="middle">Li2SiO3</entry><entry align="center" valign="middle">Li2SiO3 Li2Si2O5 *</entry></row><row><entry valign="middle"><b>Third heat treatment: 10'/850 °C</b></entry><entry align="center" valign="middle">Li2Si2O5 Li3PO4*</entry><entry align="center" valign="middle">Li2Si2O5 Li3PO4*</entry><entry align="center" valign="middle">Li2Si2O5 Li3PO4*</entry><entry align="center" valign="middle">Li2Si2O5 Li3PO4*</entry><entry align="center" valign="middle">Li2Si2O5 Li3PO4*</entry><entry align="center" valign="middle">Li2Si2O5 Li3PO4*</entry><entry align="center" valign="middle">Li2Si2O5 Li3PO4*</entry><entry align="center" valign="middle">Li2Si2O5 Li3PO4*</entry></row></tbody></tgroup></table></tables><tables id="tabl0009" num="0009"><table frame="all"><title><b><u>Table II</u></b></title><tgroup cols="6"><colspec colnum="1" colname="col1" colwidth="93mm" /><colspec colnum="2" colname="col2" colwidth="12mm" /><colspec colnum="3" colname="col3" colwidth="12mm" /><colspec colnum="4" colname="col4" colwidth="12mm" /><colspec colnum="5" colname="col5" colwidth="12mm" /><colspec colnum="6" colname="col6" colwidth="12mm" /><thead><row><entry align="center" valign="middle">Example</entry><entry align="center" valign="middle">1</entry><entry align="center" valign="middle">2</entry><entry align="center" valign="middle">3</entry><entry align="center" valign="middle">4</entry><entry align="center" valign="middle">6</entry></row></thead><tbody><row><entry valign="middle">CR-Value BS-5612 (1978)</entry><entry align="center" valign="middle">40.4</entry><entry align="center" valign="middle">37.0</entry><entry align="center" valign="middle">50.0</entry><entry align="center" valign="middle">59.3</entry><entry align="center" valign="middle">58.8</entry></row><row><entry valign="middle">Opalescence</entry><entry align="center" valign="middle">No</entry><entry align="center" valign="middle">No</entry><entry align="center" valign="middle">No</entry><entry align="center" valign="middle">No</entry><entry align="center" valign="middle">No</entry></row><row><entry valign="middle">Chemical stability in Acetic acid (24h/ 80°C, mass loss in µg/cm²)</entry><entry align="center" valign="middle">9</entry><entry align="center" valign="middle">18</entry><entry align="center" valign="middle">48</entry><entry align="center" valign="middle">3</entry><entry align="center" valign="middle">9</entry></row><row><entry valign="middle">Chemical stability in Saliva (7d/60°C, mass loss in µg/cm²)</entry><entry align="center" valign="middle">13</entry><entry align="center" valign="middle">17</entry><entry align="center" valign="middle">28</entry><entry align="center" valign="middle">27</entry><entry align="center" valign="middle">17</entry></row></tbody></tgroup></table></tables><tables id="tabl0010" num="0010"><table frame="all"><title><b><u>Table III:</u></b></title><tgroup cols="2"><colspec colnum="1" colname="col1" colwidth="27mm" /><colspec colnum="2" colname="col2" colwidth="55mm" /><thead><row><entry valign="top" /><entry align="center" valign="top"><u>Composition of artificial saliva</u></entry></row><row><entry valign="top">Component</entry><entry align="center" valign="top">Amount in mg in a total of 500 ml H₂O</entry></row></thead><tbody><row><entry>NaCl</entry><entry align="center">125.64</entry></row><row><entry>KCl</entry><entry align="center">963.9</entry></row><row><entry>NH₄Cl</entry><entry align="center">178.0</entry></row><row><entry>CaCl₂·2H₂O</entry><entry align="center">227.8</entry></row><row><entry>KSCN</entry><entry align="center">189.2</entry></row><row><entry>CO(NH₂)₂</entry><entry align="center">200.0</entry></row><row><entry>Na₂SO₄·10H₂O</entry><entry align="center">336.2</entry></row><row><entry>NaHCO₃</entry><entry align="center">630.8</entry></row><row><entry>KH₂PO₄</entry><entry align="center">654.5</entry></row></tbody></tgroup></table></tables><tables id="tabl0011" num="0011"><table frame="all"><title><b><u>Table IV</u></b></title><tgroup cols="5"><colspec colnum="1" colname="col1" colwidth="50mm" /><colspec colnum="2" colname="col2" colwidth="21mm" /><colspec colnum="3" colname="col3" colwidth="21mm" /><colspec colnum="4" colname="col4" colwidth="21mm" /><colspec colnum="5" colname="col5" colwidth="21mm" /><thead><row><entry valign="top">Example</entry><entry align="center" valign="top">9</entry><entry align="center" valign="top">10</entry><entry align="center" valign="top">11</entry><entry align="center" valign="top">12</entry></row></thead><tbody><row><entry /><entry align="center" /><entry align="center" /><entry align="center" /><entry align="center" /></row><row><entry><b>SiO₂</b></entry><entry align="center">74.37</entry><entry align="center">72.89</entry><entry align="center">72.21</entry><entry align="center">71.40</entry></row><row><entry><b>K₂O</b></entry><entry align="center">3.26</entry><entry align="center">3.18</entry><entry align="center">3.16</entry><entry align="center">3.13</entry></row><row><entry><b>Li₂O</b></entry><entry align="center">15.44</entry><entry align="center">15.13</entry><entry align="center">14.99</entry><entry align="center">14.79</entry></row><row><entry><b>Al₂O₃</b></entry><entry align="center">3.55</entry><entry align="center">3.48</entry><entry align="center">3.45</entry><entry align="center">3.41</entry></row><row><entry><b>P₂O₅</b></entry><entry align="center">3.38</entry><entry align="center">3.31</entry><entry align="center">3.28</entry><entry align="center">3.22</entry></row><row><entry><b>ZrO₂</b></entry><entry align="center">0.00</entry><entry align="center">2.01</entry><entry align="center">2.91</entry><entry align="center">4.05</entry></row><row><entry /><entry namest="col2" nameend="col5" align="left">All values above in wt.-%</entry></row><row><entry><b>SiO₂:Li₂O (Mol.-ratio%)</b></entry><entry align="center"><b>2.39</b></entry><entry align="center"><b>2.40</b></entry><entry align="center"><b>2.39</b></entry><entry align="center"><b>2.40</b></entry></row><row><entry namest="col1" nameend="col5" align="left" /></row><row><entry><b>Cycle A:</b></entry><entry namest="col2" nameend="col5" align="center">(1) 500°C/ 10 min + ( 2) 650°C/ 20 min + (3) 850°C/10min *)</entry></row><row><entry><b>Biaxial Flexural Strength /MPa</b></entry><entry align="center">786 +/- 92</entry><entry align="center">515 +/- 54</entry><entry align="center">522 +/- 82</entry><entry align="center">479 +/- 36</entry></row><row><entry><b>Contrast Ratio</b></entry><entry align="center">0.80</entry><entry align="center">0.56</entry><entry align="center">0.43</entry><entry align="center">0.36</entry></row><row><entry namest="col1" nameend="col5" align="left" /></row><row><entry><b>Cycle B:</b></entry><entry namest="col2" nameend="col5" align="center">(1) 500°C/ 10 min + (2) 700°C/ 20 min + (3) 850°C/10min *)</entry></row><row><entry><b>Biaxial Flexural Strength /MPa</b></entry><entry align="center">828 +/- 104</entry><entry align="center">659 +/- 75</entry><entry align="center">608 +/- 90</entry><entry align="center">694 +/- 113</entry></row><row><entry><b>Contrast Ratio</b></entry><entry align="center">0.83</entry><entry align="center">0.63</entry><entry align="center">0.53</entry><entry align="center">0.41</entry></row><row rowsep="0"><entry align="right"><b>*)</b></entry><entry namest="col2" nameend="col5" align="left">(1) Nucleation in the glass</entry></row><row rowsep="0"><entry /><entry namest="col2" nameend="col5" align="left">(2) Crystallization of Li-Metasilicate</entry></row><row><entry /><entry namest="col2" nameend="col5" align="left">(3) Crystallization of Li-Disilicate from Li-Metasilicate</entry></row></tbody></tgroup></table></tables>