fig2 gives an overview of an exemplary process to prepare a dental restoration according to an embodiment of the invention . typically , the process starts with the preparation of the frame 22 made from a blank 20 . the blank 20 is typically manufactured by pre - sintering a ceramic powder to form a machinable block for example according to a method as disclosed in ep 1712534 a1 and us 2006 / 0082033 a1 . a frame precursor 21 is then machined from the blank 20 based on the desired shape of the frame 22 , for example milled with a cnc milling machine . finally the frame precursor 21 is usually sintered to form the frame 22 . the shape of the frame 22 may be designed with cad software by an operator . the operator typically designs the frame 22 based on scan data obtained from capturing of a patient &# 39 ; s teeth , for example by scanning a plaster model of the patient &# 39 ; s teeth . it is also possible to make a model of the dental restoration and use the captured shape of the model for the design . such model may , for example , be a wax model which is manually shaped by a dental technician . alternatively , intra - oral scanning may be used , in which an image is captured directly from the patient &# 39 ; s teeth , rather than from a model . these techniques are familiar to persons of skill in the field . the preparation of the facing 32 will now be described . a blank 30 is provided made from an open - celled pre - sintered material from which a facing precursor 31 is formed . the facing precursor 31 is for example formed by milling it from the blank 30 with a cnc milling machine . the shape of the facing precursor 31 may also be designed with cad software . for example , the interior shape of the facing precursor may be designed based on the exterior shape of the frame 22 , and the exterior shape of the facing precursor may be designed based on the exterior shape of the dental restoration 40 . the shape of the frame 22 may be captured by scanning an available frame , or it may be available in the form of cad data if the frame was designed with cad software , for example . further the shape of the frame 22 may be obtained partially or entirely from a database that includes standard frame shapes . the final shape of the dental restoration 40 may be designed based on a captured image or data set representing the shape of the patient &# 39 ; s teeth , or based on a captured image or data set representing the shape of a model of the dental restoration , or based on shapes obtained from a library of images or data sets representing the shapes of various teeth . on the basis of the shape of the frame and the shape of the final restoration the shape of the facing may be generated by the cad system , so that the facing with its outer shape fits adjoining and cooperating teeth in the patient &# 39 ; s mouth . further , the shape of the facing is generated so that an inner surface of the facing fits with the outer shape of the frame . the milled facing precursor 31 is then combined with the frame 22 , and the facing - frame assembly is subsequently finally sintered to obtain the dental restoration 40 . the sintering of the facing precursor 31 which is placed on the frame 22 is preferably performed at a temperature lower than the melting temperature of the frame 22 , so that the frame 22 substantially maintains its shape and mechanically supports the facing precursor 31 that is softened during sintering . because the facing precursor typically shrinks during sintering the shape of the facing precursor usually is proportionally enlarged relative to the shape of the final facing . alternatively , the facing 32 is sintered before it is combined with a sintered frame ( diagram of the alternative not shown ). in this case the facing precursor is milled to substantially its final shape . the dental restoration 40 may be finally shaped , could be sandblasted and / or the surface may be at least partially finished with glaze . fig3 a and 3 b are schematic cross - sectional views showing an exemplary representation of a dental restoration that includes a crown 4 made from a frame 2 and a facing 3 . fig3 a illustrates the assembly of the crown 4 onto to a prepared natural tooth 1 . the natural tooth 1 is prepared so that it consists essentially of healthy tooth substance and provides a solid support for receiving the crown 4 . this preparation is typically done by removing any caries - infected tooth substance , and shaping the tooth by grinding to create a structure allowing for the support of and bonding to a dental restoration . the crown 4 is then assembled on the prepared tooth by mating opposed surfaces of the crown 4 with the surfaces of the prepared tooth 1 . fig3 b shows the restored tooth . the crown 4 is affixed to the natural tooth 1 , for example , with adhesive cement of the type available from 3m espe dental of seefeld , germany , under the designation ketac ™ cem plus . as shown in fig4 a - 4 e , the dental restoration may be used to replace natural tooth substance to different extents , up to replacing an entire tooth or several teeth . fig4 a again shows crown 4 of fig3 with the frame 2 and the facing 3 . fig4 b shows a bridge 14 that basically consists of two crowns 4 ′ connected with a pontic 15 that is designed to replace an entire absent tooth . the bridge has a frame 12 and attached to it a facing 13 prepared and applied in accordance with the invention . therefore the bridge 14 spans an area where a natural tooth is missing . typically a bridge extends from a prepared natural tooth , to a pontic , and then to another prepared natural tooth . a bridge can also be designed to replace two or more missing teeth , for example by including two or more pontics connected to each other in the same manner as the single pontic described above . a bridge may also have only one crown attached to only one natural tooth , for example in case a second natural tooth is not available or not suitable for attachment . fig4 c shows as a further example of a dental restoration an abutment 44 having a facing 43 and a frame 42 prepared and applied in accordance with the invention . the abutment 44 is attached on a dental implant 41 that , for example , may be implanted in a patient &# 39 ; s jaw . an abutment typically replaces an entire natural tooth and may be an alternative to a bridge . as an advantage relative to a bridge the abutment does not require the preparation of adjoining natural teeth and therefore helps to maintain the patient &# 39 ; s natural teeth . an abutment may also replace two or more natural teeth and may be attached to two or more dental implants 41 , however preferably two implants may provide sufficient fixation of an abutment replacing more than two teeth . the abutment may be connected with a pontic or may be part of a bridge , for example a bridge that extends from a dental implant , includes a pontic , and is connected to a crown that is bonded to a prepared tooth . such a bridge may also have more than one pontic . finally , fig4 d shows an inlay 54 with a facing 53 attached to a frame 52 , and fig4 e shows an onlay 64 with facing 63 attached to a frame 62 as further examples . the facings 53 , 63 are prepared and applied in accordance with the invention . inlays and onlays are typically used as smaller dental restorations . typically an inlay replaces a part of an occlusal surface of a tooth and an onlay in addition replaces at least one side surface of a tooth . inlays and onlays are typically alternatives to dental fillings , and are made for example of hardenable filling materials . they usually provide better aesthetic characteristics and better durability than normal filling materials . although specific embodiments of the present invention have been described above in detail , it will be understood that this description is merely for purposes of illustration . various modifications of and equivalents corresponding to the disclosed aspects of the preferred embodiments described above may be made by those skilled in the art without departing from the spirit of the present invention , which is defined by the following claims . a zirconia frame for a first upper human molar crown has been manufactured by milling a pre - sintered blank of lava ™ frame crown using a milling machine available under the designation lava ™ form from 3m espe ag , germany . the frame has been sintered with a heating rate of 10 kelvin per minute up to a temperature of about 1500 ° c . and a dwell time of about 120 minutes . the finished frame had a density of about 6 . 07 g / cm 3 . a facing precursor for a first upper human molar crown was milled out of a blank of pre - sintered and open - celled commercially available glass ceramic material using the lava ™ form milling machine . the blank was uniaxially pressed from a powder of a glass ceramic material as specified below . the pressed blank had a pressed density which was in the range of about 1 . 34 g / cm 3 and about 1 . 42 g / cm 3 . the pressed blank was pre - sintered at ambient pressure of about 101 . 3 kpa in a process that comprised the steps of : ( 1 ) exposing the blank to a temperature that was increased from room temperature of about 23 ° c . to about 540 ° c . at a heating rate of about 10 kelvin per minute ; ( 2 ) exposing the blank to the temperature of about 540 ° c . for a dwell time period of about 30 minutes ; ( 3 ) exposing the blank to a temperature that was increased from the temperature of about 540 ° c . to a temperature of 580 ° c . at a heating rate of about 10 kelvin per minute ; and ( 4 ) exposing the blank to the temperature of about 580 ° c . for a dwell time period of about 30 minutes . the blank was made of the reference glass ceramic material as specified herein . the facing precursor and the frame have been assembled with an amount of slurry arranged between . the slurry was prepared from the same glass ceramic powder as used for the facing precursor which was mixed with a liquid comprising about 0 . 9900 % by weight poliglykol 4000 p ( available from clariant , under the prod . code 109467 ), about 49 . 50 % by weight 1 , 2 - propandiol ( available from eissler sientific ) and de - ionized water ad to 100 % by weight . the ( glass ceramic ) powder to liquid ratio was about 2 : 1 by weight . this slurry was applied to both parts with a slight surplus eventually needed for filling a gap between the facing precursor and the frame . the so formed dental restoration precursor was dried at a drying temperature of about 60 ° c . for a drying time of about 3 minutes and then moved into a chamber of a furnace that was preheated to about 400 ° c . the time required for moving the dental restoration precursor into the furnace chamber was about another 3 minutes in which the dental restoration precursor was initially exposed to the drying temperature , and finally to the 400 ° c . present in the furnace chamber . the furnace chamber then was heated to the sintering temperature of about 790 ° c . at a heating rate of about 45 kelvin per minute . during the heating time period ( the time period in which the temperature was increased to the sintering temperature ) the furnace was evacuated from an initial ambient pressure of about 101 . 3 kpa to about 35 kpa . after the heating time period the dental restoration was exposed to the sintering temperature of about 790 ° c . for a dwell time of about 1 minute under ambient pressure of about 101 . 3 kpa . the sintering temperature of 790 ° c . was selected in accordance with the melting point of the material used for the facing precursor . in the example a sintering furnace was used which is available under the designation “ dekema austromat 3001 ” from dekema dental keramikofen gmbh , freilassing , germany . a zirconia frame for a first upper human molar crown has been manufactured by milling a pre - sintered blank of lava ™ frame crown using the lava ™ form milling machine . the frame has been sintered with a heating rate of 10 kelvin per minute up to a temperature of about 1500 ° c . and a dwell time of about 120 minutes . the finished frame had a density of about 6 . 07 g / cm 3 . a facing precursor for a first upper human molar was manufactured by grinding an “ e - max cad block lt a1 / c14 ” supplied by ivoclar vivadent ag , schaan , liechtenstein in a cerec inlab supplied by sirona ag , bensheim , germany . the facing precursor and the frame have been assembled with an amount of slurry arranged between . the slurry was prepared from a glass ceramic powder available under the designation “ empress add - on powder ” from the ivoclar vivadent ag , schaan , liechtenstein and a liquid available under the designation “ e - max ceram buildup fluid all - round ” from the ivoclar vivadent ag , schaan , liechtenstein . the powder to liquid ratio was about 2 : 1 by weight . this slurry was applied to both parts with a slight surplus eventually needed for filling a gap between the facing precursor and the frame . the so formed dental restoration precursor was dried at a drying temperature of about 60 ° c . for a drying time of about 3 minutes and then moved into a chamber of a furnace that was preheated to about 400 ° c . the time required for moving the dental restoration precursor into the furnace chamber was about another 3 minutes in which the dental restoration precursor was initially exposed to the drying temperature , and finally to the 400 ° c . present in the furnace chamber . the furnace chamber then was heated to the sintering temperature of about 850 ° c . at a heating rate of about 45 kelvin per minute . during the heating time period ( the time period in which the temperature was increased to the sintering temperature ) the furnace was evacuated from an initial ambient pressure of about 101 . 3 kpa to about 35 kpa . after the heating time period the dental restoration was exposed to the sintering temperature of about 850 ° c . for a dwell time of about 1 minute under ambient pressure of about 101 . 3 kpa . the sintering temperature of 850 ° c . was selected in accordance with the melting point of the material used for the facing precursor . for this and the further comparative examples the same sintering furnace as in example 1 was used (“ dekema austromat 3001 ” from dekema dental keramikofen gmbh , freilassing , germany ). comparative example 1 . 1 was repeated using the same type of materials and in the same way , but in comparative example 1 . 2 with a drying time of about 6 minutes rather than 3 minutes . example 1 was repeated , but with the facing precursor obtained from previously sintered material . therefore the frame and the facing precursor were prepared from the same type of material and in the same way as in example 1 . however , the facing precursor had a material structure that was generally free of cells prior to assembly with the frame . thus the material structure of the facing precursor prior to assembly to the frame was outside the range of an open - celled material as referred to in this specification . the facing precursor was made from a blank that was sintered in a process that comprised the steps of : ( 1 ) exposing the blank to a temperature that was increased from room temperature of about 23 ° c . to about 540 ° c . at a heating rate of about 10 kelvin per minute at ambient pressure of about 101 . 3 kpa ; ( 2 ) exposing the blank to the temperature of about 540 ° c . for a dwell time period of about 30 minutes at ambient pressure of about 101 . 3 kpa ; ( 3 ) exposing the blank to a temperature that was increased from the temperature of about 540 ° c . to about 800 ° c . at a heating rate of about 10 kelvin per minute at reduced pressure of about 35 kpa ; and ( 4 ) exposing the blank to the temperature of about 800 ° c . for a dwell time period of about 30 minutes at ambient pressure of about 101 . 3 kpa . the obtained blank was glued on a stub so that it could be processed on the cerec inlab machine available from sirona ag , bensheim , germany . the facing precursor then was machined from the blank by grinding , assembled to the frame , and the so - formed dental restoration precursor was sintered . the assembly and the sintering process were conducted in accordance to example 1 . in comparative example 2 . 1 a drying time of 3 minutes was used . in comparative example 2 . 2 the comparative example 2 . 1 was repeated with a drying time of 6 minutes rather than 3 minutes , and in comparative example 2 . 3 with a drying time of 9 minutes . results of the comparative examples 1 . 1 , 1 . 2 , 2 . 1 , 2 . 2 and 2 . 3 relative to example 1 fig5 shows a cross - section of a dental restoration which was manufactured according to comparative example 1 . 1 . the figure shows a frame 112 and a facing 113 which are joined as approximately indicated by line 119 . reference number 118 depicts exemplary areas in the dental restoration which comprise a defective material structure . in this case such defective material structure comprises cracks and voids that appear as bright patterns in the photograph . fig6 shows a cross - section of a dental restoration which was manufactured according to comparative example 1 . 2 . the line 129 again indicates the area at which a frame 122 joins with a facing 123 . also in this example the dental restoration has areas of defective material structure which comprises cracks and voids , again visible as bright areas . some areas of defective material structure are by way of example depicted by reference number 128 . as can be seen from fig5 and 6 the defective areas are located adjacent joint 118 , 128 between the frame 112 , 122 and the facing 113 , 123 , respectively . fig7 shows a cross - section of a dental restoration which was manufactured according to comparative example 2 . 1 . similar to the examples shown in fig5 and 6 also the dental restoration shown in fig7 has areas of defective material structure , for example indicated by reference number 148 . such areas again are located adjacent a joint between a frame 142 and a facing 143 . further fig8 shows a cross - section of a dental restoration which was manufactured according to comparative example 2 . 2 , with a frame 152 , a facing 153 and a joint between the frame 152 and the facing 153 , approximately indicated by line 159 . fig8 also shows defective areas , for example as depicted by reference number 158 . fig9 shows a cross - section of a dental restoration which was manufactured according to example 1 . the dental restoration shown appeared to be substantially free of defective areas . the transition ( approximately indicated by line 169 ) between the frame 162 and a facing 163 appeared to be generally free of voids . fig1 shows a cross - section of a dental restoration which was manufactured according to comparative example 2 . 3 . a frame 132 joins with a facing 133 in the proximity of line 138 . the dental restoration shown appeared to have less defective areas than the dental restorations shown in fig5 to 8 . fig1 is a diagram representing along the vertical axis the porosity or open porosity according to the mercury porosimetry as specified in din 66133 . along the horizontal axis a maximum pre - sintering temperature t p is represented . the diagram further shows a curve 170 that is based on measurements of the porosity or open porosity depending on the pre - sintering temperature t p . the pre - sintering temperature referred to in the diagram is the maximum temperature at which samples used for the measurements were pre - sintered . the samples used were uniaxially pressed from a powder of the reference glass ceramic material as specified herein . the pressed samples had a pressed density which was in the range of about 1 . 34 g / cm 3 and about 1 . 42 g / cm 3 . each pressed sample was pre - sintered at ambient pressure of about 101 . 3 kpa in a process that comprised the steps of : ( 1 ) exposing the sample to a temperature that was increased from room temperature of about 23 ° c . to about 540 ° c . at a heating rate of about 10 kelvin per minute ; ( 2 ) exposing the sample to the temperature of about 540 ° c . for a dwell time period of about 30 minutes ; ( 3 ) exposing the sample to a temperature that was increased from the temperature of about 540 ° c . to the pre - sintering temperature t p ( as represented in the diagram ) at a heating rate of about 10 kelvin per minute ; and ( 4 ) exposing the sample to the temperature of about the maximum temperature t p for a dwell time period of about 30 minutes . fig9 , based on example 1 , shows a dental restoration which appeared to be substantially free of defective areas adjacent the joint between the frame and the facing . fig5 to 8 , based on the comparative examples , in contrast show such defective areas . one substantial difference between the example 1 ( fig9 ) and the comparative examples 1 . 1 , 1 . 2 ( fig5 ) is the facing precursor being comprised of an open - celled material in example 1 , whereas in the comparative examples the facing precursor is made of a material that is generally free of cells , or at least is comprised of or consists of a closed - celled material structure . another substantial difference between the example 1 ( fig9 ) and the comparative examples 1 . 1 , 1 . 2 ( fig5 ) is the type of material used for the facing ( or the facing precursor ). on the other hand , comparative examples 2 . 1 , 2 . 2 generally correspond to comparative examples 1 . 1 , 1 . 2 respectively , but with comparative examples 2 . 1 , 2 . 2 using the same type of material like example 1 . the substantial difference between the example 1 and the comparative examples 2 . 1 , 2 . 2 ( fig7 ), is the facing precursor being comprised of an open - celled material in example 1 , whereas in the comparative examples the facing precursor is made of a material that is generally free of cells , or at least is comprised of or consists of a closed - celled material structure . therefore it is believed that the open - celled structure of the facing precursor rather than the type of material contributes to achieve a dental restoration being substantially free of defective areas . it is believed that in example 1 components of the slurry ( for example the liquid ) used for assembly of the facing precursor and the frame can escape through the open - celled material during drying , and probably at the beginning of the sintering process . it is further assumed that components of the slurry if trapped within the dental restoration may cause defective areas as shown in fig5 to 8 , but not visible in fig9 . support may be given by comparative example 2 . 3 ( fig1 ) in which a long drying time may have caused components of the slurry to escape prior to sintering the dental restoration precursor . the example shown in fig1 shows a dental restoration that is generally free of defects which may be a result of the long drying time that may have enabled the liquid of the slurry to escape prior to sintering . therefore another conclusion may be that the drying time may be reduced due to the facing precursor being comprised of an open - celled material . it was also found that as an advantage a facing precursor comprised of an open - celled material may help to reduce the milling time because milling of a sintered glass ceramic tended to cause chippings at the facing precursor so that the milling speed preferably would be reduced to avoid such chippings .