Patent Publication Number: US-2005115460-A1

Title: Method for molding dental restorations and related apparatus

Description:
RELATED APPLICATIONS  
      This application is a continuation in part application of Ser. No. 10/274,661 filed Oct. 19, 2002, which is a continuation in part application of application Ser. No. 09/779,465 filed Feb. 9, 2001, which claims of priority of provisional application Ser. No. 60/184,741 filed Feb. 24,2000. Application Ser. No. 09/779,465 is further a continuation-in-part application of application Ser. No. 09/695,189, filed Oct. 24, 2000, which is a continuation in part of application Ser. No. 09/235,171, filed Jan. 22, 1999, which is a divisional application of Ser. No. 08/927,774, filed Sep. 11, 1997, U.S. Pat. No. 5,897,885, which is a divisional application of Ser. No. 08/417,682, filed Apr. 6, 1995, U.S. Pat. No. 5,702,514, which is a continuation-in-part of application Ser. No. 08/250,926, filed May 31, 1994, U.S. Pat. No. 5,507,981. 
    
    
     FIELD OF THE INVENTION  
      The present invention relates to a process for the molding of metal free dental restorations such as crowns, bridges, inlays, onlays, etc., from glass-ceramic materials. The invention is also concerned with apparatus which permits dental restorations to be easily molded from glass-ceramic materials and, lastly, the invention relates to the resulting dental restorations.  
     BACKGROUND OF THE INVENTION  
      In the prior art, various methods have been used to form metal free dental restorations from glass-ceramic materials. Glass ceramic materials because of their strength, translucency, non-toxicity and other physical properties are ideal materials for use in forming dental restorations. Because of their suitability, glass-ceramic materials have been used to form dental restorations for at least eighty years. In the prior art, the most widely used means for forming dental restorations from mixtures of glass and ceramic materials is a process which utilized a slurry of glass and ceramic particles. In this process, a die is formed which is an exact replica of the remaining portion of the tooth to which the restoration is to be secured. It is understood by one skilled in the art that the remaining portion of the tooth has been prepared by the dentist in such a manner that the attachment of the restoration is facilitated. To start the process, the dentist takes an impression of the prepared tooth to create a negative impression of the prepared tooth or teeth. This negative impression is then packed with a material to form a positive impression of the prepared tooth or teeth. This positive impression is called a die. Platinum foil is then pressed over the die to form a matrix which is essentially a foundation on which the dental restoration is built. In order to form up the dental restoration, many layers of a slurry of a particulate glass-ceramic material are applied to the platinum foil matrix. As multiple layers of the slurry are built-up and dried, a semi-solid structure is formed which can be carved into shape of the desired dental restoration. Once the desired shape is achieved, the structure is then removed from the die. At this stage, the structure is referred to as a green structure. The structure is then fired. During the firing process, the particulate glass-ceramic material fuses into a solid mass. Because the green structure is formed from multiple layers of the dried glass-ceramic material, uneven fusing may result. As a result of this uneven fusing, the physical properties of the finished restoration may be detrimentally affected, thereby creating an inferior dental restoration. Further, as can be seen from the above description, the overall process is very labor intensive.  
      Other methods for forming the green restoration have been considered in the prior art, for example, in U.S. Pat. No. 2,196,258, a mixture of particulate glass and ceramic materials which incorporates a binder is packed into a flexible mold to form a green structure which is then fired to form a finished structure. Again, because the process entails the fusing of particulate material, uneven fusing may result and, hence, a weakened and inferior dental restoration may result.  
      To overcome the problems as described above, the trade has recognized that in order to produce strong, translucent metal free dental restorations, it would be desirable to form these restorations directly from a homogenous, molten glass-ceramic material. It was realized that it may be possible to produce a satisfactory restoration by forcing a molten or plastic glass-ceramic material into a mold having a cavity in the form of the desired dental restoration. The prior art further recognized that the glass-ceramic material could be introduced into the cavity when the glass-ceramic material was in the liquid or in the plastic state.  
      A constant goal of the prior art as described above was to effect the molding process in a quick and efficient manner, and in a manner that produces a dental restoration that has excellent definition and fit. In dental restorations, definition is extremely important. In order to have a satisfactory restoration, the finest details of the original tooth must be reproduced. For example, for a dental restoration to be successful, the margins must be sharp and well defined. It is in this area that the prior art molding processes are deficient, in that it was not possible to achieve the desired degree of definition.  
      Further, it is desirable to produce a dental restoration in a short period of time in order to efficiently utilize the overhead of the dental laboratory, and in order to minimize the labor content of the dental restoration.  
      Dental laboratories are not typically well funded operations. Therefore, in order to keep costs to a minimum, it is highly desirable that a suitable process for forming dental restorations utilize equipment which is relatively inexpensive. While the above described process fits this requirement, the below described DI-COR process does not.  
      As is discussed above, there are several prior art processes for the manufacture of dental restoration from glass-ceramic materials. A recent addition to the prior art is the DI-COR process as sold by Dentsply International, Inc., of York, Pa. In this process, a dental restoration is formed by centrifugal casting of a molten glass-ceramic material. This process is further described in U.S. Pat. No. 4,431,420, issued Feb. 14, 1984, and related patents. Centrifugal casting has been extensively used in the casting of metals principally by the lost wax process. Further, this process has been imminently successful for hundreds of years for use in conjunction with metals. This success results from the fact that molten metals have very low viscosity and high density in the molten state, hence, they function very well in centrifugal casting processes. That is, because molten metals have a high density and a very low viscosity in the molten state, centrifugal force is adequate for purposes of injecting the molten metal into a preformed mold cavity. In an attempt to produce dental restorations which have high definition, the above-mentioned DI-COR process uses centrifugal force to form the desired dental restorations from a molten glass-ceramic material. Molten glass-ceramic materials have a much higher viscosity, and a much lower density when compared to molten metals. For this reason, it is not possible to consistently drive a molten glass-ceramic material by centrifugal force alone into a mold in order to produce a satisfactory dental restoration. That is, a molten glass-ceramic material cannot be driven by centrifugal force into a mold cavity with sufficient force in order to always get the required definition, which is necessary to form a satisfactory dental restoration. It is well recognized by one skilled in the art, that in order to have a satisfactory dental restoration, excellent definition must be achieved in order to recreate the desired margins, which are needed for the proper fit of a dental restoration into the human mouth.  
      Further, the DI-COR process is deficient as to the coloration of the glass-ceramic material utilized. The resulting DI-COR dental restoration had an undesirable white color and must be glazed in order to produce satisfactory human coloration. As a result, the coloration is only on the surface of the dental restoration. If adjustment by grinding is needed in the final installation of the restoration into the human mouth, the glazing is removed, thereby exposing the whitish base which contrasts with the glaze. This contrast is very unsatisfactory from an esthetic point of view.  
      In contrast to this deficiency, the restoration of the subject process is adapted to utilize glass-ceramic materials, wherein the coloration of the resulting dental restoration throughout approximates human tooth coloration. Hence, if grinding is necessary in final fitting, contrast between the surface of the dental restoration and the underlying base is not observed.  
      Undesirable contrast can also result from normal wear, wherein as a result of the grinding action of one tooth against another, the glaze is worn away. Again, this is not a problem in this invention as the preferred glass-ceramic material has a uniform, natural coloration throughout. It should also be noted that the restoration of the subject invention may be glazed to achieve the exact shade desired.  
      In contrast to the above-discussed prior art processes, the process of the subject invention utilizes a positive, mechanically applied force for purposes of injecting the molten dental glass-ceramic material into the preformed mold cavity.  
      For a dental glass-ceramic material to be satisfactory for use in the formation of dental restorations, the material should incorporate many of or all of the following properties: 
      (1) It must be inert and non-toxic in an oral environment.     (2) It must have sufficient structural integrity to resist the forces of mastication, and, generally, must have a 3-point M.O.R. of at least 30,000 PSI.     (3) It should be capable of being formed into forms which are compatible with the human anatomy using simple equipment.     (4) It should have esthetic qualities (coloration similar to human teeth with a slightly translucent appearance), which are compatible with human teeth, and, hence, should be monolithic or glazable.     (5) The glass-ceramic material must not absorb moisture or stain, and it must be stress corrosion resistant.     (6) The glass-ceramic material should have wear characteristics which are similar to natural human teeth, and should be compatible with other dental materials.     (7) The glass-ceramic must have dimensional stability and resist thermal shock during processing, and, in particular, it must have dimensional stability during subsequent heat treating processes wherein recrystallization is effected.     (8) The glass-ceramic material should be compatible from a thermal expansion point of view with metals, stains, glazes, etc., as are conventionally used-to-form dental restorations.     (9) In order to create an esthetically pleasing dental restoration, it may be necessary to alter the final dental restoration to the exact shape and shade desired. In order to effect these alterations, the dental restoration must be heated to a temperature of at least 750° C. for each operation. Therefore, a satisfactory glass-ceramic material must be capable of withstanding multiple heat cycles to at least 750° C.     (10) A suitable glass-ceramic material must be capable of retaining its structural integrity during heat treating.     (11) A suitable glass-ceramic material should have: 
        a. Coefficient thermal expansions (C.T.E.) of 9 to 14.5×10 −7 /° C.;     b. Translucency of 2.5 to 4.0 on a visible scale of 0 (clear) to 5 (opaque) and overall beauty;     c. M.O.R. of at least 30 K.S.I. average;     d. Ability to be heat treated to 750-950° C.;     e. Structural integrity during heat treatment;     f. Meltability and formability;     g. Chemical durability in an oral environment.    
       

      The subject invention includes glass-ceramic materials which meet the above set forth criteria.  
     SUMMARY OF THE INVENTION  
      In accordance with the above description, it is obvious that in accordance with the prior art, it is difficult, if not impossible to form top quality dental restorations by molding glass-ceramic materials in an inexpensive and efficient manner.  
      The process, apparatus, compositions and dental restoration of the subject invention provide improvements over the prior art. By use of the process of the subject invention, glass-ceramic materials can be readily molded into dental restorations. Further, the apparatus of the subject invention is relatively inexpensive and is easy to utilize. The compositions of the subject invention are highly advantageous in that they produce esthetically pleasing dental restorations which are chemically inert in the human mouth, and have outstanding strength properties. Further, the compositions of this invention are advantageous in that when dental restorations formed from these compositions are heat treated, they maintain their structural integrity. Further, these dental restorations are capable of withstanding multiple heat cycles to at least 750° C., possess thermal expansions which are compatible with the existing porcelains and therefore, these dental restorations can be readily altered using conventional porcelain materials.  
      Likewise, dental restorations in accordance with the present invention, have acceptable coloration after heat treating, and can be used at that stage without further cosmetic treatment in the mouth. In order to enhance the esthetic properties of the resulting dental restoration, the dental restoration can be readily altered using porcelain materials to achieve any desired effect.  
      Accordingly, it is an object of the present invention to provide an efficient process whereby dental restorations may be molded from glass-ceramic materials.  
      It is a further object of the present invention to provide glass-ceramic dental restorations which have outstanding strength and are esthetically pleasing.  
      It is also an object of this invention to provide glass-ceramic compositions which are suitable for the formation of dental restorations which have a high degree of micro-structural control during the development of the crystals, thereby permitting great flexibility in forming these dental restorations.  
      It is still another object of this invention to provide a glass-ceramic material which will maintain its structural integrity during heat treating, and, in particular, will not slump or sag during heat treating outside of the investment.  
      Another object of the present invention is a glass-ceramic material which is suitable for the formation of dental restorations, and which is capable of maintaining its translucency during multiple firing cycles to approximately 750° C.  
      Still another object of this invention is a process for forming a ceramic dental restoration wherein the process comprises:  
      (1) Placing a glass-ceramic material in a heat-pressure deformable crucible;  
      (2) Heating the crucible and glass-ceramic material to a temperature at which said crucible becomes heat-pressure deformable, and the glass-ceramic material is moldable;  
      (3) Bringing the heated crucible into contact with a mold having a preformed cavity therein;  
      (4) Continuing to move the crucible into contact with the mold, thereby causing the crucible to deform against the mold and causing the moldable glass-ceramic material to be injected into said cavity, thereby forming a dental restoration;  
      (5) Cooling the mold and the ceramic dental restoration therein;  
      (6) Removing the formed ceramic restoration from the mold;  
      (7) Heat treating the dental restoration; and  
      (8) Finishing the dental restoration.  
      Also, and object of this invention is to form a heat deformable crucible having a base and a side portion, wherein said crucible is formed by the heat sintering of a particulate mixture of glass and a metal oxide. A preferred composition comprises from about 27 to 31 percent glass, and from about 69 to about 73 percent of a metal oxide.  
      Another object of this invention is a dental restoration which is formed from a glass-ceramic material having the following composition (by weight percent):  
                                                      Li 2 O    8-17           Al 2 O 3     1.5-7.0           SiO 2     60-85           Na 2 O   0-2           K 2 O   0-2           P 2 O 5     1.5-5             ZrO 2     0-3           CaO   0-1           BaO + SrO + La 2 O 3      0-12           Coloring Oxides   0-5           Glass Modifiers   Balance                      
 
      Still another object of this invention is a dental restoration which is formed from a glass-ceramic material having the following composition (by weight percent):  
                                                      Li 2 O   14-15           Al 2 O 3     4.5-5.0           SiO 2     67-71           Na 2 O   1-2           K 2 O   1.5-3.5           P 2 O 5     3-4           B 2 O 3     0-2           CaO     0-1.25           BaO   1.25-3             Tb 4 O 7     0-1           CeO 2     0-1                      
 
      Lastly, an object of the present invention is to create a Lithium Disilicate glass-ceramic material which has excellent chemical durability, and as such, will not deteriorate when exposed to the fluids in the human mouth.  
      These, and other objects, features, and advantages of the present invention, will become apparent from the detailed description herein.  
      The advantages of the present invention can be more clearly understood from the following description taken together with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a flow chart showing the steps of the preferred embodiment of the process of this invention;  
       FIG. 2  shows a sectioned lost wax mold for use in the invention;  
       FIG. 3  shows a sectioned structure illustrating the heating step of this invention;  
       FIG. 4  shows a sectioned structure illustrating the initial contact of the crucible with the mold;  
       FIG. 5  shows a sectioned structure illustrating the partial sealing of the crucible against the mold;  
       FIG. 6  shows a sectioned structure illustrating the complete sealing of the crucible against the mold, and the injection of the molten glass-ceramic into the mold cavity;  
       FIG. 7, 7   a  and  7   b  are sectioned side view showing the composite apparatus of this invention;  
       FIG. 8  shows a sectioned alternate mold;  
       FIG. 9  shows a sectioned alternate mold; and  
       FIG. 10  shows the mold of  FIG. 9  being filled with a molten glass-ceramic material. 
    
    
      It should be noted that  FIGS. 2 through 10  are schematic representations of apparatus which may be useful in this invention.  
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       FIG. 1  generally described the overall process of the subject invention, wherein the dental restorations are formed by the molding of a glass-ceramic material. As can be seen in  FIG. 1 , the first step is the formation of a mold having a suitable mold cavity therein. To start the process whereby the mold cavity is formed, the tooth or teeth in the human mouth are prepared by the dentist by procedures which are commonly known in the prior art. By using impressions which are provided by the dentist, a positive wax form is then formed and positioned in a mold usually called a ring. A semi-liquid investment material is then poured around the positive wax form. Once the investment material sets up and cures, the resulting mold is placed in a furnace and heated, thereby causing the wax to melt and run out of a sprue hole which is integral with the mold. The finished mold is then complete. This constitutes the formation of a mold by the lost wax process.  
      In the second step of  FIG. 1 , a button of a glass-ceramic material is placed in deformable crucible  8 , as is illustrated in  FIG. 3 . The button of a glass-ceramic material constitutes a small cylinder of the glass-ceramic material which usually weighs about six grams, and has a diameter of about two cm, and a thickness of about one cm. These buttons of glass-ceramic materials are a convenient shape, whereby these materials may be manufactured, sold and used. It is understood by one skilled in the art that a sufficient amount of the glass-ceramic material must be used in order to form the desired dental restoration.  
      In the third step of  FIG. 1 , crucible  8  and the button of the glass-ceramic material  7  is uniformly heated. The glass-ceramic material is heated to a temperature above its liquidus temperature, as will be described in greater detail herein below. Further, the heat-pressure, deformable crucible  8  will be described in greater detail herein below.  
      In the process of this invention, the preformed mold can incorporate multiple cavities in order that more than one dental restoration can be produced during one cycle of the subject process.  
      In steps four, five and six of  FIG. 1 , the heated crucible and glass-ceramic material are moved into contact with the mold by mechanical means in a manner which will be described in greater detail relative to the discussion of  FIGS. 3 through 6  herein below.  
      In accordance with steps  7  and  8  of  FIG. 1 , the mold is cooled and the solidified glass-ceramic casting is removed from the investment by mechanical means. The dental restoration is then cut off the sprue and finished by heat treating, polishing, and/or glazing in order to achieve the desired esthetic effect.  
      The process of this invention may utilize both glass and glass-ceramic materials for purposes of forming the desired dental restoration.  
      For strength and esthetic reasons, it is preferred that a glass-ceramic material be utilized. When a glass-ceramic material is utilized, the button of glass-ceramic material  7  is initially placed in crucible  8  in a glass phase. As heat is applied to crucible  8 , glass-ceramic material  7  becomes a ceramic via the formation of a crystalline phase. As the heating is continued, the glass matrix slowly dissolves the crystalline phase, thereby causing the glass-ceramic material  7  to re-enter the glass phase. The temperature at which the crystal phase completely dissolves in the glass matrix is defined as the liquidus temperature of the glass-ceramic material. In accordance with this invention, it is found desirable to heat the glass-ceramic material to a temperature above the liquidus temperature in order to eliminate any possibility of the existence of a crystalline phase in the molten material. Further, at this temperature, the glass-ceramic material has a viscosity of about log 3 to about log 4P, which is suitable to allow the molten glass-ceramic material to be readily injected into the mold cavity  5 . The viscosity at this temperature is defined as the working range.  
      Throughout this application, the viscosity of the molten glass-ceramic materials and of the soften crucible will be defined as the log of the respective viscosity in poises. Further, the unit poise will be abbreviated as “P”. For example, if the viscosity of a molten glass-ceramic material is 10 4  poises, the viscosity will be indicated by log 4P.  
      While  FIG. 1  illustrates the broad process of the subject invention, the individual steps as illustrated in  FIG. 1  are defined in greater detail in the description of  FIGS. 2 through 10 .  
      In  FIG. 2 , it can be seen that mold  4  has been formed by the placement of an investment material  10  in ring  12 . Prior to the placement of investment material  10  in ring  12 , ring  12  is positioned around wax form  14 , the upper extremity of which is shaped in the form of a desired dental restoration  16 . Wax form  14  is only shown in an outline form, as  FIG. 2  illustrates a finished mold  4  after wax form  14  has been burnt out.  
      The procedure whereby mold  4  is formed is generally known in the prior art, and has been used for centuries to form lost wax molds for use in metal casting procedures. As is stated above in the preparation of mold  4 , wax form  14  is positioned in ring  12 . An investment slurry  10  is then slowly poured into ring  12 , usually with the aid of vibratory techniques to insure that investment  10  completely fills ring  12 , and in particular, completely encases wax form  14 . Again, these procedures are commonly known in the prior art.  
       FIGS. 3, 4 ,  5  and  6 , show the general process of the subject invention in a schematic form.  
      Referring to  FIG. 3 , it can be seen that mold  4  having cavity  14  is positioned in the vicinity of crucible  8 . Positioned in crucible  8  is glass-ceramic material  7 , the details of which will be described herein. Crucible  8  is further positioned on a ceramic base  21  which is positioned on ram  22 , whereby crucible  8  can be moved in relation to mold  4 . An electric heating element  24  is further provided whereby heat can be applied to crucible  8 . Details of the heating of glass-ceramic material  7  will be discussed herein below.  
      During the heating of crucible  8 , glass-ceramic material  7  is converted from the solid state to the liquid state, and crucible  8  is converted from a solid, brittle state to a plastic state wherein it is heat-pressure deformable. Once the glass-ceramic material  7  reaches the desired working range, ram  22  is actuated as is illustrated in  FIG. 4 , thereby causing crucible  8  to move upwardly and to come into contact with the lower extremities of mold  4 . The actuation of ram  22  is effected by a power source not shown.  
      As can been seen in  FIG. 5 , as the upward movement of ram  22  continues, crucible  8  continues to deform against mold  4  causing the upper extremities of crucible  8  to seal against mold  4  at interface  28 . Further, it can be seen that molten glass-ceramic material  7  has started to flow into mold cavity  14 .  
      Referring to  FIG. 6 , as the travel of ram  22  continues, the deformation of crucible  8  against mold  4  is completed, and cavity  14  is completely filled with glass-ceramic material  7  as a result of the applied pressure of ram  22 .  
      Subsequent to the procedures as are illustrated in  FIGS. 3 through 6 , mold  4  is then cooled and the desired dental restoration is removed from the investment  10 . Once the dental restoration is removed from investment  10 , the dental restoration is cut apart from the sprue, and is heat treated and finished by polishing and glazing, in order to achieve the desired esthetic effect.  
      Further, after formation, dental restoration  26  is heat treated in such a manner that its strength and other properties are enhanced by the formation of crystals in a glass matrix. The details of heat treating are described below.  
       FIG. 7  illustrates apparatus  32  which may be utilized to effect the composite process which is illustrated in the flow chart of  FIG. 1 . This apparatus generally comprises support frame  30 , heat source  33 , mold retaining means  34 , and a plunger  36 . Support frame  30  generally comprises the outside frame of apparatus  32 .  
      Mold retaining means  34  comprises a bar  50  into which is threaded mold clamp  35 . As can be seen, bar  50  permits mold  4  to be locked to support bar  44 . Support bar  44  further incorporates an aperture  56  which is slightly smaller than the diameter of mold  4  and slightly larger than the diameter of crucible  8 . By the actuation of ram  36 , crucible  8  is moved up into contact with mold  4  in a manner which is similar to that as is described above in conjunction with  FIGS. 4, 5  and  6 . The apparatus of  FIG. 7  further incorporates a heat source  33 , which in the illustrated embodiment comprises an electric resistance heating element  37 . In the preferred embodiment, the heating element is molybdium disilicide. Heat source  33  is controlled by a power control source  58 .  
      As is illustrated in  FIGS. 3 and 7 , an electric resistance heater may be utilized to effect the heating of crucible  8 , and hence, glass-ceramic material  7 . In addition to the electric resistance heating as illustrated, heating may be effected via induction heating, gas torch heating, or any other appropriate means.  
      The apparatus further may incorporate a rotating means (not shown), whereby ram  36  may be rotated during the heating process in order to effect a more uniform heating of crucible  8  and glass-ceramic material  7  contained therein. The rotating means may be an electric, pneumatic or hydraulic motor. Ram  36  is further provided with means for effecting its upward movement, which in the preferred embodiment, is a pneumatic cylinder  39 .  
      Further, referring to  FIG. 7 , it can be seen that apparatus  32  incorporates a plurality of structural insulating members  41 ,  43 ,  45 ,  47  and  49 , which support and contain heating element  37 , and contain the heat created during the operation of the element  37 . These structural insulating members are formed from ceramic fiber board.  
      Likewise, it can be seen that support bar  44  incorporates an aperture  56  which has a waist section  55 . Waist section  55  is advantageous in that when crucible  8  deforms against mold  4 , the deforming upper extremities of crucible  8  are prevented from downward movement at the constriction of waist  55 , thereby causing the molten glass-ceramic material to be efficiently injected into the preformed cavity  14  in mold  4 . That is, because when the upper extremities of crucible  8  are caused to solidify in the vicinity of waist  55 , the downward movement of the molten glass-ceramic material is prevented, thereby causing the glass-ceramic material to be injected into mold cavity  14 .  
       FIGS. 7   a  and  7   b  represent the preferred apparatus  31  for use in accordance with this invention. Most of the components of the preferred apparatus  31  are identical or similar to the components from apparatus  32 ; primarily, in that a movable arm  38  is provided for wherein a preheated mold  4  may be brought into position for the molding process and positioned against furnace base  41 . Movable arm  38  may be connected to a central pivot point whereby it may be swung into position or it can be laterally moved into position.  
      Prior to moving arm  38  into position, as is shown in  FIG. 7 , a ram  22  is extended in order to place crucible  8  in heat source  33 , whereby glass-ceramic material  7  may be melted. Once this melting is effected, ram  22  is withdrawn and movable arm  38  is swung into place to the position as is illustrated in  FIG. 7   b . In this position, the forming sequence, as is shown in  FIGS. 2 through 6 , can be completed.  
      Relative to apparatus  31  and  32 , it is understood by one skilled in the art that crucible  8  may be placed in position on ceramic base  21  either manually or by automatic means.  
      The injecting of the glass-ceramic components of this invention can be effected using the apparatus as is illustrated in  FIGS. 2 through 10 , and is described herein above. In addition, a commercially available press as is sold under the trademark AUTOPRESS™ Plus, and as sold by Jeneric/Pentron of Wallingford, Conn., may be utilized. This system does not utilize a deformable crucible.  
      It is preferred that the glass-ceramic components of this invention be pressed under a vacuum of about 2 to about 8 bar with the preferred vacuum being 7 bar.  
      The above described  FIGS. 1 through 7   b , illustrate the formation of a dental restoration. In addition to being useful in the formation of essentially complete dental restorations, the composition and process of the subject invention may be used to form coping over which porcelain materials may be applied to alter and shade the dental restoration.  
      In the prior art, metal copings are extensively utilized. These copings are covered with layers of porcelain materials which are applied to the metal copings for purposes of forming a composite dental restoration which comprises a metal base and a porcelain exterior portion. Composite structures are advantageous in that a metal coping significantly enhances the strength of the resulting dental restoration. Metal copings are disadvantageous in that they are opaque and they have a color which contrasts with the color of a natural tooth, and further, they have toxicity problems in some instances. The use of the subject invention to form an all glass-ceramic dental restoration is advantageous in that the coping is essentially the same color as a natural tooth, and in particular, it is essentially the same color as the porcelain from which the exterior portion of the dental restoration is formed. Further, the all ceramic structure is advantageous in that problems resulting for certain patients who are allergic to certain metals are eliminated. In this procedure, metal free crowns and bridges of outstanding strength and esthetic properties are achieved. The outstanding esthetic properties result from the fact that these glass-ceramic copings can be utilized as a base over which a variety of different porcelains can be fired in order to achieve the most delicate coloration and esthetic properties.  
      As is stated above, the process of the invention may utilize both glass and glass-ceramic materials. Preferred glass-ceramic compositions for use in accordance with the invention are listed in Tables I to IVb. All components listed in Tables I to IVb are in weight percent.  
      The subject invention can utilize any suitable glass-ceramic material. As is shown in Tables I to IVb, preferred glass-ceramic materials for use in this invention are lithium disilicate glass-ceramic materials. In these materials, Li 2 O 2(SiO 2 ) constitutes the crystalline phase of the heat treated glass-ceramic material. Lithium disilicate glass-ceramic materials are particularly suited for use in the invention in that they are non-toxic, they resist thermal shock, have excellent strength, are corrosion resistant, and they produce dental restorations which approximate human coloration, are translucent, and esthetically pleasing. Further, lithium disilicate glass-ceramic materials are advantageous in that they maintain their structural integrity during heat treating, given that they do not slump or sag during proper heat treating.  
      Other lithium disilicate glass-ceramic materials which may be used in the process of this invention are disclosed in U.S. Pat. No. 5,219,799, issued Jun. 15, 1993.  
      Lithium disilicate glass-ceramic materials may utilize P 2 O 5  as a nucleating agent. Other nucleating agents are TiO 2  and ZrO 2 .  
      Glass-ceramic compositions which may be useful in this invention are as per Table I. The percentages of Table I and the other tables herein are in weight percent.  
                           TABLE I                                      Li 2 O    8-17           Al 2 O 3     1.5-5.0           SiO 2     60-85           Na 2 O   0-2           K 2 O   0-2           P 2 O 5     1.5-5             ZrO 2     0-3           CaO   0-1           BaO + SrO + La 2 O 3      0-12           Coloring Oxides   0-5           Glass Modifiers   Balance                      
 
      More specific, glass-ceramic compositions which may be useful in the invention are as per Table II. In particular, the compositions of Table II are useful in conjunction with either high or low fusing porcelains which may be used to alter, shade or glaze the dental restorations of this invention.  
                           TABLE II                                      Li 2 O     10-13.5           Al 2 O 3     2-3           SiO 2     70-84           K 2 O   0-1           P 2 O 5     1.5-4             ZrO 2     0-1           BaO + SrO + La 2 O 3     .5-4            CaO   0-1           Coloring Oxides   0-5                      
 
      In some instances, it is desirable to form a dental restoration which can be altered or glazed with a low fusing porcelain. For purposes of this application, a low fusing porcelain is defined as a porcelain which fuses at a temperature of about 700° C.  
      Specific glass-ceramic compositions which are useful in conjunction with low fusing porcelains are as per Table III.  
                           TABLE III                                      Li 2 O    9-13           Al 2 O 3     1.5-4             SiO 2     65-84           Na 2 O   0-1           K 2 O   0-1           P 2 O 5     1.5-4             ZrO 2     0-1           BaO + SrO + La 2 O 3      0-12           CaO   0-1           Coloring Oxides   0-5                      
 
      Specific glass-ceramic compositions which may be used in this invention are shown as per the attached Tables IV, IVa, and IVb this invention is shown as per the attached Tables.  
                                                       TABLE IV                              1   2   3   4   5   6   7   8   9   10                                                                 SiO     81.0   84.0   84.0   83.0   82.0   84.0   81.0   83.0   83.0   81.5       Al O     3.5   3.0   3.0   3.0   4.0   3.3   3.3   3.5   3.5   3.0       Li O   11.0   10.5   10.5   10.5   10.5   10.0   11.0   9.0   8.0   12.0       P O     3.0   2.0   2.0   2.0   2.0   1.5   1.5   3.0   3.0   2.0       ZrO     1.0   0.5       0.5   0.5       2.0           0.5       TiO     0.4                   0.1   1.0   0.5               C O   0.5   0.25   0.25   0.25   0.25   0.25   0.25   0.25   0.25   0.25       MnO         0.5   0.5   0.5   0.5               0.5   0.5       NiO                             0.06                   K O           0.5   0.5   0.5   1.0           1.0   0.5       N O   0.5                   0.2   0.2   0.5   0.5           Fe O                         0.09   0.09   0.09               B O                                               M O                               1.0       C O                                   1.0                                                                     11   12   13   14   15   16   17   18   19   20                                                                 SiO     80.0   86.0   84.0   81.0   82.0   82.0   81.0   78.5   80.6   72.5       Al O     6.0   1.5       3.0   3.3   3.5   3.0   2.5   2.3   2.5       Li O   10.5   10.5   11.0   11.0   10.0   10.0   11.0   10.5   12.5   10.5       P O     2.0   2.0   3.0   1.0   1.5   2.2   3.0   2.0   2.0   2.0       ZrO     0.5       1.0   2.0       2.0   1.0   0.5   1.0   0.5       TiO             0.4   0.4       0.5   0.4   0.1   0.1   0.1       C O   0.25       0.50   0.50       0.25   0.50   0.35   0.35   0.42       MnO     0.5   0.5                               0.75       NiO                                 0.02   0.02       K O   0.5                           0.5   0.5       N O                   0.2   3.0       Fe O                         0.09       B O                   3.0           3.0   0.5   9.0       M O       C O                               0.4   0.4   0.4                  
 
     
       
         
           
               
               
               
             
               
                   
                 TABLE IVA 
               
               
                   
                   
               
               
                   
                   
               
             
            
               
                   
                 Li 2 O 
                 14-15 
               
               
                   
                 Al 2 O 3   
                 4.5-5.0 
               
               
                   
                 SiO 2   
                 67-71 
               
               
                   
                 Na 2 O 
                 1-2 
               
               
                   
                 K 2 O 
                 1.5-3.5 
               
               
                   
                 P 2 O 5   
                 3-4 
               
               
                   
                 B 2 O 3   
                 0-2 
               
               
                   
                 BaO 
                 1.25-3.0  
               
               
                   
                 Tb 4 O 7   
                 0-1 
               
               
                   
                 CeO 2   
                 0-1 
               
               
                   
                 CaO 
                   0-1.25 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE IVb 
               
               
                   
                   
               
               
                   
                   
               
               
                   
                 21 
                 22 
                 23 
                 24 
                 TOLERANCE 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 SiO 2   
                 70.6 
                 68.7 
                 68.8 
                 68.8 
                 ±1 
               
               
                   
                 Al 2 O 3   
                 4.8 
                 4.8 
                 4.8 
                 4.8 
                 ±.3 
               
               
                   
                 Li2O 
                 14.7 
                 14.4 
                 14.5 
                 14.4 
                 ±1 
               
               
                   
                 P 2 O 5   
                 3.6 
                 3.3 
                 3.3 
                 3.3 
                 ±.25 
               
               
                   
                 CaO 
                 0.5 
                 1.0 
                 1.0 
                 1.0 
                 ±2 
               
               
                   
                 K 2 O 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 ±.25 
               
               
                   
                 Na 2 O 
                 1.4 
                 1.5 
                 1.5 
                 1.4 
                 ±.25 
               
               
                   
                 BaO 
                 1.4 
                 2.8 
                 2.8 
                 2.8 
                 ±.3 
               
               
                   
                 B 2 O 3   
                 0.9 
                 — 
                 1.3 
                 1.2 
                 ±.3 
               
               
                   
                 Tb 4 O 7   
                 — 
                 0.7 
                 — 
                 — 
                 ±.05 
               
               
                   
                 CeO 2   
                 — 
                 0.7 
                 — 
                 — 
                 ±.05 
               
               
                   
                   
               
            
           
         
       
     
      In the formation of a dental restoration by a molding process it is desirable to have a glass ceramic composition which will readily flow into a mold in order to create the finest details of the dental restoration. This fact is true regardless of the molding process which is utilized.  
      The compositions of this invention may be pressed into a dental restoration in either a glass or a glass ceramic phase. When the pressing is effected in the glass phase the resulting dental restoration is subsequently heat treated to convert the structure to a glass ceramic phase.  
      In some cases it is desirable to press the compositions of this invention as a glass ceramic. The ability to press or inject the composition of this invention as a glass ceramic into the complex shapes of dental restoration is facilitated by the presence of residual glass in the range of from about 15 to 60 percent by volume. This residual glass functions as a matrix for the lithium disilicate crystalline phase.  
      For purposes of forming dental restoration, in accordance with this invention, when the composition is in the glass ceramic phase the glass ceramic pellets or buttons are preheated to a temperature of from about 850° C. to 950° C., with a most preferred preheat temperature being less than 930° C.  
      The compositions of Tables IVa and IVb have been modified in accordance with the prior art in order to increase their flowability, as a glass ceramic, in comparison to the compositions of Tables III and IV, comparing the composition of tables III and IV to the compositions of Tables IVa and IVb it can be seen that in the latter mentioned tables the composition of these tables incorporate B 2 O 3 . In the compositions of Tables IVa and IVb the B 2 O 3  functions as a glass modifier for the glass phase of the glass ceramic. In this regard the B 2 O 3  softens the glass matrix and increases its flow ability thereby aiding the flow ability of the crystal phase in the molding process. This known property of B 2 O 3  produces a superior glass ceramic for use in accordance with this invention.  
      Zinc oxide is still another oxide which is safe in a dental environment which can be used to soften the glass matrix in order to facilitate the flow of the crystal phase during the molding operation.  
      The compositions as listed above incorporate Li 2 O which again softens the glass matrix and aides in crystal flow. The compositions of this invention incorporate substantial quantities of Li 2 O which likewise can function as a softener for the glass matrix. In the composition of this invention it is thought that the Li 2 O does not soften the glass matrix as nearly all of the Li 2 O is used in the formation of the crystal phase.  
      The fact that B 2 O 3  lowers the viscosity (softens) silicate glass systems and produces glasses which have low thermal expansions and high chemical resistance is discussed in  SILICATE GLASS TECHNOLOGY METHODS  by Clarence L. Babcock, John Wiley &amp; sons 1977 at page 23.  
      Relative to the above discussion of the fact that Li 2 O, Na 2 O, K 2 O and B 2 O 3  function to decrease viscosity and thereby soften glasses see also MODERN GLASS PRACTICE by Samuel R. Scholes, Industrial Publications, Inc. 1952 at page. 17.  
      Further as these references disclose that it is known in the prior art that the oxides of sodium and potassium act as glass modifiers and thereby aid in crystal flow during the molding of a glass ceramic. In this regard comparing the compositions of Tables III and IV with the compositions of Tables IVa and IVb it can be seen that in the compositions of Tables IVa and IVb the percentages of Na 2 O and K 2 O have been increased, compared to the compositions of Tables III and IV. Again in accordance with known procedures this increase in the concentration of Na 2 O and K 2 O softens the glass matrix and thereby aids in the flow ability of the crystal phase.  
      In summary the compositions of Tables IVa and IVb are altered with glass modifiers in such a manner as to aid in the flow sability of the crystal phase. It is understood by one skilled in the art that glass modifies other than these discussed above can be utilized.  
      As to a general range of compositions which are preferred for use in conjunction with this invention, the glass-ceramic compositions as defined in Tables IVa and IVb are preferred.  
      The most preferred glass-ceramic composition for use in accordance with this invention is the glass-ceramic composition as is defined by compositions 21-24 in Table IVb.  
      The glass-ceramic composition as defined by Tables I to IVb above are particularly advantageous over the prior art in that:  
      (1) They are stronger than the prior art glass-ceramic compositions;  
      (2) Those compositions which incorporate Tb 4 O 7  and cerium oxides are fluorescent in ultra violet light;  
      (3) The expansion rates of many of these glass-ceramic compositions are compatible with the expansion rate of existing altering porcelain materials; and  
      (4) They retain their structural integrity at higher temperatures than those of the prior art, thereby permitting alterations at higher temperatures.  
      Buttons of glass-ceramic materials in accordance with this invention are formed by first preparing a master batch of the components of the particular glass-ceramic desired. This batch is used to form a homogenous glass-ceramic which, upon cooling, is ground into a powder. Portions of this powder are then sintered to form buttons of the desired size and weight. Relative to these sintered buttons, the applicant has found that dental restorations which are formed from solid, non-sintered buttons are stronger than those formed from sintered buttons. These buttons can assume the shape of blocks which may be milled into dental restorations by conventional, three-dimensional milling procedures in accordance with known prior art procedures.  
      A preferred process for forming a dental restoration in accordance with this invention is as follows;  
      a. melting a mixture of metal oxides, which are capable of forming a Lithium Discilicate glass ceramic, to form a glass composition.  
      b. quenching said glass composition,  
      c. heat treating said glass composition to convert it into a glass ceramic,  
      d. grinding said glass ceramic into a powder.  
      e. modifying the glass ceramic powder with one or more glass modifiers, if desired,  
      f. compacting and sintering the powder into a button,  
      g. heating the button to a temperature wherein it becomes semi plastic and,  
      h. pressing the button into a preformed mold having a negative cavity in the shape of the desired dental restoration.  
      Another preferred process for forming a dental restoration in accordance with this invention is as follows; 
          a. melting a mixture of metal oxides, which are capable of forming a Lithium Disilicate glass ceramic, to form a glass composition,     b. cast a button of said glass composition,     c. heat treating said button to convert it into a glass ceramic,     d. heating the button to a temperature wherein it becomes semi plastic and,     e. pressing the button into a preformed mold having a negative cavity in the shape of the desired dental restoration.        

      In the formation of a glass ceramic which is useful in this invention the component metal oxides are melted together to form a glass melt. This glass melt is then heat treated to form a crystalline glass ceramic phase which is contained in a glass matrix. In accordance with this invention the percentage of the lithium disilicate crystalline phase in the glass matrix can be from about 30 to about 80 weight percent, with a more preferred range being from about 40 to about 60 weight percent with a most preferred range being from about 45 to about 55 weight percent.  
      The size of the glass ceramic crystals in the glass phase can be varied. The size of the glass ceramic crystals affects the strength of the resulting glass ceramic. The size of the glass ceramic crystals can be affected by the heat treating process i.e. the temperature of the heat treating, the ramping of the heat treating, the soaking of the heat treating etc.  
      For use in accordance with this invention it has been found that the size of the glass ceramic crystal can be from about 0.5 to about 20 microns.  
      As is listed in Tables I to IVb, coloring oxides may be added to the glass-ceramic material in order to achieve the desired coloration of the glass-ceramic dental restorations.  
      Suitable coloring oxides for use in glass-ceramic compositions which can be used in this invention, some of which are illustrated in Tables I to IVb, are SnO 2 , MnO, CeO, FeO 2 , Ni 2 O, V 2 O, Cr 2 O 3 , TiO 2 , etc. These coloring oxides may be used singularly or in combination.  
      In addition to coloring oxides, the glass-ceramic compositions of Tables I to IVb may incorporate fluorescing agents and opacifiers. After formation of a glass-ceramic restoration in accordance with the process of this invention, the resulting dental restoration is heat treated in order to effect the formation of crystals in the dental restoration. This crystal formation is generally referred to as heat treating and enhances the physical and esthetic properties of the dental restoration. A suitable heat treating sequence for use with the glass-ceramic compositions of Tables I to IVb is as per Table V below.  
      For the glass compositions which may be used in this invention, the best nucleating temperatures are about 25° to 50° C. above the upper annealing point of the glass-ceramic. It has also been determined that a slow increase in temperature from just above the upper annealing point to some 50° C. or higher, produces the best results in achieving maximum nucleation. It is also well known that the temperature then must be raised to a higher temperature to affect crystallization. This temperature depends upon the composition of the particular glass-ceramic material being used.  
      If the heating sequence in the heat treating stage is improper or is incorrectly controlled, the dental restoration may slump or deform. It is understood by one skilled in the art that an optimum heat treating procedure should be developed for each particular glass-ceramic material in order to allow the dental restoration to maintain its structural integrity during the heat treating process.  
      In the preferred embodiment, the heat treating cycle is such that in the crystalline phase, the glass-ceramic material incorporates a large number of fine crystals which are evenly dispersed throughout the glass matrix. It has been found that when the crystalline phase is very fine and evenly dispersed, a dental restoration of maximum strength results. Further, a fine crystalline structure tends to produce translucent dental restorations.  
      From the above discussion, it is obvious that dental restorations produced in accordance with this invention can be heat treated after the dental restoration is removed from the investment material and the sprue is cut away.  
      In accordance with an alternate preferred embodiment of this invention, the heat treating of the dental restoration may be effected while the dental restoration is still in the invested state. That is, heat treating can be effected while the dental restoration is still encased in the investment material.  
      In accordance with still another embodiment, the dental restoration can be removed from the investment material in which it was formed and finished or partially finished prior to heat treating. For heat treating, the material is then reinvested in an investment material which enhances the heat treatment process. After reinvestment, the heat treating is carried out in accordance with the procedure described above.  
      When the dental restoration is heat treated in the invested state, the shrinkage of the dental restoration is minimized, if not eliminated.  
      In heat treating, and, in particular, in the formation of a crystalline phase, shrinkage of the dental restoration occurs. This shrinkage can amount to 3 percent. Naturally, this shrinkage is undesirable as it adversely affects the fit of the dental restoration back into the patient&#39;s mouth.  
      As a way of overcoming the shrinkage problem, it is within the scope of this invention to invest the wax form in an investment material which expands when it hardens to produce an oversized mold cavity. This oversized mold cavity naturally produces an oversized dental restoration. This oversized dental restoration is then shrunk back to the correct size during the heat treating process.  
      For example, since the glass-ceramic material shrinks about 3 percent during the heat treating process, the original wax form may be invested in an investment which expands by 3 percent when it hardens. Using this procedure, a dental restoration which is 3 percent oversized results. This oversized dental restoration is then heat treated, whereupon it shrinks about 3 percent to produce a finished dental restoration of the correct size.  
      In order to achieve proper fit, if the restoration is to be heat treated after removal from the investment, an investment material which expands on hardening and heating by about 3 percent must be used. Investment materials which are useful with metals are not particularly suited for use in the invention as they only expand about 1.75 percent. Investments which expand about 3 percent upon hardening and heating, and hence permit this type of process, are manufactured by Whipmix Corporation of Louisville, Kentucky, and sold under the designation GIJM 2-23-94; 1.  
      The preferred heat treating process for use with the glass-ceramic compositions of Tables I to IVb is as per Table V.  
                           TABLE V                                      Heat Dental Restoration to   500° C.           Ramp or Soak at   500° to 600° C. for 2-6 hrs.           Ramp at   600°-925° C. for 2-3 hrs.           Soak at   925° C. for 1-2 hrs.           Cool to   Room Temperature                      
 
      After heat treating, the resulting dental restoration may be further finished. This finishing may include a step wherein the dental restoration is altered with one or more porcelains in order to achieve the precise shape, shade and shading desired. For those glass-ceramic materials of Table I to IVb, it is desired that the C.T.E. of the altering porcelain material be slightly more or less than the C.T.E. of the glass-ceramic material. This process wherein the C.T.E. is measured is defined in the description of the examples herein below.  
      Deformable crucible  8  is a critical part of the subject invention. Crucible  8  in its preferred embodiment has a circular base and, hence, is generally cylindrical. However, it is understood by one skilled in the art, that the crucible of this invention can have configurations other than circular.  
      The crucible is formed by heat sintering a particulate mixture of components, such as, fused silica, aluminum oxide, zirconium oxide, magnesium oxide with a glass such as borosilicate glass, soda lime glass, bottle glass, window glass, etc., clay or other materials that those skilled in the art of crucible making might use to form a crucible suitable for use with a glass-ceramic material. In the broad concept of this invention, one forms the crucible of a mixture of materials, such that at the temperature where the glass-ceramic material is in the working range of about log 3 to log 4P, the crucible has a viscosity of about log 5 to log 7P.  
      Crucibles for use in this invention are formed by slip casting. The formation of crucible by slip casting is well known to those skilled in the art, and is described in greater detail in the examples below.  
      In the description as set forth above, the compositions which are used to form crucibles which are useful in this invention, may utilize a wide range of materials. While many glasses can be used to form crucibles which are useful in the subject invention, because of their toxicity, glasses which contain heavy metals, such as lead, cadium, etc., should not be used.  
      An alternate method for the formation of crucibles for use in this invention, is by powder pressing and then sintered.  
      In this invention, the composition from which the crucible is formed contains materials such that it will be heat-pressure deformable at the desired temperature, i.e., about log 3 to log 4P, working temperature of the glass-ceramic. The heat-pressure deformation properties of the crucibles used in this invention is to be contrasted with the crucibles of the prior art which are designed to be rigid at the working temperature of the glass-ceramic material which is contained therein.  
      In the alternate structure, as is shown in  FIGS. 8, 9 , and  10 , the sealing of the deformed crucible against the mold is enhanced. In the structure as shown, mold  64  incorporates a circular depression  66  having vertical walls  68  and  70 , to which crucible  8  may seal. Vertical walls  72  and  74  of mold  65  may further incorporate a plurality of annular grooves  76 . As is further shown in  FIG. 10 , the sealing of crucible  8  against mold  65  is enhanced by the flowing of the upper extremities of crucible  8  into annular grooves  76  during the deformation process.  
      Further dental restorations can be formed from the glass ceramic compositions discussed above by milling blocks or copings of glass ceramic materials. This milling can be effected on the glass ceramic block or coping prior to or after said block or coping is treated to form the crystalline phase. Prior to heat treating to form the crystalline phase, the block is referred to as being in the green phase or being green. In the green state the material is in a glass phase. After formation of the crystalline phase, the glass ceramic is much harder and stronger when compared to the green phase. This heat treating to form a crystalline phase is often referred to as ceraming.  
      A complete dental restoration can be milled inside and out in accordance with this invention.  
      As per this aspect of the invention a glass ceramic block or coping is retained in a vise. The block or coping may be in the green or ceramed state. A milling head is then positioned in proximity to the block or coping of glass ceramic material. A computer guided mill is then brought into contact with the block of glass ceramic material.  
      The positioning of the mill is controlled by a stilus or probe which is adapted to take readings off of a model of the dental restoration which is being duplicated. Computer controlled milling processes and apparatus therefore are often referred to as CNC processes and machines.  
      Dental restorations in accordance with this invention may be milled from probe readings that are taken from models or from readings that are taken directly from a patients mouth. In the case of models, they are normally formed from plaster type materials which are poured into impressions. The impressions are negative cast of the patients mouth.  
      The milling of a dental restoration may be effected from data taken from the patients mouth by the selective positioning of a probe in the patients mouth or from a model.  
      That is the milling of the dental restoration may be effected from data which is gathered from a model or from the patients mouth.  
      In either case multi plane reference points are established. The surfaces of the model or patients mouth is then mapped out using these reference points. Once the mouth is mapped out a dental restoration can be milled which fit onto the prepared tooth surface and duplicate the exterior surfaces of the tooth or teeth which are being replicated in the dental restoration, which is being formed.  
      In all cases the positioning of the milling head during the milling process is computer controlled from data gathered during the mapping stage.  
      After the inner and outer surfaces of the dental restoration, which is being reproduced, are mapped out, a milling head is brought into contact with the coping or glass ceramic block. The milling of the dental restoration is then effected using conventional CNC grinding apparatus, wherein the positioning of the grinder is controlled by the data which was gathered during the mapping process.  
      In the formation of the model and during the conversion of the glass coping, to a glass ceramic, the shrinkage of the glass ceramic material in the ceraming process must be compensated for. This compensation can be effected by choosing a suitable investment material which shrinks or expands to a degree which compensated for the degree to which the dental restoration shrinks during the ceraming process, i.e. if the glass ceramic shrinks three percent during the ceraming process a dental technician would utilize an investment compound, to form the model, which expands by three percent. In this manner when the final dental restoration is formed the shrinkage during the ceraming process is compensated for.  
      Human teeth are not homogenous in color or in their optical properties. Generally a tooth becomes lighter in color and more translucent as the distance from the gum increases. In order to duplicate this natural color variation dental restorations in accordance with this invention may be milled from blocks of ceramic or glass ceramic materials which are segmented as to optical properties, i.e. the upper portion of the block may be more translucent than the lower portion of the block. This layer structure allows for the milling of dental restorations that start out being more natural in optical properties.  
      It is understood by one skilled in the art, that the final colorations of any milled dental restoration may be effected by glazing with glazing porcelains.  
      Glass ceramic compositions as are set forth herein are particularly suited for the formation of blocks or copings, which can be subsequently milled into finished glass ceramic dental restorations. Said blocks or copings can be formed by direct casting from the molted glass or prepared by pressing as set forth above in steps a-e.  
     EXAMPLES  
      The present invention is illustrated by the following examples. However, these examples are not to be construed as limiting the invention.  
      Dental restorations were prepared in accordance with the below listed examples. In these examples, wax models of a tooth were prepared. A sprue was then attached to the wax model. The wax model with the sprue attached was then placed in an investment ring. An investment material was then prepared by mixing 90 grams of Kerr Thermovest with 17 ml of a mixture of 2 parts of Thermovest liquid with 1 part water. The resulting mixture was then mixed to a uniform consistency. The mixed investment was then vibrated into the investment ring and around the wax model. The mold was allowed to dry and harden overnight. The investment ring was then placed in a burn out furnace at room temperature, and the temperature raised to 600° C. whereupon the wax model was burned out of the investment material.  
      The crucibles as used in these examples were prepared by slip casting. In this process, a female plaster of Paris mold was prepared by mixing 1247.4 grams of plaster of Paris with 946 ml of water to a uniform consistency. The resulting mold was allowed to harden for 36 hours. A slurry of a particulate crucible formulation mixture, as is identified in the examples below, was then placed in the preformed plaster of Paris mold. The green crucible was removed from the mold and fired in a furnace for 15 minutes at 1100° C. to sinter the crucible to a hardened, more durable form.  
      The slurry used to form the crucible was formed by mixing 1 pound of particulate material with 160 ml water.  
      Because of the hygroscopic nature of the plaster of Paris mold, the particulate crucible formulation slurry coagulates in the plaster of Paris mold. When the desired crucible wall thickness was achieved, the remainder of the slurry was poured out of the mold resulting in a green crucible structure which was allowed to dry.  
      In the listed examples and in this application, the coefficient of thermal expansion (C.T.E.) was measured from room temperature to 250° C., and is reported in units of ×10 −7 /° C .  
      The translucency of the resulting dental restoration was measured by visual inspection given a value of 0 to 5, wherein value of 0 was deemed to be perfectly clear and 5 was deemed to be opaque.  
      In the below listed examples, for purposes of injecting the molten glass and glass-ceramic material into the mold, a pressure of 30 P.S.I. was utilized, with the exception of examples 26 and 27, wherein a pressure of about 50 P.S.I. was utilized.  
      The referenced heat treating sequence for all examples, except examples 26 and 27, is in accordance with Table V above.  
      In the below examples, the glass-ceramic test rods for measuring modules or rupture (M.O.R.) and thermal expansion (C.T.E.) were formed along with color tab test samples. These test rods and color tab samples were subjected to the heat treatment process of Table V, whereby the glasses were crystallized unsuitable to glass-ceramics. The test rod samples were 0.120″×1.25″. The examples also record the visual appearance of each glass-ceramic and values of various properties exhibited by the glass-ceramic, such as, linear coefficient of thermal expansion (C.T.E.), reported in terms of ×10 −7 /° C., modules of rupture (M.O.R.), cited in terms of K.S.I. (thousands of pounds per square inch), as determined in accordance with measurement techniques conventional in the art. K.S.I. may be converted to its metric equivalent MPA by dividing K.S.I. by 0.145.  
      The components used in these examples are as follows:  
      a. Thermovest and Therovest liquid as sold by Kerr Manufacturing Co., Romulus, Mich.  
      b. 3I fused silica as sold by Harbison Walker Refractories Division of Indresco, Inc., Pittsburgh, Penn.  
      c. SP921 TF (borosilicate glass) as sold by Specialty Glass, Inc., of Oldsmar, Fla., and has the following compositions in weight percent:  
                           TABLE VI                                      SiO 2     78           B 2 O 3     15           Al 2 O 3     2.5           Na 2 O   4.5                      
 
     Example 1  
      An attempt was made to form a glass-ceramic dental restoration in accordance with the following procedure. A mold was prepared in accordance with the procedure described above.  
      By use of slip casting techniques, a crucible was prepared in accordance with the procedure described above by mixing 90.9 grams of SP921 TF glass in 363.2 grams of 3I fused silica with 160 ml of water. After slip casting and drying, the crucible was sintered at a temperature of 1100° C. A glass button weighing 6 grams was then placed in the crucible and preheated to a temperature of 650° C.  
      The composition of the glass-ceramic material used is in accordance with composition #1 of Table IV.  
      The preheated crucible and glass-ceramic material were then placed in an apparatus which is similar to that shown in  FIG. 7 , and heated to a temperature of 1400° C. for a period of 10 minutes. The ram was then actuated, and the crucible was brought into contact with the mold in accordance with the general procedure as is illustrated in  FIGS. 3 through 6 . The crucible cracked, and, hence, no dental restoration was formed.  
     Example 2  
      A glass-ceramic dental restoration was prepared in accordance with the following procedure. A mold was prepared in accordance with the procedure described above.  
      By use of slip casting technique, a crucible was prepared in accordance with the procedure described above by mixing 136.2 grams of SP921 TF glass and 317.8 grams of 3I fused silica with 160 ml of water. After slip casting and drying, the crucible was sintered at a temperature of 1100° C. A glass button weighing 6 grams was then placed in the crucible and preheated to a temperature of 650° C.  
      The composition of the glass-ceramic material used is in accordance with composition #1 of Table IV.  
      The preheated crucible and glass-ceramic material were then placed in an apparatus which is similar to that shown in  FIG. 7 , and heated to a temperature of 1400° C. for a period of 10 minutes. The ram was then actuated, and the crucible was brought into contact with the mold in accordance with the general procedure as is illustrated in  FIGS. 3 through 6 .  
      The dental restoration was then removed from the investment material, cut apart from the sprue, and heat treated.  
      The resulting dental restoration had excellent definition and a M.O.R. of 41 K.S.I., a C.T.E. of 148, a translucency of 3.6, and a softening temperature of 975° C.  
      During the forming process, the crucible retained its structural integrity, and formed an effective seal with the mold.  
      Further, during the heat treating process, the finished dental restoration retained its structural integrity and did not slump or deform during heat treating.  
      Additional tests were conducted, wherein it was determined that the crucible of this example deformed and sealed well at 1375° C. and 1425° C.  
     Example 3  
      A mold was prepared in accordance with the procedure described above.  
      By use of slip casting technique, a crucible was prepared in accordance with the procedure described above by mixing 181.6 grams of SP921 TF glass and 272.4 grams of 3I fused silica with 160 ml of water. After slip casting and drying, the crucible was sintered at a temperature of 1100° C. A glass button weighing 6 grams was then placed in the crucible and preheated to a temperature of 650° C.  
      The preheated crucible without glass-ceramic material was then placed in an apparatus which is similar to that shown in  FIG. 7 , and heated to a temperature of 1400° C. for a period of 10 minutes.  
      During the heating process, the crucible lost its structural integrity, and was not capable of effecting a seal with a mold. Therefore, no molding process was carried out.  
     Example 4  
      A mold was prepared in accordance with the procedure described above.  
      By use of slip casting technique, a crucible was prepared in accordance with the procedure described above by mixing 227 grams of SP921 TF glass and 227 grams of 3I fused silica with 160 ml of water. After slip casting and drying, the crucible was sintered at a temperature of 1100° C. A glass button weighing 6 grams was then placed in the crucible and preheated to a temperature of 650° C.  
      The preheated crucible and glass-ceramic material were then placed in an apparatus which is similar to that shown in  FIG. 7 , and heated to a temperature of 1400° C. for a period of 10 minutes.  
      During the heating process, the crucible melted, and, hence, was not capable of forming an effective seal with the mold. Therefore, no molding process was carried out.  
     Example 5  
      A glass-ceramic dental restoration was prepared in accordance with the following procedure. A mold was prepared in accordance with the procedure described above.  
      By use of slip casting technique, a crucible was prepared in accordance with the procedure described above by mixing 118 grams of SP921 TF glass and 336 grams of 3I fused silica with 160 ml of water. After slip casting and drying, the crucible was sintered at a temperature of 1100° C. A glass button weighing 6 grams was then placed in the crucible and preheated to a temperature of 650° C.  
      The composition of the glass-ceramic material used is in accordance with composition #1 of Table IV.  
      The preheated crucible and glass-ceramic material were then placed in an apparatus which is similar to that shown in  FIG. 7 , and heated to a temperature of 1400° C. for a period of 10 minutes. The ram was then actuated, and the crucible was brought into contact with the mold in accordance with the general procedure as is illustrated in  FIGS. 3 through 6 .  
      The dental restoration was then removed from the investment material, cut apart from the sprue, and heat treated.  
      The resulting dental restoration had excellent definition and physical properties, as are reported in Example 2.  
      During the forming process, the crucible retained its structural integrity, and formed an effective seal with the mold. Further, during the heating process, the finished dental restoration retained its structural integrity, and did not slump or deform. Further tests were conducted on crucibles having the composition as set forth above, wherein it was determined that the crucible cracked, and, hence, was not functional at 1375° C. An additional test demonstrated that the crucible of this example deformed and sealed at 1425° C.  
     Example 6  
      A glass-ceramic dental restoration was prepared in accordance with the following procedure. A mold was prepared in accordance with the procedure described above.  
      By use of slip casting technique, a crucible was prepared in accordance with the procedure described above by mixing 127.1 grams of SP921 TF glass and 336.9 grams of 3I fused silica with 160 ml of water. After slip casting and drying, the crucible was sintered at a temperature of 1100° C. A glass button weighing 6 grams was then placed in the crucible and preheated to a temperature of 650° C.  
      The composition of the glass-ceramic material used is in accordance with composition #1 of Table IV.  
      The preheated crucible and glass-ceramic material were then placed in an apparatus which is similar to that shown in  FIG. 7 , and heated to a temperature of 1400° C. for a period of 10 minutes. The ram was then actuated, and the crucible was brought into contact with the mold in accordance with the general procedure as is illustrated in  FIGS. 3 through 6 .  
      The dental restoration was then removed from the investment material, cut apart from the sprue, and heat treated.  
      The resulting dental restoration had excellent definition and physical properties, as are reported in Example 2.  
      During the forming process, the crucible retained its structural integrity, and formed an effective seal with the mold. Further, during the heating process, the finished dental restoration retained its structural integrity, and did not slump or deform. Further tests were conducted on crucibles having the composition as set forth above, wherein it was determined that the crucible would deform and seal at both 1375° C. and 1425° C.  
     Example 7  
      A glass-ceramic dental restoration was prepared in accordance with the following procedure. A mold was prepared in accordance with the procedure described above.  
      By use of slip casting technique, a crucible was prepared in accordance with the procedure described above by mixing 145.3 grams of SP921 TF glass and 308.7 grams of 3I fused silica with 160 ml of water. After slip casting and drying, the crucible was sintered at a temperature of 1100° C. A glass button weighing 6 grams was then placed in the crucible and preheated to a temperature of 650° C.  
      The composition of the glass-ceramic material used is in accordance with composition #1 of Table IV.  
      The preheated crucible and glass-ceramic material were then placed in an apparatus which is similar to that shown in  FIG. 7 , and heated to a temperature of 1400° C. for a period of 10 minutes. The ram was then actuated, and the crucible was brought into contact with the mold in accordance with the general procedure as is illustrated in  FIGS. 3 through 6 .  
      The dental restoration was then removed from the investment material, cut apart from the sprue, and heat treated.  
      The resulting dental restoration had excellent definition and physical properties, as are reported in Example 2.  
      During the forming process, the crucible retained its structural integrity, and formed an effective seal with the mold. Further, during the heating process, the finished dental restoration retained its structural integrity, and did not slump or deform. Further tests were conducted on the crucible of this example, wherein it was determined that the crucible functioned at 1375° C. However, at 1425° C., the crucible slumped, and, hence, could not be used in the process of this invention.  
     Example 8  
      A glass-ceramic dental restoration was prepared in accordance with the following procedure. A mold was prepared in accordance with the procedure described above.  
      By use of slip casting technique, a crucible was prepared in accordance with the procedure and description shown above in connection with example 2. A glass button weighing 6 grams was then placed in the crucible, and preheated to a temperature of 650° C.  
      The composition of the glass-ceramic material used is in accordance with composition #2 of Table IV.  
      The preheated crucible and glass-ceramic material were then placed in an apparatus which is similar to that shown in  FIG. 7 , and heated to a temperature of 1400° C. for a period of 10 minutes. The ram was then actuated, and the crucible was brought into contact with the mold in accordance with the general procedure as is illustrated in  FIGS. 3 through 6 .  
      The dental restoration was then removed from the investment material, cut apart from the sprue, and heat treated.  
      The resulting test samples and dental restoration had excellent definition and a M.O.R. of 46.7 K.S.I., a C.T.E. of 138, a translucency of 2.75, and a softening temperature of 975° C.  
      During the forming process, the crucible retained its structural integrity, and formed an effective seal with the mold. Further, during the heat treating process, the finished dental restoration retained its structural integrity, and did not slump or deform during heat treating.  
     Example 9  
      A glass-ceramic dental restoration was prepared in accordance with the following procedure. A mold was prepared in accordance with the procedure described above.  
      By use of slip casting technique, a crucible was prepared in accordance with the procedure and description shown above in connection with example 2. A glass button weighing 6 grams was then placed in the crucible, and preheated to a temperature of 650° C.  
      The composition of the glass-ceramic material used is in accordance with composition #3 of Table IV.  
      The preheated crucible and glass-ceramic material were then placed in an apparatus which is similar to that shown in  FIG. 7 , and heated to a temperature of 1400° C. for a period of 10 minutes. The ram was then actuated, and the crucible was brought into contact with the mold in accordance with the general procedure as is illustrated in  FIGS. 3 through 6 .  
      The dental restoration was then removed from the investment material, cut apart from the sprue, and heat treated.  
      The resulting test samples and dental restoration had excellent definition and a M.O.R. of 43 K.S.I., a C.T.E. of 141, a translucency of 3.0, and a softening temperature of 975° C.  
      During the forming process, the crucible retained its structural integrity, and formed an effective seal with the mold. Further, during the heat treating process, the finished dental restoration retained its structural integrity, and did not slump or deform during heat treating.  
     Example 10  
      A glass-ceramic dental restoration was prepared in accordance with the following procedure. A mold was prepared in accordance with the procedure described above.  
      By use of slip casting technique, a crucible was prepared in accordance with the procedure and description shown above in connection with example 2. A glass button weighing 6 grams was then placed in the crucible, and preheated to a temperature of 650° C.  
      The composition of the glass-ceramic material used is in accordance with composition #4 of Table IV.  
      The preheated crucible and glass-ceramic material were then placed in an apparatus which is similar to that shown in  FIG. 7 , and heated to a temperature of 1400° C. for a period of 10 minutes. The ram was then actuated, and the crucible was brought into contact with the mold in accordance with the general procedure as is illustrated in  FIGS. 3 through 6 .  
      The dental restoration was then removed from the investment material, cut apart from the sprue, and heat treated.  
      The resulting test samples and dental restoration had excellent definition and a M.O.R. of 48 K.S.I., a C.T.E. of 133, a translucency of 2.5, and a softening temperature of 975° C.  
      During the forming process, the crucible retained its structural integrity, and formed an effective seal with the mold. Further, during the heat treating process, the finished dental restoration retained its structural integrity, and did not slump or deform during heat treating.  
     Example 11  
      A glass-ceramic dental restoration was prepared in accordance with the following procedure. A mold was prepared in accordance with the procedure described above.  
      By use of slip casting technique, a crucible was prepared in accordance with the procedure and description shown above in connection with example 2. A glass button weighing 6 grams was then placed in the crucible, and preheated to a temperature of 650° C.  
      The composition of the glass-ceramic material used is in accordance with composition #5 of Table IV.  
      The preheated crucible and glass-ceramic material were then placed in an apparatus which is similar to that shown in  FIG. 7 , and heated to a temperature of 1400° C. for a period of 10 minutes. The ram was then actuated, and the crucible was brought into contact with the mold in accordance with the general procedure as is illustrated in  FIGS. 3 through 6 .  
      The dental restoration was then heat treated.  
      The resulting test samples and dental restoration had excellent definition and a M.O.R. of 42 K.S.I., a C.T.E. of 140, a translucency of 2.75, and a softening temperature of 975° C.  
      During the forming process, the crucible retained its structural integrity, and formed an effective seal with the mold. Further, during the heat treating process, the finished dental restoration retained its structural integrity, and did not slump or deform during heat treating.  
     Example 12  
      A glass-ceramic dental restoration was prepared in accordance with the following procedure. A mold was prepared in accordance with the procedure described above.  
      By use of slip casting technique, a crucible was prepared in accordance with the procedure and description shown above in connection with example 6. A glass button weighing 6 grams was then placed in the crucible, and preheated to a temperature of 650° C.  
      The composition of the glass-ceramic material used is in accordance with composition #6 of Table IV.  
      The preheated crucible and glass-ceramic material were then placed in an apparatus which is similar to that shown in  FIG. 7 , and heated to a temperature of 1400° C. for a period of 10 minutes. The ram was then actuated, and the crucible was brought into contact with the mold in accordance with the general procedure as is illustrated in  FIGS. 3 through 6 .  
      The dental restoration was then heat treated.  
      The resulting test samples and dental restoration had excellent definition and a M.O.R. of 41.5 K.S.I., a C.T.E. of 162, a translucency of 4.0, and a softening temperature of 950° C.  
      During the forming process, the crucible retained its structural integrity, and formed an effective seal with the mold. Further, during the heat treating process, the finished dental restoration retained its structural integrity, and did not slump or deform during heat treating.  
     Example 13  
      A glass-ceramic dental restoration was prepared in accordance with the following procedure. A mold was prepared in accordance with the procedure described above.  
      By use of slip casting technique, a crucible was prepared in accordance with the procedure and description shown above in connection with example 6. A glass button weighing 6 grams was then placed in the crucible, and preheated to a temperature of 650° C.  
      The composition of the glass-ceramic material used is in accordance with composition #7 of Table IV.  
      The preheated crucible and glass-ceramic material were then placed in an apparatus which is similar to that shown in  FIG. 7 , and heated to a temperature of 1400° C. for a period of 10 minutes. The ram was then actuated, and the crucible was brought into contact with the mold in accordance with the general procedure as is illustrated in  FIGS. 3 through 6 .  
      The dental restoration was then removed from the investment material, cut apart from the sprue, and heat treated.  
      The resulting test samples and dental restoration had excellent definition and a M.O.R. of 53.5 K.S.I., a C.T.E. of 136, a translucency of 4.0, and a softening temperature of 975° C.  
      During the forming process, the crucible retained its structural integrity, and formed an effective seal with the mold. Further, during the heat treating process, the finished dental restoration retained its structural integrity, and did not slump or deform during heat treating.  
     Example 14  
      A glass-ceramic dental restoration was prepared in accordance with the following procedure. A mold was prepared in accordance with the procedure described above.  
      By use of slip casting technique, a crucible was prepared in accordance with the procedure and description shown above in connection with example 2. A glass button weighing 6 grams was then placed in the crucible, and preheated to a temperature of 650° C.  
      The glass-ceramic material used was DI-COR, sold by the Dentsply Corporation, as is described above. DI-COR is thought to be fluoro-mica glass-ceramic.  
      The preheated crucible and glass-ceramic material were then placed in an apparatus which is similar to that shown in  FIG. 7 , and heated to a temperature of 1400° C. for a period of 10 minutes. The ram was then actuated, and the crucible was brought into contact with the mold in accordance with the general procedure as is illustrated in  FIGS. 3 through 6 .  
      The dental restoration was then heat treated in the investment.  
      While the glass-ceramic properly filled the mold, the resulting dental restoration cracked as a result of shrinkage during the heat treating process. This cracking is thought to be a result from the fact that the forming investment was not suitable for use in the heat treating sequence.  
     Example 15  
      A glass-ceramic dental restoration was prepared in accordance with the following procedure. A mold was prepared in accordance with the procedure described above.  
      By use of slip casting techniques, a crucible was prepared in accordance with the procedure and composition described above in connection with example 2. A glass button weighing 6 grams was then placed in the crucible, and preheated to a temperature of 650° C.  
      The composition of the glass used in weight percent was:  
                           TABLE VII                                      SiO 2     50.2           B 2 O 3     8.6           AlF 3     .7           Al 2 O 3     16.0           BaO   4.75           ZnO   19.75                      
 
      The preheated crucible and glass-ceramic material were then placed in an apparatus which is similar to that shown in  FIG. 7 , and heated to a temperature of 1400° C. for a period of 10 minutes. The ram was then actuated, and the crucible was brought into contact with the mold in accordance with the general procedure as is illustrated in  FIGS. 3 through 6 .  
      A satisfactory dental restoration resulted, which was then removed from the investment material, and cut apart from the sprue.  
      The resulting test samples and dental restoration had excellent definition and a M.O.R. of 4.0 K.S.I., a C.T.E. of 56.0, and a translucency of 0.  
      During the forming process, the crucible retained its structural integrity, and formed an effective seal with the mold. Because the material used was a glass, the resulting dental restoration was not heat treated.  
     Example 16  
      A glass-ceramic dental composition was prepared and heat treated in accordance with this invention. The composition of the glass-ceramic material is in accordance with composition #8 of Table IV.  
      After melting, test samples were prepared in accordance with the procedure described above. The resulting test samples had a M.O.R. of 30 K.S.I., a C.T.E. of 157, a translucency of 40, and a softening temperature of 925° C.  
      The test samples cracked during heat treating, therefore, the composition was deemed to be unsuitable as a material for use in preparation of dental restoration. Accordingly, further tests were not carried out and a dental restoration was not formed.  
     Example 17  
      A glass-ceramic dental composition was prepared and heat treated in accordance with this invention. The composition of the glass-ceramic material is in accordance with composition #9 of Table IV.  
      After melting, test samples were prepared in accordance with the procedure described above. The resulting test samples had a M.O.R. of 24 K.S.I., a C.T.E. of 145, and a softening temperature of 975° C.  
      Because the test samples had a M.O.R. of only 24, and the glass-ceramic was hard to melt and form, the composition was deemed to be unsuitable as a material for use in preparation of dental restorations. Accordingly, further tests were not carried out and a dental restoration was not formed.  
     Example 18  
      A glass-ceramic dental composition was prepared and heat treated in accordance with this invention. The composition of the glass-ceramic material is in accordance with composition #10 of Table IV.  
      After melting, test samples were prepared in accordance with the procedure described above. The resulting test samples had a M.O.R. of 41.4 K.S.I., a C.T.E. of 129, a translucency of 2.5, and a softening temperature of 975° C.  
      Because the test samples had a C.T.E. of less than 130, the composition may not be suitable as a material for use in preparation of dental restorations. Accordingly, further tests were not carried out, and a dental restoration was not formed.  
     Example 19  
      A glass-ceramic dental composition was prepared and heat treated in accordance with this invention. The composition of the glass-ceramic material is in accordance with composition #11 of Table IV.  
      After melting, test samples were prepared in accordance with the procedure described above. The resulting test samples had a M.O.R. of 35 K.S.I., a C.T.E. of 80, a translucency of 3.5, and a softening temperature of 975° C.  
      Because the test samples had a C.T.E. of 80, the composition was deemed to be unsuitable as a material for use in the preparation of dental restorations. Accordingly, further tests were not carried out, and a dental restoration was not formed.  
     Example 20  
      A glass-ceramic dental composition was prepared and heat treated in accordance with this invention. The composition of the glass-ceramic material is in accordance with composition #12 of Table IV.  
      After melting, test samples were prepared in accordance with the procedure described above. The resulting test samples had a M.O.R. of 31 K.S.I., a C.T.E. of 127, and a translucency of 2.5.  
      Because the test samples had a C.T.E. of only 127, the composition may not be suitable as a material for use in preparation of dental restorations. Accordingly, further tests were not carried out, and a dental restoration was not formed.  
     Example 21  
      A glass-ceramic dental composition was prepared and heat treated in accordance with this invention. The composition of the glass-ceramic material is in accordance with composition #13 of Table IV.  
      After melting, test samples were prepared in accordance with the procedure described above. The resulting test samples had a M.O.R. of 32 K.S.I., a C.T.E. of 256, and a translucency of 5.0.  
      Because the test samples had a C.T.E. of 256, the composition was deemed to be unsuitable as a material for use in preparation of dental restorations. Accordingly, further tests were not carried out, and a dental restoration was not formed.  
     Example 22  
      A glass-ceramic dental composition was prepared and heat treated in accordance with this invention. The composition of the glass-ceramic material is in accordance with composition #14 of Table IV.  
      After melting, test samples were prepared in accordance with the procedure described above. The resulting test samples had a M.O.R. of 13 K.S.I., a C.T.E. of 126, and a translucency of 4.5.  
      Because the test samples had a low C.T.E. and a low M.O.R., the composition was deemed to be unsuitable as a material for use in preparation of dental restorations. Accordingly, further tests were not carried out, and a dental restoration was not formed.  
     Example 23  
      A glass-ceramic dental composition was prepared and elevated in accordance with this invention. The composition of the glass-ceramic material is in accordance with composition #15 of Table IV.  
      After melting, test samples were prepared in accordance with the procedure described above. The resulting test samples had a translucency of 5.0.  
      Because the test samples turned opaque during heat treating, the composition was deemed to be unsuitable as a material for use in preparation of dental restorations. Accordingly, further tests were not carried out, and a dental restoration was not formed.  
     Example 24  
      A glass-ceramic dental restoration was prepared in accordance with the following procedure. A mold was prepared in accordance with procedure described above.  
      By use of slip casting techniques, a crucible was prepared in accordance with the procedure and composition described above in connection with Example 6. A glass button weighing 6 grams was then placed in the crucible and preheated to a temperature of 650° C.  
      The composition of the glass-ceramic material used is in accordance with composition #16 of Table IV.  
      The preheated crucible and glass-ceramic material were then placed in an apparatus which is similar to that shown in  FIG. 7 , and heated to a temperature of 1425° C. for a period of 10 minutes. The ram was then actuated, and the crucible was brought into contact with the mold in accordance with the general procedure as is illustrated in  FIGS. 3 through 6 .  
      The dental restoration was then removed from the investment material, cut apart from the sprue, and heat treated.  
      The resulting test samples and dental restoration had excellent definition, and a M.O.R. of 35 K.S.I., a C.T.E. of 148, a translucency of 3.5, and a softening temperature of 950° C.  
      During the forming process, the crucible retained its structural integrity and formed an effective seal with the mold. Further, during the heating process, the finished dental restoration retained its structural integrity and did not slump or deform.  
      The investment material utilized was Whipmix GIJM 2-23-94; as described herein above. The investment material had an expansion rate of about 3 percent. As a result of this expansion rate, the resulting dental restoration had excellent fit.  
     Example 25  
      A glass-ceramic dental restoration was prepared in accordance with the following procedure. A mold was prepared in accordance with procedure described above.  
      By use of slip casting techniques, a crucible was prepared in accordance with the procedure and composition described above in connection with Example 6. A glass button weighing 6 grams was then placed in the crucible and preheated to a temperature of 650° C.  
      The composition of the glass-ceramic material used is in accordance with composition #17 of Table IV.  
      The preheated crucible and glass-ceramic material were then placed in an apparatus which is similar to that shown in  FIG. 7 , and heated to a temperature of 1425° C. for a period of 10 minutes. The ram was then actuated, and the crucible was brought into contact with the mold in accordance with the general procedure as is illustrated in  FIGS. 3 through 6 .  
      The dental restoration was then heat treated in the investment.  
      The resulting test samples and dental restoration had excellent definition, and a M.O.R. of 40 K.S.I., a C.T.E. of 138, a translucency of 3.5, and a softening temperature of 950° C.  
      During the forming process, the crucible retained its structural integrity and formed an effective seal with the mold. Further, during the heating process, the finished dental restoration retained its structural integrity and did not slump or deform.  
      The investment material utilized was Thermovest, and after heat treating, was removed and the resulting dental restoration had excellent fit.  
     Example 26  
      A glass-ceramic dental restoration was prepared in accordance with the following procedure. A mold was prepared in accordance with procedure described above.  
      By use of slip casting techniques, a crucible was prepared in accordance with the procedure and composition described above in connection with Example 6. A glass button weighing 6 grams was then placed in the crucible.  
      The composition of the glass-ceramic material used is in accordance with composition #18 of Table IV.  
      The crucible and glass-ceramic material were then placed in an apparatus which is similar to that shown in  FIGS. 7   a  and  7   b , and heated to a temperature of 1425° C. for a period of 10 minutes. The ram was then actuated, and the crucible was brought into contact with the mold in accordance with the general procedure as is illustrated in  FIGS. 3 through 6 .  
      The dental restoration was then removed from the investment material, cut apart from the sprue, and heat treated.  
      The resulting test samples and dental restoration had excellent definition, and a M.O.R. of 47 K.S.I., a translucency of 4.0, and a softening temperature of 950° C.  
      During the forming process, the crucible retained its structural integrity and formed an effective seal with the mold. Further, during the heating process, the finished dental restoration retained its structural integrity and did not slump or deform.  
      The investment material utilized was Whipmix GIJM 2-23-94; as described herein above. The investment material had an expansion rate of about 3 percent. As a result of this expansion rate, the resulting dental restoration had excellent fit.  
     Example 27  
      A glass-ceramic dental restoration was prepared in accordance with the following procedure. A mold was prepared in accordance with procedure described above.  
      By use of slip casting techniques, a crucible was prepared in accordance with the procedure and composition described above in connection with Example 6. A glass button weighing 6 grams was then placed in the crucible and not preheated.  
      The composition of the glass-ceramic material used is in accordance with composition #19 of Table IV.  
      The crucible and glass-ceramic material were then placed in an apparatus which is similar to that shown in  FIGS. 7   a  and  7   b , and heated to a temperature of 1425° C. for a period of 5 minutes. The ram was then actuated, and the crucible was brought into contact with the mold in accordance with the general procedure as is illustrated in  FIGS. 3 through 6 .  
      The dental restoration was then removed from the investment material, cut apart from the sprue, and heat treated.  
      The resulting test samples and dental restoration had excellent definition, and a M.O.R. of 52 K.S.I., a translucency of 3.5, and a softening temperature of 950° C.  
      During the forming process, the crucible retained its structural integrity and formed an effective seal with the mold. Further, during the heating process, the finished dental restoration retained its structural integrity and did not slump or deform.  
      The investment material utilized was Whipmix GIJM 2-23-94; as described herein above. The investment material had an expansion rate of about 3 percent. As a result of this expansion rate, the resulting dental restoration had excellent fit.  
      Examples 26 and 27 represent the preferred process and glass-ceramic composition in accordance with this invention. As is mentioned, the resulting dental restorations of these examples were heat treated. The heat treating procedure used for examples 26 and 27 is per Table VIII.  
                           TABLE VIII                                      Heat Dental Restoration to   450° C. at furnace rate           Ramp or Soak at   450° to 550° C. for 6 hrs.           Ramp at   550°-850° C. for 2 hrs.           Hold at   850° C. for 45 minutes           Ramp at   850°-900° C. at furnace rate           Soak at   900° C. for 15 minutes           Cool to   Room Temperature                      
 
      After heat treating the dental restorations of examples 26 and 27, they were further finished by altering the shape and sh0ade with a high fusing porcelain sold under the trademark CERAMCO II, and a low fusing porcelain sold under the trademark FINAL TOUCH. Both of the high and low fusing porcelains fired and adhered well. Likewise, both porcelains had thermal expansions which were compatible with the glass-ceramics from which the dental restorations were formed.  
      The CERAMCO II and FINAL TOUCH porcelains as used herein are manufactured and sold by Ceramco, Inc., of Six Teri Lane, Burlington, N.J.  
      Relative to the dental restorations as were produced in accordance with examples 26 and 27 above, it should be noted that these restorations were exceptionally strong in that they had M.O.R. ratings of about 50 K.S.I.  
      It should be noted that the M.O.R. ratings of examples 26 and 27 do not correlate with the M.O.R. ratings as are specified relative to the other examples, in that a different test protocol was use for measuring the M.O.R. of examples 26 and 27. The M.O.R. for examples 26 and 27 were tested on a Lloyd Instrument Type No. TG-18, as manufactured by John Chatillon &amp; Sons, Inc., of Greensboro, N.C. These measurements were made in accordance with I.S.O. test no. 6872, using cylindrical bars of 0.125″ diameter with a 1″ span, 3 point bending with a cross head speed of 1 mm per minute.  
      In contrast, the M.O.R. measurements of the other examples were made on a piece of equipment which was manufactured by the Applicant. This piece of equipment has characteristics which are similar to those of the above-described Lloyd Instrument Type No. TG-18, and used the same test bars. Generally, it could be said that the M.O.R. rating of those examples other than examples 26 and 27 appear to be low.  
      For a correlation between the M.O.R. test protocol of examples 26 and 27, and the test protocol of the other examples, Table IX should be referred to.  
                       TABLE IX                           Reported M.O.R.               As Per Test Used   M.O.R. as Per       Example No.   For Examples 1-25   I.S.O. Test No. 6872                  26   42   47       27   45   52                  
 
     Example 28  
      A glass-ceramic dental restoration was prepared in accordance with the following procedure. A mold was prepared in accordance with the procedure described above.  
      By use of slip casting techniques, a crucible was prepared in accordance with the procedure and composition described above in connection with example 6. A glass button weighing 6 grams was then placed in the crucible and preheated to a temperature of 650° C.  
      The composition of the glass-ceramic material used is in accordance with composition #10 of Table IV.  
      The preheated crucible and glass-ceramic material was then placed in an apparatus which is similar to that shown in  FIG. 7 , and heated to a temperature of 1425° C. for a period of 10 minutes. The ram was then actuated, the crucible was brought into contact with the mold in accordance with the general procedure as is illustrated in  FIGS. 3 through 6 .  
      The dental restoration was then heat treated.  
      The resulting test samples and dental restoration had excellent definition, and a C.T.E. of 111, a translucency of 3.5, and a softening temperature of 850° C.  
      During the forming process, the crucible retained its structural integrity and formed an effective seal with the mold. Further, during the heat treating process, the finished dental restoration retained its structural integrity and did not slump or deform.  
     Example 29  
      A glass-ceramic dental restoration was prepared in accordance with the following procedure. A mold was prepared in accordance with the procedure described above.  
      The composition of the glass-ceramic material used is in accordance with composition 21 of Table IVb.  
      The glass-ceramic button was placed in an AUTOPRESS™ Plus press as sold by Jeneric/Pentron of Wallingford, Conn., and preheated to a temperature of 850° C.-950° C. for a period of 10 minutes. The ram was then actuated, and the button was then pressed into the mold.  
      The dental restoration was then removed from the investment material, cut apart from the sprue, and made ready for applying dental porcelains.  
      The glass-ceramic material as used in this example had a C.T.E. (250-500° C.), 10.3±0.5×10 −6 , and a glass transition T-re° C. of 500±10.  
     Example 30  
      A glass-ceramic dental restoration was prepared in accordance with the following procedure. A mold was prepared in accordance with the procedure described above.  
      The composition of the glass-ceramic material used is in accordance with composition #22 of Table IVb.  
      The glass-ceramic button was placed in an AUTOPRESS™ Plus press as sold by Jeneric/Pentron of Wallingford, Conn., and preheated to a temperature of 850° C.-950° C. for a period of 10 minutes. The ram was then actuated, and the button was then pressed into the mold.  
      The dental restoration was then removed from the investment material, cut apart from the sprue, and made ready for applying dental porcelains.  
      The glass-ceramic material as used in this example had a C.T.E. (25°-500° C.), 10.6±0.5×10 −6 , and a glass transition T-re° C. of 500±10.  
     Example 31  
      A glass-ceramic dental restoration was prepared in accordance with the following procedure. A mold was prepared in accordance with the procedure described above.  
      The composition of the glass-ceramic material used is in accordance with composition #23 of Table IVb.  
      The glass-ceramic button was placed in an AUTOPRESS™ Plus press as sold by Jeneric/Pentron of Wallingford, Conn., and preheated to a temperature of 850° C.-950° C. for a period of 10 minutes. The ram was then actuated, and the button was then pressed into the mold.  
      The dental restoration was then removed from the investment material, and cut apart from the sprue.  
      The glass-ceramic material as used in this example had a C.T.E. (250-500° C.), 10.4±0.5×10 −6 , and a glass transition T-re° C. of 500±10, a 3 point bend strength per I.S.O. 6872 MPa of 315±24, a solubility per I.S.O. 9693 ng/cm° of 60, and a solubility per I.S.O. 6872 ng/cm° of 73.  
     Example 32  
      A glass-ceramic dental restoration was prepared in accordance with the following procedure. A mold was prepared in accordance with the procedure described above.  
      The composition of the glass-ceramic material used is in accordance with composition #24 of Table IVb.  
      The glass-ceramic button was placed in an AUTOPRESS™ Plus press as sold by Jeneric/Pentron of Wallingford, Conn., and preheated to a temperature of about 930° C. for a period of 10 minutes. The ram was then actuated, and the button was then pressed into the mold.  
      The dental restoration was then removed from the investment material, and cut apart from the sprue.  
      The glass-ceramic material as used in this example had a C.T.E. (25°-500° C.), 10.3±0.5×10 −6 , and a glass transition T-re° C. of 500±10, a 3 point bend strength per I.S.O. 6872 MPa of 300±30, a solubility per I.S.O. 9693 ng/cm° of less than 100, and a solubility per I.S.O. 6872 ng/cm° of less than 100.  
      It should be understood that the invention is not limited to the embodiment shown and described in  FIGS. 1 through 10 , and examples 1 through 32, since the process and composition parameters can be varied, and the configuration of the apparatus can be altered without departing from the scope of the invention.  
      The above description and drawings are illustrative only, since modifications could be made without departing from the present invention, the scope of which is to be limited only by the following claims.