Abstract:
A process for the formation of dental restorations from glass-ceramic materials and the resulting dental restorations. In this invention, a dental restoration is prepared by placing a glass-ceramic material in a heat-pressure deformable crucible. Heat is then applied to the crucible in order to bring the glass-ceramic material to a working range at temperatures above its liquidus temperature. The crucible in which the glass-ceramic material is placed has heat-pressure deformation properties which are matched to the working temperature of the glass-ceramic material being heated. The heat deformation properties of the crucible must be such that when the glass-ceramic material in the crucible is in the working range the crucible is heat-pressure deformable without rupturing. Once the glass-ceramic material is heated to its working temperature, the crucible is brought into contact with a mold having a preformed cavity therein, the cavity being in the shape of the desired dental restoration. As the distance between the heated glass-ceramic material and the mold is decreased, the crucible is deformed to form a seal with the mold thereby facilitating the injection of the molten glass-ceramic material into the mold cavity. An interactive seal between the crucible and the mold may be provided for. The composite process may be carried out in a vacuum. The resulting dental restoration has superior optical esthetic and strength properties. The invention further includes the heat-pressure deformable crucible which is useful in the process of this invention and the related apparatus.

Description:
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 utilizes 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 from 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 the 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. As a result an inferior dental restoration results. 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 material 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, 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, as 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 overhead 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 DICOR process does not. 
     As is discussed above, there are several prior art processes for the manufacture of dental restorations from glass-ceramic materials. A recent addition to the prior art is the DICOR process as sold by the Densply Corporation 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 DICOR 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, 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 DICOR process, is deficient as to the coloration of the glass-ceramic material utilized. The resulting Dicor 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, where 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 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 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 MOR of at least 30,000 PSI. 
     3. Should be capable of being formed into forms which are compatible with the human anatomy using simple equipment. 
     4. 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. Further, the glass-ceramic material must not absorb moisture or stain and it must be stress corrosion resistant. 
     6. Likewise 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 likewise must have dimensional stability and resist thermal shock during processing and in particular it must have dimensional stability during subsequent heat treating processes wherein recrystalization is effected. 
     8. Further, 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 is necessary to glaze and stain the final dental restoration to the exact shade desired. In order to effect this glazing straining etc. the dental restoration must be heated to a temperature of about 950° C. for each staining or glazing operation. Therefore, a satisfactory glass-ceramic material must be capable of withstanding multiple heat cycles to about 950° C. 
     10. It must be capable of retaining its structural integrity during heat treating. 
     11. In summary a suitable glass-ceramic material should have; 
     A. Thermal expansions of 130° to 150°×10 /° C. 
     B. Translucency of 2.5 to 3.5 on a visible scale of 0 (clear) to 5 (opaque) and overall beauty. 
     C. M.O.R. of at least 30,000 psi average. 
     D. Ability to be heat treated to 925°-950° C. 
     E. Structural integrity during heat treat 
     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 about 950° C. possess thermal expansions matching existing porcelains and therefore, these dental restorations can be readily stained or glazed using conventional 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 glazed or stained 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 an apparatus for the molding of glass-ceramic dental restorations which is easy to use, is lab friendly, and which is reasonably priced. 
     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 the forming of these dental restorations. 
     It is still another object of the present 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 inventions 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 950° C. 
     Lastly, an object of the present invention is to create a 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 advantageous 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 is a sectioned side view showing the composite apparatus of the invention. 
     FIG. 8 shows a sectioned alternate mold. 
     FIG. 9 shows a sectioned alternate mold. 
     FIG. 10 shows the mold of FIG. 9 being filled with a molten glass-ceramic. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 generally describes the overall process of the subject invention wherein 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 produced of the desired dental restoration. This positive wax form is then 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 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 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 to 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 the 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 is heated. As the heating of the glass-ceramic material 7 continues, crystal formation is effected whereby glass-ceramic material 7 becomes a ceramic via the formation a of crystalline phase. As the heating is continued, the glass matrix slowly dissolves the crystalline phase thereby causing the glass-ceramic material 7 to reenter 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 4 P 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 glass material is 10 4  poises the viscosity will be indicated by log 4 P. 
     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 FIG. 7. 
     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 have 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 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 be 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 further 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 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 molybdenum 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 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 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 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. 
     The above described FIGS. 1 to 7 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 copings over which porcelain may be built up and fired by another method such as the slurry method, as is described hereinabove. In the prior art, metal copings are extensively utilized. These copings are covered with layers of low fusing porcelain which is 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 disadvantagous 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 advantagous 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 advantagous 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 results 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 and II. All components listed in Tables I and II are in weight percent. 
     While the subject invention can utilize any suitable glass-ceramic material as is shown in Table I and II preferred glass-ceramic materials for use in this invention are lithium disilicate glass-ceramic materials. In these materials Li 2  O2(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 humane coloration and are translucent and esthetically pleasing. Further, lithium disilicate glass-ceramic materials are advantageous in that they maintain their structural integrity during heat treating, in that they do not slump or sag during proper heat treating. 
     Other lithium disilicate glass-ceramic materials which may be used in this invention are disclosed in U.S. Pat. No. 5,219,799 issued Jun. 15, 1993. 
     Lithium dissicate glass-ceramic materials may utilize P 2  O 5  as a nucleating agent. Other nuculating agents are TiO 2  and ZrO 2 . 
     Glass-ceramic compositions which may be useful in this invention are as per Table I. 
     
                       TABLE I______________________________________Li.sub.2 O       8-12Al.sub.2 O.sub.3             2-5.0SiO.sub.2       75-85Na.sub.2 O      0-2K.sub.2 O       0-2P.sub.2 O.sub.5 1.5-5TiO.sub.2       0-2ZrO             0-3Coloring Oxide  0-5______________________________________ 
    
     Specific glass-ceramic compositions which may be useful in the invention are as per Table II. 
     
                                           TABLE II__________________________________________________________________________COMPOSITION #      1  2  3  4  5  6  7  8  9  10 11 12 13 14 15 16 17__________________________________________________________________________SiO.sub.2  81.0         84.0            84.0               83.0                  62.0                     84.0                        81.0                           83.0                              83.0                                 81.5                                    80.0                                       86.0                                          84.0                                             81.0                                                82.0                                                   82.0                                                      81.0Al.sub.2 O.sub.3      3.5         3.0            3.0               3.0                  4.0                     3.3                        3.3                           3.5                              3.5                                 3.0                                    6.0                                       1.5   3.0                                                3.3                                                   3.5                                                      3.0Li.sub.2 O 11.0         10.5            10.5               10.5                  10.5                     10.0                        11.0                           9.0                              8.0                                 12.0                                    10.5                                       10.5                                          11.0                                             11.0                                                10.0                                                   10.0                                                      11.0P.sub.2 O.sub.5      3.0         2.0            2.0               2.0                  2.0                     1.5                        1.5                           3.0                              3.0                                 2.0                                    2.0                                       2.0                                          3.0                                             1.0                                                1.5                                                   2.2                                                      3.0ZrO.sub.2  1.0         .5    .5 .5    2.0      .5 .5    1.0                                             2.0   2.0                                                      1.0Tio.sub.2  .4             .1 1.0                           .5             .4 .4    .5 .4CeO        .5 .25            .25               .25                  .25                     .25                        .25                           .25                              .25                                 .25                                    .25   .5 .5    .25                                                      .5MmO.sub.2     .5 .5 .5 .5          .5 .5 .50                                       .5NiO.sub.2                    .06K.sub.2 O        .5 .5 .5 1.0      1.0                                 .5 .5Na.sub.2 O .5             .2 .2 .5 .5                .2 3.0Fe.sub.2 O.sub.3          .09                        .09                           .09                     .09BaO                                                  3.0MgO                             1.0CaO                                1.0__________________________________________________________________________ 
    
     As is listed in Tables I and II, coloring oxides may be added to the glass-ceramic material in order to achieve the desired coloration of the glass-ceramic dental restoration. 
     Suitable coloring oxides for use in glass-ceramic compositions which can be used in this invention, some of which are illustrated in Table I and II are SnO 2 , MnO, CeO, Fe 2  O 3 , Ni 2  O, V 2  O 3 , Cr 2  O 3  CeO etc. These coloring oxides may be used singularly or in combination. 
     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 and II is as per Table III 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 anneal point of the glass-ceramic. It has also been determined that a slow increase in temperature from just above the upper anneal point to some 50° C. 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 treat cycle is such that in the crystalline phase the glass-ceramic material incorporates a large number of fine crystals which are evenly dispersed through out 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. 
     Whenever 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 patients 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. That is the investment material expands when it hardens to produce an oversize mold cavity. This oversize mold cavity naturally produces an oversize dental restoration. This oversize 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 using an investment material which expands about 3 percent when it hardens. Using this procedure a dental restoration which is 3 percent oversize results. This oversize 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 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 hence permit this type of process are manufactured by Whipmix Corporation of Louville, Ky. and sold under the designation GIJM 3-23-94; 1. 
     The preferred heat treating process for use with the glass-ceramic compositions of Table I and II is as per Table III 
     
                       TABLE III______________________________________Heat Dental Restoration To              500-600° C.Ramp or Soak at    500-600° C. for 2-6 hoursRamp at            600-925° C. for 2-3 hoursSoak at            925° C. for 1-2 hoursCool to            Room Temperature______________________________________ 
    
     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 that 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 4 P the crucible has a viscosity of about log 5 to log 7 P. Crucibles for use in this invention are formed by slip casting. The formation of crucibles 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, cadmium 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 4 P 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. 
     EXAMPLES 
     The present invention is illustrated by the following Examples which, however, are not to be construed as limiting the invention to their details. 
     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 gram 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. where upon the wax model was burnt 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 hyorosiopic 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 the Thermal Expansion (C.T.E.) was measured from room temperature to 250° C. and is reported in units of ×10 /° C. 
     The modulus of rupture (M.O.R.) was measured and reported in term of K.S.I. 
     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 ceramic material into the mold a pressure 30 P.S.I. was applied to the Ram. 
     The referenced heat treating sequence is in accordance with Table III above. 
     In the below Examples the glass-ceramic test rods for measuring modules of rupture (MOR) 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 III whereby the glasses were crystallized in situ to glass-ceramics. The test rod samples were 0.120&#34;×1.25&#34;. 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. Exp.), reported in terms of ×10/° C. modules of rupture (MOR), cited in terms of K.S.I., as determined in accordance with measurement techniques conventional in the art. 
     The components used in these Examples are as follows: 
     Thermovest and Therovest Liquid as sold by Kerr Manufacturing Co. Romulus, Mich. 48174. 
     3I Fused Silica as sold by Harbison Walker Refactories division of Indresco Inc., Pittsburg, Pa. 15223. 
     SP921 TF (borosilicate glass) as sold by Specialty Glass Co. of Oldsmar, Fla. and has the following compositions in weight percent. 
     
         ______________________________________   SiO.sub.2         78   B.sub.2 O.sub.3         15   Al.sub.2 O.sub.3           2.5   Na.sub.2 O           4.5______________________________________ 
    
     Example 1 
     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 described above by mixing 90.8 grams of SP921TF glass-ceramic 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 II. 
     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 1400° 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 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 techniques, a crucible was prepared in accordance with the procedure described above by mixing 136.2 grams of SP921TF glass and 317.8.2 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-ceramic 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 II. 
     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 1400° 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 removed from the investment material cut apart from the sprue and heated treated. 
     The resulting dental restoration had excellent definition and physical properties. 
     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° and 1425° C. 
     Example 3 
     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 181.6 grams of SP921TF 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-ceramic button weighing 6 grams was then placed in the crucible and preheated to a temperature of 650° C. 
     The preheated crucible with 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 techniques, a crucible was prepared in accordance with the procedure described above by mixing 227 grams of SP921TF 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-ceramic 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 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 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 techniques, a crucible was prepared in accordance with the procedure described above by mixing 118 grams of SP921TF 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-ceramic 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 II. 
     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 1400° 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 removed from the investment material cut apart from the sprue and heat treated. 
     The resulting dental restoration had excellent definition. 
     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. 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. Additional test should 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 techniques, a crucible was prepared in accordance with the procedure described above by mixing 127.1 grams of SP921TF 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-ceramic 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 II. 
     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 1400° 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 removed from the investment material cut apart from the sprue and heat treated. 
     The resulting dental restoration had excellent definition. 
     During the forming process, the crucible retained its structural integrity and formed an effective seal for the mold. Further, during the heat treating process, the dental restoration retained its structural integrity and did not slump or deform. Additional tests were conducted on the crucible of this example wherein it was determined that the crucible would deform and seal at both 1375° 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 techniques, a crucible was prepared in accordance with the procedure described above by mixing 145.3 grams of SP921TF 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 II. 
     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 1400° 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 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 accordance with example 1. 
     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 dental restoration retained its structural integrity and did not slump or deform. Additional 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 techniques, a crucible was prepared in accordance with the procedure and description described above in connection with Example 2. A glass-ceramic 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 II. 
     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 1400° 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 removed from the investment material cut apart from the sprue and heated treated. 
     The resulting test samples and dental restoration had excellent definition and 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 techniques, a crucible was prepared in accordance with the procedure and description described above in connection with Example 2. A glass-ceramic 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 II. 
     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 1400° 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 removed from the investment material cut apart from the sprue and heated treated. 
     The resulting test samples and dental restoration had excellent definition and 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 techniques, a crucible was prepared in accordance with the procedure and description described above in connection with Example 2. A glass-ceramic 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 II. 
     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 1400° 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 removed from the investment material cut apart from the sprue and heated treated. 
     The resulting test samples and dental restoration had excellent definition and a M.O.R. of 48 K.S.I., 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 techniques, a crucible was prepared in accordance with the procedure and composition described above in connection with Example 2. A glass-ceramic 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 II. 
     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 1400° 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 M.O.R. of 42 K.S.I., a C.T.E. of 140 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 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 techniques, a crucible was prepared in accordance with the procedure and composition described above in connection with Example 6. A glass-ceramic 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 II. 
     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 1400° 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 M.O.R. of 41.5 K.S.I., a C.T.E. of 162 a translucency of 4 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 techniques, a crucible was prepared in accordance with the procedure and composition described above in connection with Example 6. A glass-ceramic 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 is in accordance with a composition #7 of Table II. 
     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 1400° 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 removed from the investment material cut apart from the sprue and heated 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 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 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 techniques, a crucible was prepared in accordance with the procedure and composition described above in connection with Example 2. A glass-ceramic 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 as sold by the Densply Corporation and as is described above. Di-Cor is thought to be flouro-mica glass-ceramic. 
     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 1400° 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 in the investment. While the glass-ceramic properly filled the mold the resulting dental restortion cracked as a result of shrinkage during the heat treating process. This cracking is thought to 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 III______________________________________   SiO.sub.2         50.2   B.sub.2 O.sub.3         8.6   AlF.sub.3         .7   Al.sub.2 O.sub.3         16.0   BaO   4.75   ZnO   19.75______________________________________ 
    
     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 1400° 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. 
     A satisfactory dental restoration resulted which was then removed from the investment material cut apart from the sprue. 
     The resulting test samples and dental restoration had excellent definition and a M.O.R. of 4 K.S.I., a C.T.E. of 56 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 in accordance with this invention. The composition of the glass-ceramic material is in accordance with the composition No. 8 of Table II. After preparation, test samples were prepared in accordance with the procedure described above. The resulting test samples had a K.S.I., a C.T.E. of 157, a translucency of 30 and a softening temperature of 925° C. 
     Because the test samples cracked during heat treating therefore 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 16 
     A Glass-ceramic dental composition was prepared in accordance with this invention. The composition of the glass-ceramic material is in accordance with the composition No. 8 of Table II. After preparation, test samples were prepared in accordance with the procedure described above. The resulting test samples had a C.T.E. of 157, and a softening temperature of 925° C. 
     Because the test samples cracked during heat treating therefore 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 17 
     A Glass-ceramic dental composition was prepared in accordance with this invention. The composition of the glass-ceramic material is in accordance with the composition No. 9 of Table II. After preparation, 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 therefore 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 in accordance with this invention. The composition of the glass-ceramic material is in accordance with the composition No. 10 of Table II. After preparation, 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 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 19 
     A Glass-ceramic dental composition was prepared in accordance with this invention. The composition of the glass-ceramic material is in accordance with the composition No. 11 of Table II. After preparation, 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 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 in accordance with this invention. The composition of the glass-ceramic material is in accordance with the composition No. 12 of Table II. After preparation, 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 127 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 21 
     A Glass-ceramic dental composition was prepared in accordance with this invention. The composition of the glass-ceramic material is in accordance with the composition No. 13 of Table II. After preparation, 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., and a C.T.E. of 256. 
     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 in accordance with this invention. The composition of the glass-ceramic material is in accordance with the composition No. 14 of Table II. After preparation, 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 C.T.E. and a M.O.R. of less than required therefore 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 in accordance with this invention. The composition of the glass-ceramic material is in accordance with the composition No. 15 of Table II. After preparation, 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 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 5. A glass button weighing 6 grams was then placed in the crucible and preheated to a temperature of 450° C. 
     The composition of the glass-ceramic material used is in accordance with composition #16 of Table II. 
     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 removed from the investment material cut apart from the sprue and heated treated. 
     The resulting test samples and dental restoration had excellent definition and a M.O.R. of 35 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 heat treating process, the finished dental restoration retained its structural integrity and did not slump or deform during heat treating. 
     The investment material utilized was Whipmix GTJM 3-23-94; As described herein above. The investment material had an expansion rate of about 3%. 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 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 5. 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 II. 
     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 in the investment material. 
     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 heat treating process, the finished dental restoration retained its structural integrity and did not slump or deform during heat treating. 
     The investment material utilized was Thermovest and after heat treating was removed and the resulting dental restortion had excellent fit. 
     It should be understood that the invention is not limited to the embodiment shown and described in FIGS. 1-10 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.