Abstract:
Porcelain compositions, especially dental porcelain compositions, which contain leucite crystals in controlled amounts in a glassy phase matrix to permit selective adjustment of the thermal coefficient of expansion of the porcelain compositions, including methods of preparing and using same and products derived therefrom.

Description:
FIELD OF THE INVENTION 
     This invention relates to leucite-containing porcelain compositions and to methods of preparing the same. More particularly, this invention relates to porcelain compositions, especially dental porcelain compositions, which contain leucite crystals in controlled amounts in a glassy phase matrix to permit selective adjustment of the thermal coefficient of expansion of the porcelain compositions. 
     Although not limited thereto, the invention has particular applications to a series of distinct dental porcelain powders comprising a glassy phase matrix having a predetermined index of refraction and fusion temperature, and in which there is a dispersed leucite phase to impart to the powder a predetermined thermal coefficient of expansion. The invention also contemplates blends of such leucite-containing porcelain powders, each of which comprises a different amount of leucite so as to exhibit a different thermal coefficient of expansion, and to dental porcelain made from such blends. In a particularly suitable application, the invention also contemplates dental products and their preparation, especially such products having metal substrates secured to a leucite-containing porcelain covering wherein the porcelain exhibits a firing temperature in the range of from about 815° C. to about 1315° C. and a preselected thermal coefficient of expansion ranging from 8×10 -6  to about 20×10 -6  in/in/°C. at 500° C. 
     BACKGROUND OF THE INVENTION 
     The use of porcelain compositions in the field of dentistry is well known, as are porcelain compositions themselves and procedures for preparing porcelain compositions. See, for example, U.S. Pat. No. 3,423,828 to Halpern, which discloses a synthetic resin denture base comprising a major portion of porcelain particles; U.S. Pat. No. 3,464,837 to McLean, which discloses a porcelain-containing material that is suitable for use as a dental enamel veneer; and U.S. Pat. No. 3,052,982 to Weinstein, which relates to fused porcelain-to-metal teeth. Similarly, porcelains, including dental porcelains, which contain some quantity of leucite are known. See, for example, 82 Chemical Abstracts 63365m, which relates to leucite glass-ceramic enamel compositions which may be bonded to gold alloy substrates; and U.S. Pat. No. 4,101,330 to Burk, which discloses the preparation of a porcelain or ceramic body from a ceramic raw material consisting essentially of at least about 78.1 percent by weight nepheline syenite, about 3 to 7 percent by weight of pre-formed leucite particles and about 0 to 15.5 percent by weight of at least one modifier selected from the class consisting of oxides and oxide precursors of potassium, sodium, and lithium. The ceramic raw materials are such that when fused to form a fired ceramic body, the resulting body comprises a modified nepheline syenite glassy phase having leucite particles dispersed therein. 
     Other prior art which relates generally to dental and/or porcelain compositions, some of which may contain leucite crystals, include U.S. Pat. No. 4,120,729 to Smyth, entitled &#34;Novel Low Temperature Maturing Dental Glaze;&#34; 81 Chemical Abstracts 81713g, an abstract entitled &#34;Enamels with High Thermal Expansion Doefficient;&#34; U.S. Pat. No. 4,337,316 to Votava, entitled &#34;Sanitary Ware and Process of Production;&#34; U.S. Pat. No. 3,775,164 to Smith, entitled &#34;Method of Controlling Crystallization of Glass;&#34; and U.S. Pat. No. 3,615,765 to Bystrova, entitled &#34;Glaze for Ceramic Parts and Articles.&#34; 
     Dental porcelains generally may be classified as higher fusing porcelains, i.e., those porcelains fusing above about 1000° C., and lower fusing porcelains, i.e., those porcelains fusing below about 1000° C. The higher fusing porcelains generally exhibit resistance to thermal stress and mechanical shock and to erosion by mouth fluids, and have been fused to metals having a compatible thermal coefficient of expansion, such as the platinum-iridium alloys. The lower fusing porcelains have been used for using to lower melting substrates, such as gold alloys, but there have been problems, at least in part, because of the disparity between the thermal coefficient of expansion of the gold alloys and the lower fusing porcelains. 
     Attempts have been made to match a given porcelain material to a given dental support metal so as to increase the compatibility between their respective thermal coefficients of expansion. These attempts have included the preparation of specifically formulated and prepared porcelains to be used with dental metals having a specific narrow thermal expansion range. For example, in U.S. Pat. No. 3,052,982 to Weinstein, there is disclosed a technique for preparing porcelain coverings for metal supports, wherein the porcelain is tailored in each case to have a thermal coefficient of expansion which is compatible with that of the metal being used for the support. The porcelains disclosed in this patent are prepared by mixing predetermined amounts of components prepared in part from different feldspars and glasses, the composition and relative amounts of the components being responsible for the physical characteristics of the porcelain products. 
     While techniques of the above type have been used with some success in the art, they often result in porcelains which have expansion characteristics that vary with firing conditions. 
     Accordingly, it is an object of the present invention to provide dental porcelain compositions which, when fused, provide a porcelain body having controlled coefficient of thermal expansion. 
     It is another object of the invention to provide dental porcelain compositions which selectively and readily can be tailored for use with a variety of commercially available metal support materials. 
     Still another object of the invention is to provide an efficient technique for matching the coefficient of thermal expansion of a dental porcelain composition with that of a metal support structure to which the porcelain composition is to be fused. 
     Another object of the invention is to provide a dental porcelain material having all the characteristics necessary for advantageously producing an aesthetic dental restorative. 
     Yet another object of the invention is to provide a unique system of leucite-containing raw materials which can be blended and fused to form a dental porcelain having a preselected coefficient of thermal expansion. 
     Still another object of the invention is to provide a facile technique for providing a range of porcelain materials, each having a preselected coefficient of thermal expansion and each having a firing temperature on the order of about 700° C. to about 1315° C. 
     Yet another object is to enable the modification of a naturally occurring potash feldspar or glass of the same composition to form a series of thermal expansion control, leucite-containing frits. 
     Still another object is to control the coefficient of thermal expansion of a dental porcelain through the use of porcelain-forming raw materials comprised of intentionally graded leucite-containing frits prepared from a feldspar material. 
     These and other objects and advantages of the present invention are achieved, in a broad sense, by first providing a series of leucite-containing, glass-ceramic frits, each of the series containing a different amount of leucite and thus exhibiting a different coefficient of thermal expansion. The series of glass-ceramic frits can be blended selectively with a matrix glass to control the thermal expansion coefficient of the glass matrix/glass-ceramic blend. The matrix glass in combination with the glass-ceramic frits is selected to govern the firing temperature, glass transition temperature, viscosity, and translucency of the resulting system. 
     In one aspect of the invention, the glass matrix is comprised of a mixture of two glasses. Preferably, one of the glasses would be a relatively higher melting glass having a fusion range on the order of from about 750° C. to about 845° C., preferably 800° C. to about 830° C., and the other would be a relatively lower melting glass having a fusion range in the range of from about 700° C. to about 825° C., preferably 785° C. to about 815° C. In most cases, the difference in fusion range between the lower melting glass and the higher melting glass is at least about 10°-15° C. The mixture or glasses generally would possess the desired balance between fluidity and stiffness, and surface properties attributable to the two glasses comprising the mixture. The glasses also would be selected such that the refractive index of the fusion product of their mixture would be about the same as that of the leucite-containing glass-ceramic frits, and such that the coefficient of thermal expansion of their fusion products would be in the range of from about 9×10 -6  to about 10×10 -6  in/in/°C. at 400° C. Although a number of glasses satisfy the above selection criteria, suitable examples of matrix glasses are shown in Table 1. 
     
                       TABLE 1______________________________________     Higher Melting                   Lower Melting     Matrix Glass  Matrix GlassConstituents     Percent by Weight                   Percent by Weight______________________________________SiO.sub.2 67.0-73.0     61.0-66.0Al.sub.2 O.sub.3     7.2-9.3       12.3-14.8CaO       0.2-1.0       2.3-4.5Na.sub.2 O     12.5-14.4      8.5-10.2K.sub.2 O 6.2-7.1       5.0-5.9Sb.sub.2 O.sub.3     0.1-0.3        0.1-0.35Fe.sub.2 O.sub.3     0.1           0.1PbO       0.1           0.1Li.sub.2 O.sub.3        1.5-3.5BaO                     0.5-1.0B.sub.2 O.sub.3         0.2-0.6MgO                     0.8-2.0______________________________________ 
    
     Thus, in one aspect of the invention the relatively higher melting glass matrix material may comprise a frit having a composition consisting essentially of, by weight percent, about 67.0 to 73.0 percent silicon dioxide, 7.2 to 9.3 percent aluminum oxide, 0.2 to 1.0 percent calcium oxide, 12.5 to 14.4 percent sodium monoxide, and 6.2 to 7.1 percent potassium oxide, and 0.1 to 0.3 percent antimony(III) oxide, and the second or relatively lower melting glass matrix material may comprise a frit having a composition consisting essentially of by weight percent, about: 61.0 to 66.0 percent silicon dioxide, 12.3 to 14.8 percent aluminum oxide, 2.3 to 4.5 percent calcium oxide, 8.5 to 10.2 percent sodium monoxide, 5.0 to 5.9 percent potassium oxide, 1.5 to 3.5 percent lithium oxide, 0.5 to 1.0 percent barium oxide, 0.2 to 0.6 percent boric oxide, 0.8 to 2.0 percent magnesium oxide, and 0.1 to 0.35 percent antimony (III) oxide. 
     Generally speaking, the matrix glasses may be mixed over a fairly wide range of higher to lower melting glass. However, it is normally preferred that the higher to lower melting glass be mixed in a ratio of from about 1:10 to 10:1 with ratios of from about 1:2 to 2:1 being most suitable. 
     The leucite-containing, glass-ceramic fritted materials which are to be blended with the matrix glass are themselves comprised of a glassy phase matrix in which is dispersed particles of leucite crystals. The series of leucite-containing frits are graded in the sense that each frit contains a different amount of leucite particles and thus a different coefficient of thermal expansion. For example, it is contemplated that a first member of the series might contain 10 percent by weight leucite, that a second member might contain 20 percent by weight leucite, that a third member might contain 40 percent by weight leucite, and so on, with the balance of each member being the glassy phase matrix. It is an important feature of the present invention that the glassy phase matrix can be derived from a feldspathic material which is the same feldspathic material that is used to prepare each member of the series. As used in this specification and claims, the term feldspathic material is meant to describe naturally occurring feldspars as well as glasses and mixtures of oxides having substantially the same chemical composition as such naturally occurring feldspars. 
     In one embodiment of the invention, the series of leucite-containing, glass-ceramic frits is prepared by doping or adding potassium nitrate, potassium carbonate, potassium silicate, or some other equivalent potassium sorce to a feldspathic material such as a potash feldspar, in separate batches, adding a different amount of potassium nitrate to each separate batch. The potassium nitrate-doped batches are then heated in a furnace at a temperature of from about 1120° C. to about 1650° C. or potentially as high as the equipment will allow for a period up to about eight hours. During this heating period, all of the feldspar melts and leucite crystals begin to precipitate. The leucite particles generally are in the micron size range, for example, on the order of about 2 to 50 microns. The resulting frit is cooled and pulverized to a size on the order of -200 mesh for admixture with the matrix glasses described above. 
     In another embodiment, a frit is prepared from an undoped potash feldspar using the same procedure outlined above. except that no source of potassium is added to the feldspar. The resulting frit, which may be referred to as a zero-doped frit, may be mixed with the matrix glasses described above. 
     The composition of a series of leucite-containing, glass-ceramic frits prepared in accordance with the invention may vary over relatively wide limits with respect to the amount of leucite contained therein. However, since all of the members of the series are prepared by doping a feldspathic material in a systematic fashion, the overall composition of the various members of the series will be essentially the same, except for the ratio of leucite to residual glass. In addition, those members of the series having higher percentages of leucite will have correspondingly higher coefficients of thermal expansion. It is this difference in thermal expansion which enables the use of the compositions of this invention to prepare dental porcelains having controlled expansion characteristics. The composition of a series of leucite-containing glass-ceramic frits prepared in accordance with a preferred aspect of the invention is shown in Table 2.  Also shown in Table 2 is the stoichiometric composition of leucite and a frit prepared only from a potash feldspar and free from detected leucite. Table 3 illustrates the coefficient of thermal expansion of the frits obtained by doping the potash feldspar shown in Table 2 with 0, 2, 4, 6, 9, and 11 percent by weight potassium nitrate, respectively, based on the total composition. It will be appreciated that other potassium nitrate dopant levels or the use of other potassium sources may be used in accordance with this invention as well. It will be appreciated, also, that feldspathic materials other than a naturally occurring potash feldspar may be used. 
     
                                           TABLE 2__________________________________________________________________________Potassium Nitrate Doped Feldspar Series          All Constituents in % by WeightComposition    SiO.sub.2             Al.sub.2 O.sub.3                 FeO                    CaO                       Na.sub.2 O                           K.sub.2 O                              Other__________________________________________________________________________Potash feldspar          64.80             18.85                 0.01                    0.05                       3.20                           12.92                              0.17Potash Feldspar + 2% KNO.sub.3          64.19             18.67                 0.01                    0.05                       3.17                           13.74                              0.17Potash Feldspar + 4% KNO.sub.3          63.57             18.50                 0.01                    0.05                       3.14                           14.57                              0.16Potash Feldspar + 6% KNO.sub.3          62.93             18.31                 0.01                    0.05                       3.11                           15.43                              0.17Potash Feldspar + 9% KNO.sub.3          61.95             18.02                 0.01                    0.05                       3.06                           16.76                              0.16Potash Feldspar + 11% KNO.sub.3          61.25             17.84                 0.01                    0.04                       3.02                           17.67                              0.16Stoichiometric Leucite          55.10             23.30         21.60__________________________________________________________________________ 
    
     
                       TABLE 3______________________________________Frit Thermal Expansion Data(× 10.sup.-6 in/in/°C.)           TE @ 25° C.Composition       400° C.                      500° C.                               600° C.______________________________________*Potash Feldspar  15.13    15.31    15.44*Potash Feldspar + 2% KNO.sub.3             14.7     15.83    15.44             14.32    15.53    14.92**Potash Feldspar + 4% KNO.sub.3             16.62    18.55    17.58             16.65    18.64    18.77             16.76    18.72    --*Potash Feldspar + 6% KNO.sub.3             17.19    20.21    20.77**Potash Feldspar + 9% KNO.sub.3             16.21    18.51    23.16**Potash Feldspar + 11% KNO.sub.3             16.35    18.52    24.56             16.48    18.63    24.98______________________________________ *Fired in Research Kiln **Fired in Factory Kiln 
    
     In one preferred embodiment, a porcelain material, which is suitable for fusion to a metal substrate, is prepared using a two component glass matrix and two or more, but most preferably two, leucite-containing, glass-ceramic frits to control the thermal expansion. The two component glass matrix, which is comprised of a higher melting glass and a lower melting glass, as described above, is blended into separate batches with the two or more glass-ceramic frits to form a series of master frits containing different amounts of leucite crystals. Each master frit thus would be prepared from the two components of the matrix glass and one of the glass-ceramic frits, including, if desired, a zero-doped frit. The porcelain material would then be made by blending together a mixture of two or more master frits. Accordingly, in the case where two master frits are employed, the resulting porcelain would comprise four components, namely: the two glasses comprising the matrix glass and two leucite-containing, glass-ceramic frits. The selection of the master frits would be made so as to obtain the desired coefficient of thermal expansion in the final porcelain. Generally speaking, the resulting porcelain materials selectively would exhibit a coefficient of thermal expansion of from about 10 to about 19 in/in/°C. at 500° C. This would enable a manufacturing technician to prepare quite readily from a stock of a relatively few master frits porcelain materials that would be suitable for use with any dental alloy. 
     The master frits may comprise a majority of glass matrix material or a majority of glass-ceramic frit as is desired. However, the amount of glass matrix to glass-ceramic used to prepare the master frits generally is in the range of from about 1:10 to 10:1 by weight, and preferably is in the range of from about 1:2 to 2:1. 
     In another embodiment, the porcelain material can be prepared simply by blending and then firing a mixture of the two glass components comprising the glass matrix and two or more, but preferably two, glass-ceramic frits of different leucite content. Again, if desired, one of the glass-ceramic frits may be prepared from a zero-doped feldspar or from another suitable potassium-doped or zero-doped feldspathic material. In this embodiment, the amounts of the various components generally would be such that the ratio of higher melting to lower melting glass of the glass matrix would be from about 1:10 to 10:1, preferably from about 1:2 to 2:1; the ratio of the two different glass-ceramic frits would be from about 1:10 to 10:1, preferably from about 1:2 to 2:1; and the ratio of the glass matrix (both glasses combined) to the glass-ceramic frit material (all glass-ceramic frit components combined) would be from about 1:10 to 10:1, preferably from about 1:2 to 2:1. Generally speaking, the glass matrix (both glasses combined) will comprise from about 20 to about 80 percent by weight of the total weight of the various components. 
     It will be appreciated that the present invention enables the preparation of a wide variety of dental prosthetic devices which require a porcelain veneer to be bonded to a metal support, usually through an opaque layer. These devices which include, for example, fixed or removable bridgework, crowns, and the like can be prepared with relative ease and exceptional reproducibility with respect to matching the coefficient of thermal expansion of the porcelain veneer material with that of the metal support. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Briefly stated, the porcelain compositions of this invention are prepared from a mixture of a glass matrix material and a glass-ceramic frit material, the latter material containing leucite particles and being used for the purpose of controlling the thermal expansion characteristics of the final porcelain composition. As indicated above, the glass matrix material preferably is prepared from a blend of two glass compositions, one having a higher melting point than the other. The two glass compositions are selected such that the combination of the glass matrix and the selected glass-ceramic frits exhibit the desired firing temperature, glass transition temperature, viscosity, and translucency. 
     The glass-ceramic frits which are selected for admixture with the glass matrix preferably are prepared by potassium-doping a naturally occurring potash feldspar with potassium nitrate or an equivalent potassium source, and then heating the doped feldspar to a temperature of from about 1120° C. to about 1315° C. The heated mixture is held for a period of from about zero to about eight hours, during which time the feldspar forms a glassy phase in which 2 to 50 micron sized leucite crystals precipitate. The resulting glass-ceramic is then cooled and pulverized. The amount of leucite particles dispersed in the glassy phase of the glass-ceramic frits depends upon the amount of potassium nitrate dopant that is used, and in a preferred embodiment, frits are prepared using dopant levels of about 2, 4, 6, 9, and 11 percent potassium nitrate. Obviously, other dopant levels can be used, and in some cases it may be desirable to use a zero-doped material. 
     The total amount of glass-ceramic frit that is mixed with the total amount of glass matrix may vary, but generally, the ratio of glass-ceramic to glass matrix is from 10:1 to 1.10, and preferably from 2:1 to 1:2. Likewise, the ratio of the two glasses comprising the glass matrix generally may vary over the range of from about 10:1 to 1:10, and preferably from about 2:1 to 1:2. 
     The four components of the final porcelain material, i.e., the two glasses of the glass matrix and two glass-ceramic frits having different amounts of leucite, may be blended as such and fired to form the porcelain. However, it is more preferred to form master frits by blending separate batches of glass matrix (containing both the higher and lower melting glasses) and a single glass-ceramic frit. The final porcelain composition is then prepared by mixing two or more master frits which contain different amounts of leucite. 
     The mixture of the four porcelain components, or the mixture of at least two master frits, may then be used to cover a metal support to form a dental prosthetic device in the usual manner. 
    
    
     The following examples illustrate the porcelain compositions of the present invention, including the glass matrix materials and the glass-ceramic frits from which they are formed. Also illustrated is the use of the porcelain compositions in combination with metal substructures or supports of the type which are used in dental prosthetic devices. The examples are merely illustrative of preferred embodiments of the invention and are not to be deemed as limitative thereof. All percentages and parts are given by weight unless noted otherwise. 
     EXAMPLE 1 
     A matrix glass was prepared by mixing 25 parts of glass A and 30 parts of glass B noted in Table 4. The glass A and the glass B were in the form of -200 mesh powder. 
     
                       TABLE 4______________________________________Component   % by Weight, A                    % by Weight, B______________________________________SiO.sub.2   69.79        63.84Al.sub.2 O.sub.3       9.09         13.51CaO         0.79         2.77Na.sub.2 O  13.45        9.39K.sub.2 O   6.66         5.45Sb.sub.2 O.sub.3       0.22         0.22Li.sub.2 O               2.26BaO                      0.78B.sub.2 O.sub.3          0.31MgO                      1.47______________________________________ 
    
     To the above mixture was added 25 parts of a glass-ceramic frit prepared from the feldspar noted in Table 2 doped with 9 percent potassium nitrate and 20 parts of a glass frit prepared from the same feldspar doped with 4 percent potassium nitrate. The glass-ceramic frits were prepared by firing the doped feldspar to a temperature of about 1230° C. and holding the heated mixture for about three hours until the leucite crystals precipitate, whereafter the mixture was cooled and ground to -200 mesh. 
     The glass matrix/glass-ceramic mixture was blended to form the desired porcelain product. The fusing point of the final product was about 954° C. and its coefficient of thermal expansion was about 1.2×10 -6  in/in/°C. The porcelain product was fused to the alloys shown in Table 5 by different laboratory technicians during the preparation of a three unit bridge product. Of seventeen products so prepared, sixteen proved to be visually acceptable only one was visually unacceptable. The unacceptable product was prepared with the Olympia® alloy; three other repetitions with the Olympia® alloy were acceptable. 
     
                                           TABLE 5__________________________________________________________________________                     Cracks Observed With                     Unaided Eye                     Number             Type of of        NoDental Alloy     Product Of             Alloy   Repetitions                           Cracks                               Cracks__________________________________________________________________________Biobond ® II     Dentsply             Nickel- 2     0   2     International             ChromeBiobond ® C &amp; B     Dentsply             Nickel- 1     0   1     International             ChromeCobond ®     Dentsply             Cobalt- 3     0   3     International             ChromeWill-Ceram ® W-1     Williams Gold             Platinum-Silver                     2     0   2Option ™     Ney     Palladium-                     3     0   3             CopperOlympia ®     Jelenko Gold-   4      1* 3             PalladiumAthenium ™     Williams Gold             Palladium-                     2      1**                               1             Copper__________________________________________________________________________ *Technique related **Cracks were not observed on receipt of fused bridge product from the dental laboratory. 
    
     EXAMPLE 2 
     Two master frits containing 55 parts glass matrix and 45 parts of a glass-ceramic frit were prepared. The glass matrix was comprised of 25 parts of glass A (Table 4) and 30 parts of glass B (Table 4). The glass-ceramic frit used to prepare the first master frit was prepared in accordance with Example 1 from the potash feldspar shown in Table 2 doped with 4 percent potassium nitrate. The second glass-ceramic frit was prepared in the same manner from the same feldspar doped with 9 percent potassium nitrate. Both of the master frits contained leucite in a 5 to 10 micron particle size range dispersed in the residual glassy phase. The master frit prepared from the 9 percent potassium nitrate-doped feldspar exhibited a thermal expansion coefficient which was significantly higher than that of the master frit prepared from the 4 percent potassium nitrate-doped feldspar. 
     A porcelain product was prepared from a mixture of equal parts of the first and second master frits. The porcelain product, which had a fusion temperature of about 955° C. exhibited a coefficient of thermal expansion which was intermediate that of the respective master frits. The porcelain product is suitable for fusing to the alloys indicated in Table 5. 
     EXAMPLE 3 
     Example 2 was repeated except that the porcelain material was prepared by firing a mixture comprised of the first and second master frits in a ratio of 3.5:1 instead of 1:1. The thermal coefficient of expansion of the resulting porcelain was about 11.9×10 -6  in/in°C. The porcelain product is suitable for fusing to palladium-gold alloy having a relatively low expansion behavior, such as W-3, a product of Williams Gold having 48.5 percent gold, 39.5 percent palladium, and 10.5 percent indium. 
     EXAMPLE 4 
     Example 2 was repeated except that the second master frit was prepared from a glass-ceramic derived from a 2 percent potassium nitrate-doped feldspar instead of a 4 percent doped potassium nitrate feldspar. 
     A porcelain product was prepared from a 2:1 mixture of the first and second master frits. The porcelain product was compatible to the W-3 gold-palladium alloy noted above. 
     EXAMPLE 5 
     Example 2 is repeated except that the first master frit is prepareed from glass-ceramic derived from a 6 percent potassium nitrate-doped feldspar instead of a 9 percent potassium nitrate-doped feldspar. 
     A porcelain product was prepared from equal parts of the first and second master frits. Upon fusing to Cobond, a cobalt-chrome alloy, the porcelain contained several cracks that were visible by the naked eye. 
     It is preferred by those skilled in the art that porcelain veneers should be in a state of compression on their outer surface to take full advantage of the best qualities of ceramic systems which are far stronger in compression than in tension. The present invention comprises several glass ceramic frits of different and distinct thermal expansions which are blended in specific predetermined ratios to produce a procelain product of desired thermal expansion. The thermal expansion of the porcelain product may thus be optimized to provide compatibility with the various types of alloys intended for porcelain veneering. For example, to provide compatibility with alloys of regular to relatively high expansion behavior, such as Biobond®II, Cobond®, Will-Ceram® W-1 and Option™. 
     The porcelain product comprises two different glass ceramic frits blended to produce a thermal expansion of 12.2×10 -6  in. per in. per °C. Where the product is intended for use with relatively low expansion behavior, alloys such as Olympia®, Will-Ceram®W-3, ratio of high to low expansion frits is altered to produce a porcelain product with a thermal expansion of 11.9×10 -6  in. per in. per ° C. 
     This invention contemplates the use in dentistry of alloys intended for porcelain veneering which might exhibit thermal expansions either higher or lower than those currently in use. 
     It is to be understood that the invention is not confined to the particular forms shown, described, and exemplified herein, the same being merely illustrative and that the invention may be carried out in other ways without departing from the spirit and scope thereof as set forth in the accompanying claims.