according to an embodiment of the present invention , titanium or a titanium alloy is used as the metal base for a dental restoration . titanium and most of its alloys are extremely biocompatible , and provide the strength and longevity necessary for dental restorations . unlike precious metals such as gold , titanium and its alloys are relatively inexpensive . current , commercially available dental porcelains are not compatible with titanium or its alloys because thermal expansion and fusion temperatures of these porcelains are too high . the present invention also provides a dental porcelain which has a thermal expansion coefficient close to that of titanium ( approximately 9 . 91 × 10 - 6 in / in /° c . room temperature to 570 ° c .) and a fusion temperature of less than 850 ° c ., preferably less than 800 ° c . when heated to more than 800 ° c ., titanium readily oxidizes and also transforms from an alpha crystalline structure to a less - useful beta crystalline structure . therefore , when using a titanium metal base , it is desired to employ a dental porcelain having a fusion temperature of less than 800 ° c . ( approximately 1470 ° f .). the present invention also provides a dental porcelain which absorbs oxygen rather than produces oxygen to even further minimize the amount of oxidation of the titanium . of the titanium alloys useful in accordance with the present invention titanium 6 . 4 is a preferred alloy . titanium 6 . 4 comprises 6 percent aluminum , 4 percent vanadium and the remainder is titanium . other titanium alloys may be used provided they have low fusing temperatures and thermal expansion coefficients . another preferred alloy comprises 5 percent aluminum and 2 . 5 percent tin . preferred dental porcelains according to the present invention preferably comprise oxides of silicon , boron , sodium , potassium , cerium , zinc , titanium , zirconium and tin . various combinations of these oxides may be employed according to the present invention provided they form porcelains having a thermal expansion coefficient close to that of titanium and a fusion temperature of less than 800 ° c . silicon dioxide is a major component of porcelains according to the present invention . silicon dioxide has a fusion temperature of approximately 1700 ° c ., much too high to be compatible with titanium independently . according to the present invention , porcelains comprising greater than about 50 percent silicon dioxide are preferred . porcelains having silicon dioxide present in an amount of between about 58 and about 65 percent by weight are more preferred , while those having about 61 percent are most preferred . aluminum oxide may also be used in porcelains according to the present invention , however , it is not necessary . if used , amounts between about one and about four percent by weight are preferred , about two percent being most preferred . in order to reduce the fusion temperature of the porcelains of the present invention , boron oxide ( b 2 o 3 ) and sodium oxide ( na 2 o ) may be added to the porcelain mixtures . boron oxide is also used to increase the durability of a dental porcelain . preferably , boron oxide is present in an amount of between about seven and about 13 percent by weight of the porcelain mixture although amounts of up to 30 percent may be used . most preferably , boron oxide is present in an amount of about nine percent by weight . sodium oxide is preferably present in an amount of up to about 12 percent by weight , most preferably , about eight percent . if no sodium oxide is used , the weight percent of boron oxide , potassium oxide or both may be increased . according to the present invention , potassium oxide ( k 23 o ) may be used and is preferably present in an amount of up to about eight percent by weight . preferably , potassium oxide is present in an amount of about six percent by weight . if no potassium oxide is used , the weight percent of boron oxide , sodium oxide , or both may be increased . to increase the bonding strength of porcelains according to the present invention , cerium oxide and zinc oxide may be added to the porcelain mixtures prior to dissolving . only a small amount of cerium oxide is necessary to greatly improve bonding strength . cerium oxide is preferably present in an amount of between about one and about three percent by weight , two percent being more preferred . zinc oxide is preferably present in greater amounts such as about eight to about eleven percent by weight although amounts of at least eight percent by weight may be used . about nine percent by weight zinc oxide is most preferred . titanium oxide is also useful in porcelains according to the present invention amounts of titanium oxide up to about four percent by weight may be used . the amount of titanium oxide preferred is between about two and about four percent by weight , three percent being most preferred . zirconium oxide and a complex of yttrium oxide and cerium oxide are optional ingredients . if used , the zirconium oxide is present in an amount of up to about one percent by weight , preferably about 0 . 5 percent by weight . zirconium oxide is primarily used as an opacifier for the porcelain . the yttrium oxide - cerium oxide complex may be used to add fluorescence to the porcelains . if used , amounts of up to only about one percent by weight are necessary to provide dentition having natural - looking fluorescence . about 0 . 3 percent by weight of the complex is most preferred . the above mixtures make up body porcelains which may be used in combination with a titanium or titanium alloy base . preferred body porcelain materials of the present invention have the following compositions : ______________________________________weight percent morecomponent broad preferred preferred______________________________________sio . sub . 2 50 - 70 58 - 65 61al . sub . 2 o . sub . 3 0 - 4 1 - 4 2b . sub . 2 o . sub . 3 7 - 33 7 - 13 9na . sub . 2 o 0 - 20 6 - 12 8k . sub . 2 o 0 - 20 5 - 8 6ce . sub . 2 o . sub . 3 0 - 3 1 - 3 2zno 8 - 11 8 - 11 9tio . sub . 2 2 - 4 2 - 4 3zro . sub . 2 0 - 1 0 - 1 . 5y . sub . 2 o . sub . 3 -- ce . sub . 2 o . sub . 3 0 - 1 0 - 1 . 3______________________________________ the body porcelain may be formed by mixing the various oxides together with the exception of the opacifier and fluorescing agent which are added later . the mixture of various oxides is then heated until all the oxides melt and dissolve . this temperature may exceed 1500 ° c . for some combinations of the oxides . once the oxides melt and fuse together , a uniform clear molten mass is produced . the mass is then cooled to form a clear glass which has a randomly oriented atomic structure and little , if any , crystalline arrangement . the glass is then crushed to form a powder having an average particle size of up to about 70 microns . preferably , the particles have an average size of between about 15 and about 20 microns . the opacifier and fluorescing agents may then be added if desired . the powder may then be wetted and applied in the form of a paste to a metal base or an opaque ceramic and heated to about 800 ° c . where it fuses to form a dental restoration . temperatures as low as about 780 ° c . may be used to fuse the body porcelain . typically , the body porcelain is applied to a layer of opaque porcelain which has been painted onto a metal base . the body porcelain may be used by itself , however , greater amounts of pigments would need to be added to the body porcelain so as to disguise the metal base . in order to improve the bonding between the metal and the porcelain , an opaque porcelain is preferably prepared and used to prime the metal base for bonding with the body porcelain . the opaque porcelain comprises approximately 70 to 80 percent by weight body porcelain . most preferably , the opaque porcelain comprises about 75 percent by weight body porcelain . the remainder of the opaque porcelain comprises at least one member selected from the group consisting of titanium oxide , cerium oxide , stannic oxide and zinc oxide . most preferably , all 4 oxides are combined to make up the remaining 20 to 30 percent by weight of the opaque porcelain . when all four oxides are used , the titanium oxide is preferably present in an amount of between about four and about six percent by weight , most preferably about five percent by weight . the cerium oxide is preferably present in an amount of between about four and about six percent by weight , most preferably about five percent by weight . the stannic oxide is preferably present in an amount of between about eight and about 12 percent by weight , most preferably , about 10 percent by weight . the zinc oxide is preferably present in an amount of between about four and about six percent by weight , most preferably , about five percent by weight . in a preferred embodiment , the opaque porcelain comprises seventy - five percent by weight body porcelain and twenty - five percent by weight a mixture of titanium oxide , cerium oxide , stannic oxide and zinc oxide . these 4 oxides are pulverized and mixed with powdered body porcelain and then fused to form the opaque porcelain . when the opaque porcelain is fused to the metal base prior to fusing the body porcelain to the metal base , fusing of the body porcelain to the base is greatly improved . after coating the metal with the opaque porcelain , the body porcelain is then coated on the opaque porcelain and the entire structure is heated to near 800 ° c . preferred opaque porcelain materials of the present invention have the following compositions : ______________________________________weight percent morecomponent broad preferred preferred______________________________________body porcelain 70 - 80 70 - 80 71tio . sub . 2 0 - 30 4 - 6 5ce . sub . 2 o . sub . 3 0 - 30 4 - 6 5sno . sub . 2 0 - 30 8 - 12 10zno 0 - 30 4 - 6 5______________________________________ to form the opaque porcelain , about 70 to 80 percent by weight of the body porcelain in a powdered form is mixed with about 20 - 30 percent by weight of a mixture of the four oxides mentioned above with respect to the opaque porcelain . this new mixture is then heated to about 1000 ° c ., below the melting point fuses with the oxide particles to form a hard mass which is opaque . the hard mass is then crushed to form opaque porcelain particles of about the same size as desired for the body porcelain or finer . particle sizes of about 15 to 20 microns ar preferred . the opaque may be applied to a metal base and fused at temperatures as low as about 800 ° c . when applying the opaque porcelain to the metal base , the porcelain is typically first mixed with a liquid vehicle so that it may be applied to the base . the liquid vehicle may comprise water , organic solvents , or other liquid carriers which are inert to the reaction between the porcelain and the titanium . one preferred liquid vehicle is smooth touch ™ ( available from jeneric / pentron , wallingford , conn .) which comprises water with wetting agents . upon heating to fusion temperatures , the liquid typically evaporates and is preferably completely removed from the system . when dental restorations according to the present invention are formed in a furnace , it is preferable to provide a vacuum in the furnace so as to minimize the presence of oxygen . if present , oxygen will oxidize the titanium , particularly on exposed surfaces , which may weaken the bond between the titanium and the porcelain . nitrogen in the air also reacts with titanium to form titanium nitride . under vacuum conditions , the amount of nitrogen which can react with titanium may also be minimized . the use of a vacuum in a furnace also decreases the amount of bubbles formed in the porcelain . the dental restorations and porcelains of the present invention may be fired in conventional dental furnaces under much lower temperatures then generally needed for current , commercially available porcelains . oxidation of the titanium may also be minimized if the titanium metal base is completely covered with the porcelain upon application of the porcelain to the base . this reduces the amount of titanium surface area available on which oxygen can react . also , porcelains according to the present invention tend to absorb oxygen and / or shield the alloy from nitrogen or oxygen which further decreases the amount of titanium oxidation and nitride formation . pigments may be added to either the body porcelain or the opaque porcelain to form naturally looking restorations of various shades . these pigments are commercially available and well known to those of skill in the art . although the methods and compositions in accordance with the present invention have been described in connection with preferred embodiments , it will be appreciated by those skilled in the art that additions , modifications , substitutions and deletions not specifically described may be made without departing from the spirit and scope of the invention defined in the appended claims .
porcelain compositions suitable for use as porcelain layers on metal base dental restorations have fusion temperatures of about 800 ° c . and lower . the compositions may be employed as coatings on titanium and titanium alloy bases since the compositions have thermal expansion values close to those of titanium and its alloys . a method is also provided for forming dental restorations with relatively inexpensive and biocompatible metal bases and low fusing dental porcelains .