The considerations which must be taken into account in the fabrication of a dental ceramic are presented in my U.S. Pat. No. 4,046,732 which issued on Sept. 6, 1977, and said patent is hereby incorporated by reference as if fully presented herein. As stated in said patent, the dark color of the substrate, usually a platninum alloy or stainless steel, must be masked. This is generally accomplished through the use of an opaque ceramic in the form of a base coat directly upon the metal substrate. Conventionally, this undercoat is applied to the metal from a dispersion in distilled water and must be baked to a hard condition prior to application of the porcelain body over the undercoat. In the absence of such an intervening firing, application of the porcelain body in distilled water will disturb the undercoat so that the benefit thereof is destroyed. This difficulty arises due to the fact that the dried ceramic coating reverts to the powdery state upon evaporation of the distilled water in which it is dispersed during the application thereof.
A further difficulty arises from the fact that the ceramic body shrinks during firing. As a result, the undercoat moves away from the gingival portion of the metallic substrate exposing same to view. As is evident, it is necessary that methods be provided for applying the two coatings and carrying out the firing thereof so that the entire metallic substrate is hidden from view.
The binders or dispersants taught in the aforenoted patent have proved to be effective in solving a number of the problems associated with hiding of the substrate and application of the porcelain thereover. However, the compositions presented in the present disclosure achieve the same objectives more simply and provide the firing at a lower temperature. The matter of the temperature is particularly important since the present compositions require firing at temperatures as high as 1900.degree. F. The time required to reach this temperature is undesirably great, and, firing at such a high temperature shortens the life of the furnace appreciably. As is evident, then, it would be desirable to be able to overcome these difficulties, both for the purpose of achieving a superior product and for decreasing the cost of manufacture.
USPTO Backgrounds
USPTO Backgrounds