A technique of printing on a glass sheet a so-called thick film paste prepared by milling a silver powder with an organic binder, followed by baking at a temperature of from 400.degree. to 800.degree. C., is widely used for various industrial applications where a conductive circuit on a glass sheet is desired, such as glass panel heaters, antifogging circuit-provided window glass, automobile rear windows, electromagnetic wave-shielding glass and various electronic display tubes such as fluorescent display tubes.
When the glass surface is observed from the side opposite to the baked circuit side i.e. from the rear side, if a white or yellowish brown bright colored pattern distinctive from the background exists, the commercial value or appearance of the product is substantially impaired in many cases.
In such applications, there has been an increasing demand to make the color of the silver-type conductor circuit to be a desired color such as a dark color or a black color, and various attempts have been made in this respect.
For example, it has been attempted to incorporate various substances such as pigments in a silver-type thick film paste.
However, substances which can be incorporated to a silver-type thick film paste which is subjected to baking in air at a high temperature, are practically limited to metal oxides. Such metal oxides are insulators in most cases, and those which are comparable to silver in their electrical conductivity, are not industrially or economically readily available. Accordingly, it has been common that the function as a conductive thick film paste is substantially impaired even when the color can be made black by the incorporation of such a substance.
Further, it is conceivable to incorporate various noble metal powders to silver. However, no substance has been discovered which is capable of suppressing the color development of silver on a glass sheet and at the same time capable of maintaining the conductivity and the economical feasibility.
Further, a method is conceivable in which a separate thick film paste is preliminarily printed for the opacifying purpose, followed by printing the silver-type conductor. However, such a method is industrially difficult and lacks in the practical applicability or the economical feasibility especially for large-sized mass production products such as automobile rear windows.