Painted glass objects were decorated with a brush or a pen once their final shape had been attained. After being painted, they were fired in a kiln at temperatures that permanently fixed the designs on the surface without compromising the object’s shape.
Stained (or luster-painted) glass was produced in Egypt and Syria from the seventh through the ninth century. It was painted with pigments containing silver and/or copper and fired in a kiln at a low temperature. Glass thus treated cannot really be considered lustrous, because the pigment was “absorbed” beneath the surface through a chemical reaction and permanently colored—or stained—the glass, becoming part of its atomic structure.
Most stained objects are in pale-colored glass decorated in a monochrome brownish or yellowish pigment; there was a brief period when colored glass or colored decorative patterns were favored before the monochrome style regained its appeal. Silver-based paints first turn yellow, then progressively amber and deep brown; copper-based pigments quickly become red or ruby-colored, but their firing is difficult to control in a kiln (silver was often added for this reason). Yellow and orange stains can also be obtained from both silver and copper. By applying pigments to both sides of open-shaped vessels, glassmakers highlight details or outlines and exploit the transparent glass wall to create subtle shading effects. Proper control of firing time and temperature are critical to achieve the desired results; even today this aspect remains one of the most challenging in the production of stained glass.