Painting a design onto glass with glass paint and then firing it in a kiln is called enameling.
In stained-glass work, enamel is a powdered or finely prepared glass-based material mixed with a medium and brushed onto the surface. Firing melts the material so it bonds permanently to the sheet. Artists can use it for outlines, facial features, shading, lettering, and other details that cannot be produced simply by selecting colored glass.
Enameling differs from silver staining, which uses silver compounds to create yellow or amber effects after firing. It also differs from leading, which is the structural assembly of glass pieces with lead came.
Because firing can alter the color and transparency, test pieces and controlled kiln temperatures are important parts of professional practice.