Silver stain
Silver stain, also called yellow stain, is a solution of a silver compound that is painted on one surface of the glass and then fired at a modest temperature. The silver migrates into the surface and turns it yellow, anywhere from pale lemon to deep orange depending on the mixture and the firing.
It was introduced in the early fourteenth century and changed the way windows were made. Before it, a yellow detail, such as hair, a crown or a halo, needed a separate piece of yellow glass held in lead. With silver stain it could be painted onto a single piece of glass, and a clear piece of glass could be given both yellow and white areas. Painted on blue glass, it gives green.
It is applied to the outside face of a panel, away from the paint on the inside, so that it can be fired without affecting other colours.