This window is particularly effective due to the atmospheric background and shadowy figures created with glass painting. The method harks back to European cathedral windows - using a mixture of ground pigments and glass - that is applied with special techniques, and then fired in a kiln to temperatures ranging from 1050-1350 degrees. When the glass itself just begins to soften and slump, the paint fuses into the surface, guaranteeing permanence against all but the most aggressive abrasion.
Indeed, it is the metal - the lead came - that is the most fragile part of a stained glass window, and most subject to the deterioration wrought by time and improper cleaning methods. We always advise our clients that "benign neglect" is the best upkeep of their stained glass, and most certainly to never clean the stained glass with modern cleansers like Windex spray.