They are the main kind of medieval visual art that has survived in Western Europe, together with illuminated manuscripts. As a result, stained glass windows have been referred to as "illuminated wall ornaments." They were made from stained and colored sheets of glass set into frames made of wood or metal wire and covered with enamel or paint. The glass was often imported from France and Germany. Stained glass is so named because the colors used to stain the glass fade over time due to light exposure. However, the colors remain fixed once they're applied. In the 11th century, stained glass began to be used instead of polychromy (many colors) on body armor and weapons. This display of color was thought to make them less threatening to angels and saints who were expected to protect warriors from harm. In the 12th century, artists began to use colorful glass in panels or scenes instead of monochromatic glass for body armor and weapons. These panels could be attached to buildings as window displays or used in church interiors. In the 13th century, artists began to use more expensive materials such as ruby, emerald, and sapphire glass to create scenes or objects that would not be possible using cheaper colored glass.