These versatile discs are flat, clear window glass onto which multicolored figures and scenes are fired on at 1200 degrees so that the subjects actually fuse to the glass to become part of the surface.
Some appear to have the design painted and fired onto the front surface, and some have a decal against a white background fused on the back surface.
You can wrap a single one in lead or foil and hang it to catch the sun in your window, or you can incorporate it into a larger project by adding a border.