Revealing the Tin Side of Float Glass with Ultraviolet Light
When plate glass (AKA: FLOAT GLASS) is manufactured, one side of touches molten tin which leaves an invisible coating on the glass. The silver stain reacts differently with the tin & non-tin side. A short-wave ultraviolet light will reveal the tin coating which fluoresces with a milky white haze.
Beveled glass is cut from float glass and the manufacturers do not check to see where the tin side is. These two bevels would take stain differently.
In this example the tin side was on top and has been partially removed in the beveling process.