Last Updated: Sep 29, 2016 Views: 90
I checked with a retired Corning scientist who gave the following explanation and made a graphic (attached) to illustrate his point:
"There is an optical explanation for this increase in brilliance. Without the overlay, incident light can only reflect off the colored glass based upon the law of reflection where the reflected angle is the same as the incident angle. When a transparent layer is placed on top, there are many more possible light paths depending on the distribution of light from the illumination source and the shape and roughness of the tesserae. This will enhance the amount of light reflected in a particular direction and change the appearance. The figure below illustrates the difference using one possible light path through the transparent overlay. If the sides of the tesserae are rough rather than smooth, a wider distribution of reflected angles will result. Any roughness at the interface between the transparent and colored glass will also increase the distribution of angles. It is this distribution of angles that produces the brilliance when compared with the colored glass alone. Regularly shaped pieces such as those made by machine will have a more limited number of possible light paths and therefore reduced brilliance."
I hope this helps!