When glass weathers, it forms layers. These layers are so thin, they cause light interference, like that caused by a film of oil on water. Refraction creates these colors, there are no colors in the layers themselves.
Title supplied by Robert H. Brill.
Reference list A : Glass studies, in: Brill, Robert H. Chemical analyses of early glasses, A-128
Digitized from a slide by Boston Photo Imaging; Corning Museum of Glass; 2011.