The enamel-like shell covering the vessels is very brittle and susceptible to flaking. As the outer shell breaks away, it exposes the flimsier individual silica films beneath and disrupts their integrity: the coatings become patchy and variable in thickness. To the human eye, the patches scatter light in such a way as to produce a brilliant peacock feather look that we call iridescence.
The environment in which the glass vessels are stored today must remain moist or else the layers of silica gel will dry out and crumble. All the weathering layers are fragile and tend to break away if the glass vessel is handled too often.
During the 1880's, Louis Comfort Tiffany created an iridescent glass which he called favrile glass. He achieved the iridescent appearance, however, by addition of metal oxides and variation of furnace temperature.