Ancient Glass Blog of The Allaire Collection

WHAT IS THE IRIDESCENCE ON ANCIENT GLASS ?

Posted in Uncategorized by Allaire Collection of Glass on May 19, 2014

What is iridescence on ancient glass?

 The iridescence on ancient glass was unintentional unlike what is found on modern Tiffany, Loetz, and Steuben glass. Caused by weathering on the surface, the iridescence, and the interplay of lustrous, changing colors, is due to the refraction of light by thin layers of weathered glass. How much a glass object weathers depends mainly on burial conditions and to a lesser extent the chemistry of it. These conditions are humidity, heat and type of soil the glass was buried in. The chemistry of ancient glass though basically the same as our soda glass differed in the purity of raw materials and compositional ratio.  There were also differences in flux alkali used such as natron (sodium carbonate) or potash (potassium carbonate). Generally glass made in the Western Provinces with potash has less iridescence than glass from the Eastern Mediterranean areas using natron. At the same time burial conditions also were different. Natural iridescence is sometimes found on modern glass bottles from digs in the back yards of old houses or pulled out of river beds. The word iridescence comes from Iris, the Greek Goddess of rainbows and refers to rainbow-like colors seen on the glass which changes in different lighting.  It is simply caused by alkali (soluble salt) being leached from the glass by slightly acidic water and then forming fine layers that eventually separate slightly or flake off causing a prism effect on light bouncing off and passing through the surface which reflects light differently, resulting in an iridescent appearance. Modern iridescence sometimes called iris glass is made by adding metallic compounds to the glass or by spraying the surface with stannous chloride or lead chloride and reheating it in a reducing atmosphere. Examples below are ancient glass from the Allaire Collection with natural iridescence.

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