Ancient Glass Blog of The Allaire Collection


Posted in Uncategorized by Allaire Collection of Glass on March 26, 2020

What is iridescence on ancient glass?

 The iridescence on ancient glass was unintentional unlike what is found on modern Tiffany, Steuben glass from USA. 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. Another type of iridescence can form on the inside of a vessel called patina (see 23R below).

Examples below illustrate ancient glass from the Allaire Collection with natural iridescence.

What is iridescence on modern glass

Modern iridescence sometimes called iris glass is made by adding metallic compounds to the glass or by spraying the surface with tin chloride also called stannous chloride (SnCl₂) or lead chloride (PbCl2) and reheating in an annealing oven in a reducing atmosphere. A reducing atmosphere (redox) is produced in a fuel (oil) fired annealing oven by partly closing the draft and depriving the kiln of oxygen or adding carbon monoxide, nitrogen to replace the oxygen in the kiln. This reducing atmosphere on the glass surface in the presence of metallic compounds produce a rainbow effect called iridescence. The rainbow like effect that changes according to the angle from which it is viewed or the angle of incidence of the source of light. On ancient glass, natural iridescence is caused by interference effects of light reflected from several layers of weathering.


Examples below of Steuben, Tiffany, Loetz illustrate the iridescence on modern glass.



One Response

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  1. wynkin said, on March 26, 2020 at 6:53 am

    Some beauties there!

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