WHAT IS THE IRIDESCENCE ON ANCIENT GLASS ?

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. Another type of iridescence can form on the inside of a vessel called patina (see 23R below). Modern iridescence sometimes called iris glass is made by adding metallic compounds to the glass or by spraying the surface with stannous chloride (SnCl₂) or lead chloride (PbCl₂)  and reheating in an annealing oven in a reducing atmosphere. A reducing atmosphere is produced in a fuel fired annealing oven by reducing the draft and depriving the kiln of oxygen or adding carbon monoxide, nitrogen to replace to oxygen in the kiln. . 

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

24R Roman double balsamarium 4-5th C

12R Piriform unguentarium 1st Century

19R Blown ribbed bowl 1st Century H: 6 cm, D: 8 cm

9R Roman mold-blown perfume bottle 3-4th Century

46R Roman tall-neck bottle 2-3rd Century 19.7 cm

32R Roman candlestick balsamarium 2-3rd Century 18.5 cm

23R Roman Zig Zag Pitcher 4th Century

4R Roman cylindrical bottle 3rd Century 9.8 cm