Twisted glass bracelet, 300-500 AD

Twisted glass bracelet, 300-500 AD
Dating:300 AD–500 AD
Origin:Mediterranean Basin, Eastern Mediterranean
Material:Glass (all types)
Physical:6.5cm. (2.5 in.) - 15 g. (.5 oz.)

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  Originally crafted from translucent dark purple glass, and now enhanced by the iridescence brought about by aging, this bracelet was probably made in the Eastern Mediterranean basin in the fourth or fifth century AD.

“Twisted bracelets are unlikely to have appeared before the 4th century CE. The monochrome twisted pieces remained popular throughout most periods and at most places, but may have been especially common in Syria and Asia Minor” (Spaer 2001:194).

Israel Museum #77.12.758

Glass Iridescence
The iridescent effect that so often enhances immeasurably the beauty of ancient glass was not planned by ancient glass artisans. Instead, it is the combined result of weathering processes and the properties of light. The rainbow effect you commonly experience in daily life, such as on soap bubbles or drops of oil spread on water, stem from the same action: light bouncing on a extremely thin transparent film.

When a glass bottle is new, there is no such thin film. The wall of the bottle is homogenous. But “as glass is exposed to water in its burial environment, some of its [chemical] components can be dissolved by the water and carried away (leached out). This generates a thin surface layer of glass that has a different composition that the undegraded bulk of glass. Often, there is a think layer of air between the corroded surface and the bulk” (Bezúr 1999).

When ordinary white light strikes the bottle, some of the rays bounce off the top surface of the thin film, and some go through the thin film and then bounce off the glass-air interface between the thin film and the underlying glass. When the rays coming back from the bottom of the thin film reemerge into open air, they combine with those that simply bounced off the surface. But since they have been delayed by their additional travel, their waves are no longer in phase (in synch). When these two streams of out-of-phase white light combine, some of the wavelengths cancel out (and therefore those colors disappear), and other wavelengths are reinforced (and therefore those colors become very intense), thus turning white light into vivid random colors.

Glass artists of the late 19th Century, such as Louis Comfort Tiffany, admired the iridescence of Roman glass, and devised ways to produce it deliberately by placing the glass piece while still very hot in an oven filled with vapors (tin and iron chlorides) that would alter the surface and create a thin film of different composition, yielding an iridescent effect that did not require a thousand years to develop.

A more thorough technical discussion of the phenomenon by Aniko Bezúr of the University of Arizona Department of Materials Science and Engineering is available from

Bibliography (for this item)

Spaer, Maud
2001 Ancient glass in the Israel Museum: beads and other small objects. The Israel Museum, Jerusalem. (194)

Bibliography (on Glass Iridescence)

Bezur, Aniko
1999 Online Notes on Iridescence ( University of Arizona, Department of Materials Science and Engineering, Tucson, AZ.

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