Answered By: Regan Brumagen
Last Updated: Jul 19, 2016     Views: 194

From Dr. Robert Brill, Research Scientist Emeritus:

In my opinion (and there are those who might disagree), there are only two types of glass that can be correctly and appropriately described as "dichroic".

The first type is the group of Roman glasses exemplified by the Lycurgus Cup.  I know of a total of eight or nine such pieces.  All but the Lycurgus Cup are only fragments.  These glasses exhibit true dichroisim; they have different colors when viewed by transmitted or reflected light.  In the case of the Lycurgus Cup, it is a pea-soup green by reflection and magenta by transmission.  The other examples show different color pairs, but owe their dichroism to the same chemical effect.  All of these glasses contain minute levels of colloidal gold and/or silver.  Their dichroism is the result of light scattering.  It is the effect known as Mie scattering, in contrast to Rayleigh scattering which is commonly observed in nature.  We have duplicated all of the coloring pairs found in the fragments mentioned above in the laboratory by introducing minute levels of gold and/or silver into glasses. 

The second type of dichroism is that exhibited by glasses containg Rare Earth or (sometimes) Transition Element oxides.  These glasses have very sharp cut-off peaks in the visible range of their transmission curves.  Their dichroism is seen only in transmitted light; it does not involve reflectance.  The interactions of their sharp cut-off transmission spectra, with light sources having different color temperatures ("warmer" or "cooler" sources), along with the varying sensitivities of the human eye to different wavelengths of visible light, result in our perceiving different colors when the glasses are illuminated under different conditions.  That is a complicated way of saying that if pieces of such glasses are viewed against (for example) a fluorescent light tube or an incandescent light bulb, our eyes see the same glass as having two different colors.  Dichroism in rare earth glasses goes back to the Moser glasses, which, I believe, date from about 1890–1910.

These two quite different physical and chemical effects are both logically described as being dichroic.  While there are numerous other captivating color effects observed in other types of both ancient and modern glasses, none of them, in my opinion, should be called "dichroic".  Such effects as iridescence and fluorescence are caused by entirely unrelated physical and chemical effects and do not even fit the derivation of the word "dichroic".  That holds for the iridescence caused by weathering on ancient glasses as well as for the iridescence of thin surface films deposited on modern glasses by sprays or vapor deposition. 

The only marginally logical additions might be luster glasses (and glazes) that have been accidentally overfired.  Such glasses sometimes show turbidity or halation surrounding the normally yellow or amber transparent stains that we call "luster" on Islamic glasses or "silver-stained" on stained glass windows.  The African beads that I have seen that show a marked bluish turbidity with an orangy transmission, look to me like high-lime glasses that have simply become devitrified by prolonged heating.  They show an appearance similar to that seen on 17th–19th century bottle glasses that have been accidentally reheated.  It might be stretching a point a little, but I suppose those beads could be called dichroic.  Some African beads have been said to contain uranium.  If so, that could contribute to their color, but would not necessarily make them dichroic.

The explanations of the colors of dichroic glasses is a relatively complicated subject.

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