QUESTION: I am trying to find an explanation why the overlaying of a colour with transparent glass should enhance its brilliance. I have carried out experiments and this effect does in fact happen every time I fuse transparent glass over coloured glass. I have looked at refraction and reflection but still cannot find the answer.
“Bright colours including gold leaf were fused between two layers of glass to create little glass cubes (tesserae) to enhance their brilliance.”
Is there any way you can help?
ANSWER: It would really help to have both a picture and a description of what you use to measure "brilliance." Also, have you tried putting a layer of water or oil between the pieces of glass? You could be seeing a couple of things that I can think of. You could be seeing multiple reflection between the panes of glass in the air layer that separates the panes, meaning multiple chances for the light to interact with the pigment. An oil layer should drastically reduce this effect. You could also (or instead) be seeing a reduction in the overall light transmission (well, that's for sure, I'm just not certain it's responsible for the effect you mention). With lower overall light transmission between the two panes, it could be that the intensity of the colored light relative to the overall light appears to be higher. That would result in the appearance of a richer color.
Another possibility is psychology. There's an amazing amount of pre-processing that goes on inside the human brain. For example, the moon appears much larger closer to the horizon, but it's a purely psychological effect. It could be that reduction in light or the appearance of the surface of the clear pane makes the color appear richer or more full somehow, which is why I ask how you measure "brilliance."
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QUESTION: Thank you most kindly for your quick reply.
My actual research relates to Byzantine Mosaics where a quote states, “Bright colours including gold leaf were fused between two layers of glass to create little glass cubes (tesserae) to enhance their brilliance." I have also found References to roman techniques of burnishing, grinding smooth, waxing and polishing "not only to bring out the colour" which are common throughout the writings of early art so there is little doubt that this enhancement exists. The only scientific reference is that refraction in the 13th Century but I cannot see how this can be so.
I have not used any instrument to check for brilliance but in my own experiments I and others can see a clear difference in colour (it seems brighter and more intense.
So, no scientific equipment at all, just human observation. Gold leaf would increase the intensity of reflected light inside at some points, which would enhance "brilliance" internally. Such references are all subject to human interpretation, as I said. It definitely doesn't increase the intensity of light to add a clear layer. It's either psychological or physical, but you're going to have to determine for yourself which is which since I haven't seen photos of your experiments.