Topic: Alexandrite Bird Bowl, Why green?

[johnphilip]

Thanks Frank et al, Great all i needed to know for now,nice to have so many ologists in one place.JPH

[mrvaselineglass]

svazzo: in looking at your picture, does it look green to the eyes when you saw it, or was it something that just showed up in the pic?  the reason i ask is that i was looking at your pictures again, and the white background turned sepia-toned in the pic that shows the green bowl.  sometimes my camera does that same sepia thing.  just curious if it was the eyes or the pic only that shows green.

[Martin Brown]

What you have is colour changing under different light spectra. e.g. Neodymium

Neodymium absorbs yellow/green parts of the spectrum and as that is the main component of fluorescent light is naturally has a dramatic effect on it. It can also be used in low proportions to improve the clarity of green tinted glass without any colouring effects, no doubt some difference would be visible under fluorescent but possibly invisible.

Almost certainly it is neodymium doped glass. It looks very green under mercury fluorescent lights & CFLs, pink under warm incandescent and halogen and anything from pink to neutral grey in sunlight. Used in combination with other glass doping it can accentuate the colours or mask them. It very strongly filters yellow light.

A quick and dirty test is to look at it illuminated by the light of a yellow low pressure sodium street lamp (or even better look through the piece at one). Neodymium safety glasses are used by glassblowers to see into sodium glare in the working flame.

An easier test at home is to find something with a yellow indicator LED on it and look through the pink glass at that. You should see a big difference in brightness if it is Nd doped glass which severely attentuates a narrow band of yellow light. A CD can be used to provide a reflection spectrum to characterise glass colour transmission qualitatively (Nd should show a very sharp dark band in the yellow and another in the mid green).

Regards,
Martin Brown

[Frank]

Another way of checking when you only have photographs is to correct the colour balance in both images, this will compensate for the colour of the lightsource but will not affect, so much, the colour change effect.

I think that eliminates Neodymium

[svazzo]

Hi Frank,
I am not sure i understand what you mean about correcting the color balance.
The picture on the bottom was taken at daylight, but under 1 of those energy efficient lights.
You cant see the entire photo, but in the background the door and bedroom is a normal colors. They didnt change the color in the photo to yellow. Only the paper I put the bird on looked more yellow because it isnt a true white or bright white.
Also, I dont think color balance on photos gives you the correct result.
I have tried color balance on many of my photos and it really distorts the color dramatically sometimes, depending on how many parts are light or dark, what the main color in the photo is, etc.
In the bottom photo you changed the brightness and contrast really drastically, lightening up the entire body of the bowl. Of course it will look more white/clear that any other color when you are doing that. You can really tell because the photo is so pixilated now. If you play with color balance enough you can turn any photo any color you really wanted to. I do have a background in digital photography, so i know it can be done... anyway.. still not sure I see why you are eliminating  Neodymium.
I am not sure I understand yet.

[Frank]

Neodymium gives a blue colour under fluorescent light, no other colour is possible. There is clearly something in the glass that is reacting yellow and in the manipulation this concentrates on the densest part, so what you need to know is what fluoresces yellow, chances are it was used as a decolourant.

If you match one colour, ideally white, in photos taken under different types of light you get a relationship between all the colours. It does not make a difference if the image is partial, as long as the relevant part is identical. The reason being that the camera records the visible light and that is what is being changed when the light source is changed. This was fresh in my mind as when I transferred an old article from one website to another and reworked the photographs - which had had a comment about how those taken under tungsten and daylight brought out colours you could not see. Passing them through the white balancer, got rid of those odd colours and while there was still some difference it was negligible.

I have been using this photoshop plug-in for a couple of years now and have found it to be excellent and accurate.

[Martin Brown]

Hi Frank,
I am not sure i understand what you mean about correcting the color balance.
The picture on the bottom was taken at daylight, but under 1 of those energy efficient lights.
{snip}
see why you are eliminating  Neodymium.
I am not sure I understand yet.

Neither do I. Looking green under a CFL or standard fluorescent light tube is a hallmark of Neodymium doped glass (or in the old days before they could separate all the rare earth elements didymium glass). It usually looks pink under most continuum thermal filament lights.

The definitive test is find a shovelware aluminium coated CD, place it in sunlight to get a visible reflection spectrum and put the piece between you and the spectrum. If you see almost no yellow light (a black band) and a weaker dark band in the green then it is definitely neodymium doped glass - no question. There are other iridising surface techniques that could produce the same colour shift but the colour would vary with the angle of incidence of the light. The image you showed looks to me like Nd doped bulk glass. The reason it looks green under fluorescent light (or energy saving CFL) is that the very strong Hg green emission line isn't attenuated so the colour balance is wildy shifted by filtering out all the yellow.

Regards,
Martin Brown

[svazzo]

Hi Martin,
I am not very well versed in the technical aspects of glass and light properties, but I think i understand more now what Frank was saying about the balance. I see now that he meant take 1 color (white) and make it the same in both photos... Working in photoshop I know you can turn any photo any color, so that is why I dont mess around with color balance. It is very difficult to maintain consistant in every photo because of what I mentioned earlier about the amount of the main color in a photo, how light or dark it is, etc.
Would it not be called Neodymium even if the color under regular light is the same as those others, and having the property of changing its color under a secondary light source?
I guess the main question is if Neodymium is only called that by a Result Specific change in the color of the glass?
Meaning it can only turn 1 color, not another?

Still this confuses me more, since it was mentioned earlier in the topic that other glass companies made this type of glass specifically to turn other colors.

Javier

[Frank]

When it was used as a decolourant their is some illumination effect but not noticeable normally. When used as colourant the designer would know of the effect and exploit it as a feature rather than a distortion, and now people collect it.

[Martin Brown]

Would it not be called Neodymium even if the color under regular light is the same as those others, and having the property of changing its color under a secondary light source?
I guess the main question is if Neodymium is only called that by a Result Specific change in the color of the glass?
Meaning it can only turn 1 color, not another?

Still this confuses me more, since it was mentioned earlier in the topic that other glass companies made this type of glass specifically to turn other colors.

It is somewhat confusing. Part of it is to do with how the eye perceives colour (and cameras attempt to mimic this). A "white minus yellow" filter is not something you normally encounter. Nd doped glass filters are sold to photographers to enhance contrast in reds and greens (and used in the front of TV sets to filter unwanted yellow output in the phosphors). I should declare an interest here - I sell filters to astronomers for photography to help combat low pressure sodium light pollution. The following website shows the spectra characteristic of a Nd doped glass (although for historical reasons he calls it a Didymium filter since the glass usually has a mix of mostly Neodymium and a trace of Praesodymium). The ratios of these affecting exactly how the glass will look (especially in artificial fluorescent light).

http://home.freeuk.com/m.gavin/grism2.htm
(the Didymium spectrum is right down at the bottom right with HPS & LPS sodium above and below, CFL & standdard fluorescent further up the page)

Looking at a spectrum through your glass piece will show if it has the characteristic pure yellow light blocking effect.
Wiki is surprisingly good in explaining this unusual doped glass too.  The name didymium is perpetuated by glassblowers.

http://en.wikipedia.org/wiki/Didymium
or
http://www.answers.com/topic/didymium
(longer)

I have samples of Nd doped glass to hand with different doping levels and thicknesses. To my eyes under CFL light weak Nd doping looks grey or pale turquoise, and as the concentration is increased it looks progressively greener, and then goes that sort of yellow khaki mustard colour beloved of German fashion designers. I don't have anything with more in than that. They all look progressively darker shades of pink in full spectrum light. I suspect some of the wide variety comes from whether or not the glass is pure neodymium doped (very expensive in the early days, impossible before 1885) or doped with a didymium mix of Nd & Pr and the ratio used. I'd expect most glass before WWII to use didymium rather than pure neodymium since it was only after the atomic age that it became relatively cheap to separate them. Add the possibility of additional ionic colours in the mix and you can have quite a few permutations. Hope this helps.

Regards,
Martin Brown