Curious Chemicals

Dichroic Glass

What is remarkable about some glass is the ability for it to change colour under certain circumstances. These generally fall under four categories; Uranium, Selenium, Cadmium (all being affected by UltraViolet: UV, or 'black' light) and Neodymium.

Uranium/Vaseline

The most common type of Dichroic glass that is immediately identifiable by the ability to glow a lurid green when exposed to a UV (black) light. The glass is often green or yellow, but not always: sometimes what looks like clear glass with a faint green tinge can actually contan some uranium. Uranium glass can also glow under normal daylight; probably signifying a higher content.

In a very few cases some blue glass will actually glow faintly green. However, this does not necessarily indicate the presence of uranium as other elements can react similarly. Essentially, the only way to genuinely prove or disprove the presence of uranium in glass is by using a geiger counter!

The term Vaseline also confuses the issue. In the US this commonly refers to glass that is a citric green or yellow (after Vaseline pots) and also contains uranium. In Europe this can just mean the yellow or green citric colouration of the glass, so for this reason I prefer to use the terms individually to classify the glass, or both terms if they apply!

Selenium

Associated with some red glass and tends to glow orange under UV light. Selenium was originally used as a decolourant to remove the green tinge caused by the natural iron oxides found in glass. Often used with Cadmium (below).

Cadmium

Cadmium Sulphides are associated with making yellow glass and this causes it to glow yellow under UV light.

When used in conjunction with Selenium this can result in a salmon-pink glow.

I believe Cadmium is no longer considered for glass production due to the environmental hazards.

Neodymium

I certainly won't labour the point, but referring to Judith Van Buskirk-Gugudan excellent article on this subject covers just about everything there is to know about this type of glass. Including mentions of all the known makers and photographed examples.

To summarise: Neodymium glass is often lilac coloured in normal daylight and changes to a pale-blue under fluorescent (but not incadescent) light.

There are scarcer examples that are clear, slightly grey or amber-brown that can change to green.

And finally... Trichroic?

Neodymium glass can be found relatively easily, but how about some that also contains Selenium as well? Such items do exist, and I suppose should be called 'Trichroic', including two that are:

• lilac in daylight
• green under fluorescent
• red-orange under UV

It is also worth noting that the green is quite bright and not too dissimilar from the glow given off with uranium glass.

Hints & Tips

Always carry a UV light with you, so that glass can be easily and swiftly tested 'on the hoof'. Tiny keyring UV lights work very well and can be easily obtained.

Testing Neodymium glass outdoors can be difficult; at car-boot sales, for example. However, while lilac-coloured glass is often Dichroic glass this is not sure-fire guarantee, of course! A standard bright-white LED torch can help but somewhere dark to test the glass is advisable.