2 .Material Origins - Dichoric Glass

The word “dichroic” is derived from two Greek words “di” meaning “two” and “chroma” for colour. So “dichoric” plainly means “two-coloured”. This is the most distinctive feature of dichoric glass- that it seems to have more than one colour, especially when viewed from varied angles.The origins of this type of glass are ancient and are found in many civilizations throughout history.

So what exactly is dichroic glass and how did it end

up in the world of art?

The first and oldest man-made type of dichroic glass uses metals, such as gold and silver that are evenly mixed throughout the glass to form a type of colloidal dispersion. The metal particles are so small, about 5-200 nanometers, and dispersed so well throughout the glass, that the glass appears entirely homogeneous and monochromatic.

A perfect example of this would be The Lycurgus Cup created in the 4th century AD in Italy. In reflected light the glass appears green. In transmitted light the glass appears red. The cup is in fact on display at the British Museum in London.

The Lycurgus Cup, 4th Century AD, Italy. It appear green (left) when illuminated from the outside and red (right) when illuminated from the inside)

The more recent type of dichroic glass uses a vacuum deposition process to add layers of various metallic oxides on top of the glass such as titanium, chromium, aluminium, zirconium, or magnesium; or silica that are vapourised by an high-voltage electronic beam in a vacuum chamber. The molecules are deposited onto the surface of a blank sheet of usually clear or black glass. The finished glass can have as many as 15 to 50 layers of these materials, yet the thickness of the total coating is approximately 30 to 35 millionths of an inch, so that’s  about 760 to 890 nm. The coating that is created is very similar to a gemstone and by controlling the thickness of the coating, different colors may be obtained.

The history of the latter type of dichroic glass goes back to the 1950's and 60's when NASA’s Department of Defence, developed this as a shield against cosmic radiation. An example of this is the golden sheen on an astronaut's face shield is a dichroic coating meant to protect against the harsh glare of unfiltered sunlight.

Dichroic glass does not use paints, dyes, gels or any standard colurings agents to create colour anymore than a prism does. The fantastic colours are created through the manipulation of light. The multi-coloured effect is the result of complex light interactions called "thin film physics". Thin-film physics are also responsible for rainbow patterns in a soap bubble, the swirling colours of an oil slick floating on a puddle and the dramatic reflections in dragonfly wings.

Uses of Dichoric Coating

Optical use- it is used in microscopes, LCD progectors, 3D movies and sunglasses.

It has also become a popular material used by artists and designers through dichroic coating manufacturers.

A good example of and artist who uses dichoric glass is Chris Wood, who uses it to create installations in urban areas, creating a play with light in building, walls and indoor spaces.

                     Petrie, Chris Wood, St Helens & Knowsley Teaching Hospital, St Helens.

Light Garden, London Bridge Hospital- Chris Wood