Thursday, 22 January 2009

Coloured Glass

Glass is coloured by (1) impurities in the batch ingredients, or(2) by one of three processes:

a. using a dissolved metallic oxide to impart a colour throughout
b. forming a dispersion of some substance in a colloidal state, and
c. suspending particles of pigments to form opaque colours.

A few historical examples:
The name of a colour was often used to describe a certain kind of glass, as in the Black bottle: a term for bottles of dark green or dark brown glass, the dark colour of which protected the contents from light. Often the "colour" black that appeared with reflected light was caused by the combination of iron, found in the sand used to make the batch, and the sulphur found in the smoke from the coal used to melt the batch. "Black" glass was first made in England in the mid-17th century.

Often, the name of the mineral added to give the glass its colour, as in Uranium glass (glass coloured with uranium oxide) was used. This brilliant yellow-green glass was first made in the 1830s.

Sometimes, a combination of both the additive and colour, as in Gold ruby glass was used. This is a deep red glass coloured by the addition of gold chloride to the batch. The method of making gold ruby glass was perfected shortly before 1679.

Optical terms can be used to describe the glass, as in dichroic glass. This is glass that is one colour when seen by reflected light and another colour when light shines through it (this is sometimes due to the presence of minute amounts of colloidal gold).

Iridescent glass is a deliberate effect (visually similar to the shimmering rainbow effect seen on the surface of soap bubbles, oil slicks, or fish scales). This is achieved by the introduction of metallic substances into the batch or by spraying the surface with stannous chloride or lead chloride and reheating it in a reducing atmosphere. On ancient glass, iridescence is caused by interference effects of light reflected from several layers of weathering products.

Iron can produce greens, iron and sulphur can produce ambers and browns, copper can produce light blues, cobalt produces very dark blue, manganese can produce shades of amethyst colour, tin can produce white, lead antimony can produce yellow and various metals produce reddish glasses.

A decolorizer is a substance (such as manganese dioxide or cerium oxide) used to remove or offset the greenish or brownish colour in glass that results from iron impurities in the batch or iron or other impurities in the pot or elsewhere in the production process.

The full article can be seen at the Corning Museum of Glass