Glass

Glass consists of four principal components

a former
alkali flux
stabiliser
colourants/opacifiers

Former

Silica, which is added as sand.

Alkali flux

Soda or potash e.g. wood ash

Stabiliser

Lime or magnesia. Calcareous sand is rich in marine shells, and therefore lime.

Colourants/Opacifiers

These can be naturally present in the glass due to impurities in the raw materials, e.g. in the case of green/blue-green glass which results from the presence of iron in the sand.

Other colourants are likely to be deliberate additions to the glass melt of small quantities of mineral-rich material or in some cases slags from metalworking processes.

The elements in ancient glass that affect its appearance are mainly iron, manganese, cobalt, copper, tin and antimony.

The presence or absence of lead is also important, while it doesn’t produce a colour itself (except in the form lead-tin oxide or lead-antimony oxide) it can change the hue of other colourants.

In addition when added to opaque glasses it ensures that the colourants form in a controlled way and are uniformly distributed.

Opacity in glass can be due to a number of factors; intensity of colour, bubbles in the glass or the inclusion of opacifying agents, such as tin and antimony.