How Coloured Glass Is Made: Oxides, Silica and Heat

Clear glass is made mostly from sand (silica), with soda ash to lower the melting temperature and lime to make it durable. Coloured glass is the same material with something added. A very small amount of a metal compound, often well under one per cent of the batch, is enough to turn a whole sheet blue, green, purple or red. Making the colour consistently is a matter of chemistry, temperature and experience, and it is the reason a good sheet of coloured glass has always been valued.

How colour gets into glass

There are two main ways. In the first, the metal dissolves in the molten glass as ions, and each ion absorbs some colours of light and lets others through. In the second, the metal forms extremely small particles, measured in billionths of a metre, that are scattered through the glass and filter the light. The first gives the blues, greens, purples and browns. The second gives the strongest reds and the yellows.

What each colour comes from

Blue: cobalt oxide gives a deep blue, and only a tiny amount is needed. Copper compounds give lighter blues and turquoise.
Green: chromium gives a clear emerald green. Iron gives paler bluish and yellowish greens, and copper can give greens depending on the glass.
Purple: manganese gives purple and amethyst.
Yellow and amber: cadmium compounds give yellow and orange. Iron combined with sulphur and carbon gives amber and brown.
Red and pink: see the next section.
Opal and white: opacifiers such as tin oxide or fluorine compounds form tiny crystals that scatter light, so the glass is cloudy or milky instead of clear.
Fluorescent greens and yellows: uranium was used in some older glass in small quantities. It is now rare and tightly controlled.

Does gold make red?

Yes. Gold gives one of the oldest and most prized reds, known as gold ruby. A very small amount of gold chloride, only about one part in a hundred thousand, is added to the batch. The glass often comes out of the furnace almost colourless. It is then reheated, a step called striking, which lets the gold form particles of exactly the right size. The size of the particles decides the colour, from a pale pink or cranberry to a deep ruby red, so the heat treatment matters as much as the gold.

Gold ruby is expensive because of the gold and difficult because of the control it needs. Other reds are made more cheaply. Copper can give a dense red, which was the usual red in medieval windows. Selenium combined with cadmium gives a bright red or orange. Cadmium is toxic, so its use is regulated and some makers have moved to alternatives.

Melting and forming

The raw materials are weighed and mixed into a batch. The batch is melted in a clay pot or a tank furnace at roughly 1,400 to 1,500 degrees Celsius, together with cullet, recycled broken glass of the same colour, which helps it melt. The molten glass is held until bubbles clear and then formed:

Mouth-blown (antique) glass: a cylinder is blown, opened and flattened, as described under window glass. It has the slight ripples and bubbles people look for in restoration.
Rolled glass: molten glass is passed between rollers, and the rollers can impress a texture. Most cathedral, opalescent and many textured sheets are made this way.
Fused sheet: in kiln work the sheet must be made to a tested rate of expansion so that different colours can be melted together.

Afterwards the glass is annealed, cooled slowly in a lehr so that it does not hold stress and crack, and then cut into sheets and checked.

Pot metal, flashed and painted glass

Pot metal is glass coloured all the way through while molten in the pot. It is the glass of the great medieval windows.
Flashed glass is clear or lightly coloured glass with a thin layer of a stronger colour on one face. Red is often made this way because a full thickness of red would be almost black. A flashed layer can be removed by acid or abrasion to leave a pattern, which is how a single sheet can carry two colours.
Painted and stained glass: detail is added by painting with a mixture of ground glass and metal oxide, which is then fired on in a kiln. Silver stain is a different process: silver compounds painted on the surface and fired turn the glass yellow to orange.

Why colour varies

The same ingredients do not always give the same result. The final colour depends on the base glass, the melting temperature, how much oxygen is in the furnace, how thick the sheet is and how long it is held hot. Small changes give a different shade, so a maker cannot be sure of the colour until the glass has cooled, and one batch may differ from the next. This is why glaziers and artists buy all the glass for a window from one batch and why restoration glass is matched by eye.

Terms such as pot metal, flashed glass and silver stain are explained in the Glass Glossary.

How Coloured Glass Is Made