The short answer is that real coloured glass does not fade in the way that fabric or a printed poster does. A medieval window that has been in place for 700 years still has its blue, because the colour is part of the glass itself and sunlight cannot bleach it. But glass can still change, and a window can look as if it has faded for other reasons. This page sets out which is which.
Colour that is part of the glass
In pot metal glass the colour comes from metal compounds dissolved in the glass or from tiny metal particles held in it, as described in how coloured glass is made. These are stable at everyday temperatures and under sunlight, so blues, greens, ambers and reds from the furnace do not fade. The same applies to flashed glass, where the colour is a thin layer fused on to the sheet, and to silver stain and fired paint, which are melted into the surface in a kiln.
The glass that does change colour
There is one well-documented case. Until the early twentieth century, glassmakers added manganese to counteract the green tint that iron gives to ordinary glass. In strong sunlight, and especially ultraviolet light, that manganese slowly changes and the glass takes on a faint purple or amethyst tint. The effect is called solarization. It is why some old window panes and bottles are pale purple. In tests, some pieces changed noticeably in a few weeks of exposure and others took a year or more, and the colour weakens with depth from the surface. The change can be reversed by reheating the glass in a kiln, but this is for specialists and is rarely worth it. It affects clear or pale glass more than a window of deep colours.
Reasons a window looks faded
- Dirt. Grime, soot, algae, mould on the inside from condensation, and mineral crusts on the outside cut down the light and mute the colours. Careful cleaning can bring a window back (see how to clean decorative glass).
- Loss of paint. Detail is painted on glass and fired, but it can wear away or flake, especially where the firing was poor. Glass with lost paint appears paler and less detailed. Lost paint is very hard to restore.
- Surface corrosion. Some medieval glass was made with a high proportion of potash and not enough lime, and it is vulnerable to damp and pollution. The surface becomes pitted and cloudy and the colour looks dull. Corrosion is a chemical reaction of the glass itself, not fading.
- Lead bloom. Old lead can develop a white powdery oxide that spreads on to the glass and makes it look dusty.
- Cold paint and transfers. Colour that is painted on, but not fired in a kiln, can fade or flake. Modern craft paints, transparent enamels not fired, and printed transfers are examples. These are not the same as the stained glass of a traditional window.
- Light levels. The colour looks different at different times of day, and an added screen, a new building opposite or protective glazing outside can change it.
Films and applied colour
Colour films and other applied colours are different from glass coloured in the furnace. They contain dyes or pigments, so their long-term stability depends on the quality of the film and, for overlay, on the glass unit that protects them. Quality films with UV stabilisers are made to last for many years, and a film inside a sealed unit is well protected. This is covered in overlay colour films, UV protection and durability. Traditional coloured glass that is in good condition is not affected in this way.
Protecting old glass
- Protective glazing. Conservators often fit a clear protective glazing layer outside a historic window, with ventilation, to shield the glass from weather and impact while keeping the view.
- Keep it ventilated. Condensation on the inside does more harm than sun, because it feeds mould and corrosion.
- Do not over-clean. Cleaning painted glass too hard removes paint for good.
- Seek advice early. Cracks, bowing lead and white bloom are signs to call a specialist.
Related pages
- How coloured glass is made
- How to clean decorative glass
- Conservation and restoration
- Medieval stained glass and the origins of leaded lights
Terms are explained in the Glass Glossary.