A fading issue

As glass becomes more prolific in the built environment, Phil Brown, European regulatory marketing manager at Pilkington UK, discusses how sunlight exposure can cause the colours of fabrics, paints and other materials to fade over time – and outlines what measures glaziers can take to help prevent it.

The growing accessibility of advanced glazing with thermal insulation and solar control properties has opened up new ways for architects to fill rooms with natural light. But, as building designers create more bright and airy spaces for us to live and work in, we need to consider the cases where sunlight exposure needs to be minimised to prevent irreversible damage being caused to objects.

Sunlight causes dyes in fabrics, paints and other materials to fade over time, which can be a major issue, especially in historic buildings, museums or art galleries where artefacts and paintings can be damaged. Naturally, for homeowners, interior designers, and collectors of valuables, it can be necessary to limit sunlight exposure in certain locations.

In the glass and glazing industry, it’s often thought that using laminated or tinted glass will prevent coloured pigments from fading. However, the cause of the fading and the effect it has on any given item are complex, and there’s no one-size-fits-all solution.

Firstly, we need to understand what causes objects to fade. A material’s colour is caused by its chemical makeup, and specifically chromophores – part of a molecule or atom that absorbs light. Chemicals absorb light at different wavelengths, and this results in the balance of colours reflected by objects, determining their appearance.

Over time, exposure to light reduces absorption by the chromophores making the object appear whiter in colour. This effect is known as photodegradation.

Direct daylight typically has the strongest effect because it contains high levels of UV radiation, the most energetic end of the light spectrum, but all light, even from artificial sources or having passed through a UV filter, will cause some degree of photodegradation.

The speed that this occurs at varies dramatically depending on the type and duration of exposure, and by controlling these factors in the right way, fading can be greatly reduced. Also, the ambient temperature is important, as changes involved in fading tend to proceed more rapidly at higher temperatures.

Selecting the best glass for preventing sunlight damage is not as simple as it may appear. Specifications for glass to reduce fading commonly refer to UV transmittance, with tinted glass generally allowing less UV light to pass through than clear, monolithic glass. Laminated glass also tends to block more UV than tinted glass, due to the presence of the polyvinylbutyral (PVB) interlayer.

For guidance, 6mm clear glass typically has a UV transmittance of 57%, and 6mm blue-tinted glass transmits 18%. Meanwhile, 6.4mm clear laminated glass transmits just 3% of UV rays, while 7.5mm clear laminated glass transmits none at all.

While different types of glass can help control UV transmittance, the issue remains complex. Some dyes and pigments in materials fade relatively slowly, others can fade very quickly. The narrow wavelength band of the UV transmission may not necessarily relate to their anti-fade properties. Some materials can fade noticeably just from the effect of visible light.

Some North American and international standards have tried to address this by introducing more sophisticated parameters such as a ‘damage factor’ for materials. These evaluate the transmittance of different types of glass not only in the UV region but also for visible light.

However, these have never really become established in the UK or mainland Europe, and UV transmittance, with its known shortcomings, continues to be the most prevalent glass property quoted.

The only way to eliminate 100% of light-induced changes in materials is to protect them from all light. In museums and art galleries, some rather elaborate precautions may be taken, such as restricting viewing times and using specially designed low-energy light sources. Of course, these may not be practical in domestic and retail environments.

However, the following are some helpful tips to reduce the risk of fading:

Reduce the ambient temperature, as fading tends to occur more rapidly at higher temperatures – so solar control glass, which helps prevent heat gain in rooms, may help

Avoid local overheating from lamps

Use laminated glass, with as thick an interlayer as possible

Reduce exposure to UV light and the blue end of the visible spectrum

Reduce overall levels of illumination, both daylight and artificial – particularly avoiding direct sunlight

Use shades or curtains

Reduce the time that the material is exposed to the light source

So, while tinted and, in particular, laminated glass can offer considerable benefits in blocking UV light, there is far more to fading than meets the eye.

As awareness of the advanced properties modern glazing grows in the architecture community, so will its specification in new builds and projects. But, glaziers need to make sure that efforts to create bright spaces aren’t hampered by possible damage posed to valuable goods.