Why do glow-in-the-dark pictures gradually fade?

Chemists and museum specialists are trying to understand why this happens. New findings presented at the American Chemical Society’s ACS Fall 2026 conference point to an unexpected culprit: for some phosphorescent pigments, prolonged high humidity may prove to be just as significant a factor in their deterioration as exposure to light.

This is particularly important for museums. Until now, great attention has been paid to limiting lighting when preserving vibrant modern artworks. Now, researchers are showing that the microclimate must also be controlled.

Why some paints are simply bright, whilst others glow after the lights are switched off

Ordinary paint appears coloured because it absorbs some of the light falling on it and reflects the rest.

With luminescent materials, the mechanism is more complex: they absorb light energy and then re-emit it.

However, fluorescent and phosphorescent materials work in different ways.

Fluorescent paints, such as DayGlo, look particularly bright under certain lighting conditions, including ultraviolet light. They absorb radiation and almost immediately re-emit some of the energy as visible light.

But switch off the light source — and the effect disappears almost immediately.

Phosphorescent materials are able to retain some of the energy they have absorbed for longer. Once the light has been switched off, they gradually release this energy and continue to glow in the dark for a while.

This is the principle behind the glow-in-the-dark star stickers familiar to many, elements of road safety markings and certain art materials.

New experiments by Sarah Schmidtke Sobek’s team at The College of Wooster focus primarily on precisely these types of phosphorescent pigments with afterglow.

Why does the glow fade over time?

No pigment remains chemically unchanged forever.

Light, oxygen, humidity, temperature and the interaction of different components within the material gradually trigger chemical reactions. These can alter both the substances responsible for the glow and the environment in which these substances are found.

The result becomes visually apparent.

The colour may darken. The fluorescent effect may weaken. And phosphorescent material, after being ‘charged’ by light, will no longer glow as brightly or for nearly as long.

For ordinary paint, a change in shade already poses a problem for conservators.

But in the case of luminescent art, it is not simply the colour that is lost. The artistic effect itself—which may have been central to the artist’s vision—disappears.

Humidity proved to be more important than the scientists had expected

Sobek and her colleagues first determined the chemical composition of the luminescent materials, and then studied their photophysical properties — that is, how they absorb energy, store it and emit light.

The researchers discovered that many phosphorescent pigments differ significantly in composition from conventional fluorescent dyes.

Glow-in-the-dark materials often use inorganic compounds and mineral components, whereas daytime fluorescent pigments may contain organic dyes.

One of the most unexpected preliminary findings was the effect of high humidity.

According to the researchers, prolonged exposure to a humid environment can play just as significant a role – and, under certain conditions, an even greater one – in the degradation of phosphorescent material as prolonged exposure to ordinary light.

This is a significant consideration for museum storage.

It is not enough simply to reduce the brightness of the lamps or shorten the exposure time. If the material is sensitive to moisture in the air, the artefact may require strict control of relative humidity.

There is an important limitation here.

The new findings were presented at the ACS Fall 2026 scientific conference, rather than in a separate published paper providing a full description of the experiments.

The American Chemical Society’s press release does not specify exact humidity values, the duration of all tests, the number of samples studied, or the percentage of luminescence loss.

Therefore, the statement:

‘humidity destroys luminescent paints faster than light’

would be too strong.

It would be more accurate to say that, for some of the phosphorescent materials studied, high humidity proved to be a significant factor in ageing and, in certain experiments, may have had a greater effect than light.

For other chemical compositions, the result may be different.

Sometimes the paint darkens, even though the main dye is still intact

Sobek’s team has been investigating another type of material for several years now — bright daylight-fluorescent pigments.

This work began, in particular, with the collection of the American designer Stephen Sprouse at the Indianapolis Museum of Art at Newfields. Sprouse made extensive use of extremely bright pinks, greens and other DayGlo colours.

It turned out that this type of paint may consist of a complex mixture of several substances.

It contains dyes, a polymer base and special components that enhance visual brightness — optical brighteners.

These absorb ultraviolet radiation and re-emit part of the energy in the visible spectrum, making the surface appear brighter.

However, the different components of the mixture age at different rates.

Researchers have found that optical brighteners can degrade before the main colourant.

As a result, the surface darkens and looks less striking, even though the pigment itself, which determines the main colour, may still be present.

For a restorer, simply matching the colour is not enough

This presents an unusual challenge when restoring modern works of art.

Suppose a restorer needs to restore a damaged section of a bright pink fluorescent work.

It is possible to find a new paint that, under normal daylight, matches the old one almost perfectly.

But under ultraviolet light, it will become obvious that the two areas glow quite differently.

Even if a perfect match is achieved today, the problem does not end there: the original material continues to age, so in a few years’ time the old and restored sections may once again differ in colour and intensity of glow.

Consequently, conservators must assess not only the visible colour of the material, but also how it behaves under different types of lighting.

A single bright paint may contain several chemical systems at once

Previous research by the team has shown just how complex industrial fluorescent pigments can be.

To determine their composition, the scientists had to use several methods at once — including chromatography, mass spectrometry and infrared spectroscopy.

Even materials that look almost identical on the surface may contain different dyes and polymer matrices.

Furthermore, manufacturers may change their formulations over time.

This means that a modern work of art cannot be reliably restored simply by finding a paint of a similar shade in a shop.

From a chemical point of view, it may turn out to be a completely different material, which will age in a completely different way.

Scientists are also studying materials with long-lasting afterglow

As part of the current project, researchers are working specifically with phosphorescent systems containing lithopone — a historically well-known white pigment.

Student Eleanor Fleming is using photographic methods to measure how long such materials continue to glow after exposure to visible light.

The researchers are interested in how the intensity and duration of the glow change as the material ages.

However, ordinary lithopone on its own should not automatically be considered a ‘glow-in-the-dark paint’. It is the specific phosphorescent formulations in which it is incorporated that are of interest.

The same chemistry is used far beyond the confines of museums.

Fluorescent and phosphorescent substances are used wherever it is necessary to attract attention quickly or maintain visibility in the dark: in emergency signs, clothing, road markings and technical signage.

Therefore, understanding the causes of degradation is of interest not only to conservators.

If scientists can identify which components of luminescent systems are particularly sensitive to water, light or other conditions, manufacturers will potentially be able to create more durable materials.

The researchers even mention runway surfacing and markings as possible future applications.

But this is still a long way off. The new experiments do not involve testing a finished surface on an actual runway.

It is impossible to completely halt the chemical ageing of materials.

Even under ideal museum conditions, molecules gradually change. The task for specialists is to understand the mechanism of degradation and slow it down as much as possible.

This is particularly important for luminous art.

A work of art may appear physically intact: the canvas is not torn, the paint has not flaked off, and the image remains recognisable.

But if the substances responsible for fluorescence or afterglow have broken down, part of the original artwork has, in effect, already disappeared.

This is precisely why scientists are investigating not only what colour the paint was, but also what it did with light.

New findings show that the answer depends on a complex interplay between chemical composition and the environment. Light remains one of the main factors in ageing, but for some materials, ordinary water in the air may prove to be a hidden enemy.

Source

The new findings were presented on 25 August 2026 at the American Chemical Society’s ACS Fall 2026 conference as part of the ‘Chemistry Behind Art and Art Conservation’ symposium.

Paper: “Glow in the light & dark: Investigations of the photochemistry of emissive pigments used in art”.

The research is being conducted by Sarah Schmidtke Sobeck of The College of Wooster, Gregory Smith of the Indianapolis Museum of Art at Newfields, and their colleagues.