It is a well-known fact that the colour of paintings changes over time due to a build-up of dust on the surface or to alterations in some of the paint layer's components. As a result, some old works of art may now look very different from the way they looked when the artist created them.
Old works are not the only ones affected by chromatic changes. Some relatively recent ones are also affected by them. It therefore seems that some knowledge of the behaviour of the materials is an important step towards preventing the evolution of the chromaticity of contemporary works of art and towards finding out how the chromaticity of works of a certain age have evolved.
Almost all the materials in painting, which can be very diverse, can suffer from some type of alteration that is reflected by chromatic change. That change may not be apparent just by looking at the painting, since the effect depends on many factors like lighting, the colours of each area (the human eye is more sensitive to some than others), the history of the work, the materials used, the layout of the materials in each area, etc.
We can therefore see that many factors influence the chromatic stability of paintings and that the problem is very complex. Nevertheless, a study of chromatic evolution can be approached from various different yet complementary angles: experiments on the works (monitoring and analytical control of the chromaticity) and experiments on each of the materials in them (a study of the optical behaviour of each material).
The least stable materials in a painting are its organic components with complex molecules. Their structures change when they come into contact with the air, since the chemical links forming the molecules change. The speed of that process is conditioned (accelerated) by the effect of electromagnetic radiation, which adds energy to the system.
The behaviour of some commercial binders commonly used in contemporary paintings was analysed in this study. For that purpose, mixtures of some of those materials were exposed to daylight under forced conditions (described later) and their optical behaviour was determined. Those data allow us to estimate the potentially different behaviours of the range of products tested and to extrapolate what would happen to a painting that has some of those binders in it.