FTIR investigations of paint degradation in art conservation

How do art conservators know the way the old masters mixed their paints? FTIR spectroscopy is one of the most commonly used techniques to determine the chemical composition of paints, binders and fillers. But why is this so important when you are restoring art?

Deterioration of artwork

People like admiring paintings in art galleries – so much has been clear for centuries. But rarely, we know which method or which materials an artist used to create his/her work. Even if it says „oil painting by Rembrandt“ the components are usually much more complex than it seems.

Unfortunately, it is precisely this complexity that leads to the paintings slow but inevitable deterioration by different causes. For example chemical changes, the influence of sunlight, material interactions, or a poor choice of materials used by the artist.

Of course, artists rarely knew how the choice of colors or binders would affect their artworks in the future. One example of this is the use of titanium dioxide in paintings created from the early 1920s onwards.

As it turns out, titanium dioxide (TiO₂) facilitates the degradation of linseed oil, a painting medium, which may lead to cracking and yellowing. To better understand this effect, researchers in art conservation set out to determine the effect of visible light and the TiO₂ mineralogy on its reactivity.¹

The method of choice? FTIR spectroscopy of course!

Art conservation enhanced by FT-IR!

In their experiments pure linseed oil, as well as a mixture of linseed oil with rutile or anatase (two modifications of TiO₂) was subjected to visual and UV light. The aging process was then monitored over a time span of 49 days with Brukers LUMOS II FTIR microscope in ATR mode.

But how exactly does the degradation process show in the spectra? There are three telltale spectral markers that the scientists looked out for.

1. Formation of an IR band „shoulder“ at 1710 cm⁻¹, indicating free fatty acid formation
2. Growth of peaks from 1670 cm⁻¹ to 1610 cm⁻¹, indicating autooxidation of paint
3. Peak decrease at 1168 cm⁻¹, indicating degradation of triglyceride ester linkage bond

In the image below, the changes to pure linseed oil, triggered by aging under visible light, are shown. All spectral markers show changes but they are not very pronounced.

In contrast, changes in the mixtures of linseed oil with rutile or anatase over time are significant (see image below).

At 1710 cm⁻¹ a new peak has formed which only showed as a small shoulder in the linseed control. Furthermore, a strong shoulder appeared in the range from 1670 cm⁻¹ to 1610 cm⁻¹. Last but not least a clear decrease in the intensity of the peak at 1168 cm⁻¹ occurred. These results show that TiO₂ has a significant negative effect on the stability of linseed oil based paint.

Conclusion

Studies about art conservation are crucial to understand why and how paintings loos their charisma or even become irretrievably damaged. For studies like this but also the development of methods for the conservation of works of art, our LUMOS II FTIR microscope is the optimal choice.

Do you want to know more about how Brukers FTIR solutions can help in preserving artwork? Then have a look at our blog entries evolving around historic monuments and cultural heritage materials.

Reference

(1) Schmitt, T., Rosi, F., Mosconi, E. et al. New insights into the deterioration of TiO₂ based oil paints: the effects of illumination conditions and surface interactions. Herit Sci 10, 99 (2022). https://doi.org/10.1186/s40494-022-00733-2