Old masters in the spotlight

White specks are appearing in the UK’s national collection of priceless masterpieces, but do they threaten the paintings’ futures? Catherine Higgitt and John Plater take up the story.

Look beyond the image in certain old master works and something strange appears to be happening to the paint’s surface. Tiny ’pimples’ are often seen, which can deform the surrounding area, erupting up through the layers to appear as opalescent white spots or inclusions on the surface. Although these inclusions are not always immediately apparent, the examination of paintings with a microscope reveals their presence. But do they present a problem? Are they a threat to the paintings themselves? Many explanations for the origin of paint inclusions have been proposed over the years. Perhaps they were always intended to be present, added to alter the texture of the paint or for special optical effect. Alternatively, they may be an unintended consequence of the use of a particular ingredient or technique. More alarmingly, the inclusions might indicate that the paintings are undergoing a serious deterioration process, quite different from the usual changes that occur with time. Such a process might, if left unchecked, eventually lead to the paintings being permanently disfigured.

In an attempt to understand the nature and origin of these blemishes, the National Gallery in London, UK, has been carrying out a detailed study of a number of paintings showing this phenomenon (ranging in date from 13th to late 18th century). However, this is not the first such investigation into the nature of these inclusions. In the 1970s, Belgian conservation scientist, L?opold Kockaert, was the first researcher to attempt to investigate inclusions in old-master paintings. Using microchemical staining tests, he concluded that they were pockets of protein that had become trapped within the paint. Traditionally, paints were prepared by grinding a pigment with various natural binders. These binders included drying oils, such as linseed and walnut, and egg tempera, which is a mixture of egg yolk and water. Kockaert concluded that the inclusions arose because the paintings were made using a binder that was an emulsion of egg (an aqueous medium) in oil (a non-aqueous medium).

Kockaert’s explanation is supported to some degree by experiments using gas chromatography (GC), a technique which has been used to analyse samples of paint since the 1960s. By examining a paint sample’s organic-acid content, the type of binder used in its manufacture (egg tempera or drying oil) can be determined. When they are fresh, drying oils contain high levels of polyunsaturated and saturated fatty acids. Over time, any polyunsaturated acids degrade, by cross-linking and oxidation reactions, to leave three distinct types of monomer: the C₁₈ and C₁₆ fatty acids, stearic acid and palmitic acid respectively; and the C₉ dicarboxylic acid, azelaic acid. Azelaic acid, not present in fresh oils, is the major decomposition product of the polyunsaturated acids found in drying oils. In sharp contrast to this, azelaic acid is not formed when egg tempera ages, since there are no polyunsaturated acids in fresh egg.

Paint samples taken from areas containing inclusions often show low levels of azelate (when studied by GC), which seems to support Kockaert’s conclusion that the use of an emulsion of egg and oil was responsible for the formation of the blemishes. However, somewhat confusingly, samples from elsewhere in the paint layer, or from other areas on the same painting have frequently been shown to contain only drying oil and no egg tempera at all.

Attempts to characterise inclusions have always been hampered by their small size (less than 0.1 mm across) and the very limited amount of material available from valuable and irreplaceable works of art. The complex pigment and binder mixtures used by artists, combined with the changes that occur within paint as it ages and deteriorates, also present difficulties. Then there is the additional problem that, over the centuries, a painting will have accumulated non-original materials, such as varnishes, glue layers or repaint. This all helps to confuse the analysis.

However, by the 1990s, with the availability of more sophisticated analytical techniques, it was possible to overcome some of these problems and a different explanation of the inclusion phenomenon became apparent. Scientists at the National Gallery, and elsewhere, used infrared microscopy to show that inclusions contain lead compounds. Specifically ’lead white’, a basic lead carbonate used as a pigment since antiquity, and lead carboxylates, known more commonly as ’lead fatty-acid soaps’, resulting from the reaction of lead-containing pigments with oils.

Armed with this knowledge, lead soaps of long-chain fatty and dicarboxylic acids were prepared and characterised in a collaboration between the National Gallery and the University of Aberdeen. It was through this shared interest in synthesis and structural characterisation that we (the authors) started to work together.

Microscopic examination of paint samples smaller than a pinhead reveals that inclusions tend to appear as white opalescent masses within the film. At the higher magnifications afforded by scanning electron microscopy (SEM), the inhomogeneous nature of the inclusions is clearly visible. Using an energy-dispersive X-ray analyser (EDX), lead was the only element detected in the inclusions. In back-scattered electron micrographs, more highly scattering, electron-rich regions appear lighter in colour. Analysis of paint samples using this technique shows that inclusions consist of lead-rich regions with a lamellar structure, surrounded by less-scattering regions with a lower lead content.

So, what of Kockaert’s earlier theories? Infrared microscopy shows that, in fact, no protein is present in the inclusions or in the paint surrounding them. This effectively rules out the use of an emulsion of egg tempera and oil and implies that the medium was always oil, despite the low azelate levels often detected by GC. The IR spectra also confirm that many inclusions contain lead white in addition to lead carboxylates or soaps. It seems most likely, then, that the inclusions are agglomerations of lead soaps, formed by the reaction of red lead or lead-tin yellow with the oil medium - what is known more commonly to chemists as a saponification process.

The IR microscope also allowed us to explore the distribution of components within individual inclusions. For example, in Lorenzo Costa’s A Concert, analysing the lead-tin yellow highlight on the woman’s brocade sleeve (see front cover) reveals relatively large inclusions. They have clearly defined opaque centres that are surrounded by translucent haloes. By comparing the intensities of infrared bands of lead carboxylates and carbonates it is possible to determine that the haloes are rich in lead soaps while the more opaque centres have high concentrations of lead carbonates. Thus the inclusions appear to be mainly regions of lead carboxylate (with a translucent appearance), while the opaque material seen in some inclusions is lead white, consistent with the variations in lead content noted in electron micrographs.

Other research groups have reported similar results to this, but the precise composition of the inclusions, and the reason for the unusual azelate levels detected, was still not fully explained. But the sharp eyes and skilful hands of colleagues at the National Gallery’s scientific department have revealed the first direct evidence for the nature of the inclusions.

After much patient study, a small sample of lead-tin yellow paint from the small tree in Meindert Hobbema’s The Avenue at Middelharnis provided a crucial clue. In this particular sample, the inclusions were not so opaque and they contained little lead white. This meant that we were finally able to use the IR microscope to obtain wonderfully clear spectra. These reveal that the sample (and others examined subsequently) contains only lead palmitate and stearate and no lead azelate.

Having identified what inclusions contain, we now have to tackle a more important question: why do they form in the first place? Chemists have long known that lead-containing pigments form lead soaps on reaction with fatty acids and oils (indeed it is this property that is believed to give such paints their desirable properties). If the pigment has been well dispersed in the binding medium, these soaps might be expected to be uniformly distributed throughout the film. What is so unusual about the paintings containing inclusions is that the saturated fatty-acid soaps appear to be aggregating together. Exactly what triggers this process, and the precise mechanism by which it occurs, is still unknown but some of the factors involved are becoming clearer and it is obvious that there must be a considerable driving force behind the process, since inclusions can grow so large that they distort surrounding layers.

Test reconstructions made at the National Gallery demonstrate that the formation of inclusions is a slow process. Formation takes place via a slow migration of material, followed by coagulation. It is quite possible that these processes are driven by the changes that occur as the film gets older. As drying oil ages the degree of cross-linking increases and, as oxygen becomes incorporated and degradation occurs, the film becomes more polar. As this happens, inclusions may form as a kind of phase separation, linked to the increasing incompatibility between the oil matrix and more mobile components such as free fatty acids and lead carboxylates. The absence of lead azelate in these agglomerations suggests that the mobility of the acid components is very important. Azelaic acid’s short hydrocarbon chain and multiple carboxylic acid groups means it is more polar than its monocarboxylate equivalents, and therefore much less mobile. The same can also be said for the corresponding lead salt. This migration of the more mobile carboxylic acids from elsewhere in the film is likely to account for the apparent lowered azelate levels that are often measured by GC: in a sample containing a high proportion of inclusions, measured levels of the more mobile palmitate and stearate compounds are enhanced, relative to the sluggish azelates. The high degree of intermolecular order that is likely to be present in the lead carboxylate material in the inclusions may also help to drive the separation of the soaps from the more amorphous oil matrix, and would also tend to disfavour the incorporation of lead azelate into these regions.

From studies undertaken in Amsterdam it also appears that other pigments (such as 19th century zinc-based pigments) may give rise to the phenomenon. Indeed zinc carboxylate inclusions have been observed in works by Vincent van Gogh, including Sunflowers at the National Gallery. Further, these studies suggest that the process may still be on-going, particularly in 19th century works.

Fortunately, it appears that the answer is ’no’ because, although the chemistry going on inside a paint film is very complicated, and inclusion growth is slow, it seems likely that it will also be self-limiting: we do not observe the blemishes getting progressively worse with time, ie there are no more inclusions in older pictures than in more recent ones.

An issue which commonly challenges the museum and art-gallery community is one of restoration. And indeed, with respect to the formation of inclusions in works of art we have to ask ourselves an important question: should attempts be made to arrest, prevent or reverse the process of inclusion formation if it is an unwanted deterioration process? Since the formation of inclusions is a natural part of the many changes that occur to a painting as it ages, intervention seems undesirable, particularly as the phenomenon is not fully understood and changes to the delicate chemical balance in the film could lead to far more damaging results. However, this does not mean that the study of inclusions in paintings is not important. It is vital that the phenomenon is recognised and monitored. The presence of inclusions in a paint layer may influence the conditions under which paintings are stored and exhibited and could affect decisions regarding other conservation or restoration work being carried out. Armed with this knowledge, conservators and scientists can ensure that these unique and treasured masterpieces are preserved for future generations.

Acknowledgements

Catherine Higgitt is a higher scientific officer at the National Gallery in London, UK. John Plater is a senior lecturer in chemistry at the University of Aberdeen.

Further Reading

• C Higgitt, M Spring and D Saunders, Pigment-medium Interactions in Oil Paint Films containing Red Lead or Lead-tin Yellow, National Gallery Technical Bulletin, Volume 24, 2003, National Gallery Company Ltd, 75
• M J Plater et al, Polyhedron, 2003, 22, 3171
• Details of other technical examinations carried out at the National Gallery can be found in the National Gallery Technical Bulletin, which is published each summer, and in the catalogues produced by the National Gallery to accompany exhibitions, such as the recent show Titian and Art in the Making: Underdrawings in Renaissance Paintings
• Artists’ Pigments: A Handbook of Their History and Characteristics, Volumes 1-3, National Gallery of Art, Washington, distributed by Oxford University Press