Paintings are made of molecules. If your reaction to this truism is “So what?” you share the mentality of most art historians and museum curators. If artists did not care about molecules, they ask in annoyance – and indeed before the 19th century no artist could even know they existed – then why should art historians?
Whether artists knew about them or not, however, there are excellent reasons for art historians to care about molecules. Old master paintings as we see them in museums owe a lot to the molecule. Virtually all the art we see betrays traces not only of gross disfigurement due to moisture, abrasion, vandalism and ill-advised corrective treatments, but also of more insidious molecular processes. Chemical reactions can alter the way a painting looks from the inside out; leaving no obvious indication that anything has changed at all.
Since 1995, the Netherlands Organization for Scientific and Scholarly Research (NWO) has been sponsoring the first massive, coordinated effort to identify and analyze these effects. The project is called MOLART: “Molecular aspects of ageing in painted works of art.” The coordinator of MOLART is Jaap Boon, who in his daily life is an analytical mass spectrometrist. He works at the FOM Institute for Atomic and Molecular Physics, FOM standing for Fundamenteel Onderzoek der Materie. (Fundamental Research of Matter. People who break out in a rash from acronyms are advised to stay away from this kind of work.) MOLART is a five-year project, which will be rounded off in the year 2000. Regular progress reports have been issued, which can be consulted on Internet at http://www.amolf.nl/departments/molart.
As an art historian, I cannot claim always or even often to understand exactly what MOLART is discovering. The (English-language) cumulative Progress Report 1995-1998 is full of sentences such as: “HPSEC of artificially aged tempera samples indicated that the formation of cross-linked lipid oligomers correlates with the oxygenation.” It does not particularly help to know that HPSEC stands for High Performance Size Exclusion Chromotography. But there are finds that even a layman can think he understands, and some of these are a revelation. The part that got to me concerned the mobility of fatty acids, which are present in the oils that were used in oil paint from the 15th century on. As Boon explained it to me, most fatty acid molecules bond with the metallic elements in pigments, forming a stable compound. However, some of them do not. These molecules are released and they migrate spontaneously through the painting. Those which move toward the front can leave the paint layer and go on into the varnish. This means that when varnish is removed, as is the general practice in restoring old paintings, a certain amount of original material is removed with it. (Varnish removal sounds more innocuous than it is. It has been estimated of French Impressionist paintings that in this treatment the paint and varnish layers lose 30 percent of their weight.
This brings us to the crux of the matter. The killer sentence in the progress report is this: “Many of the traditional conservation methods are now thought to have side effects which require study on the molecular level.” If even varnish removal has unacknowledged dangers, what to think of such interventions as the Pettenkofer process, patented in 1867, for “regenerating” discolored varnish to make it more transparent. This is done by exposing a painting in a closed space to the vapor of alcohol and a South American resin called Copaiba balsam. In paintings that were treated this way in the past, we now know, “the use of common solvents for cleaning and varnish removal often leads to a loss of original paint material.”
The section on this key issue contains a small contradiction. Page 94 of the report states that a trained eye can recognize the minute telltale signs of an old Pettenkofer treatment. However, on page 95 we read: “The author (Sibylle Schmitt) participated in the investigation of Rembrandt’s ‘The anatomy lesson of Dr. Tulp’ at Mauritshuis, Den Haag. In spite of the fact, that numerous treatments including the use of Copaiba balsam in the past have been documented, no actual remaining evidence could be found,” not even with the microscope or through chemical analysis, let alone the naked eye. This suggests that a higher level of risk may be involved in routine cleaning procedures than even MOLART has been able to detect.
The MOLART report leaves me in no doubt that the study of paintings at the molecular level is sensible and fruitful. It might even be a scientific gold mine. The doubt with which it does leave me is different. Research of this kind is always going to discover ever more and never fewer symptoms of ageing – and unwanted side effects of conservation practices. This is in fact the pattern of the history of art restoration at large. When you speak to an art restorer, you will always hear about new advances in knowledge that make present-day restoration so much safer than in the past. Their predecessors, they will say, however gifted and conscientious they were, simply knew too little about the effects of their procedures. With the increased pace of conservation studies, this generational dialectic has been accelerated. MOLART not only speeds this process up even more; it also enlarges its scope into vast new domains. Jaap Boon may be right when he says that MOLART enables “more and more decisions to be made on the basis of knowledge available beforehand.” But whether the speed-up in knowledge will lead to a slow-down in the aging process of paintings remains to be seen.
© Gary Schwartz 1999. Published in Dutch in Het Financieele Dagblad, 24 July 1999, p. 28. Published on the Schwartzlist 24 June 2021.