It is well-known to conservators and the public alike that external forces—air pollution and acid rain, physical damage, bacterial or fungal growth, exposure to light or moisture—can damage these masterpieces, but what about internal issues? Most of us have never stopped to think that a painting could be eating itself alive!
It’s a big jump from true science to science fiction, and paintings aren’t really alive; ergo, they cannot eat anything, let alone themselves. However, the metaphor is accurate. Parts of the oil binder and the metal pigment colorants in oil paints, such as zinc or lead white, can react to form a serious problem—metal carboxylates, or colloquially, soaps. These soaps transform the previously colored paint into something translucent. They “eat away” the original paint and wreak havoc by popping up either to the surface as protrusions, cracks, or hazy films, or between layers of paint causing separation or delamination. Ironically, metal soaps—especially aluminum stearate—are commonly added during the paint manufacturing process to produce specific properties (e.g., wettability, texture, or consistency) in the final product. Aluminum stearate, in particular, increases the paint’s wettability and produces a final consistency akin to soft butter. Once the presence of these soaps has been noticed in a painting, we are left with a few questions: 1) Are these soaps just a part of the original paint or are they the result of an internal chemical reaction? 2) How are these soaps distributed? 3) Are they dangerous to the paintings, and if so, how dangerous?
Gabrieli et al. have tried to answer these questions with micro-attenuated total reflection Fourier transform infrared spectroscopy (micro-ATR-FTIR). FTIR works on the principles of induced molecular vibrations. Since molecules are very small, it doesn’t take a whole lot to get them to move—a beam of infrared light will do the trick! Certain groups of molecules, called functional groups, move in different ways. These movements produce peaks with specific shapes and locations in a spectrum that can be used like molecular fingerprints to identify compounds.
Figure 1 shows the FT-IR spectra for three paint samples from Jackson Pollock’s Alchemy. Using micro-ATR–FT-IR, the authors were able to create 2-D “maps” showing the distribution of aluminum stearate (from the original paint) and zinc stearate (produced within the paint by chemical reaction) by integrating the characteristic peaks corresponding to these two compounds for all of the spectra taken across an surface area of paint. A visual map is produced by assigning a color scale to the integral values, in which blue is low/zero and red is high. Figures 2b and 2c show the resulting heat maps for zinc soaps and aluminum soaps, respectively, over the same spatial area. These images help to answer questions 1 and 2. Both zinc and aluminum soaps are present in the paint, and the zinc soaps seem to form around the aggregates of aluminum soaps.
Question 3 is harder to definitively answer. The zinc soap distributions for six different Alchemy samples are shown in Figure 3. There are two different sizes and arrangements of aggregations: 1) A1, A2, and B2 show smaller, more dispersed aggregates that are visually similar to the 30 day aged model samples made by the authors; 2) A3, B1, and B3 show fewer, but larger, soap aggregates that are attributed here to a more advanced stage of saponification. Logistically, the larger an aggregate grows, the more likely it is to cause damage to the surrounding area. Now that we know this process is happening and is at least partially fueled by the paints’ composition, we must find a way to prevent (or at least minimize) future damage.
All figures reproduced/adapted with permission from Gabrieli, F.; Rosi, F.; Vichi, A.; Cartechini, L.; Buemi, L. P.; Kazarian, S. G.; Miliani, C. Analytical Chemistry 2016, 89 (2), 1283–1289. Copyright American Chemical Society. Alchemy is copyright 2009 Pollock-Krasner Foundation / Artists Rights Society (ARS), New York.