New Insights into the Cleaning of Paintings
SCMC-0003 SCMC-0003
66 • smithsonian contributions to museum conservation FIGURE 13. (left) Multivariate statistical results approximating the data set by matrix multiplication of pure component concentration images and (right) their spectral signatures. Using a spatial simplicity model, the top image represents PbCO 3 , and the bottom image corresponds to a new Pb association formed in the umber paint layer (Keenan, 2009). solution was the one that generates high image contrast in the X-Y spatial dimensions (Keenan, 2009). The pure spectral components derived from this analysis include a chemically complex Pb-bearing component rich in C and O as well as containing Mg-Al-Si-Ca and Fe (see Figure 13). The Pb compound extends from the Pb carbonate interface to some tenths of micrometers into the paint layer. A one-dimensional analysis of the spectral image data additionally supports the presence of Pb well into the paint layer. Integrated net counts (characteristic minus background radiation) for Pb derived from an area at the interface region for the Pb M a X-ray line are plotted orthogonal to the interface and distinctly show nonzero Pb net intensities within the burnt umber layer (Figure 12). DISCUSSION Because lead is not known to be present in burnt umber at any appreciable concentration, an explanation for the Pb-bearing phase within the paint after 2.5 years is best described by cation migration. During the drying process, the basic lead carbonate is apparently solubilized, hypothetically by minor amounts of hydrolyzing oil, at the interface and migrates via diffusion into the umber paint layer. The presence of clays in this layer may trap the migrating lead compound to form a spectrally distinct association, as evidenced by the presence of the Si-Al-Mg-Ca. Whether this is merely a physical adsorption, typical of clays, or a new compound that integrates all these elements plus the Pb-Fe-C-O is yet to be determined. Although the exact nature of this phase is currently not known, carbon comprises ~39% (by weight) of its composition, suggesting an oil or fatty acid lead salt plus the ferruginous clay mixture. The possibility has been considered that the Pb signal detected is an artifact caused by secondary characteristic fluorescence of Pb in Pb carbonate by characteristic Ca and Fe radiation in the burnt umber. However, considering the low concentration of Ca in the paint and the large offset in energies between the Fe K line and Pb M X-ray lines, these possibilities appear remote. Secondary fluorescence of Pb by continuum radiation in the paint layer is perhaps more likely. However, the distribution of the Pb-bearing phase is not concentrated at the interface and tails- off in intensity as a function of distance, as one might expect if this phenomenon were responsible for producing the Pb spectral signature observed. Moreover, the 1-D profile of Pb net intensities do not decrease sharply as a function of distance from the Pb carbonate interface, further suggesting that evidence for cation migration during the drying process has been established. CONCLUSIONS The results obtained from this long term study can be summarized as follows: • The early drying of oil paints is due to autoxidation, but it does not necessarily result in a tough, durable film. • The later film-forming processes are largely the result of metal ions from pigments, added driers, or even contamination from other materials inducing cross-linking and polymerization.
number 3 • 67 • Not all pigments contribute to the development of good paint films, and some, such as zinc oxide, can actually have detrimental effects, causing brittleness and even delaminating the paint layers. • Pigments such as the earth colors, organic dyes, and blacks fail to induce a durable film, and this is a result of hydrolysis of the paints. • Without pigments supplying “active” metal ions, film formation in drying oils is poor. • Different pigments have dramatically different effects on film formation and the durability of both oil and alkyd paints. • In the competing processes of polymerization, cross-linking, and hydrolysis, hydrolysis can be dominant in paints made with many pigments, including the earth colors. • Mixing paints made with active pigments can increase the durability of paints with less active pigments. • One layer of paint in a painting can affect other adjacent layers both positively and adversely. The paint chosen for the ground layer can affect the long-term stability of the painting. This study has confirmed that ion migration occurs and has shown that this migration may result in the formation of new associations for the case of a burnt umber paint applied over a lead white paint. Research into the ion migration process is just beginning, and far more research is required to elucidate it. REFERENCES Bennett, E. F. 1941. A Review of Driers and Drying. Brooklyn, N.Y.: Chemical Publishing Co. Keenan, M. R. 2009. Exploiting Spatial-Domain Simplicity in Spectral Image Analysis. Surface and Interface Analysis, 41:79–87. Marling, P. E. 1927. Accelerators and Inhibitors of Linseed Oil Drying. Canadian Chemistry and Metallurgy, 11:63–66. Morgan, H. H. 1951. Cobalt and Other Metals as Driers in the Paint and Allied Industries. Paint Manufacture, 21:239–248, 259. Olson, J. 1921. Cobalt and Its Compounds. Monthly Digest National Varnish Manufacturers’ Association, 17:2–12. Stewart, W. J. 1967. “Driers.” In Treatise on Coatings. Volume 1: Film-Forming Compositions, ed. R. R. Myers and J. S. Long, part 1, pp. 211–223. New York: Marcel Dekker. Tumosa, C. S., and M. F. Mecklenburg. 2005. The Influence of Lead Ions on the Drying of Oils. Reviews in Conservation, 6:39–47. van den Berg, J. D. J. 2002. Analytical Chemical Studies on Traditional Linseed Oil Paints. Ph.D. thesis, University of Amsterdam, Amsterdam.
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number 3 • 67<br />
• Not all pigments contribute to <strong>the</strong> development <strong>of</strong> good<br />
paint films, and some, such as zinc oxide, can actually have<br />
detrimental effects, causing brittleness and even delaminating<br />
<strong>the</strong> paint layers.<br />
• Pigments such as <strong>the</strong> earth colors, organic dyes, and blacks<br />
fail to induce a durable film, and this is a result <strong>of</strong> hydrolysis<br />
<strong>of</strong> <strong>the</strong> paints.<br />
• Without pigments supplying “active” metal ions, film formation<br />
in drying oils is poor.<br />
• Different pigments have dramatically different effects on film<br />
formation and <strong>the</strong> durability <strong>of</strong> both oil and alkyd paints.<br />
• In <strong>the</strong> competing processes <strong>of</strong> polymerization, cross-linking,<br />
and hydrolysis, hydrolysis can be dominant in paints made<br />
with many pigments, including <strong>the</strong> earth colors.<br />
• Mixing paints made with active pigments can increase <strong>the</strong><br />
durability <strong>of</strong> paints with less active pigments.<br />
• One layer <strong>of</strong> paint in a painting can affect o<strong>the</strong>r adjacent layers<br />
both positively and adversely. The paint chosen for <strong>the</strong><br />
ground layer can affect <strong>the</strong> long-term stability <strong>of</strong> <strong>the</strong> painting.<br />
This study has confirmed that ion migration occurs and has<br />
shown that this migration may result in <strong>the</strong> formation <strong>of</strong> new<br />
associations for <strong>the</strong> case <strong>of</strong> a burnt umber paint applied over a<br />
lead white paint. Research <strong>into</strong> <strong>the</strong> ion migration process is just<br />
beginning, and far more research is required to elucidate it.<br />
REFERENCES<br />
Bennett, E. F. 1941. A Review <strong>of</strong> Driers and Drying. Brooklyn, N.Y.: Chemical<br />
Publishing Co.<br />
Keenan, M. R. 2009. Exploiting Spatial-Domain Simplicity in Spectral Image Analysis.<br />
Surface and Interface Analysis, 41:79–87.<br />
Marling, P. E. 1927. Accelerators and Inhibitors <strong>of</strong> Linseed Oil Drying. Canadian<br />
Chemistry and Metallurgy, 11:63–66.<br />
Morgan, H. H. 1951. Cobalt and O<strong>the</strong>r Metals as Driers in <strong>the</strong> Paint and Allied<br />
Industries. Paint Manufacture, 21:239–248, 259.<br />
Olson, J. 1921. Cobalt and Its Compounds. Monthly Digest National Varnish Manufacturers’<br />
Association, 17:2–12.<br />
Stewart, W. J. 1967. “Driers.” In Treatise on Coatings. Volume 1: Film-Forming<br />
Compositions, ed. R. R. Myers and J. S. Long, part 1, pp. 211–223. <strong>New</strong><br />
York: Marcel Dekker.<br />
Tumosa, C. S., and M. F. Mecklenburg. 2005. The Influence <strong>of</strong> Lead Ions on <strong>the</strong><br />
Drying <strong>of</strong> Oils. Reviews in Conservation, 6:39–47.<br />
van den Berg, J. D. J. 2002. Analytical Chemical Studies on Traditional Linseed Oil<br />
Paints. Ph.D. <strong>the</strong>sis, University <strong>of</strong> Amsterdam, Amsterdam.
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