CHARACTERIZATION BY FTIR OF THE EFFECT OF LEAD WHITE ON SOME PROPERTIES OF PROTEINACEOUS BINDING MEDIA
SILVIA A. CENTENO, MARCELO I. GUZMAN, AKIKO YAMAZAKIKLEPS, & CARLOS O. DELLA VÉDOVA
FTIR measurements showed that lead white reduces hydrogen bonding around the peptide C=O and N-H groups by removing water molecules and induces a small change in the protein's secondary structure due to aggregation. The water structure around the peptide linkage temporarily recovers when the samples are conditioned in a high-humidity environment (90% RH), but the low-humidity structure is restored after five days of exposure to standard humidity conditions (45–50% RH). Therefore, humidification is not enough to restore the mechanical properties of the paint films containing lead white in the long term. Massicot was found to promote a similar change as lead white around the peptide C=O but had a less drastic effect on the N-H groups.
The characterization of this aspect of the degradation process suggests that the addition of carbohydrates to proteinaceous materials used in consolidation treatments of lead white flaking areas might be beneficial for the long-term stability of the damaged areas, though the chemical and color properties of the consolidant mixtures should be systematically evaluated first.
The authors would like to acknowledge Marjorie Shelley, conservator in charge, and Margaret Lawson, associate conservator, at the Sherman Fairchild Center for Works on Paper and Photograph Conservation (MMA) for their support and helpful discussions; the A. W. Mellon Foundation for the fellowship awarded to Marcelo I. Guzman; and Dr. Masanori Sato, conservation scientist in charge, Nara National Cultural Properties Research Institute, Japan, who initiated investigations into media problems of lead white paints in aqueous binders in collaboration with AkikoYamazaki-Kleps.