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
Lead white, 2PbCO3•Pb(OH)2, can be considered the most important of all white pigments from a historical point of view. It is mentioned in almost all lists of pigments from ancient times to the present (Gettens et al. 1993). It has been used in a variety of media, conferring them with different properties. When used ground in oil media, it is known to accelerate the drying of the oil, giving a product that forms a hard film and that is more resistant than most other oil-pigment mixtures. Changes in the chemical properties of linseed oil mixed with lead white have been characterized by Fourier transform infrared spectroscopy (FTIR) and attributed to the saponification of esters and the deprotonation of carboxylic acids to form lead carboxylates (Meilunas et al. 1990). Using gas chromatography–mass spectroscopy (GC-MS), van den Berg et al. (1999) reported that, as lead white can enhance the photochemical degradation of oil binders, small breakdown products like formic acid are formed and these products interfere with the metal carboxylates already present, leading to an increased extractability of organic fractions and the consequent release of the pigment and loss of mechanical stability of the paint film. Boon et al. (2002) further characterized the dissolution, metal soap formation, and remineralization processes that occur in lead-pigmented ground/intermediate oil paint layers using FTIR, scanning electron microscopy–energy dispersive x-ray spectroscopy (SEM-EDX), and secondary ion mass spectroscopy (SIMS).
Cracking, flaking, and separation of the paint film from the substrate have been reported by several conservators on areas containing lead white on parchment and other works of art (Drayman 1968-69; Bykova et al. 1976; Quandt 1992). Lawson and Yamazaki-Kleps (2002) attributed the problems ranging from friability to cracking, tenting, and cleavage observed in the manuscript “The Belles Heures of Jean of France, Duke of Berry” to the interaction of the lead white with the binding media. However, no systematic study on the effect of the pigment on the properties of proteinaceous or polysaccharide binders commonly used in medieval illuminated manuscripts has been carried out.
Lawson and Yamazaki-Kleps (2002) carried out accelerated aging tests in a fadeometer on samples of proteinaceous, gum, and cellulose derivative binders and their mixtures with lead white. Aging in a fadeometer was found to promote yellowing in all the protein–lead white mixtures. The mixtures containing Kremer gelatin presented the smallest color change of all the binders tested. In the particular case of the medieval manuscript TheBelles Heures of Jean of France, Duke of Berry, yellowing is not apparent in the passages containing lead white, probably because the folios have been kept protected from exposure to light. However, as mentioned above, selective cracking of these areas occurs when compared with paint passages containing other pigments in the same folio.
In this study, mixtures of lead white with different proteinaceous binders were analyzed by FTIR to determine the effect of the pigment on the hydration properties of the fresh binders. Glair, i.e., egg white, and animal glue were commonly mentioned in medieval treatises as binders (see, for example,Thompson 1956, 51–61; Merrifield 1999, 232, 316), so glair prepared by the whipping method, rabbit skin glue, bone glue, and gelatin were chosen for analysis. In addition, whipped glair, rabbit skin glue, and gelatin mixed with the yellow lead–containing pigment massicot, PbO, were studied by FTIR to compare their properties to those of the samples containing lead white.
The composition of gelatin, glair, and animal glue has been published by several authors (Mills and White 1999 and references therein). Gelatin is produced by partial hydrolysis of collagen in either an alkaline or acid medium, both giving a partially denaturated protein as a product. These treatments result in the formation of a high proportion of α-chains. Linked α-chains, such as β-chains (α-chain dimers), nonhydrolyzed or partially renaturated triple helices, and polypeptides resulting from the degradation of α-chains are also present in gelatins (Dupont 2002). Rabbit skin glue and bone glue are generally prepared by the digestion of the animal tissue in hot water, and their main component is also gelatin, whereas the main component of glair is ovalbumin, a glycoprotein (Mills and White 1999 and references therein).