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Analytical Chemistry applied to Art Studies

Chemical analysis applied to problems in art have focused in three major areas – 1) determination of forgeries (e.g., is the painting completely forged or merely touched up?), 2) degradation studies since knowing the process of degradation for a painting allows for suitable methods to be developed to prevent accelerated damage of the object and 3) conservation studies. This project developed at RPI focused on the latter – conservation studies. In particular, this project is focused on conservation of historical paintings in NY State.

Conservation studies are highly local. To truly conserve an object that has cultural as well as historical value requires an accurate, detailed knowledge of the material used in the creation of the object. This is a difficult challenge that requires both an understanding of the historical context of the object as well as general scientific knowledge to begin the conservation process. Furthermore, many of the objects that have to be conserved are of significant age (100 years old or greater). The significant object age can cause problems since oftentimes the historical records are either incomplete or missing. The effects of chemical aging (deterioration of paint and binder, etc.) combined with incomplete knowledge of storage conditions for the painting throughout its lifetime are common problems that painting conservators face on a daily basis. In addition to these environmental factors, pigment and binder creation were often individualistic preferences of the artist and are tremendously variable from one artist to the next. Painters often created their own pigments and rarely kept records. Moreover, the aging processes that occur during the painting lifetime are actually initiated from the time the first layeris applied. Paint drying can occur within a few hours and depends on numerous environmental factors – humidity, temperature, light intensity and the specific pigments or other additives used. Even after drying, chemical reactions continue throughout the lifetime of the painting. These chemical reactions cause discoloration, yellowing, fading, and darkening of the pigments.

Appreciating the heterogeneity of a painting’s physical structure is critical for approaching the chemical analysis of these samples [11]. A painting consists of many superimposed layers placed onto a support that is either a wooden panel or a canvas as shown in Figure 1 [12]. The first layer is what is known as the "ground" layer and is sometimes called the "binder" or the support layer. The binding layer can be made up of either carbohydrates or proteins. Then paint layers are applied and may consist of several layers for large areas coupled with detailed regions and highlights. The paint layer can contain inorganic as well as organic components. The heterogeneity provides locally distributed micro-and nanostructures throughout the paint layer. It is common to apply a varnish layer after pigment drying to provide gloss as well as saturate the paint colors. Thus, the end result is a multi-layered sample that becomes blended. Furthermore, painted objects are not homogenous and adjacent areas on the painting can be quite different from each other with respect to their paint layers.

Figure 1. Photo of a microscopic image of paint layers in a sample. The beige area represents the bottom most layer and most likely is the binder. The upper layers (blue/grey) are paint layers. Note the heterogeneity within each of these layers and how they blend at the intersection (denoted with * and quite dark). Reproduced with permission from Anal. Chem. 2003, 75, 716-722, figure 5, Inset A. Copyright 2003 American Chemical Society