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1047-Y04-01

Delamination of Oil Paints on Acrylic Grounds Yonah Maor, and Alison Murray Art Conservation Program, Queen's University, Kingston, K7L 3N6, Canada ABSTRACT In a set of composite samples of oil or alkyd paints, over acrylic grounds, naturally aged for eight years, some of the samples delaminated. Samples were analyzed with X-ray fluorescence (XRF), inductively coupled plasma (ICP), Fourier transform infrared - attenuated total reflectance (FTIR-ATR) and atomic force microscopy (AFM), as well as other techniques not detailed in this paper. Results indicate the main cause of delamination is metal soaps in the oil paint and particularly zinc soaps. The ground is a minor consideration as well, rougher grounds providing better adhesion than smooth ones. INTRODUCTION Since the 1950s the traditional lead white in oil ground has mostly been abandoned in favor of titanium white in acrylic or oil-modified alkyd grounds. The acrylic grounds are used for both oil and acrylic paints. The bond between an acrylic ground and an oil paint is of a mechanical nature, rather than there being a chemical bond: the oil paint enters crevices in the rough acrylic ground and interlocks as it dries. Painting in oil over acrylic is usually considered a safe practice. In some cases, however, there has been severe delamination a few months or years after painting (see figure 1). This paper examines a set of composite samples, some of which are delaminating, in order to find what the delaminating samples have in common. Once theories were established, severely delaminated paintings were examined as well.

Delaminated area

Figure 1. Michael Skalka, Colarado Lagoon, Long Beach, CA, Oil and alkyd on canvas, 12”x16”, 2006. Photo taken immediately after painting. Detail of delamination with ground showing, taken Nov. 2007

EXPERIMENT Samples This research made use of samples prepared in 1999 by Marion Mecklenburg at the Smithsonian Institution. The samples were combinations of eight different acrylic grounds (table I) and 20 paints, on Mylar® (polyethylene terephthalate). The paints were either oils or alkyds, from several manufacturers, and included the following pigments: verdigris, titanium white, lead white (also called flake white), raw sienna, cobalt blue, ultramarine, light red oxide, yellow ochre, terre verte, Indian red and burnt sienna. This paper focuses on ten paints where there were samples on all the different grounds. None of the samples with the other ten paints delaminated. The samples were prepared identically and stored together so that variables related to technique or environment may be ruled out. Table I. Ground and paints in composite samples A B C D E F

Ground Aaron Brothers White Gesso W&N* Clear Gesso Base for Acrylics W&N Acrylic Gesso Primer Utrecht Artists’ Acrylic Gesso Utrecht Professional Acrylic Gesso Liquitex Acrylic Gesso

G H

Dick Blick Artists’ Acrylic Gesso Grumbacher Artists’ Acrylic Gesso

Paint 1 Custom manufactured Verdigris in oil 2 W&N, Titanium White in oil 3 Grumbacher, Flake White in oil