Effects of Humidity on Gessoes for Easel Paintings
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Effects of Humidity on Gessoes for Easel Paintings Michael Doutre1, Alison Murray1, and Laura Fuster-López2 1
Art Conservation Program, Queen’s University, Kingston, Ontario, K7L 3N6 Instituto Universitario de Restauracion del Patrimonio, Universidad Politecnica de Valencia, Valencia, Spain, 46022 2
ABSTRACT Gessoes are widely used in easel painting as grounds or preparatory layers; in art conservation, gessoes are employed as infill materials to level a loss in the paint surface in preparation for inpainting. The goal of this investigation was to establish the relationship between the mechanical behavior of various gessoes when exposed to different relative humidities (25%, 50%, and 100%) and to compare modern commercial gesso products with a traditional gesso. The materials included two commercial artists’ acrylic gessoes (composed of largely titanium dioxide and aqueous dispersions of acrylic polymers), two commercial spackling compounds frequently used in the conservation of easel paintings, and a traditional gesso (calcium carbonate and rabbit skin glue). Uniaxial tensile testing was used to characterize the elastic modulus, strain at failure, and ultimate tensile strength (UTS) of the materials. By understanding the physical limits of these materials under different conditions, damage to artworks and the failure of conservation treatments containing these types of materials may be prevented or reduced. INTRODUCTION Various materials are used to fill losses in a painting and thereby maintain the artist’s original intent. The additional material does, however, make the entire painting system more complex. Any fill material should react to changes in the environment, for example changes in relative humidity (RH), in a manner that does not compromise the surrounding original material. A complete characterization of gessoes at different relative humidities is therefore important. Historically, gessoes were made of a white pigment, such as chalk or gypsum, and a binder, often rabbit skin glue. The 1950’s saw the emergence of the widespread application of commercial emulsified polymers and acrylic artists’ materials. Since then, interest has grown in the use of modern polymeric materials in art conservation, both as a response to the growing number of art works made using modern materials that require conservation and because modern materials are used in treatments. Compared with traditional materials, these new polymers have greater reversibility and their chemical and physical properties can be better controlled [1]. Often the properties of the traditional and modern materials differ widely. Identifying and understanding where and how the forces are distributed among the various layers is key for properly handling an artwork. Any materials applied during
conservation must not contribute additional stress. Fuster-López described the effect of various relative humidity situations on paintings [2]. For paintings executed on canvas supports, at standard RH levels, the primary forces are concentrated in the size lay
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