Fine Pore Structure Characterization in Two Gessoes Using Focused Ion Bean Scanning
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Fine Pore Structure Characterization in Two Gessoes Using Focused Ion Bean Scanning Michael Doutre1, Ashley Freeman1, Brad Diak2, Alison Murray1, George Bevan1, and Laura Fuster-López3 1 Art Conservation Program, Queen’s University, Kingston, Ontario, K7L 3N6 2 Mechanical and Materials Engineering, Queen’s University, Kingston, Ontario, K7L 3N6 3 Instituto Universitario de Restauracion del Patrimonio, Universidad Politecnica de Valencia, Valencia, Spain, 46022 ABSTRACT The movement of fluids through a porous medium is a function of the material type and the size and morphology of the voids. In the conservation of painted artworks, the movement of materials (for example, cleaning solutions) is a major factor in how a work reacts to treatments and a large influence on how the work will change with time. Of particular importance in the conservation of painted surfaces is the preparatory layer. This is a highly active transport medium because it is generally highly porous and a comparatively large and uniform component of a painted surface. In this work, a gesso film of calcium carbonate and rabbit skin glue, typical of the preparatory layer of many painted works, and an acrylic-based gesso film were imaged by focused ion beam (FIB) scanning. The gessoes were milled and scanned with gallium ions serially, in sequential planes orthogonal to the plane of the film. This yielded quantifiable measurements of the fine internal structures at a resolution far higher than previously reported for this type of material. This enabled a greater understanding of the geometry of the internal surfaces, increasing the understanding of the mechanics of capillary flow and diffusive behavior in this extremely common and significant material. INTRODUCTION The structure of a painting generally comprises many different materials organized as layers on a support, including a size for the support layer, the preparatory layer, and the pictorial layer. Traditionally, the preparatory layer, also referred to as the gesso or ground layer, consists of a white pigment, such as chalk or gypsum, bound with an adhesive, such as hide glue. More recently, modern commercially available materials have been used as the adhesive to bind different pigments, such as an acrylic adhering titanium dioxide particles. While the chemical and physical characteristics of the preparatory layer were the central focus of prior research, more recent studies have examined the diffusive characteristics [1,2,3] of aqueous treatments and the internal morphology of the preparatory layer. The resulting information may help to determine how the gesso layer interacts with the paint layer; how the gesso layer may facilitate movement of mobile applied materials, such as paint and cleaning solutions; and how physical damage may occur and spread. Another application is that the gesso forms a reactive layer to environmental changes in temperature and relative humidity. Numerous techniques can visualize the internal morphology of artworks. Computed tomography (CT) is a non-destructive te
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