Article: Eraser cleaning of gypsum plaster: Evaluating damage potential using reflectance transformation imaging Author(s): Kathryn Brugioni Source: Objects Specialty Group Postprints, Volume Twenty-Two, 2015 Pages: 205-224 Editors: Emily Hamilton and Kari Dodson, with Sarah Barack and Kate Moomaw, Program Chairs ISSN (print version) 2169-379X ISSN (online version) 2169-1290 © 2016 by The American Institute for Conservation of Historic & Artistic Works 1156 15th Street NW, Suite 320, Washington, DC 20005 (202) 452-9545 www.conservation-us.org Objects Specialty Group Postprints is published annually by the Objects Specialty Group (OSG) of the American Institute for Conservation of Historic & Artistic Works (AIC). It is a conference proceedings volume consisting of papers presented in the OSG sessions at AIC Annual Meetings. Under a licensing agreement, individual authors retain copyright to their work and extend publications rights to the American Institute for Conservation. This article is published in the Objects Specialty Group Postprints, Volume Twenty-Two, 2015. It has been edited for clarity and content. The article was peer-reviewed by content area specialists and was revised based on this anonymous review. Responsibility for the methods and materials described herein, however, rests solely with the author(s), whose article should not be considered an official statement of the OSG or the AIC. ERASER CLEANING OF GYPSUM PLASTER: EVALUATING DAMAGE POTENTIAL USING REFLECTANCE TRANSFORMATION IMAGING KATHRYN BRUGIONI Practically all methods for dry-cleaning gypsum plaster involve abrasion on some scale. Previous studies of abrasion-cleaning methods have successfully quantified damage potential by imaging test coupons in a scanning electron microscope; however, this instrumentation is not available to everyone. Furthermore, the photomicrographs used show a level of detail that will never be perceptible to the human eye. In the absence of scanning electron microscope analysis, a more accessible method for measuring surface abrasion should be assayed: reflectance transformation imaging. This study explores the extent to which reflectance transformation imaging can reveal these surface changes to gypsum plaster caused by multiple dry-cleaning materials. Reflectance transformation imaging captures were compared to scanning electron microscope photomicrographs, which allowed for a calibration of the data collected with reflectance transformation imaging. KEYWORDS: Gypsum plaster, Dry-cleaning, Eraser cleaning, Reflectance Transformation Imaging 1. INTRODUCTION Being moldable, carvable, and paintable, gypsum plaster has been used since the beginning of human history. For all of plaster’s versatility and stability, the surfaces of artifacts made thereof can become embedded with dust and grime. A porous material that is often of a uniform color, plaster is one of the most difficult materials to clean successfully without solubilizing the substrate, without creating tide lines, without driving soiling into its pores, and without abrading the surface. To avoid the problems associated with introducing liquids into a plaster-cleaning system, erasers are often employed to dry-surface-clean such objects. After a discussion of the material terminology and principles involved, this article will describe the various precedents in eraser cleaning and the application of this technique to plaster. The use of reflectance transformation imaging (RTI) to evaluate damage potential of such treatments will then be considered in comparison to less accessible methods, such as SEM. The research presented at the 2015 OSG Tips Session was undertaken in conjunction with a gypsum-plaster treatment, performed at the Conservation Center of the Institute of Fine Arts, New York University, for which an eraser cleaning was indicated. The treatment of this object was employed as proof of concept of the preceding research. 1.1 TERMINOLOGY Any cleaning method must be designed with the nature of the material in mind; however, there is often confusion over the definition of the word plaster. In certain contexts, its meaning depends on material function, whereas in others, its meaning depends on its chemistry. Many discipline-specific lexicons, such as the Getty Research Institute’s Art and Architecture Thesaurus Online (Plaster 2004; Stucco 2004) and the Museum of Fine Arts Boston’s CAMEO (Plaster 2013; Stucco 2013), tend to define plaster and stucco differently in relation to the more chemically specific terms,lime plaster and gypsum plaster. As such, conflations of such terms commonly occur. Although this study was borne of the study and treatment of a gypsum plaster object, the following procedures and findings could be applied to materials of comparable porosity and hardness. AIC Objects Specialty Group Postprints, Vol. 22, 2015 205 13_Brugioni.indd 205 24/01/2017 12:58 206 1.2 CHEMISTRY AND STRUCTURE OF GYPSUM PLASTER Gypsum plaster is mined as the mineral gypsum, calcium sulfate dihydrate (CaSO4 · 2H2O). After mining, it is ground, purified, and calcined. This process typically forms the “plaster” product, calcium sulfate hemihydrate (CaSO4 · ½ H2O), which when mixed with water reforms the dihydrate. The free water (the water in excess of the waters of hydration) evaporates after application or molding of the paste, leaving a hard, carvable, and paintable material, which is reusable if refired. The most common crystal forms are alpha and beta forms, the former being more acicular and prismatic, and the latter being tabular (Christensen, Jensen, and Nonat 2010). Different manufacturing processes create different polymorphs or different ratios thereof, and these crystal structures impart different properties on the final product (Singh and Middendorf 2007). Alpha-type crystals pack more tightly, forming a less porous, harder, heavier, and less-compressible material that is preferred for sculpture and for detailed mold making. Beta-type crystals form a lighter, more porous, less-durable plaster, favored for certain construction applications (Kogel et al. 2006). Various additives, including driers, retardants, accelerants, thickeners, plasticizers, and deflocculants, may be used to modify the microstructure of the hardened gypsum; reduce or increase its compressive strength; or modify curing time, density, compressibility, and porosity (Hummel et al. 2003; Singh and Middendorf 2007; Guan et al. 2010). More than one plaster type may be present on the same object in the form of repairs, revisions, or where different plaster properties are needed. Accordingly, cleaning results may vary between different objects or across one surface. 2. PRECEDENTS IN ERASER CLEANING No matter the formulation of the material used, pores form in cast plaster due to the imperfect packing of crystals and the gradual evaporation of free water from the substrate. This porosity allows soiling—in the form of grease and dust—to become easily ingrained in and below the surface of the plaster (fig. 1). Gypsum plaster is also slightly soluble in water (approximately 2.4 g/L at 208C), and aqueous treatment of any soiling carries the risk of driving surface dirt farther into the substrate (American Chemical Society 2006) (fig. 2). Because of its porosity and water sensitivity, plaster is often dry-cleaned with erasers to clear soiling without irreversibly driving it into the pores of the plaster. It is essential, however, that the correct eraser product be chosen, as most plasters are deceptively soft and can be scratched by a fingernail. Popular in the realm of paper conservation (Pearlstein et al. 1982), eraser cleaning has been adapted for the cleaning of stone by Williams and Lauffenburger (1995). As with any treatment protocol, it is important to first ensure the suitability and stability of the materials used for treatment and then to evaluate their damage potential both physically and chemically. Pearlstein et al. (1982) and Williams and Lauffenburger (1995) juxtaposed three primary types of erasers: those based on rubber (containing rubber, drying oils, sulfur, and abrasives), factice (containing vulcanized vegetable or animal oils cross-linked with sulfur bonds), and vinyl (usually containing polyvinyl chloride, phthalate plasticizer, and calcium carbonate) (AIC, Book and Paper Group 1992). These studies consider the chemical composition of the various eraser types, their degradation products, their working properties, their efficacy in treatment, and their mechanical damage potential. Although such products should be periodically reevaluated to account for any changes in formulation, the findings detailed in the preceding references provide a point of departure for future testing. In brief, these studies caution against the use of products based on factice and rubber, as they leave considerable amounts of residue, and this residue then degrades to sulfurous by-products on treated surfaces. These evaluations also establish the importance of screening erasers for the presence of abrasives, which are included to increase grating action, and colorants, which can leave surface streaks. These ingredients are not appropriate in the context of a conservation treatment. Brugioni AIC Objects Specialty Group Postprints, Vol. 22, 2015 13_Brugioni.indd 206 24/01/2017 12:58 207 Fig. 1. Soiling, in the form of grease and dust, can become embedded in the porous plaster surface, causing considerable disfigurement. (Courtesy of Badde 2009, 26; Library Company of Philadelphia 2014) Fig. 2. UV photograph