New Insights into the Cleaning of Paintings

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116 • smithsonian contributions to museum conservation FIGURE 1. Wang Youshen (China, born 1964), Yu Gong and His Later Generations (Yu Gong he tade zizi sunsun), 1986. Oil on masonite, 94 × 79 cm. Courtesy of the Queensland Art Gallery; gift of Nicholas Jose and Claire Roberts through the Queensland Art Gallery Foundation 2008. (left) Photograph of the painting by Natasha Harth and (right) photograph of the ultraviolet fluorescence image by Gillian Osmond. CLEANING On the basis of information presented by Richard Wolbers during a cleaning workshop held at Queensland Art Gallery, Brisbane, in July 2008, surface conductivity and pH were measured across colors to inform design of a cleaning system. Wolbers recommends cleaning oil films with a solution in a pH range of 5.5–8.5 (one to two units above the surface measurement) and conductivity 10–20 times the surface readings. These parameters are to optimize soil removal without disrupting the paint surface. A solution with a suitable conductivity also aims to leave the paint surface electrically neutral and less likely to attract dust. Measurements of the painting were taken before cleaning (Table 1). Both paint and dirt solubility were tested with water adjusted to pH 7.5 using triethanolamine, diluted to 1,000 µS/cm conductivity. The increased conductivity appeared to reduce the water sensitivity of the paint but required rinsing, which was not appropriate for the fragile surface. Wolbers was consulted about ways to modify this cleaning system so it would not require rinsing. Phosphate was selected as a suitable buffer and ionic species. Sodium was recommended as the counter ion as it has a single positive charge and produces an inert complex with the cleaned oil surface (R. Wolbers, personal communication, 3 September 2008). A solution of 0.05% dibasic sodium phosphate in deionized water was prepared. The pH was adjusted to 7.5 with hydrochloric acid. The solution was diluted to a conductivity of 1000 µS/cm and tested with good results. Areas previously considered too sensitive to clean with deionized water were able to be surface cleaned with no visible solubility issues. After cleaning, surface measurements showed a fairly consistent pH in the range 6–7, with conductivity readings indicating a reservoir of surface ions as planned (Table 1). In the future, monosodium phosphate will be used in preference to the dibasic form enabling pH to be adjusted with sodium hydroxide, thereby avoiding the presence of chloride ions. CONCLUSIONS The response of the painting to cleaning implies that its water sensitivity is at least partially related to ionic disruption of the paint. Deionized water acts as a hypotonic solution,
number 3 • 117 Table 1. Surface measurements carried out before and after cleaning. The areas measured are identified by paint color and location, i.e., BR = bottom right, CR= center right. Yellow paint was physically fragile and therefore was undercleaned, reflected in postcleaning readings; postcleaning measurements were avoided altogether in the second area of yellow paint. Measurement Before cleaning Red (BR) Red (CR) Yellow (BR) After cleaning Before cleaning After cleaning Before cleaning After cleaning Yellow (CR) before cleaning Green (BR) Before cleaning After cleaning Conductivity (µS/cm) 52 42 127 154 59 107 23 200 167 pH 7.1 6.4 5.5 6.2 6.6 5.7 5.3 6.4 6.6 extracting ions from the paint film to reach equilibrium, effectively solubilizing vulnerable paint. It is unclear if there is any connection to ongoing research by others investigating water sensitivity of modern oil paints attributable to phenomena such as hydrolysis of additives in the film (Mills et al., 2008). Prior to this treatment, paintings were frequently cleaned with deionized water alone to avoid a rinsing stage. This case study demonstrates that customizing deionized water is a relatively simple way to reduce some potential impacts of surface cleaning, and this approach has now become routine practice. John Drennan, Centre for Microscopy and Microanalysis, University of Queensland. REFERENCE Mills, L., A. Burnstock, F. Duarte, S. de Groot, L. Megens, M. Bisschoff, H. van Keulen, and K. J. van den Berg. 2008. “Water Sensitivity of Modern Artists’ Oil Paints.” In Preprints ICOM-CC 15th Triennial Conference New Delhi, pp. 651–659. New Delhi: Allied Publishers. Acknowledgments The authors thank Richard Wolbers, University of Delaware, Bronwyn Ormsby, TATE, for the FTIR analysis undertaken, and
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Smithsonian Institution Scholarly P
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smithsonian contributions to museum
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Contents PREFACE ACKNOWLEDGMENTS
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number 3 • v Extended Abstract—
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viii • smithsonian contributions
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The Removal of Patina Stephen Gritt
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number 3 • 3 and conceals its bea
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number 3 • 5 REFERENCES Algarotti
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8 • smithsonian contributions to
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Surface Cleaning of Paintings and P
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number 3 • 19 methods for cleanin
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number 3 • 21 not the varnish its
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Extended Abstract—A Preliminary I
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number 3 • 25 FIGURE 3. Detail of
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Extended Abstract—A New Documenta
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number 3 • 33 FIGURE 2. Recording
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Oil Paints: The Chemistry of Drying
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number 3 • 53 FIGURE 3. Weight ch
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number 3 • 55 FIGURE 7. Formation
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number 3 • 57 FIGURE 11. Differen
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The Influence of Pigments and Ion M
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number 3 • 61 FIGURE 3. Mechanica
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number 3 • 63 FIGURE 7. Mechanica
Page 77 and 78: number 3 • 65 observation that co
Page 79: number 3 • 67 • Not all pigment
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Page 101 and 102: Characterization and Stability Issu
Page 103 and 104: number 3 • 91 FIGURE 2. Film form
Page 105 and 106: number 3 • 93 (Saunders, 1973). T
Page 107: number 3 • 95 allows identificati
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Page 119 and 120: Sensitivity of Oil Paint Surfaces t
Page 121 and 122: number 3 • 109 Analytical Methods
Page 123 and 124: number 3 • 111 The method for ass
Page 125 and 126: number 3 • 113 FIGURE 6. Secondar
Page 127: Extended Abstract—The Effect of C
Page 137 and 138: Multitechnique Approach to Evaluate
Page 139 and 140: number 3 • 127 RESULTS AND DISCUS
Page 141 and 142: number 3 • 129 FIGURE 4. The SEM
Page 143 and 144: number 3 • 131 FIGURE 7. (top) St
Page 145 and 146: number 3 • 133 FIGURE 10. Stress-
Page 147 and 148: Extended Abstract—A Preliminary S
Page 149: number 3 • 137 Acknowledgments An
Page 159 and 160: Cleaning of Acrylic Emulsion Paints
Page 161 and 162: number 3 • 149 advocates of publi
Page 163 and 164: number 3 • 151 2.5, 3.5, 4.5, 5.5
Page 165 and 166: number 3 • 153 ethoxylate units i
Page 167 and 168: number 3 • 155 FIGURE 5. Golden A
Page 169: number 3 • 157 alien und Methoden
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166 • smithsonian contributions t
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Extended Abstract—The Removal of
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number 3 • 171 RESULTS AND DISCUS
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number 3 • 173 FIGURE 3. The SEM
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Extended Abstract—Cleaning Issues
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number 3 • 177 Acknowledgments Th
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Effects of Commercial Soaps on Unva
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number 3 • 187 The aqueous and or
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number 3 • 189 all tests. The res
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number 3 • 191 palmitic and stear
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FIGURE 5. Scanning electron microsc
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number 3 • 195 FIGURE 6. Confocal
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Extended Abstract—Tissue Gel Comp
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number 3 • 199 • The capillary
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Extended Abstract—Use of Agar Cyc
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number 3 • 207 FIGURE 4. The FTIR
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Extended Abstract—The Schlürfer:
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number 3 • 223 FIGURE 3. Removal
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Extended Abstract—Surface Cleanin
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number 3 • 227 Table 3. Cleaning
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Extended Abstract—Laser Cleaning
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Ending Note: Summary of Panel Discu
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Index TABLES in BOLD; Figures in It
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number 3 • 243 polyvinyl acetate
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