New Insights into the Cleaning of Paintings

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80 • smithsonian contributions to museum conservation The size layer affected the measured and observed darkening imparted by varnishing, and this was most pronounced in samples with chalk in glue binder. Grounds prepared with a cold size layer reduced absorption of the varnish into the canvas and concomitant darkening. Fluid sizing of the canvas provided some barrier to varnish absorption and darkening, whereas the unsized grounds darkened the most. A similar, though less pronounced, trend was observed and measured in samples in emulsion media. Samples containing bone black with chalk in a glue medium darkened more on varnishing than all the other samples in any medium, suggesting pigmented grounds such as those used by the Impressionists may be particularly vulnerable to darkening. Color Change on Varnish Removal FIGURE 1. Template for sample treated and untreated quadrants. This example is composed of unsized fine- weave linen, with a single priming of chalk in hide glue. DE and the percentage color change in samples after varnish removal from lined and unlined quadrants are illustrated in Figures 3 and 4. The measurable “recovery” of the unvarnished surface after varnish removal was measured by calculating the percentage change: [ ] [ ] ∆Eu – va – [ ∆Eu – v] × ∆Eu – v 100 where u is the untreated quadrant, v is the varnished quadrant, and a is the area after varnish removal. The larger the percentage change is, the more “successful” the varnish removal was in recovering the original surface. A larger percentage also reflects a significant difference in color between untreated and varnished grounds. Oil- bound samples changed the least on varnishing and lining, and thus, the small FIGURE 2. Selected DE values of samples after varnishing and varnish removal.
number 3 • 81 FIGURE 3. The difference in ∆E before and after varnish removal from ground samples. The dashed line at ∆E = 1 indicates the threshold of visible difference. FIGURE 4. Percentage change in ∆E following varnish removal. percentage changes represent a reversal of this shift. Although removing varnish from the lined samples was practically more straightforward, the darkening imparted by lining was greater than the recovery of the surface after varnish removal. Glue- bound samples showed the greatest changes (on varnishing and) after varnish removal. The DE values after varnish removal for all the other samples (with the exception of the lined samples in emulsion medium) were below 1DE and thus were not detectable by the eye. However, there were significant changes in these samples that are reflected in the calculated percentage change. The most successful varnish removal in terms of visible change was from glue- bound samples. The recovery of the surface was greatest in the lined samples, reflected in the percentage change. This follows practical observations that lining facilitated the removal of varnish in these samples, whereas it was more difficult to remove the varnish from the more absorbent unlined samples in glue medium. The greatest changes were measured between the untreated quadrants and the lined and varnished quadrants of the glue- bound samples. The average DE value of the untreated quadrant versus the lined and varnished quadrant was 14.11, reflecting the color change due to both lining and varnishing. The average DE value of the same quadrants following varnish removal was 10.06, demonstrating a visually significant reduction in darkening. Incomplete recovery of the unlined quadrant on varnish removal was evident from the comparison between the untreated quadrant and the varnished quadrant following varnish removal. Thus, it was not possible to recover the appearance of the unvarnished surface of the most absorbent samples, and varnishing of these samples was not reversible. Monitoring Varnish Removal The extent of varnish removal from oil- bound samples was most effectively monitored by change in gloss that was visible in LM and ordinary light. In UV light, samples bound in oil and emulsion media that did not contain lead white exhibited a characteristic yellow green fluorescence in the unvarnished quadrants. This cast was slightly reduced on varnishing, and when the varnish was removed using polar solvents, the fluorescent material was disturbed, and the cleaned areas appeared purple. This suggests that the solvent affected a skin of fluorescent surface material on these samples. This hypothesis was supported by the results of testing a different solvent mixture for varnish removal that did not affect the superficial fluorescent layer. Influence of Binding Medium on Varnish Removal The glue- bound samples required the most polar solvents and most repeated swab rolling to satisfactorily remove the varnish. Isopropanol and mixtures of iso- octane:isopropanol (3:1 or 2:1) were required, with an average of 25 swab rolls. The unlined quadrant required the same or a more polar solvent to remove the varnish compared with the lined quadrant of the same sample. In general, DE values decreased after varnish removal, whereas unsized samples bound in glue containing chalk, bone black, and barium sulfate measured an increase in DE between the lined and the lined plus varnished quadrants. This change reflects the combined effects of impregnation of the samples, the abrasion of the most raised areas of ground on peaks in the canvas weave, and varnish removal. Removal of varnish from samples in emulsion medium required a lower- polarity solvent mixture and less mechanical action than samples in glue medium. Mixtures of iso- octane:isopropanol (4:1 and 3:1) required 20 swab rolls, and the coarse texture of the samples on jute required 40 swab rolls to remove the varnish. Varnish removal from the unlined quadrants required the same or slightly more polar solvent mixtures than the lined quadrants of the same sample. Samples containing lead white in oil were
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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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Page 43 and 44: Extended Abstract—A New Documenta
Page 45: number 3 • 33 FIGURE 2. Recording
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Page 63 and 64: Oil Paints: The Chemistry of Drying
Page 65 and 66: number 3 • 53 FIGURE 3. Weight ch
Page 67 and 68: number 3 • 55 FIGURE 7. Formation
Page 69 and 70: number 3 • 57 FIGURE 11. Differen
Page 71 and 72: The Influence of Pigments and Ion M
Page 73 and 74: number 3 • 61 FIGURE 3. Mechanica
Page 75 and 76: number 3 • 63 FIGURE 7. Mechanica
Page 77 and 78: number 3 • 65 observation that co
Page 79: number 3 • 67 • Not all pigment
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 and 128: Extended Abstract—The Effect of C
Page 129: number 3 • 117 Table 1. Surface m
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
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number 3 • 131 FIGURE 7. (top) St
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number 3 • 133 FIGURE 10. Stress-
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Extended Abstract—A Preliminary S
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number 3 • 137 Acknowledgments An
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140 • smithsonian contributions t
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Cleaning of Acrylic Emulsion Paints
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number 3 • 149 advocates of publi
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number 3 • 151 2.5, 3.5, 4.5, 5.5
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number 3 • 153 ethoxylate units i
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number 3 • 155 FIGURE 5. Golden A
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number 3 • 157 alien und Methoden
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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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