New Insights into the Cleaning of Paintings

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90 • smithsonian contributions to museum conservation intermixable with traditional oil paints, and intermediate drying rates can be found by their mixing, so mixed- media paintings can be created without problems (Staples, 1982a). Also, alkyd paints remain soluble in turpentine and white spirits for the duration of a typically working session (6–8 hours); therefore, the initial stiffening can be reversed, and an artist can control the semidry state for long periods of time and make simple modifications. Diluting these paints in solvents can allow an artist to achieve a watercolor painting effect without significant bleeding of the colors into each other (Staples, 1982a). The type of alkyd resin used in artists’ alkyd paints is a longoil alkyd, which is an oil- modified polyester polymer that contains between 56–70 weight percent (wt %) fatty acids. The oil length is the weight percent of fatty acids in the finished resin. Alkyd resins, based on specific applications, are grouped between short oil (35–45 wt %), medium oil (46–55 wt %), long oil (56–70 wt %), and very long oil (above 70 wt %). Changing the oil length and oil type has a great impact on the properties and ultimate applications of the resins; however, many of them are out of the scope for this work, and therefore, only longoil alkyds will be discussed. The rapid touch drying of artists’ alkyds is due to solvent evaporation (physical drying) and the high molecular weight of the resin, which requires fewer cross- links to form a coherent film. The unsaturation of the fatty acids is, for the most part, unaffected during the manufacturing of the resin, so the chemical drying properties of the alkyd paints are similar to those of traditional oil paints; the crosslinking and film formation takes place by autoxidation, aided by catalysts, such as Co, Zr, and Ca. However, the higher molecular weight of alkyd resins has a disadvantage since alkyds become harder and therefore more brittle at an earlier stage of their aging process than oil colors do (Lack, 1988). Also, longoil alkyd resins tend to yellow upon aging, especially with linseed oil- based resins that have a high unsaturated fatty acid content. To overcome this, semidrying oils, such as soya, safflower, etc., can be used to manufacture alkyds. The small decrease in drying efficiency is countered by the fact that semidrying oils have a lower tendency to discolor during aging. Figure 1 summarizes alkyd properties obtained with different oils. As mentioned above, longoil alkyd film formation occurs by catalyzed autoxidation. During this process, hydroperoxides are formed and degrade into radical species that participate in crosslinking by recombination reactions resulting in ether, peroxy, and carbon- carbon cross- links (Muizebelt et al., 1994; Mallégol et al., 2000). If crosslinking proceeds to an excessive degree, b- scission can occur, forming lower molecular weight degradation products as well as a number of other oxygenated degradation species, such as carboxylic acids, aldehydes, and alcohols. The oxygenated species can change forces within the film and, to a small degree, the film surface interactions with different solvents (Ploeger et al., 2009a). The lower molecular weight species can be lost through evaporation and extraction, contributing FIGURE 1. Summary of alkyd properties obtained by varying the oil type and oil length of the resin. The shaded top right section represents the area of artists’ alkyd resins. Alkyds in the left half are generally for spray and industrial use with lacquer- type drying and curing. Alkyds in the right half are more for brushing applications and have been modified for flowability and better stability in containers. Modified after Patton (1962).
number 3 • 91 FIGURE 2. Film formation and degradation processes. Modified after van Gorkum and Bouwman (2005). FIGURE 3. Conservation problems encountered with alkyd paints and resins. From left to right: yellowing with age, brittleness (cracking), blooming effect, and wrinkling during drying. to an increased brittleness of the film since these species could otherwise act as plasticizing agents. Figure 2 describes the general film formation and degradation processes. Conservation problems encountered with alkyd paints are few but significant in their impact on the stability and appearance of the work of art (Figure 3). The main concerns are (1) pigmentdependent wrinkling and aging of the films, (2) relative humidity and temperature- dependent migration of fatty acids, and possibly other additives, resulting in a “blooming” effect, (3) significant stiffening and cracking of the films with age, and (4) rapid yellowing and darkening. This paper addresses some of the characterization and stability issues encountered with artists’ alkyd paints, which can be comparable to industrial unmodified alkyd formulations. Some practical conservation issues and solutions will be discussed, and basic storage guidelines will be proposed. The results from the artists’ alkyd paints are based on studies of Winsor & Newton Artists’ Alkyd Colours (discontinued), London Alkyd Colours (discontinued), Griffin Alkyds (currently available) and reference materials, Ferrario Alkyd (Colore Alchidico), Da Vinci Paint Co. Leonardo Oil with Alkyd (formulation from 2006), Paints Dry Quick (PDQ) Quick Dry Oil (discontinued), and various alkyd media. DISCUSSION Characterization In order to identify the presence of an alkyd resin, a number of analytical techniques can be used. One of the most common is
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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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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: Characterization and Stability Issu
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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
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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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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
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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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