Old Master Paintings: Ε. René de la Rie Science Department National Gallery of Art Washington, DC 20565
Varnishes are the most vulnerable parts of old master paintings. Until the mid-nineteenth century most oil and tempera paintings received this final transparent coating, which greatly influenced a picture's appearance and radically affected color saturation and gloss. As the techniques and materials employed by artists began to change in the late nineteenth century, the use of picture varnishes declined, although they are still used today. Several sources describe early varnish recipes (1-9). Oil varnishes, which had been described by Theophilus in the eleventh century (3), were prepared by boiling natural resins such as sandarac, gum mastic, or rosin with drying oils such as linseed or walnut oil (3-7). The "vernice liquida" frequently mentioned by Cennini in his // Libro del'Arte (8), which appeared around 1390, was a solution of sandarac in linseed oil (4, 7). White lead (basic lead carbonate) or litharge (lead monoxide) were often added to shorten the drying time. Although oil varnishes were most common, egg whites either alone or combined with honey, gum arabic, or other gums were also used (3, 5, 8). Oil varnishes were gradually replaced by "spirit varnishes" or "essential oil varnishes," which were solutions of natural resins in a volatile solvent, usually oil of turpentine. These were first mentioned in sixteenth-century Italy and were widely used throughout Europe in the seventeenth century (4). The most common resins used for spirit varnishes were still mastic, sandarac, and rosin (4), although Venice turpentine was also mentioned (6). Today, the most popular natural resin varnishes consist of dammar resin (first described by Lucanus in 1829) (10) or gum mastic dissolved in oil of turpentine.
"Restorer Jan van Dijk" by Jan ten Compe. This 1754 painting depicts a restorer with a partially cleaned painting. Restorations Environmental effects cause traditional picture varnishes to break down rapidly. Autoxidation reactions cause the thin layers of organic material to haze, crack, and yellow (1). Because the degraded varnishes obscure the images beneath them, they are removed and replaced regularly during restorations. This "cleaning" is generally accomplished using organic solvents. Al-
1228 A · ANALYTICAL CHEMISTRY, VOL. 61, NO. 21, NOVEMBER 1, 1989
though unaged spirit varnishes are soluble in oil of turpentine, more polar solvents (i.e., alcohol or acetone mixtures) are needed to remove the oxidized layers (1). Although the paint layers are often less soluble than the varnish layers, solvent action may cause swelling and leaching (11). Some paintings cannot be treated with solvents at all. Varnish deterioration— perhaps the most important problem in painting conservation—is the reason 0003-2700/89/A361-1228/$01.50/0 © 1989 American Chemical Society
ANALYTICAL APPROACH
AStudy of the Varnish Problem
Partially cleaned painting, "View on the Cannareggio, Venice," by Francesco Guardi. most restorations are necessary. At the Metropolitan Museum of Art in New York, various analytical techniques have been used to study factors that affect the optical properties of varnishes and their degradation. In addition, ways to inhibit the autoxidative degradation of natural resin varnishes using stabilizers have been investigated. New synthetic varnishes are currently being tested at the National Gallery of Art (Washington, DC) and at the Metropolitan Museum of Art in collaboration with other museums. Synthetic varnishes There has long been an interest in developing more stable coatings that remain transparent and soluble in solvents of low polarity {!). In the 1930s, it
was suggested that poly(vinyl acetate) (PVA) might be a stable coating material for old master paintings (12). Several acrylate polymers—including poly(n-butyl methacrylate) and poly(isobutyl methacrylate), which are soluble in mineral spirits, as well as Acryloid B72 (an ethyl methacrylate copolymer manufactured by Rohm & Haas)—were also introduced during the following decades and used extensively for varnishes (1). Some of these coatings are not as stable as they first appeared. For example, the poly(butyl methacrylate) coatings tend to undergo cross-linking reactions (which eventually may render them insoluble); and PVAs have relatively low glass transition temperatures, which cause them to collect dust and dirt. However, Acry-
loid B72 is extremely stable and would appear to be almost ideal. Return to natural resins Nevertheless, many painting conservators are using natural resin varnishes again, because the synthetic coatings produce an appearance different from that achieved with traditional varnishes. This subject has received relatively little attention in the past (1, 13-15), and research efforts have focused primarily on the degradative and mechanical properties of varnishes. This may in part be because of the misconception that the function of a varnish is primarily a protective one. Although varnish may protect the underlying paint to some extent, its function is primarily an aesthetic one. In the sixteenth cen-
ANALYTICAL CHEMISTRY, VOL. 61, NO. 21, NOVEMBER 1, 1989 · 1229 A
ANALYTICAL APPROACH tury, Armenini described the effect of varnishes: [They] "enliven and draw out the colors and preserve their beauty for a very long time. Varnish also has the power to bring out all the minute details in a work, making them appear very clear" (2). Varnishes have a threefold effect on the appearance of paintings: They cause the colors to be glossier, darker, and more saturated. The gloss of a surface is essentially a function of its roughness (16-21). Smooth surfaces are glossy because of a high degree of specular reflection; the amount of light reflected at the surface is small compared with that penetrating the paint layer. Microscopic and larger surface imperfections cause a reduction of gloss as a result of diffuse reflection (scattering) of light at the surface; the increased surface area causes the amount of reflected light to be relatively large. Paint surfaces in general show a combination of specular and diffuse reflections. The scattered light from a paint surface is generally white light (or at least the same color as that of the light source) because it does not penetrate the colored layer; light that penetrates the paint layer is returned as colored light. Large amounts of scattered white light cause a desaturation of the color (i.e., the color becomes paler). Therefore, smooth paint surfaces will have not only higher gloss but also more saturated colors (i.e., purer color containing less white) than microscopically rough paint surfaces. The desaturation caused by a rough surface is most noticeable in dark areas, because the amount of scattered light is proportionally high. Application of varnish
over a microscopically rough paint surface produces a smoother surface and, therefore, higher gloss and more saturated colors. The colors will also appear darker because all pigment particles are embedded in either medium or varnish, which have a higher refractive index (RI) than air. A good varnish will make subtle color differences visible (especially in dark passages) by eliminating scattered white surface light. Importance of MW and refractive index Ketone resins are oligomeric materials formed by the condensation of methyl cyclohexanones or cyclohexanone-containing structures such as:
0
0
-àîÔ η
Sold previously as AW2, MS2, and Ke tone Resin Ν and currently as Laropal K80 (manufactured by BASF), they have been widely used since the 1950s in picture varnishes (22, 23). Although they are not ideal, they have better op tical properties than other synthetic resins. They do not yellow as much as natural resin varnishes but are other wise not highly stable; they autoxidize rapidly, resulting in changes in solubil ity, cracking, and hazing. The keto groups make the resin molecules sus ceptible to photochemical scission re-
Table 1. Average molecular weights of resins used in picture varnishes3 Resin
At,
M„
M„/At,
Rosin Venice turpentine Gum mastic Dammar resin Laropal K806 Acryloid B67c Acryloid B72" 27He AYAA' AYAFf
313 344 460 488 442 10,960 11,397 38,666 31,691 51,370
534 508 1,929 1,361 680 44,764 65,128 86,445 88,567 117,697
1.71 1.47 4.20 2.79 1.54 4.08 5.72 2.24 2.80 2.29
" Determined by GPC; M, = number-average molecular weight; M,=: weight-average molecular6 weight; AVM, = polydispersity. Ketone resin (BASF). 0 Poly(isobutyl methacrylate) (Rohm & Haas). " Ethyl methacrylate copolymer (Rohm & Haas). • Poly(isoamyl methacrylate), experimental varnish. ' Polyvinyl acetate) (Union Carbide).
1230 A · ANALYTICAL CHEMISTRY, VOL. 61, NO. 21, NOVEMBER 1, 1989
actions, and the resins are also rather brittle. Reduction of the keto groups to hydroxyl functionalities greatly im proves the stability of the resins, al though the brittleness is increased. Re duction of the brittleness can be ob tained by esterifying some of the hydroxyl groups (22, 23). Ketone resins are similar to natural resins in that they both have low mo lecular weights (MW) and high RIs when compared with acrylates (see Ta bles I and II). The large difference in molecular weight results in a corre spondingly large difference in solution viscosity: Solutions of synthetic poly mers have higher viscosities than those (in the same solvent and at the same concentration) of natural or ketone res ins. Solutions of natural and ketone resins have viscosities low enough for brush application at concentrations as high as 40-50% by weight, whereas syn thetic polymers are typically applied at concentrations of 10-20% by weight. Polymeric coatings produce rougher surfaces than low MW coatings over microscopically rough substrates. Vis cous coatings reproduce the roughness of the substrate beneath it to a larger extent, thereby producing lower gloss and less color saturation than coatings of low viscosity (17). This phenomenon is attributed to the greater amount of solvent that has to evaporate from high-viscosity coatings once the var nish has become immobile ("no-flow" point). Once this point is reached, a coating will essentially reproduce the texture of the substrate beneath it be cause the varnish will shrink to a lower level above a valley than above a peak of a rough surface. The more solvent present at this stage, the more pro nounced the reproduction of the micro scopic surface imperfections in the dri ed coating will be. Using slowly evapor ating solvents, the maximum amount
Table II. Refractive indices of resins used in picture varnishes Gum mastic Dammar resin Sandarac Rosin Amber Laropal K80 27H Acryloid B72 Polyvinyl acetate) Poly(n-butyl methacrylate) Poly(isobutyl methacrylate)
1.536 1.539 1.545 1.525 1.546 1.529 1.477 1.487 1.467 1.483 1.477
Igor The of leveling can be achieved at the "noflow" point; however, this will not af fect the extent to which surface imper fections are reproduced after this point. Another location where light reflec tion can occur is at the paint-varnish interface. A varnish should have an RI that is as close as possible to the bind ing medium of the paint, and good con tact between the varnish and paint lay er should be made. Although few quan titative data are available, there are indications that varnishes with RIs of less than 1.51 can produce a noticeable amount of reflected light at the paint-
varnish interface. Thus MW and RI both play an important role in the opti cal properties of varnishes.
Dammar resin varnishes Dammar resin originates from trees of the Dipterocarpaceae family, which grow in the East Indies, particularly in Malaya and Indonesia (24, 25). The trees exude sticky by-products of their metabolism that yield the resin after the volatile components are evaporat ed. The primary constituents are triterpenoids, several of which have pre viously been identified (see Figure 1; 24-26 and references therein). A small
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• Descriptive statistics hydroxyhopanone: R=0 riyuruxynopanune. n=u 3-acetoxy-22-hydroxy-hopane:R=H3C-CO-0
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Typical structures occurring in the triterpenoid fraction of dammar resin. CIRCLE 175 ON READER SERVICE CARD ANALYTICAL CHEMISTRY, VOL. 61, NO. 21, NOVEMBER 1, 1989 · 1231 A
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polymeric fraction is also present. The resin, still used today, was praised as early as the nineteenth century (and used as a spirit varnish) for its clarity. It is more durable and less prone to yellowing than gum mastic and other resins. The degradation of dammar resin films has been investigated extensively (27); natural aging of dammar films causes yellowing, cracking, and a con siderable change in solubility. Acceler ated aging studies have been conduct ed in a fadeometer (manufactured by Atlas Electric Devices Co.), containing a xenon arc with both a borosilicate inner filter and a soda-lime outer filter, which eliminate shorter wavelength UV radiation and thus simulate day light through glass windows. In these tests, cracking and solubility changes occur, but no extensive yellowing takes place. Yellowing can be brought about by heating dammar films to 100 °C or even lower temperatures. Little change in solubility occurs under such condi tions; the change in solubility can be illustrated using mixtures of cyclohex ane and toluene or acetone and tolu ene, which represent a series of increas ing polarity (28). The solvent mixture needed to remove dammar films changes from 100% cyclohexane to about 70/30 toluene/acetone after sev eral hundred hours of light aging (Fig ure 2); thereafter, little change takes place. Much less change in solubility occurs during heat aging: Films aged at 100 °C for 500 h are soluble in 100%
toluene. Although more polar solvents are needed, the resin remains essential ly soluble, unlike some polymeric coat ings that may become insoluble be cause of cross-linking reactions. Changes in the MW distribution of dammar films during aging can be demonstrated using gel permeation chromatography (GPC). A large part of the triterpenoid fraction disappears during light aging, and material of higher MW is formed (Figure 3); less change occurs during heat aging. The small amount of material in lower MW fractions indicates the presence of ses quiterpenes. Comparison of chromatograms of yellowed films obtained with an RI detector with those obtained us ing a UV detector operated at wave lengths between 250 and 400 nm re veals that chromophores occur throughout the MW distribution. Chromophores absorbing at longer wavelengths are relatively abundant in the high-MW fraction. Useful data can also be extracted from UV-vis spectra of dammar films during aging experiments. Spectra of fresh and aged films (10-15 μτη thick, applied to quartz plates) are shown in Figure 4. During aging, absorption in the UV range increases and also occurs at increasingly longer wavelengths. Eventually, yellowing occurs and ab sorption appears in the short-wave length visible light region. Hazing and cracking result in a general decrease in transmission (increase in scattering); this shows up clearly in the long-wave-
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Unstabilized with UV filter
need
• • • • •
ANALYTICAL APPROACH
Φ
A/B 0/100
50/50 ο CO
2% Tinuvin 292 3% Tinuvin 328 with UV filter
100/0 1000
J_ 1500
2000
_L 2500
J_ 3000
_L 3500
Aging time (h) WaWflffBBB
Figure 2. Change in solubility of dammar resin films during light aging with and without a UV-absorbing filter, and with and without stabilizing additives. The solvent mixtures referred to are either mixtures of cyclohexane (A) and toluene (B) or mixtures of acetone (C) and toluene.
1232 A · ANALYTICAL CHEMISTRY, VOL. 6 1 , NO. 2 1 , NOVEMBER 1, 1989
Make an induction period. After ~300 h, ΔΑ(300) and Y for samples heat-aged at 100 °C or higher exceed those for samples aged in the xenon arc fadeometer. Both absorption in the UV range and yellowness of the films increase rapidly and suddenly during heat aging after aging under light for several hun dred hours. An Arrhenius plot of In ( Y) following 336 h of heat aging at different tem peratures versus the reciprocal of the temperature demonstrated a linear re lationship in the 65-95 °C range. At higher temperatures, however, values for In (Y) exceed those predicted by a linear relationship. Replacement of air by nitrogen virtually eliminates changes in the UV-vis spectrum of dammar films during light aging. How ever, during heat aging under nitrogen at 80 °C or 100 °C the changes are con-
length part of the visible range of the spectrum, where no absorption occurs. The change in absorbance at 300 nm (ΔΑ(300)) and a yellowness factor (Y) have been measured during accelerated aging procedures (Figure 5). In these experiments, the change in absorbance is calculated as: ΔΑ(300) = [A(300, i) - A(300,0)] where A(300, t) = absorbance at 300 nm, at time t. Yellowness factor is de fined as: Y = 100[A(420, f) - A(580, i)] For heat-aged films, ΔΑ(300) and Y were multiplied by a factor 10/d, in which d = film thickness in μπι. In light-aged films, however, chromophores developed in a thin surface lay er only; both increase rapidly without
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15
16
Retention time (min) -L
_L
J_
J_
30,000
10,000
3000
1000
300
150
Approximate molecular weight Figure 3.
Gel permeation chromatograms (using an Rl detector) of dammar resin.
Chromatograms of film before aging (1), after 353 h of heat aging at 80 °C (2) and 100 °C (3), and after 348 h of light aging (4). All samples are of equal concentration (weight to volume).
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ANALYTICAL CHEMISTRY, VOL. 6 1 , NO. 2 1 , NOVEMBER 1, 1989 · 1233 A
ANALYTICAL APPROACH siderable, resulting in values for Y that are slightly lower than those after heat aging in air. Heat aging under nitrogen at 80 °C or 100 °C after light aging under air produces large and Sudden increases in ΔΑ(300) and Y similar to those that occur when this procedure is carried out in air. Yellowed films also show considerable fluorescence under UV light; the yellow and fluorescent chromophores are light sensitive and
can be destroyed with visible light (of wavelengths >400 nm). The IR spectra of dammar films show the expected absorption for carboxylic acid, hydroxyl, and carbonyl groups. Changes take place during both heat and light aging (i.e., intensities change and the carbonyl region be comes broader and more complex). However, because the mixtures are complex, no significant information is
obtained. If films cast from alkaline so lutions are used instead, a strong in crease during light aging is demon strated for the carboxylate anion ab sorption; no significant increase occurs during heat aging. The data indicate that photochemically initiated autoxidation is followed by nonoxidative thermal reactions, which cause yellowing. Primary reac tions involved in the autoxidation of organic materials have been widely published (29). The triterpenoid frac tion of dammar resin contains tetracy clic and pentacyclic compounds that have keto, ether, tertiary carbon, and olefinic functionalities. These groups are all susceptible to photochemically initiated radical reactions. Much less is known about secondary reactions of the autoxidation process. Yellowing may be related to the occurrence of de hydration reactions, as indicated by the deviation from linearity in the Arrhenius plot for Y at temperatures above the boiling point of water. Con densation reactions may play a role as well. It is obvious that the change in solubility is caused by the formation of more polar oxidation products, partic ularly those containing carboxylic acid groups. Dammar resin varnish with stabilizers
Figure 4.
UV-vis spectra of dammar resin films.
Spectra of film before aging (1), after 323 h of light aging (2), after 336 h of heat aging at 100 °C (3), and after 341 h of light aging followed by 22 h of heat aging at 100 °C (4).
Stabilizers that would interfere with the autoxidation process have been in vestigated (30, 31). Radical scavengers and UV absorbers are among the possi bilities that have been examined. The most powerful light stabilizers available are hindered amine light sta bilizers (HALS) (32). Many of these are derivatives of 2,2,6,6-tetramethylpiperidine. HALS are oxidized rapidly to form stable nitroxyl radicals, primarily by reacting with peroxy radicals, which are abundantly produced in the autoxi dation process. 0* 1
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Ν
"-+ROO* — "
V^V
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One reason that HALS appear to be so efficient is that the nitroxyl radicals are regenerated in the following way:
Figure 5. Effect of aging on the change in absorbance and yellowness factor for dammar resins. Curve 1: Change in absorbance at 300 nm during heat aging at 100 °C. Curve 2: Yellowness factor during heat aging at 100 °C. Curve 3: Change in absorbance at 300 nm during light aging for 323 h, followed by heat aging at 100 °C. Curve 4: Yellowness factor during light aging for 323 h, followed by heat aging at 100 °C.
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 21, NOVEMBER 1, 1989 · 1237 A
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2000
|
2500
2 % Tinuvin 292 + 3 % Tinuvin 328 Aging time (h)
mm Figure 6. Change in Δ Α ( 3 0 0 ) of unstabilized dammar films; films containing 0.5, 1, and 2 % Tinuvin 2 9 2 ; and films containing 2 % Tinuvin 2 9 2 and 3 % Tinuvin 3 2 8 dur ing light aging behind a UV-absorbing filter. The right vertical axis lists the solvent mixtures needed to remove the films after ~2500 h of aging (A, cyclohexane; B, toluene: C, acetone).
Although HALS are effective in experi mental varnishes that are based on synthetic low-MW resins, the results obtained with varnishes based on natu ral resins such as dammar resin have been disappointing. Photochemical degradation occurs so easily in dammar resin that has been subjected to UV light that even the most powerful stabi lizers are ineffective. In fact, benzotriazole- and benzophenone-type UV ab sorbers break down rapidly in films of the resin, a phenomenon not observed in more stable substrates such as polyacrylates and polyolefins. UV light is the primary cause for photochemical degradation reactions, although short-wavelength visible light may cause photochemical reactions as well. To simulate conditions present in galleries not exposed to UV light, films of dammar resin were also aged under exclusion of UV light. Not surprisingly, it was found that aging of the samples was slowed down considerably. Be cause many museum galleries today have eliminated the UV component of the radiation produced by their light sources, the possibility of stabilizing dammar films under such conditions should be investigated. During light aging, ΔΑ(300) in creases slowly but eventually reaches considerably higher levels when a UV filter (cut-on wavelength 406 nm) is used. This effect is attributed to the vulnerability toward UV light of some degradation products containing UVabsorbing chromophores. Although ab
1238 A · ANALYTICAL CHEMISTRY, VOL. 6 1 , NO. 2 1 , NOVEMBER 1, 1989
sorption at 300 nm quickly reaches a maximum in the presence of UV light, degradation continues and is actually much more severe than in the absence of UV light. UV-absorbing chromo phores that are stable only when visible light is present react further in the presence of UV light, causing the ab sorption to reach a plateau relatively early. In the absence of UV light, HALS Tinuvin 292 (manufactured by CIBAGEIGY) strongly inhibits the forma tion of UV-absorbing chromophores and the change in solubility (Figure 6). The effect of HALS can be enhanced by the addition of the benzotriazoletype UV absorber Tinuvin 328. UV ab sorbers work best as protectors of lay ers beneath the layer in which they are found; this is because of their screening of UV light. However, benzotriazoleand benzophenone-type UV absorbers synergistically enhance the effect of HALS even when used together in the top layer (33). Mechanisms other than absorption of U V light, such as quench ing and radical scavenging, may play a role with both types of absorbers and could therefore explain the effect of Tinuvin 328. A short period of heat aging at 80 °C after prolonged light aging (>2000 h) causes extensive yellowing in unstabi lized films, whereas no yellowing oc curs in films containing 2% Tinuvin 292 and 3% Tinuvin 328. Thus, if photo chemical degradation is inhibited, the subsequent thermal reactions that
cause yellowing remain absent as well. Stabilization has a dramatic effect on solubility (Figure 2). For example, after more than 3000 h of light aging using a UV filter, dammar films con taining 2% Tinuvin 292 and 3% Tinuvin 328 can still be removed with 100% cyclohexane. Unstabilized films aged us ing a UV filter for a similar period re quire a mixture of 29% acetone and 71% toluene, whereas those films (with and without additives) that were light-aged without using a filter show rapid dete rioration as judged by solubility prop erties. IR spectra and GPC data of unstabi lized films show major changes despite the use of a UV filter. GC/MS shows that little of the triterpenoid fraction survives in such films (26). The IR spectra of films of dammar (containing 2% Tinuvin 292 and 3% Tinuvin 328) light-aged using a UV filter show little change from the unaged product. GPC of these films affords chromatograms that are essentially identical to those of the unaged resins. GC/MS of the triter penoid fraction of the aged material re
veals that all original components sur vive, indicating that authentic stabili zation has been achieved. Use of dammar varnish with additives is a via ble alternative for museums that have eliminated the UV component from their light sources. Future directions
We are currently investigating several commercially available synthetic resins that are more stable than ketone resins. These include hydrocarbon resins, which are often oligomers obtained from dicyclopentadiene. The most sta ble hydrocarbon resins are fully hydrogenated and free of functional groups. They dissolve in aliphatic sol vents and, because of their cyclic na ture, have relatively high RIs. We also are studying several condensation products of urea and aliphatic alde hydes, one of which has shown excep tional stability. Although work is still in progress, it is already clear that many of the resins under investigation have stabilities that are far superior to the products
currently in use by conservators. When formulated with stabilizing additives such as HALS, films of these resins are extremely stable even in the presence of UV light. Several of the resins are being used in experimental varnishes by a number of conservators. Many conservators have already expressed enthusiasm for the products; some have claimed that they have been able to achieve the same appearance with them as with natural resins, although typically the desired effects can only be achieved with synthetic resins by ap plying a final spray coating. Natural resin varnishes apparently have unique rheological properties that allow for subtle manipulation during brushing. Obviously, the complex chemical composition of these resins and the presence of both a low-MW fraction and a polymeric fraction con trol these properties. However, it is not yet clear which factors are most impor tant, and more precise information about the optical effects of varnish should be obtained. Goniophotometry (i.e., the measurement of the amount of
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reflected light as a function of the angle of reflection), reflectance spectropho tometry, and other measurements should be carried out on paint surfaces that have varying degrees of roughness and that contain varnishes of varying composition applied in different ways. The rheological properties of natural resin varnishes should be studied by viscometry at high shear rates and at high concentrations in order to under stand the effects of sustained brushing. Attempts are being made to imitate the rheological properties of dammar and mastic varnishes using mixtures of low-MW resins and polymers. Al though more work is needed, varnishes formulated with a low-MW synthetic resin, the proper solvents, a stabilizing additive, and perhaps some polymers should find their way to the conserva tion community in the near future.
References
(1) Feller, R. L.; Stolow N.; Jones, Ε. Η. On Picture Varnishes and Their Solvents; National Gallery of Art: Washington DC, 1985. (2) Armenini, G. B. On the True Precepts of the Art of Painting; Burt Franklin & Co.: New York, 1977. (3) The Strasburg Manuscript, A Medieval Painters' Handbook; Borradaile, V.; Borradaile, Ft., Trans. Alec Tiranti: London, 1966. (4) van de Graaf, J. A. Ph.D. Dissertation, University of Utrecht, The Netherlands, 1958. (5) Thompson, D. V. The Materials and Techniques of Medieval Painting; Dover: New York, 1956. (6) Eastlake, C. L. Methods and Materials of the Great Schools and Masters; Dover: New York, 1960; Vols. I and II. (7) Merrifield, M. P. Original Treatises on the Arts of Painting; Dover: New York, 1967; Vols. I and II. (8) Cennini, C. D. The Craftsman's Hand book; Thompson, D. V., Trans.; Dover: New York, 1960. (9) Ruurs, R. Maltechnik 1983, 89,169-74. (10) Lucanus, F. Schweiger's J. 1829, 55, 60-66; Feller, R. L., Trans., Carnegie Mel lon Institute. (11) Feller, R. L.; Stolow, N.; Jones, Ε. Η. On Picture Varnishes and Their Sol vents; National Gallery of Art: Washing ton, DC, 1985; pp. 47-116. (12) Gettens, R. J. Tech. Stud. Field Fine Arts 1935/6, 4, 15-27. (13) Thomson, G. Stud. Conserv. 1957, 3, 64-78. (14) Feller, R. L. Science 1957, 125, 114344. (15) De Witte, E.; Goessens-Landrie, M.; Goethals, E. J.; Van Lerberghe, K.; Van Springel, C. ICOM Committee for Con servation, 6th Triennial Meeting, Ottawa, ON, Canada, 1981; paper no. 81/16/4. (16) de la Rie, E. R. Stud. Conserv. 1987,32, 1-13. (17) Bruxelles, G. N.; Mahlman, Β. Η. Off. Dig. Fed. Paint Varn. Prod. Clubs 1954, 351, 299-314. (18) Hunter, R. S. The Measurement of Appearance; John Wiley & Sons: New York, 1975.
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(19) Judd, D. B.; Wyszecki, G. Color in Business, Science and Industry; 3rd éd.; John Wiley & Sons: New York, 1975. (20) Meeten, G. H. In Optical Properties of Polymers; Meeten, G. H., Ed.; Elsevier Applied Science: London, 1986; pp. 1-62. (21) Wilmouth, F. M. In Optical Properties of Polymers; Meeten, G. H., Ed. Elsevier Applied Science: London, 1986; pp 265333 (22) Shedrinsky, A. M. Ph.D. Dissertation, New York University, 1986. (23) de la Rie, E. R.; Shedrinsky, Α. Μ. Stud. Conserv. 1989, 34, 9-19. (24) Mills, J. S.; White, R. Stud. Conserv. 1977 22 12-31 (25) Mills' J. S.; Werner, A.E.A. J. Chem. Soc. 1955,3132-40. (26) de la Rie, E. R. Doctoral Dissertation, University of Amsterdam, 1988. (27) de la Rie, E. R. Stud. Conserv. 1988,33, 53-70. (28) Feller, R. L. ICOM Committee for Conservation; 4th Triennial Meeting, Venice, Italy, 1975; paper no. 75/22/4. (29) Scott, G. Atmospheric Oxidation and Antioxidants; Elsevier: Amsterdam, 1965. (30) de la Rie, E. R. Stud. Conserv. 1988,33, 9-22. (31) de la Rie, E. R.; McGlinckey, C. W. Stud. Conserv. 1989, 34,137-46. (32) Rozantsev, E. G.; Kagan, E. Sh.; Sholle, V. D.; Ivanov, V. B.; Smirnov, V. A. In Polymer Stabilization and Deg radation; Klemchuk, P. P., Ed.; ACS Symposium Series 280; American Chemi cal Society: Washington, DC, 1985; pp. 11-35. (33) Gugumus, F. In Developments in Polymer Stabilisation—1, Scott, G., Ed.; Applied Science: London, 1979; pp 261308.
E. René de la Rie received a doctorandus degree in physical chemistry (1975) and a doctorate {1988) from the University of Amsterdam. From 1975 to 1978, he was employed at the Central Research Laboratory, Ministry of Culture (The Netherlands). He also served as a docent and administrator for the Training Program for Conservators from 1978 to 1981. At the Metropolitan Museum of Art in New York (1981-1989), de la Rie created a scientific laboratory for the Paintings Conservation Department. He is an adjunct professor at the Conservation Center of the Institute of Fine Arts of New York University, and became head of the National Gallery's Science Department in May 1989.
