Conservation treatments of paintings on ceramic and glass: two case studies

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Conservation Treatments of Paintings on Ceramic and Glass: Two Case StudiesAuthor(s): Niccolo CaldararoReviewed work(s):Source: Studies in Conservation, Vol. 42, No. 3 (1997), pp. 157-164Published by: Maney Publishing on behalf of the International Institute for Conservation of Historic andArtistic WorksStable URL: http://www.jstor.org/stable/1506711 .

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CONSERVATION TREATMENTS OF PAINTINGS ON CERAMIC AND GLASS: TWO CASE STUDIES

Niccolo Caldararo

Summary-The treatment of flaking pigment on two paintings, one on ceramic and the other on glass, is described; a brief history of painting on glass and ceramics places these works in context. The treatments are discussed in relation to other types of treatment for paintings on similar rigid supports. The longevity of the conservation methods used is discussed, as are the results of re-examination of the objects some years after the completion of the treatments.

Introduction

This paper discusses the examination and treatment of two paintings, one on ceramic and the other painted in reverse on glass. Both paintings had suf- fered problems with the detachment of pigment lay- ers from the substrate. The treatments are evaluated in the context of the conservation literature relating to paint films on glass and ceramic substrates. However, only those treatments which maintain the structure of the artifact as a painting on glass during conservation are discussed, as opposed to those which involve the transfer of the paint layers to tem- porary supports prior to re-affixing them to the origi- nal glass [1] (see Table 1, p.162)*. In 1978 Bailey, Roth and Keck produced notes for the catalogue of an exhibition of the Mildred Lee Ward collection of reverse glass paintings [2]. Notes in the appendix warned that conservation treatments of paint films on glass should be considered temporary, since the nature of the bonding between paint film and glass was poor and relatively unstable. The present study aims to determine how temporary such treatments have been and to compare the success of various methods.

Paint on glass or high-fired ceramic surfaces is inherently fragile, unless it has been fired, as in win- dow glass for example. The main defects in painting on such surfaces are the lack of attachment of paint films, and the porosity of glass and ceramic [3]. Although the main cause of paint detachment is nor- mally the drying out of the paint medium, there are other potential reasons for paint release. In glass, the ions of various salts (especially those of sodium and potassium) tend to migrate to the surface, a condi-

tion referred to as 'weeping' or 'sweating' glass [4, 5]. In ceramics, various salts also migrate to the surface during cycling of moisture, and push off surface films during evaporation. Finally, films may release due to the condensation of moisture on the surface of glass or high-fired ceramic.

Background

Paintings or decorations on vitreous and fired ceramic surfaces originated in antiquity [6, 7]. Keiser refers to examples of miniature portraits on glass from the later Roman Empire (second or third century AD) [8], while medallions in the shape of small glass plates with painted religious themes and symbols have been found in the catacombs in Rome. Reverse glass paintings-Hinterglasmalerei in German-originated from Byzantium according to Davison and Jackson [9] who cite Irimie and Focsa [10]. Graham [11] makes the same statement but cites Vydra [12]. In arguing for a central European origin, Arteni and Sanchez Posada de Arteni [13] cite Merrifield [14] and Dancu and Dancu [15]. According to Dancu and Dancu, the technique was reintroduced into Europe in the four- teenth century from Byzantium [16]. Eswarin makes a compelling argument for a continuous develop- ment of the technique from Roman times (third or fourth century BC) to the nineteenth century [17]. The fragile link in this theory is provided by the thirteenth-century reverse-painted glass in the high altar of St Stephen's Chapel in Westminster Abbey. Another thread might lead from the monastic painters of Mount Athos to the Balkans and Venice, or monastic painters' manuals carried to Venice after the sack of Byzantium in 1204 AD. The reverse-glass painter Didron discovered several such manuals at Mount Athos in the mid-nineteenth cen- tury, the oldest of which was dated to the tenth or eleventh centuries [18, 19]. Roman examples, like

*Editor's note. Some of the solvents and materials employed in the conservation treatments described in this paper are toxic or hazardous. Manufacturers' health and safety information should be consulted prior to their use.

Received 14 June 1995 Received in revised form 9 July 1996

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the gold glass method, and mediaeval reverse engravings, both of which form part of Didron's argument for a direct link, are, however, not reverse glass paintings, as pigment is not applied directly to the reverse of the glass in either case. Even the 'pittura translucida' method, where the painting is made separately and then pasted to the glass while still wet, differs from true reverse glass painting. It may be speculated that the reverse glass painting method was derived from the tradition of the Mandylion coverlet [20], where images of Christ were carried through the streets of cities in the east- ern Roman Empire on certain religious days and the populace would wash the image or anoint it with fluids. There is some evidence that these paint- ings were framed behind glass to protect the image. Transfer of a painted image onto glass by a vapor- phase mechanism is well known [21, 22]. The appearance of this transfer image may have been considered miraculous and retained as the outline for painted enhancement, eventually resulting in reverse-glass paintings of icons.

Chinese reverse-glass paintings were introduced into Europe at the beginning of the eighteenth cen- tury [23] and were in increasing demand in Europe after the middle of the century [24]. Lowe dates glass painting on stained glass windows to tech- niques introduced in the sixteenth century [25], while enamel painting on glass vessels is said by van de Graff [26] and Drahatova [27] to have begun in Venice around the middle of the fifteenth century.

Painting on glass using oil paint is mentioned in old treatises on art, for example by Antonio da Pisa at the end of the fourteenth century [28] and by Cennini [29]. The painting method with oils was developed as an adjunct to the mixing of colors with varnish when adding colors to fired ceramics or stained glass. Mixing 'copper resinate' with oil of turpentine and turpentine resin produced a sticky pigment that was very adaptable to use on vitreous surfaces. A number of the pigments developed by glassmakers and glass painters were later adapted for use in easel painting [26]. The working method for many Northern European glass painters was first to paint the glass with a coating of nut-oil or linseed oil, let this dry in the sun and then paint in colors ground in oils or turpentine over this coating [30].

True Hinterglasmalerei must be distinguished from reverse glass prints, produced by the tech- nique known in the eighteenth century as 'back painting on glass' [31]. Prints applied to the reverse of sheets of glass are not strictly paintings; engrav- ings on paper are adhered with turpentine to a sheet of glass, the paper largely removed, and the outlines of the engraving painted and varnished to resemble an oil painting.

Case studies

Painting on a clock glass The treatment of this reverse glass painting, a coun- try scene by an anonymous artist, was undertaken in the laboratory of Mario's Conservation Services in 1977 under the direction of two conservators, Sidney Williston and Kory Berrett. The painting, measuring 32 x 52cm, was a glass panel from a New England Terry clock, dated c. 1900. A prelimi- nary report of the treatment has appeared elsewhere [32]. The conservation methods employed on this reverse glass painting formed the basis for the suc- cessful treatment of the painted ceramic artifact described in the next section.

When received by Williston and Berrett, the reverse glass painting had detached areas of pig- ment over approximately 25% of its area, with flak- ing varying from simple lifting to curling. The object before treatment is shown in Figure 1. At the suggestion of Victor Covey (National Gallery of Art, Washington), Williston and Berrett used the Pettenkofer method to soften the varnish and paint films through the action of solvent vapor. Tests on the paint film showed that a solvent mix of 50% methyl ethyl ketone (butanone) and 50% cellosolve (2-ethoxyethanol) was effective in this respect. The glass painting was therefore placed in an airtight chamber and a vessel containing the solvent mix was suspended in the chamber at a distance of 2.5cm above the painting. After four and a half hours the areas of curling and flaking had relaxed into place in contact with the glass. The glass paint- ing was then removed from the chamber and placed unpainted side down onto low density polyurethane foam. A sheet of glassine paper was placed over the

Figure 1 Reverse-painting on glass. a New England clock-case cover. Front view before treatment.

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painted side, followed by a sheet of soft felt and then weights. After four days the weights were removed and the felt lifted. The glassine adhered to the paint in only one small area. The painting was therefore left overnight exposed to air without weights; the next day the glassine was removed without incident. All but 5% of the previously detached paint had re-adhered to the glass. The remaining areas were re-attached using a cotton swab dipped into the same solvent mixture.

After six months, the areas which had lost paint prior to treatment were retouched with Magna acrylic pigments mixed with xylene (dimethylben- zene). Losses in the gilded border were restored using gold leaf applied in gelatin. The entire back of the paint layer was then covered with a green- tinted coat of Liquitex acrylic emulsion to prevent the paint from lifting.

Painting on a ceramic tile

A painting on a porcelain-like ceramic showing a domestic scene by an anonymous artist was brought to the Conservation Art Service laboratory for examination and treatment in 1990 (Figure 2). The ceramic sheet measured 10-2 x 29.8cm and was approximately lcm thick. There was no signa- ture, date or provenance, but an imprint in the ceramic on the verso read '2 G 10 30'. The painting was framed in such a manner that the filets of the frame covered approximately lcm of the margin of the painting, but one of the filets was loose and had come to rest extending over a larger area of the painting surface (see Figure 2). The painting exhib- ited, on initial viewing, a generalized crazing and flaking of the surface, with complete detachment of large flakes of the pigment layer over large areas of the image. After removal of the frame it was noticed that the flaking and crazing did not extend under the filets or under the area of the dislodged filet. These areas seemed to have been protected

Figure 2 Painting on ceramic prior to unframing and treatment. Note the dislodged filet at the top of the panel.

from whatever agent(s) had caused the damage. The painting had an 'alligator skin' pattern (that

is, one comprising squares and rectangles) of crack- ing and curling paint, that can result from a num- ber of causes. Foremost among these is the tendency for the glass to produce temperature and relative humidity gradients where water may con- dense or migrate to the surface and evaporate. When a room heats and cools during seasonal changes, such cycling can cause serious surface strains on a pigment coating the surface, especially one not soluble in, or permeable to, water. The same pattern of cracking can occur when a resin coating is applied over a paint film that is not quite dry, or where the film is weakly attached to the sur- face. As the resin coating dries it pulls the film up and apart.

The painting was mounted in a frame with a loose inner filet. The back of the painting displayed a significant 'tide line' due to contact with water or another solvent, and what appeared to be water damage. On removal of the backing paper it was found that the paper had been adhered with a heavy application of animal glue. It was initially thought that the surface appearance was the result of the object coming in contact with water, which the staining seemed to support. However, the stain and the damage to the surface did not overlap. The pigment had been coated with a substance, and subsequently a solvent had caused the coating and the pigment to be damaged. Areas of the painting which were under the filet and under the dislodged filet were not damaged, indicating that only the exposed surface had come in contact with a damag- ing agent (Figure 3). The damage was not uniform over the exposed area but was heaviest in the center left where the figures were concentrated. This fea- ture might support a theory that the painting had previously been cleaned using an aerosol or spray-

Figure 3 Painting on ceramic after unframing but before treatment. The damage to the paint layer can be seen, as can the well-preserved area that was pro- tected by the dislodged filet at the top of the panel. Note that the very light area beneath the figures is due to glare from the lights used in photography.

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delivered cleaner. While the paint layer exhibits a general craquelure pattern which might indicate the application of a coating prior to complete drying of the paint film, the rectangular flakes only appear in the center and upper regions (Figure 3).

Treatment of the ceramic painting A literature search for treatments of paint films on ceramic and glass supports led to Williston and Berrett's report from 1978 [32] and Newton's criti- cal bibliography [5]. Williston and Berrett were con- tacted concerning the stability of the object they had treated. As Williston stated that he believed that the object was still in good condition, the same method of conservation used on the clock glass painting was chosen for the ceramic painting. Rather than carry out tests directly on the work, these were undertaken on a few small detached flakes, allowing a detailed study under the micro- scope of the behavior of the paint when exposed to solvents. It was obvious that a coating of some kind had been applied to the ceramic, as a clear layer could be observed unevenly overlapping the sides of the ceramic sheet. Solubility tests on this coating indicated that it swelled in alcohols-for example, isopropanol (propan-2-ol)--and dissolved when alcohols were used in combination with tur- pentine or acetone (propanone). It appeared as if the coating had been applied over the pigment as a final layer. Visual examination could not, however, establish if a preparatory coating had been applied to the surface prior to painting. A flake of the paint was placed on a microscope slide and the effects of adding various solvents were observed. It was veri- fied that the film softened on addition of ethanol and isopropanol and that it released a milky cloud when a drop of acetone was placed on the slide next to it. Unfortunately, the nature of the paint medium could not be determined from the flakes available. Laurie's caution [33] concerning solvent vapors penetrating paint films and softening the layers and the potential for the migration of volatile fractions was borne in mind and will be discussed later.

It was decided to use a two-step process based on Doerner's version of the Pettenkofer method [34]. First, the ceramic panel was placed in a closed chamber with an open container of isopropanol. This resulted in a softening and swelling of the film at the edges. An equal amount of acetone was added approximately one hour later; after an expo- sure of 24 hours the paint film had reformed com- pletely (Figure 4). While the paint film was now re-attached, the individual* rectangular pieces of paint still did not quite meet, leaving a pattern of craquelure. As this had been the appearance of the

Figure 4 Painting on ceramic immediately after relaying the paint using vapor treatment.

painting in areas which did not exhibit lifting and flaking prior to treatment, it was felt that the gaps should not be retouched to form a continuous image. However, it was felt that some compensa- tion for areas of lost painting was in order. Areas of lost pigment were retouched with gouache and a gloss similar to the varnish coating was achieved by using gum arabic (Figure 5).

The durability of treatment methods

After the treatment of the ceramic painting had been completed, a colleague sought advice on the treatment of a reverse glass painting that was awaiting conservation, which showed similar deteri- oration. Although the Williston/Berrett method of treatment was suggested, the long-term durability of such treatments was still in question. Feller had reported that such reformed films were unstable and tended to revert to a detached condition [35]. The author was familiar with Solomon's carbon-14 tests which demonstrated that ethanol was retained in the paint film [36], but it was not thought that loss of the solvent would result in subsequent detachment. While paint films have certainly suf- fered some leaching of components from the var- nish and paint layers over time, perhaps as a result of the event which caused the initial damage, it was not considered that loss of retained solvent would

Figure 5 Painting on ceramic after retouching major areas of paint loss with gouache and gum arabic.

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render the paint film unable to adhere. Williston and Berrett waited six months before retouching and coating the paint film--a period greater than that indicated for solvent loss in Jones's article [36]. The owner of the ceramic painting was contacted six months after the treatment and the work was re-examined. It was found that the paint was still completely adhered. Feller believes that the cracked and shrunken paint film might have developed a 'memory' of its deformed structure and that, once the solvent again evaporated, it would revert to this form; this process might take as long as a year to occur [37]. In an effort to pursue this problem, the literature was re-examined. Schmitt, having access to Pettenkofer's notebooks, recently examined seven paintings which Pettenkofer had treated between 1864 and 1865 [38]. Schmitt's examination showed that the paint/varnish films had undergone consid- erable deformation during the exposure to solvent vapor, with evidence of migration of film compo- nents, especially ground and pigment, into upper layers. These paintings, however, had probably been treated with Pettenkofer's copaiba balsam addition to the surface, which was meant to help fill in the gaps between cracked sections of paint. There was no detachment or loss, and it seems apparent from Schmitt's observations that areas of detachment had been successfully filled by the treat- ment and remained stable for a considerable time. This is, nevertheless, only empirical data and from a very small sample and conclusive information can hardly be derived from it. Other efforts to reduce blanching using a similar method but with dimethylformamide vapor were also reported to be successful [39]; an examination of experimentally treated samples in a scanning electron microscope indicated that stable attachment had been attained [40].

Stolow, who has also tried to restore leached paint films, felt that the procedure could result in a coherent paint layer, especially if pressure were applied to enhance attachment [41]. He considered that the 'reforming equation' was difficult to deter- mine since there were so many variables-for exam- ple pigment composition, age, varnish, aging conditions--and the time for reversion was, per- haps, indeterminable [41]. Finally, Stolow suggested that the treatment might be more successful if it were followed by the application of a new, flexible coating as applied by Williston and Berrett. There were distinct differences between the 1990 treat- ments used by the author and that used by Williston and Berrett. The latter had used pressure after exposure to the vapor and had applied a coat- ing over the re-attached film. Another factor that might affect the stability of the treatment is the dif- ference in gas permeability between the glass of the

clock face and the glazed and fired surface of the ceramic.

Re-examination of treated objects

The surface of the ceramic painting was re-exam- ined one year after the treatment had been com- pleted. No change was apparent. The ceramic painting was then examined again three years after treatment; its condition was unchanged, except it appeared slightly less glossy than in 1990. In 1996, more than five years after treatment, there was still no change in condition. Williston was asked to re- examine the clock face he had treated and found that the paint film was stable. A comparison of photographs taken before and after treatment in 1977-78 with those taken in 1993 shows that, although there was some minor lifting of the paint, this was in areas which had not been previously detached. Overall, the treatment was still judged to be a success.

Conclusion

Reforming treatments have been in disrepute in recent years, but the results of the two treatments described in this paper indicate that under certain conditions, especially with supports which are solid and rather impervious to moisture, treatments using this method may remain stable over some time. Cooperation in comparing treatments and tracking their durability over time should be continued and repeated with other examples. Table 1 provides a summary of data on treatment type and durability based on evidence from several conservators. Conservation treatments need to be evaluated for durability; these evaluations should consider vari- ables introduced by the materials of construction, different storage conditions and possible variations in application.

Acknowledgements

This paper was organized and written with the direct aid of Sidney Williston, Kory Berrett, Robert Organ and Joanna Pessa. Each of them consented to read drafts of the paper and kindly provided detailed suggestions for corrections and changes. Carol Aiken, Karen Graham, Patricia Jackson, Laurie Hamilton and many others provided treat- ment report information and re-examination data. Special thanks to Dr Robert Feller, and to Dr Nathan Stolow for help regarding chemical ques- tions specific to films and solvents. Thanks to

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Table 1 Treatment methods and re-examination

Method Date Date of Conservator and re-examination date of report and condition

Vapor 1973a 1993; little change Williston and Berrett (1978) [32] 1979b not re-examined Aiken (no date) 1990 1996; no change Caldararo (1997)

Solvent brushed on 1980 not re-examined Aiken (no date) Adhesive brushed on 1980 not re-examined Terwen (no date)c

Bretz (no date)d Solvent brushed on followed by adhesive 1980e not re-examined Kramer (no date) Varnish followed by injection into voids 1976' not re-examined Emery (no date) Adhesive brushed on, then synthetic

fabric attached 1967 not re-examined Roth (no date)g 1980 not re-examined Aiken (no date)h

Adhesive brushed on 1976i not re-examined Graham (1976) [11] Vacuum applied 1982 1994; no change Graham (no date) Epoxy transfer 1982 not re-examined Thornton (1982) [1]

1994 not re-examined Friend (1994)

Notes (a) Williston and Berrett's method: the object is first exposed to solvent vapor in a closed container, then the object is

placed under weights before being allowed to dry in air without weights. Finally, the surface is coated with an acrylic emulsion [32].

(b) Aiken used acetone (propanone) vapor. (c) Terwen used Plexisol P550. (d) Bretz used 80% beeswax mixed with 20% Arkon P70. She also used a heated tool during the treatment. (e) Kramer used an alcohol as solvent and gelatin to re-adhere the film, except in the case of Roman glass on which

he used Mowiol (10%). (f) Emery used dammar as varnish and injected Penetrol. (g) Roth used Vinac B7 in a mixture of methanol and toluene (methylbenzene), before coating the surfaces with Lucite

44. A grey silk fabric was then applied as a backing. (h) Aiken here used Paraloid B72 dissolved in xylene (dimethylbenzene) as an adhesive and Cerex as a backing mater-

ial. (i) Graham used a number of adhesives, including Paraloid B72 in cellosolve (2-ethoxyethanol) in 1976 and PVA-

AYAC in diacetone alcohol (4-hydroxy-4-methylpentan-2-one) in 1982.

Candis Griggs, conservation assistant at Conserva- tion Art Service, for her help in the preparation of the final draft of this paper.

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Author

NICCOLO CALDARARO is director and chief conser- vator of Conservation Art Service. He received his BA in anthropology from the University of California, Berkeley in 1970 and his MA in anthro- pology with a specialization in archaeological con- servation in 1983 from San Francisco State University. At present he is Adjunct Professor of

Anthropology at SFSU and teaches a variety of courses in anthropology at other local colleges. His present research in conservation is concerned with the evolution of the decision-making process in the field, with specific focus on treatment development by different conservators dealing with similar prob- lems, and the durability of treatments over time. Mr Caldararo has been employed by a number of museums over the past 30 years including the California Academy of Sciences, the Asian Art Museum of San Francisco, the De Young Museum and the California Palace of the Legion of Honor. Address: Conservation Art Service, 893 Folsom Street, San Francisco, CA 94107, USA.

R6sume--On dicrit le traitement de pigments qui s ecaillent sur deux peintures, I'une sur ceramique et I'autre sur verre, une breve histoire de la peinture sur ces deux materiaux replagant ces oeuvres dans leur contexte. On discute les traitements relativement aux autres types de traitement des peintures sur des supports rigides similaires. On discute egalement la longevite des methodes de conservation ainsi que les resultats du reexamen de ces objets quelques annees apres la fin de leur traitement.

Zusammenfassung--Der Verfasser des Artikels beschreibt die konservatorischen Maj3nahmen zur Sicherung gefdhrdeter Farbschichten auf zwei Gemdlden, davon eines auf einer Keramik, das andere auf Glas; ein Abrij3 zur Geschichte der Malerei auf Glas und Keramiken dient der Einordnung der behandelten Kunstwerke. Die hier durchgefahrten Maj3nahmen werden im Bezug zu anderen Arten der Behandlung von Malerei auf dhnlichen festen Bildtrdgern diskutiert. Gegenstand der Erdrterung sind auch die Langzeitwirkung der durchgefiihrten Konservierung ebenso wie die Ergebnisse einer erneuten Untersuchung der Objekte einige Jahre nach Beendigung der Arbeiten.

Resumen-Se describe el tratamiento de pintura pulverulenta de dos pinturas, una sobre cerdmica y la otra sobre vidrio; un breve resumen de la pintura sobre estos tipos de soporte situta las obras en contexto. Los tratamientos se discuten en relaci6n a otros tipos de intervenciones en cuadros realizados sobre soportes rigidos similares. La longevidad de los mdetodos de conservaci6n es tratada, asi como los resultados de el re-examen de los objetos anios despues de la finalizacidn de los tratamientos aplicados.

164 Studies in Conservation 42 (1997) 157-164

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