Solvent Gels for the Cleaning of Works of Art: The Residue Question edited by Dusan Stulik, David Miller, Herant Khanjian, Narayan Khandekar, Richard Wolbers, Janice Carlson, W. Christian Petersen, Valerie Dorge, Timothy P. Whalen (Research in Conservation: J. Paul Getty Trust Publications) The cleaning of a work of art often involves removing not only dirt and grime but also unwanted layers of varnish, gilding, and paint from the work's surface. The challenge for conservators lies in finding a cleaning agent that will act on one layer without affecting the layer being preserved and without leaving any harmful residues on the cleaned work. This book, which examines gel cleaning in the treatment of paintings and painted works of art, presents the methodologies, data, and results of a collaborative project of the Getty Conservation Institute and Winterthur Museum. Among the issues covered are the theory and application of gel cleaning systems, the detection of residues left on the surfaces of objects cleaned with these systems, research into solvent-gel and solvent residues, stability of surfactants during natural and artificial aging, and recommendations for formulating gels for specific cleaning tasks.
Excerpt: In the cleaning process, conservators are guided by the principle of effectively removing a layer or layers of dirt, undesired paint layers, or varnish without damaging or altering the mechanical or chemical properties of the paint layers, the surfaces to be retained, or the substrate. In objects conservation, cleaning also can involve removing salts or grime from the substrate of porous materials such as stone. For decorative surfaces, the conservator must identify the chemical difference between layers and develop a formulation that acts only on the layer or layers to be removed. The ideal cleaning agent would be single-acting; that is, it would act on one layer (or treat several as one) without affecting the layer(s) to be preserved.
What Is a Gel?
In the broadest sense, a gel is a water-based formulation thickened with a polymer or other high molecular weight material. Thickened solutions may be fairly simple in composition, such as a water gel, or they can be more complex, such as a gel containing methyl or ethyl cellulose , propylene glycol, fumed silica, a nonionic detergent, and triethanolamine (Grissom, Power, and West 1988). Historically, "packs," "pastes," "poultices," "gels," "compresses," and "pads"`were the terms used for thickened solutions. The gel is a vehicle for carrying the "active" cleaning components to the surface to be cleaned. Organic solvents are the major active ingredients dealt with in this study. The recipe for the gel under examination is provided in the relevant chapters throughout this book.
A range of products constitute the "active" ingredients for conservation treatments. These act chemically, for example, aqueous reagents (e.g., alkaline glycerol, used to remove copper corrosion products [Scott 2002]), ammonia, other chelating agents (e.g., EDTA, ammonium citrates), biochemical agents such as enzymes, or organic solvents.
Along with the active ingredients, thickeners form the basics of gel systems. Thickeners are selected based on the properties that are most suited to a particular task: ease of mixing; the ability to hold the solution
to the surface of the object; the degree to which the cleaning action can be controlled; and the ability to completely remove the gel from the surface. Thickeners have included cellulose ethers such as methyl cellulose, hydroxypropylmethyl cellulose, carboxymethyl cellulose, and, more recently, the Carbopols, a series of polyacrylic acid polymers.
Additives may be incorporated for various reasons. Detergents increase the wetting capability to improve contact with the surface and remove dirt through micelle formation. Surfactants such as Ethomeen interact with gelling agents such as Carbopol to form the gel. Buffers maintain the solution at the optimum pH level for enzyme action or permanently alter the pH for a specific cleaning purpose.
Gelled formulations are used to lengthen solvent retention time, to control the depth of penetration by limiting capillary action, to control the cleaning process on vertical or other complex surfaces, or to increase the gel's effectiveness in extracting the soiling or stain as the gel dries. Materials are selected or modified to achieve the desired working proper-ties. Manipulation of the evaporation rate of the active cleaning agent is probably the most important property gel formulations offer.
Slowing evaporation can allow a solution to penetrate farther into a surface to solubilize material at some greater depth. Depth of penetration is essential, for example, to remove stains, salts, or inaccessible soiling embedded in porous substrates such as stone or ceramics.
A gel can be thinned to allow some mechanical action during cleaning. However, thinning carries the risk of increasing absorption of the gel media into the surface layers or substrate. This is a particularly important factor for moisture-sensitive objects as they require a limited depth of penetration.
Traditional thickening methods
Thickened or paste materials such as soap mixtures have long been used for cleaning paintings in an effort to control the action of the traditional aqueous cleaning agents.
With the introduction of organic solvents as cleaning tools in the early part of the twentieth century, wax-solvent pastes were used to control solvent migration and penetration (Gridley 1991). In this cleaning process, the wax acted as a reservoir to prevent the solvent from migrating to surrounding areas and to reduce its evaporation rate. The limitations of this approach were`the working properties of the wax-solvent formulation, the problem of wax residue, and the need to clear the residue with organic solvents.
Gels based on cellulosic products as thickening agents have been used as a form of poultice to remove grime and stains from the surface and substrate of objects. The range of cellulosic products includes methyl
cellulose, ethyl cellulose , hydroxy methyl cellulose, sodium carboxymethylcellulose, and car-boxy methyl cellulose. These formulations have shown great versatility in the cleaning of organic, inorganic, and composite objects. The high solvent to sorbent ratio of gels has contributed to their popularity (Goldberg 1989).
Aqueous Gel–based Cleaning Systems
The gel-based aqueous methods of cleaning painted surfaces introduced by Wolbers in the mid-1980s offered an optional new set of materials. More important, they offered a more comprehensive approach to selecting a system for specific cleaning problems. Although solvent gels are the focus of this research, other gel types are discussed to provide a broader context for the study.
A comprehensive explanation of cleaning actions and the role of individual components of the three aqueous cleaning systems—enzymes, resin soaps, and solvent gels—is now available to the conservation community through the publication of Cleaning Painted Surfaces: Aqueous Methods . Basically, the systems consist of a cleaning agent (organic solvent, enzyme, resin soap, etc.) held in an aqueous gel of a thickening agent, a surfactant, and a pH buffer. The gel reduces the capillary flow of the solvent and allows the cleaning agent to be applied with precise control so as not to dissolve all layers at once. In addition, a gel can be formulated to remove a specific layer, which offered tremendous advantages in that the layers can be selectively removed from the top down. For example, in theory, a more oxidized upper varnish layer (or layers) could be removed, leaving underlying layers untouched. It should be noted that solvent gels are different in concept from the other gels mentioned. In enzyme, resin soaps, and other dirt-removing formulations, the major component of the liquid phase is water that is thickened directly by a water soluble polymer thickener. By contrast, in solvent gels water is often present only in small proportions and the solvent is thickened indirectly by a polymer in an aqueous environment that is then mixed or emulsified into the solvent phase.
The systems are based on firstly identifying and understanding the layers or materials in the specific instance—those to be removed and those to remain. The first step in the identification process usually is examination of cross sections under normal and ultraviolet light, followed by the use of fluorescent dyes. Although the use of dyes (in the form of indicators or fluorescent markers) to identify binding media is not new in conservation (Johnson and Packard 1971; Martin 1977), Wolbers and Landrey improved the process so that the use of fluorescent dyes to characterize various layers formed a vital initial stage in tailoring a gel .
The main advantages offered by the gelled systems are:
control of the organic solvent evaporation rate and of capillary flow into surrounding areas and underlying layers
control of the surface contact time to increase effectiveness of the cleaning agent and reduce potential effects on the surface minimizing human exposure to toxic organic solvents
Application to conservation
One of Wolbers's early presentations on his cleaning systems was at a Washington Conservation Guild meeting in Washington, D.C., in 1986. Another was at the Wooden Artifacts Group session of the 16th Annual Meeting of the American Institute for Conservation (AIC) (Wolbers, . Wolbers presented the theoretical and practical aspects of aqueous cleaning systems more fully to the conservation community through a series of workshops entitled "New Methods in the Cleaning of Paintings" and hosted annually by the Getty Conservation Institute in Marina del Rey, California, from 1987 through 1990. A workshop was also held in 1989 at the Courtauld Institute of Art, London, which was preceded by a one-day mini-conference on the topic; and in 1990, at the National Gallery of Victoria, Melbourne, cosponsored by the GCI, and at the Canadian Conservation Institute, Ottawa.
The development, application, and acceptance (or questioning) of this gel cleaning methodology by the wider conservation community has been gradual and cumulative. Various articles by conservators and scientists who had either participated in one of the workshops or had become informed and interested in this new development appeared in North American and European professional newsletters in the late 1980s. Articles by Chris Stavroudis and Sharon Blank (1989) as well as several by Anna Southall (1988, 1989) were the first in the United States and the United Kingdom, respectively, to try to explain to the conservation community the basis of the methodology and the advantages it offered.
Application to paintings conservation
Early applications of aqueous systems to the cleaning of paintings in the Winterthur Museum collection were carried out on two nineteenth-century oil on canvas works, Winterscape: Skating on the Pond (E. Von Liebrach) and Venetian Canal Scene (unattributed). The former had undergone a number of earlier treatments and was cleaned in 1988–89 using a solvent mixture to remove the resin varnish followed by a resin soap to remove residual coating. Recovery of the aesthetic presentation was undertaken on the latter in 1990 and required three steps: an abietic acid soap was used to remove the uppermost coating, a modified formulation of the soap was applied to remove an older coating layer, and a solvent gel was used to remove the original coating protected by the frame rabbet. More detailed explanations of the methodology for choosing the specific systems for these paintings, along with their application to other paintings and objects, can be found in Cleaning Painted Surfaces .
Further, J. Paul Getty Museum paintings conservators Andrea Rothe and Mark Leonard applied one or more of the aqueous systems to a number of paintings in the Museum's collection during the early stages of their application to conservation, as a result of a workshop Wolbers organized during his guest scholarship in the Paintings Conservation lab in 1987. The paintings include The Farewell of Telemachus and Eucharis (Jacques-Louis David, 1818) (fig. 1.1) and Christ's Entry into Brussels in 1889 (James Ensor, 1888) (Plate 1), both cleaned in 1987. Samples from these two paintings (both oil on canvas) were included in the examination of surfaces treated in the past with the aqueous system, a component of the Gels Research Project discussed in chapter 6.
Following the workshops, the aqueous systems were tested for their efficacy in specific cleaning situations in many other conservation labs. In 1987 Joe Fronek, paintings conservator at the Los Angeles County Museum of Art, reported on the use of a lipase gel to remove overpaint and a xylene emulsion used to remove both a varnish from deep impasto and an attempted thinning of an Acryloid B-67 acrylic varnish. Fronek noted that (a) his lack of experience with the new methodology may have been the cause of minimal success in these cleaning situations; and, importantly, (b) "enzymes, soaps and emulsions are not necessarily a substitute for other cleaning techniques [but] more and more we are discovering their effectiveness when other systems have failed" .
Other early applications in cleaning paintings were reported by Koller (1990) and Stringari (1990). Koller used resin soaps in an attempt to remove old varnishes from flora, a nineteenth-century by an unknown artist, on which three varnishes had been identified. This: was one of the first published accounts of concerns regarding gel residues. Stringari used enzyme gel and solvent gel formulations to successfully remove surface coatings and admixtures of facing and lining residues from a triptych of van Gogh paintings that had proven very sen sitive to traditional solvent methods.
Application to furniture conservation
The main focus of research into the concerns of residue and leaching from the use of aqueous cleaning methods has been in the paintings conservation field. But the early work at Winterthur Museum and the Winterthur/University of Delaware Program in Art Conservation addressed cleaning problems in objects and furniture conservation, particularly the conservation of varnished fine furniture. Furniture conserva tors, faced with the difficulty of removing modern synthetic varnishes such as polyurethanes without damaging underlying shellac and other natural resin-based layers, looked to the use of gels as a possible solution.
One of the first reported treatments of an object with an aqueous cleaning system to solve a cleaning problem at the Winterthur Museum was on a Philadelphia piecrust tilt-top tea table, ca. 1765–80 (fig. 1.2), treated in 1988. Solvents alone did not sufficiently soften the polyurethane layer, applied in 1974, to allow for removal, and a two-step cleaning procedure was devised. The first step was a solvent gel to remove the bulk of the polyurethane and toning layer; the second was an enzyme/detergent gel to remove the residual polyurethane. (See Plate 9 and chap. 6.)
Additional examples of the use of solvent gels to selectively remove furniture coatings that could not be removed with traditional methods were reported by Susan Buck (1993) at the Society for the Preservation of New England Antiquities Conservation Center.
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