Volume 12, Number 3, Sept 1990, pp.13-16

Investigations into Techniques for the Consolidation of High Pigment Volume Concentration Paint at the Getty Conservation Institute

The problems encountered in the consolidation of painted ethnographic objects were reviewed¹ in preparation for an advanced training course for ethnographic conservators conducted at the Getty Conservation Institute, June 11 to June 29, 1990, "The Consolidation of Painted Ethnographic Objects." [Read a review of this course by Robin Chamberlin in Conference Reviews--Ed.] The course was developed under the direction of Sue Walston, coordinator for the ICOM Ethnographic Objects Working Group and previously Head of Materials Conservation at the Australian Museum. The input of the Scientific Department of the GCI was sought to overcome some specific, unresolved problems in the consolidation of these objects, which include the following:

1. The introduction of a consolidant may result in a change in appearance, seen as darkening, streaking or added gloss to what was originally a matte surface.
2. The strength of consolidation obtained for a given concentration of the commonly used consolidants has not been quantitatively studied, making comparison of consolidants difficult.
3. Although the aging (changes in properties with time) of the resins may be known, the aging of the composite consolidated paint (resin/pigment/support) has not been demonstrated.

The conservation literature was searched using keywords in the Bibliographic Database of the Conservation Information Network, BCIN, which contains citations from all of the volumes of Art and Archaeology Technical Abstracts (AATA) and the library holdings of ICCROM, in addition to material from other sources. Most previous studies in the conservation literature stressed fluctuations of the substrate (such as the dimensional changes that occur in wood with variations in moisture content resulting from changes in relative humidity) as a primary cause for delamination and cracking of paint. Different materials were not compared for their effect on the appearance of consolidated powdering paint in equal concentrations. For example, a 1% gelatin aqueous solution would be compared with a 5% solution of Acryloid B72 in toluene. The darkening that resulted from addition of a resin was often explained on the basis of a difference in the refractive indices of the pigment and the resin added.

Following consultation with coatings scientists, conservation scientists and conservators, a different focus was developed during the course of investigations at the GCI. The major factors considered were that:

1. Paint with a pigment volume concentration (PVC) greater than the critical pigment volume concentration has considerable inherent vice in both cohesion of the pigment particles and adhesion to the support.
2. This type of paint may darken as a result of the volume of added resin to achieve consolidation² due to the darkening that occurs when the void spaces between pigment particles are filled.
3. Methods of application that control the distribution of the resin in the paint or on the surface of the paint may be as important, or more important, than individual properties of different resins in achieving the desired strength of consolidation and in minimizing appearance changes.

Pigment Volume Concentration

The paint industry describes the concentration of vehicle and pigment in a coating based upon volume instead of weight, because many of the properties of coatings vary with volume and may be compared on this basis. An important consideration is the critical pigment volume concentration (CPVC). Above the CPVC, void spaces are present. Below the CPVC, the pigment particles are separated by binder. Abrupt changes in many paint properties may occur at the CPVC, including mechanical, permeability and optical properties³. In addition to becoming lighter in color with increasing void space, tensile properties decrease and porosity increases as the pigment volume concentration gets increasingly high above the CPVC.

There has been little reason for studying high PVC paint in industry because these coatings do not function in an industrial capacity (for instance, as an automobile coating) or contribute to the durability or more commonly desired optical properties in a work of art (such as an oil painting).

However, coatings with a high PVC are often encountered in painted ethnographic objects because of the indigenous technologies used in preparing the paint. Kaolin and ochres may be applied from a water slurry and contain no binder at all. Small amounts of binders may have been added or organic material of limited durability may have been used. These effects are not only important to the conservation of painted ethnographic objects. Modern works of art may have a high PVC paint when an artist specifically tried to achieve a matte appearance or experimented in novel paint formulations. Medieval paintings executed with water soluble binders are very often powdering and at a high PVC.

Feller and Kunz² have shown that the lightness of a high PVC above the CPVC paint increases with the PVC. Paint may also be matte and appear very light in color due to surface roughness. In addition to darkening from filling of void spaces, darkening, gloss or higher saturation may result when a clear coating is smoother than the original surface.

Surveys

An initial group of 14 conservators from North America and Australia were surveyed with a written questionnaire concerning the techniques and materials used to consolidate painted ethnographic objects. There was a wide variety of response and no consensus among the conservators as to how to handle this problem. The survey was then extended to contacting by phone the Ethnographic Objects Specialty List Index in the 1989-1990 AIC Directory, resulting in interviews with 86 conservators. Some general conclusions were obtained from these interviews:

1. aqueous solutions work best (noticeably of cellulose ethers and gelatin);
2. when using organic solutions, multiple applications of dilute solutions may result in less change of appearance than one application of a more concentrated solution; and,
3. in some instances, using a very low volatility solvent, such as diethylbenzene (DEB), will also minimize changes in the appearance of consolidated paint.

The reasons for the success of these methods were questioned. First, the cellulose ethers and gelatin solutions are very viscous and only small amounts of material can be used. Was their effectiveness due to low concentrations (little filling of void spaces) or lack of penetration and leveling resulting from a high viscosity? Or is the fact that water is quite polar and may have an affinity for a polar clay surface the important factor? Ethanol solutions seem also to work very well. Why do multiple applications of dilute solutions work so well? Although DEB can be very successfully used to minimize darkening and tide marks⁴, two major problems exist. One is the long drying time: the odor is disagreeable and the spreading cannot be contained to a small area. Second is the limited solubility: Acryloid B72 is soluble in DEB but many polymers, such as poly(vinyl acetate), are insoluble.

A recent study at the GCI stressed the importance of solvent "quality" on the physical properties of thermoplastic resins applied from organic solutions⁵. Solvent quality refers to the effect on the contraction or extension of a polymer chain when dissolved in a particular solvent. Films of poly(vinyl acetate), (PVAC), cast from "poor" solvents have low extensibility and high strength. In contrast, films from "good" solvents for PVAC have higher extensibility and lower strength, relative to the films cast from poor solvents. Further studies at GCI are investigating this effect on acrylic polymers used in conservation, and whether or not greater adhesive strength results from using "good" solvents for a particular polymer. These studies may affect how DEB is used in conservation, because DEB is a fairly "poor" solvent for many polymers.

Solution Properties

The study of Feller and Kunz² also indicated that the individual refractive indices of pigment and resin were relatively unimportant in the darkening that occurs in high PVC paint. Although individual resin properties may be important, as in the strength of an adhesive bond or a required flexibility of a polymer, the key to effective consolidation and minimal change of appearance in this type of paint is found in varying the parameters associated with the application procedures.

Some of the answers to the problems encountered in the consolidation of a high PVC paint are found by considering the properties of solutions. There are two important considerations: viscosity and wetting. Other than the placing of a solid, cast resin under a flake, the introduction of a consolidant or adhesive is through an aqueous or organic solution. Viscosity is the resistance of a liquid to flow. By decreasing the viscosity, a solution will flow more quickly and penetration through a porous or powdery layer will be enhanced. Or, by sufficiently increasing the viscosity, a solution can be contained with lack of penetration. Solutions of a higher viscosity will also conform more to the morphology of a rough surface, and due to lack of leveling contribute to the maintenance of a matte appearance⁶. One way of controlling the viscosity of a solution is through the volatility of the solvent. This is done either by choosing a low volatility solvent for a longer period of low viscosity or by choosing a very volatile solvent to increase the viscosity quickly. As the solvent evaporates, solutions become increasingly more viscous because the concentration of the polymer is constantly increasing. Alternatively one could work in an atmosphere saturated with the vapor of the solvent used to dissolve the resin, thus effectively inhibiting evaporation and maintaining a low viscosity⁷.

Wetting can also become important in controlling the flow of a solution. Coatings that have been soiled with oils or formulated with waxes or oils will not be penetrated effectively by aqueous solutions. The ability of a solution to wet a surface may be affected by pre-wetting the surface or by adding a surfactant to the solution.

Techniques

Undesirable changes in appearance can result from dissolution of soluble components and physical redistribution of pigment particles ⁸. Changes in surface reflection and localized concentrations of added resin may also affect appearance. The techniques discussed below relate only to the last two considerations.

A paint requiring consolidation may be in a variety of conditions, ranging from coherent and flaking to crumbling or powdering. Layers of powdering paint will require different techniques depending upon whether they are quite thick or thin. Because increased penetration is not possible in a very thin layer, lowering surface tension or viscosity may not improve the appearance of a consolidated thin layer of paint. Some techniques helpful for consolidating flaking, porous paint cannot be used with powdering paint. High PVC paint may be adjacent to, or layered over, paint with a high resin content. Several over-laid areas of paint may each exhibit different problems, such as a flaking paint over a powdering ground.

The studies at the GCI have revolved around understanding what occurs when different application procedures are used. Experimental models (facsimiles) of painted wood surfaces with particular problems were developed based upon information from the scientific, coatings and anthropological literature. These models were used to trial theoretical solutions to the modeled problems in conjunction with experienced conservators of painted surfaces. For instances when increased penetration is necessary, we have explored low volatility solvents, pre-wetting, working in an atmosphere saturated with solvent vapors, using small particle-size acrylic dispersions ⁹, surfactants, and multiple applications of dilute solutions. To decrease penetration we have considered high viscosity solutions, reforming of a solid film placed under a flake, consolidation of a flake prior to adhesion, saturation of porous flakes with a hydrophobic organic solvent and adhesion with an aqueous emulsion ¹, and multiple sprayings with a highly viscous solution made with a highly volatile solvent¹¹.

Many other factors also need to be considered: the choice of a particular resin; dirt on the surface; methods of cleaning either before or after consolidation; possibility of retreatment, the effect of pigment particle size and distribution; the requirements of different substrates; and, loss of information for future study.

A few remarks on the choice of a resin should be included here. Because there is little likelihood that a resin can be removed from a fragile surface in need of consolidation, only the most stable resins are considered: Acryloid B72, PVAC, methyl cellulose, carboxy methyl cellulose and acrylic emulsions or dispersions. And also because future removal is in doubt, thermosets are being considered because the strength resulting from low concentrations exceeds that of thermoplastic resins used in the same amount. Only the most stable thermosets, such as moisture-curable resins based upon aliphatic diisocyanates, are being considered¹². However, the possibility of retreatment is greatly diminished with a thermoset.

Dissemination of Information

Practical and theoretical aspects of these techniques, and particular physical states of a painted surface which might benefit from their use, will be presented in October at the WAAC meeting in Avalon, California. A more detailed article, which includes information shared by the participants of the training course, is in preparation for submission to the ICOM Ethnographic Working Group Newsletter. Further experimental work at GCI is planned to determine quantitatively the comparative strength of consolidation resulting from using specific resins in equal concentrations. The review of the literature is continuing and may result in a published bibliography, possibly as a supplement to AATA. There is the possibility that the course may be repeated. Also under consideration is a monograph which would explore in greater depth the practical and theoretical factors to be considered in the consolidation of high PVC paint.

References

1. E. F. Hansen, E. T. Sadoff and R. Lowinger, "A Review of the Problems Encountered in the Consolidation of Painted Ethnographic Objects and Some Potential Remedies," Preprints of the 9th Triennial Meeting of the ICOM Committee for Conservation Dresden (in press).

2. R. L. Feller and N. Kunz, "The Effect of Pigment Volume Concentration on the Lightness and Darkness of Porous Paints," in AIC Preprints of the 9th Annual Meeting, Philadelphia, Pennsylvania, ed. J. Johnson and M. Morales, (Washington, D.C.: American Institute for the Conservation of Historic and Artistic Works, 1981) 66-74.

3. T. C. Patton, Paint Flow and Pigment Dispersion, (New York: John Wiley and Sons, 1979).

4. E. C. Welsh, "A Consolidation Treatment for Powdery Matte Paint," in The AIC Preprints, 8th Annual Meeting, San Francisco, ed. M. Morales (Washington D.C.: The American Institute for the Conservation of Historic and Artistic Work, 1980), 141-150.

5. E. F. Hansen, M. R. Derrick, M. Schilling and R. Garcia, "The Effect of Solution Application on Some Physical Properties of Amorphous Thermoplastic Resins Used in Conservation: Poly (vinyl acetate)," Journal of the American Institute for Conservation, Spring, 1990 (in press).

6. R. L. Feller, N. Stolow and E. Jones, Picture Varnishes and Their Solvents, (Washington, D.C.: National Gallery of Art, 1985) 137-145.

7. E. F. Hansen, R. Lowinger and E. T. Sadoff, "Consolidation in a Vapor Saturated Atmosphere: A new technique for minimizing changes in the appearance of matte, powdering surfaces," in preparation.

8. U. Schiessl, Konservierungstechnische Beobachtungen zur Festigung Wassrig Gebundener, Kreidender Maslchichten auf Holz, Zeitschrift fur Kunsttechnologie und Konservierung, 2 (1989): 292-320.

9. S. P. Koob, "Consolidation with Acrylic Colloidal Dispersions," AIC Preprints of the 9th Annual Mtg., Phila PA, ed. J. Johnson and M. Morales, (Washington D.C.: American Institute for the Conservation of Historic and Artistic Works, 1981) 66-74.

10. R. Futernick, personal communication, June, 1990.

11. L. Mibach, personal communication, June, 1990.

12. E. F. Hansen and N. Agnew, "Consolidation with Moisture- Curable Diisocyanates: Polyureas and Polyurethanes," Preprints of the 9th Triennial Mtg. of the ICOM Committee for Conserv., Dresden (in press).