Volume 8, Number 1, Jan. 1986, pp.7-11

The History of the Use of Synthetic Consolidants and Lining Adhesives

There has been a tremendous increase in the use of synthetic materials by conservators and nowhere is this more evident than in the lining and consolidation of paintings. In the past, conservators were reluctant to turn to synthetics but this attitude appears to be shifting as a wide variety of these materials are now used in every area of conservation. The purpose of this paper is to review the most recent work and attitudes concerning the lining and consolidation of paintings. I shall begin by providing some historical background of the traditional methods and materials used for lining and consolidation.

The lining of paintings came into general use in the 18th century. The adhesives for lining were based on aqueous mixtures, usually animal glue, flour, resin and Venice turpentine. One problem with this type of lining was the potential for shrinkage of the fabric support due to the moisture inherent to the process. In addition, these glue/paste adhesives became hard and brittle. They attracted mold growth and became difficult to remove. Glue/paste linings are still in use today.

In the 19th century, beeswax-resin mixtures came into use as lining adhesives in conjunction with hand ironing to press the two canvases together. Many improvements have been made to this hot-melt lining over the years, including the introduction of the hot table (1946) and the use of vacuum pressure (1955.) The wax- resin lining method is still the dominant lining used today, at least in this country.

The advantages and disadvantages of wax-resin and glue/paste linings have been argued in literature from at least the mid-1930's and the furor has not diminished. Naturally, this critical analysis focused attention on the need for other lining adhesives and techniques. Following traditional lining procedures, the surface of the painting is left vulnerable to deformation due to the heat and pressure that is necessary to effect a bond. This is particularly true for paintings having high impasto, or for those contemporary paintings in which the paint film is much softer than that of a traditional oil painting. Also, in the wax resin method, consolidation and lining are treated as one because it is impossible for the adhesive not to penetrate the fabric support and sometimes even the ground layer. This can cause considerable darkening of the support and ground, resulting in irreparable damage. One of the first uses of synthetic materials for the lining of paintings was the substitution of synthetic waxes and resins for the beeswax and natural resins used in the traditional mixtures. These synthetics were introduced in the mid-1950's. The use of synthetic waxes and resins was an attempt to lessen the darkening effects of the traditional wax resin mixtures since the synthetic materials are paler. In addition, softer waxes could be used in order to reduce the heat necessary to achieve a satisfactory lining.

As a result of the problems presented by the traditional lining procedures, several new lining methods have been proposed which address one or more of the disadvantages mentioned above. Of course, all of these new methods have their own advantages and disadvantages. One should remember that new does not necessarily mean better. We should not discard the old procedures and pick up the new ones without serious consideration. Each new adhesive or method is just another tool to be added to our repertoire. This paper is not an attempt to champion the use of synthetic materials. The point is that we should have at our disposal as many choices as possible so that well informed decisions can be made and treatments can be tailored to address specific problems. I would like to make an observation. We have all seen good glue/paste linings and good wax-resin linings. The adhesive used or the method employed may not be as important as how well it is understood, handled and applied by the individual doing the treatment. Any adhesive, natural or synthetic, and any method can be misused and result in damage.

Probably the first synthetic used in conservation was a poly (vinyl acetate) commonly called PVA. There are numerous varieties and their use in conservation was first discussed in 1932 in "Technical Studies." Their use as lining adhesives was first considered in a 1933 article by GETTENS and STOUT in Technical Studies called, 'The Problem of Lining Adhesives for Paintings.' They made experimental observations on painting fragments lined with various model adhesives among which was PVA. In 1953, LUCAS and BROMMELLE actually wrote an article called, 'The Failure of Synthetic Materials in Picture Conservation.' They evaluated PVA emulsions plasticized with dibutyl phthalate and also PVA in toluene solutions as possible adhesives for lining, strip lining, and marouflage. They found that PVA linings failed under stress and that reversibility was difficult. Today PVA is widely used in all areas of conservation.

In the 1930's and even into the 1950's, PVA was one of the few synthetics receiving any general use in conservation. Today the conservator has access to a multitude of synthetic materials. This may only create an uncertainty with regard to the choice of the right material. Some are so new that questions concerning preparation, correct dilution, and above all correct application are still unanswered. We must be able to make educated decisions as to the appropriate use of these materials.

I am now going to review the more recent literature in which synthetics have come to the forefront in the areas of lining and consolidation. In the late 1960's, Gustave Berger began his research into a heat seal type adhesive for lining. Wax resin's staining and that it could only be used as an impregnating adhesive were the main reasons for the development of Beva. Beva is a mixture based on an ethylene-vinyl acetate copolymer. It was first presented in the Bulletin of the IIC-AG in 1970, being billed as a new adhesive for the consolidation of paintings, drawings and textiles. It was reported to be colorless and transparent, able to be applied to and removed from porous materials without impregnation. It was described as being resistant to yellowing or crosslinking, and it was to give excellent adhesion and remain soluble in low petroleum solvents. Its use as a lining adhesive for paintings was described in the above bulletin in 1971. Being a heat-sealed type adhesive, Beva requires heat and pressure to effect a good bond. It can be used with or without impregnation of the original. When used as an impregnating adhesive, its staining effect on the fabric and ground is much less than with wax-resin. Beva is now a very commonly used material. Its merits and drawbacks have been argued for many years and Berger has published numerous articles illustrating its use. The formula for Beva was published in the IIC Lisbon Congress Preprints of 1972.

Variations on the use of Beva have been made by other conservators, such as the use of webbed Beva spray applications. With this type of application, lower temperatures can be used, irregularities on the reverse of the original can be cushioned during lining and reversibility is much easier as the bond strength is reduced.

Kristin Hermann presented a paper during the Paintings Specialty Group Session at the Philadelphia AIC meeting in 1981, in which she described a nap bond drop lining process and her experiments with Beva. In these experiments she replaced those components of the commercial product which have the potential to crosslink with more stable elements. She made experimental linings using the newly formulated adhesives and suggested that conservators could alter the basic Beva formulation to fit one's particular need and preference. In addition, the nap bond lining process she described uses minimal heat and virtually no pressure. In this technique the Beva is sprayed onto the lining fabric only. This prepared fabric is then placed on the hot table and heated to 160 °. F. The painting, under Dutch method restraint, is then laid onto the now tacky adhesive. It is rolled gently with a rubber brayer to insure contact and then removed from the heat. Since the painting is in contact with heat for only a matter of minutes, the paint surface never gets very hot. In this method, bond strength is reduced, reversibility is increased and danger to the picture is drastically diminished.

Also in the IIC Lisbon Congress Preprints of 1972, Bernard Rabin published his formula for a PVA heat-seal adhesive. It is a mixture of the Union Carbide products AYAA and AYAC which had to be made by the conservator in the studio. Beva and PVA linings are similar in their methodology in that both are heat-seal systems and do not require impregnation of the original. Possibly because PVA has a long history of use in conservation, the Rabin adhesive initially obtained greater use than Beva as an alternative to wax-resin lining in this country. However, PVA linings were found to become brittle with age. In the past five years, Beva appears to have surpassed the Rabin heat-seal as the adhesive of choice for those conservators looking for an alternative, and in those cases which do not require overall consolidation/impregnation.

At the same time that Berger started working with Beva, Vishwa Mehra, an Indian working in Amsterdam, began to develop a cold lining system. Mehra maintained that heat used in conjunction with vacuum pressure was the main problem with wax-resin linings and he restricted his research exclusively to cold linings. He also developed a cold lining suction table which is a major component of his method. After experimenting with a variety of different adhesives, he settled on acrylic emulsion, Plextol B- 500, a methyl methacrylate/ethyl methacrylate copolymer, as his lining adhesive. Actually the first account of the use of this material that I found was published by another Indian, BHOWMIK, in 1970. In his article which appeared in the "Museum and Picture Gallery Baroda, Museum Bulletin, XXII Technical Issue," Bhowmik does mention Mehra's work with Plextol B-500.

Mehra's research was part of an overall review of lining by the ICOM Committee on Structural Supports of Canvas Paintings. His initial publication was an Interim Report to this ICOM Committee in 1972. This report discusses the disadvantages of conventional lining methods and outlines the criteria for his search for a new lining method. He mentioned Plextol B-500 and Mosilith DM-5 (PVA emulsion) as lining adhesives. He also presented four materials for use as consolidants in this initial report: Bedacryl, a poly (n-butyl methacrylate); Plextol B-500; Plexisol, a poly (n-butyl methacrylate); and Mowital, a poly(vinyl butyral.) The first two had undergone thorough testing and the last two were still in the process of being tested.

It is interesting to note that Mehra tested four consolidants, two of which are poly(n-butyl methacrylates.) The reason for this is that the properties of any synthetic polymer depend on the degree of polymerization--the number of monomeric units linked together to form the polymer. The chains are not all of equal length in any given preparation; hence, an average molecular weight or average degree of polymerization is reported. The average molecular weight has a great effect on many properties of the polymer including its viscosity in solution, flexibility, solubility, and glass transition temperature. Therefore it is possible that two poly (n-butyl methacrylates) could have very different properties and that only one might be suitable for use in treating a specific problem. The FELLER, STOLOW and JONES book, On Picture Varnishes and Their Solvents, is still a good reference for acquiring a basic understanding of polymers and their properties. Based on this information, it is important to remember that when preparing Treatment Reports, one should not just write that poly(n-butyl methacrylate), or some other polymer was used, but exactly which one Plexisol P-500, Elvacite 2044, Bedacryl X-122--because each will have slightly different properties.

Mehra goes on to describe a test lining in which the painting was first consolidated with 5% Bedacryl X-l22 in turpentine. This consolidant was applied to both the front and the reverse of the painting. The painting was then lined using Plextol B-500 thickened with hydroxyethylcellulose. The thickened adhesive was applied to both the reverse of the original fabric and the lining support. While still wet, the two were placed together under sandbag pressure and allowed to dry. This wet lamination type of lining requires no heat. The thickening of the adhesive helps to reduce moisture content while permitting application to the original without significant penetration of the adhesive. However, wet lamination will produce a very strong bond, especially when applied to both surfaces, and I believe that reversibility would be difficult.

At the following ICOM Meeting (Venice 1975), Mehra described a new system using Plextol B-500 as before but this time in a more controlled manner and using what he described as a nap bond. He also introduced his Low Pressure Cold Lining Table which creates pressure by the movement of air through a plenum just below the surface of the table, rather than by using a vacuum. The adhesive is thickened as before, but applied now by forcing the paste through a screen (much the same way silk screening is done) and results in a dot pattern of adhesive on the lining support. No adhesive is applied to the reverse of the painting, in contrast to his original method. By choosing a screen of specific mesh size and thickness, Mehra maintained that a regulated amount of adhesive can be applied. Theoretically, the conservator can control the strength of the bond by limiting the amount of adhesive used. Again the bond is achieved wet using the suction of the cold table now instead of weighting with sandbags. No pressure is applied to the face of the painting. In this system the amount of moisture used in lining has been drastically reduced as compared to the earlier method. This lining should be reversible by peeling because the quantity of adhesive is reduced and it is applied only to one surface. The dot pattern has disturbed many conservators but Mehra contends that by using the appropriate screen, one can obtain a continuous layer of adhesive.

I find that controlled application through a screen is difficult. But this new system is an improvement over Mehra's initial test lining and it does allow one to line a painting without heat and using little pressure on the face of the painting. Mehra also changed to the use of Plexisol P-550, a poly (n-butyl methacrylate), as an overall consolidant, instead of his initial use of Bedacryl X-122 which is also a poly (n-butyl methacrylate). In both cases the consolidant was used on both the front and back of the painting.

Up to this point all of Mehra's linings were done with some amount of moisture in the adhesive. In a 1981 article in "The Conservator," he briefly describes a dry-film system in which the Plextol B-500 is thickened with 15% toluene, applied to the lining support and allowed to dry thoroughly. Just before the painting is to be lined, the dry film is sprayed with either isopropyl alcohol or toluene to create a tacky, contact adhesive. The painting is then placed over the support and left to adhere under low pressure. This technique can be used for those paintings that might react to moisture. This type of bond, although easily reversible, is often very weak and its usefulness is therefore limited.

It appears that while Mehra was developing a cold lining method, two Danish workers, Arthur Ketnath and Bent Hacke were simultaneously developing a low temperature heat sealing system using another acrylic emulsion. This was a softer polymer called Plextol B-360, an n-butyl methacrylate/methacrylate copolymer. Both men published in 1976.

Ketnath used Plexisol P-550, as did Mehra, for overall consolidation. Ketnath's article describes a series of tests performed on Plexisol B-550. It appears to hold up under accelerated aging, resistance testing and removability testing. Plextol D-541, a semi-hard acrylic emulsion, is also suggested as an adhesive for strip lining in Ketnath's article.

Bent Hacke's work was published in the same Danish publication as Ketnath's. Hacke then follows this with two ICOM Meeting articles in 1978 and 1981 and, consequently, his work is much better known. He developed a Low Pressure Heat Seal Table and, as Ketnath, used Plextol B-360 and Plexisol P-550 for lining and consolidation respectively. The system requires low levels of heat and minimal pressure, which is a suction type rather than vacuum pressure. Plextol B-360 is tacky when dry and can be activated at only 110°.-120°. F. The Plextol is usually applied to the lining fabric only, in one to three layers. Consolidation is accomplished, if necessary, with Plexisol P-550. Hacke's lining system is based on an aqueous emulsion as was Mehra's, but now the adhesive is allowed to dry and then activated at low temperature for lining.

Plextol B-360 is distributed in this country by Fine Arts Stretchers, Inc. in New York as the Lascaux product HV-360 in already thickened, ready to use form. It has found acceptance as an easily reversible adhesive that can be used at low temperatures for those paintings where heat can deform the paint layer. However, the bond strength is low so any pronounced deformations in the paint layer or support may not be held planar by using this particular lining method. Hacke's Low Pressure Table is equipped with a humidification system so that paint layer and support deformations can be treated prior to lining. After pretreatment with moisture to relieve stress and relax deformation, Hacke claims that many paintings may need only strip lining instead of an overall lining.

One can increase bond strength of Hacke's system by mixing the Lascaux HV-360 with HV-498. Lascaux HV-498 is a similar acrylic emulsion to the HV-360 but it activates at the slightly higher temperature of 130°.-140°. F. By controlling the proportions of these two emulsions one can vary the activation temperature to suit a specific need.

One last development... Robert Fieux, an American, proposed a lining method based on electrostatic hold and a pressure sensitive silicone adhesive on Teflon-coated glass fabric. The adhesive is purchased already coated onto the Teflon-coated glass fabric so one's choice of lining support is limited. No heat and only minimal pressure is required, which is an advantage in some instances, but the bond strength is very weak. I believe that this system is very limited but I did want to mention it as an alternative in some cases.

All the above workers--Berger, Rabin, Mehra, Ketnath, Hacke, and Fieux--use synthetic fabrics as lining supports. Among these supports are glass fabric, Rhoplex-coated glass fabric, polypropylene polyester, and Teflon-coated glass fabric.

It is interesting to note that Plextol, Plexisol, Bedacryl and other synthetic resins have their equivalents in American products. However, it is often difficult to determine which American product is similar to its European counterpart. Individual testing and information from the manufacturer are necessary in making this determination. At any rate, it is not necessary to use the exact materials mentioned here or someone's pet system. These systems, materials and attitudes about lining can be used as guidelines when developing treatments in one's own work.

I would like to emphasize that very little independent testing of these synthetic materials has been done. Each researcher has tested the materials used in his own method but I believe that further, objective testing is warranted. In the Preprints of the ICC Paris Congress on Adhesives and Consolidants (1984), there are several articles by independent workers in which testing of these synthetic materials has begun. The results are encouraging and all conservators should become more familiar with this published information.

In closing I would like to quote from MRS. KECK'S 1977 article 'Lining Adhesives: Their History, Uses and Abuses' in the Journal of the AIC:

The competent practitioner is always that person who selects the procedure most appropriate to the nature of the demand. 'Appropriate' implies personal response to observed characteristics. 'Select' implies freedom to make an educated choice.

To claim superiority of any single material, method of instrumentation over all others seems to me quite as disastrous an invitation as shouting insults to passersby from the window of a glass house.

Bibliography:

1) Percival-Prescott Westby, "The Lining Cycle," Conference on Comparative Lining Techniques, National Maritime Museum, 1974. (Good Bibliography.)

2) Berger, Gustave, "Formulating Adhesive for the Conservation of Paintings." Conservation of Paintings and the Graphic Arts, IIC Lisbon Congress, pp. 613-629 (1972).

3) Mehra, Vishwa, "Comparative Study of Conventional Relining Methods and Materials and Research Towards Their Improvement." Interim Report to the ICOM Committee for Conservation, 4th Triennial Meeting, Venice, 1975. Paper No. 75/11/5.

4) Rabin, Bernard, "A Poly (Vinyl Acetate) Heat-Sealing Adhesive for Lining." Conservation of Painting and the Graphic Arts, IIC Lisbon Congress, pp. 631-635 (1972).

5) Fieux, Robert, "Electrostatic Hold as a Pressure Source in the Lining of Paintings..." Journal of the AIC, Vol. 16, No. 2, pp. 36-38 (1977).

6) Ketnath, Arthur, "Acrylic Resins for the Conservation of Paintings on Canvas with the Use of the 'Heat Seal' Method." Medolelelser om Konservering, Vol. 2, Nos. 7-8, pp. 223-235 (1976)

7) Hacke, Bent, "A Low-Pressure Apparatus for the Treatment of Paintings." Meolelelser om Konserverln, Vol 2, Nos. 7-8, pp.199- 222. (1976).

8) Brommelle, et al. (Editors) "Adhesives and Consolidants," IIC Paris Congress, 1984. (Several articles on the testing of synthetic products).

9) Baer, N.S. and Kunz, N.L., "The Lining of Paintings -- 1900 to 1975: An Annotated Bibliography." AATA Supplement, Vol. 14, No. 1, Summer 1977.

10) DeWitte, E. and Goessens-Landrie, M, "The Use of Synthetic Polymers in Conservation: An Annotated Bibliography." AATA supplement, Vol. 13, No. 1, Summer 1976 (Part I: 1932-1965) and Vol. 13, No. 2, Winter 1976 (Part II: 1966-1974).