A Consolidation Treatment for Powdery Matte Pa int

by Elizabeth C. Welsh.

In: AIC Preprints of Papers Presented at the Eighth Annual Meeting, San Francisco, California, 22-25 May 1980, pages 141 150. Published by the American Institute for Conservation of Historic and Artistic Works, Washington, D.C., 1980.

Introduction

Powdery matte paint on art objects frequently needs consolidation to strengthen the paint as a film and to help hold the paint to its substrate. Conservation literature on the subject of consolidating matte paint was reviewed and the recommendations evaluated. A treatment using Acryloid B-72 (Rohm & Haas tradename) in diethylbenzene was developed.

Basic Criteria

As with all conservation treatments involving the introduction of new substances into an art object, the consolidant must be resoluble for an indefinite period of time. It might seem an academic point to require reversibility when considering a resin or other consolidant which will soak into a paint layer. It is often felt that such a consolidant could not be removed even if desired, because it would be too disruptive to the fragile paint. However, it is possible to remove resoluble resin from paint with a poultice technique, if not by swabbing, and sometimes the need to remove the consolidant comes long after its application.

A consolidant for matte paint must also have and retain the properties of colorlessness and flexibility, and it must be free from components that may be harmful to the object initially and after aging. It must be possible to apply the consolidant using solvents, temperatures, and tools that are not harmful to the object receiving treatment. The appearance of the object should be affected as little as possible by the consolidation treatment.

Literature Review

Over the past twenty years or so, a number of treatments for consolidation of matte paint have been proposed.

Poly n-butyl methacrylate, one of the earliest acrylics to be proposed for varnishing paintings [18, 31, 37], is now widely recommended for paint consolidation [9, 27, 32, 49]. It is available as a solid; for example, Elvacite 2044--formerly Lucite 44--(Du Pont Company tradenames), or in solution; for example, Bedacryl 122x [10] (Imperial Chemical Industries, Ltd. tradename), which has been renamed Synocryl 9122x [3] (Cray Valley Products, Ltd. tradename). When starting with Bedacryl 122x, the 40% solution in xylene is diluted usually with toluene, xylene, or acetone. When starting from solids, petroleum distillates containing about 10%-35% aromatics may also be used [4, 19]. A solvent combination that has also been recommended is 1:1 ethanol:xylene [27].

Poly n-butyl methacrylate has two disadvantages. Its glass transition temperature is about 20 deg. C (68 deg. F) [4]; therefore, it is soft at room temperature and has been shown, in film form, to rapidly imbibe airborne dirt [19, 20 p.146, and 21]. It is difficult to predict whether a consolidant which is largely present within the paint layer--not on the surface--will collect dirt, but it is a hazard which must be considered. The second problem with poly n-butyl methacrylate is that it crosslinks [19, 20 p.157, 21, 23, 30, 46]. Because of its low glass transition temperature and its crosslinking, poly n-butyl methacrylate should not be considered an acceptable candidate for paint consolidation.

Soluble nylon was introduced in conservation literature in 1958 [52]. It is made by reacting nylon with formaldehyde [29, 40, 51], and its chemical name is usually given as N-methoxymethyl nylon [29, 40]. Billmeyer has stated [8 p.437], "[soluble nylons] are copolymers of undisclosed composition." The soluble nylons most used in conservation are Calaton CA and CB (formerly Maranyl C109/p and C109) (Calaton and Maranyl are Imperial Chemical Industries, Ltd., tradenames). The granular solid is dissolved in warm methanol, ethanol, or alcohol and water [29, 40, 50, 51, 52]. As a dried film, soluble nylon is permeable to water and matte in appearance [40, 51, 52]. It was said to be flexible and non-contracting [40, 51, 52], but de Witte [14] demonstrated that it appears to have greater contractile forces than originally believed. Although its aging properties, if known, have not been published, it has been recommended as a treatment for paint consolidation for many years. Experience has shown, however, that after a few years, soluble nylon coatings become murky and gray in appearance (probably partially due to picking up airborne dirt) [11] and very difficult to redissolve [14].

Cellulose ethers--methyl cellulose, hydroxyethyl cellulose, sodium carboxymethyl cellulose, etc.--are popular for paint consolidation because they seem to have very little effect on the appearance of the paint they consolidate [42]. They are dissolved or dispersed in water and/or in organic solvents, depending on the cellulose ether used. They are often used in low concentrations because many of them are very viscous. A study has been published [42] in which it was found that three particular cellulose ethers remained water soluble after lengthy exposure to ultraviolet radiation. A technical bulletin from a cellulose ether manufacturer, on the other hand, describes how some cellulose ethers may be made insoluble by the crosslinking of "free unsubstituted hydroxyl groups of the cellulose molecule"[2]. In addition to questions about their resolubility, more information is needed on the resistance of cellulose ethers to yellowing, and on the length of time for which they retain their film strength and other initial characteristics.

Polyvinyl alcohols are water soluble, and that is their chief attraction as consolidants for some kinds of paint. There is a wide range of polyvinyl alcohols available, varying in molecular weight and degree of hydrolysis [13, 25 p.17-21]. Although some work [42] seems to show that certain polyvinyl alcohols do not crosslink, other writers state many conditions under which they do [7, 8, 13, 29, 35, 36, 45]. For this reason, it seems best to avoid polyvinyl alcohols until the properties of particular varieties are more definitely established.

Polyvinyl butyral is also sometimes used as a consolidant, usually dissolved in ethanol or other alcohols. Polyvinyl butyral is generally described in industrial literature as having great toughness, stability on exposure to sunlight, and clarity [1, 8, 34]. It is susceptible to crosslinking by "heating or with a trace of mineral acid" [1, 34], and on the molecular level, there are several mechanisms for crosslinking [1]. Some conservation literature describes polyvinyl butyral as resoluble [36], but as with polyvinyl alcohols, doubt remains as to whether it has long- term reversibility [19]. It does not appear to possess characteristics for paint consolidation that cannot be surpassed by polyvinyl acetate.

Polyvinyl acetate was the first synthetic resin in conservation to receive acclaim [28, 44]. It was first shown by Thomson [46, 47] to have excellent properties. It does not exhibit crosslinking when exposed to near-ultraviolet light [17, 19, 47], it does not yellow perceptibly upon aging [21], and it is available in a range of molecular weights with corresponding ranges of viscosity and softening points [29, 39]. Its solubility grade is 89 [19, 20 p.34] and is usually dissolved in alcohols, toluene, acetone, or Cellosolve. Polyvinyl acetate emulsions have also been used for paint consolidation [12]; however, additives and residual material from the polymerization reaction present in commercial emulsions may be harmful to paint [20 p.130, 45]. Experience also shows that polyvinyl acetate emulsions are difficult to redissolve to the degree needed to remove them from paint which they have penetrated.

Elvacite 2013 (Du Pont Company tradename), a methyl/n-butyl methacrylate copolymer [4], has recently been recommended [48, 49]. It has exceptionally low viscosity [4], but it is rigid and brittle at room temperature. Its elongation at break is only 0.5% [4]. In addition, it is a copolymer whose aging properties have not been studied.

Poly methyl methacrylate has occasionally been used as a powdery paint consolidant [33]. Like Elvacite 2013, it is rigid and brittle at room temperature--properties which are not usually desirable. Plasticizers have sometimes been added to counter the inherent brittleness, but it has long been felt that materials should be selected for conservation which themselves possess the desired characteristics [19; 20 p.30, p.153, p.160].

Acryloid B-72, a ca. 70/30 ethyl methacrylate/methyl acrylate copolymer [15], has been extensively studied since 1963 [19]. Feller has described it as "the most stable thermoplastic resin, soluble in hydrocarbon solvents, that we have encountered" [21 p.205]. It is considerably more photochemically resistant than the ISO-R105 blue wool standard No. 6 [24]; it is expected to suffer a loss of no more than 20% of its essential properties in 100 years of exposure under normal museum conditions [24]. It is extremely resistant to crosslinking [17, 19], it does not yellow perceptibly upon aging [21], it has low reactivity with sensitive pigments [5], and it is flexible enough to withstand a bend around a mandrel less than 1/10-inch in diameter without cracking [20 p.124]. Its solubility grade is 80 [19], and it can be dissolved in ethanol, 1-butanol, chlorinated hydrocarbons, Cellosolve, toluene, xylene, acetone, methyl ethyl ketone, and dimethylformamide, among other solvents [6].

A recent study has pointed out an apparent difference between Acryloid B-72 from about five years ago and that being sold today. Analysis showed that the "old" Acryloid B-72 was about 68/32 ethyl methacrylate/methyl acrylate copolymer and the "new" Acryloid B-72 is of the proportion 70/30 [15]. While there are noticeable differences between the old and the new, Feller has stated that a 2% shift in the copolymer composition would not alter aging properties [16]. The Rohm and Haas Company has claimed that the composition has not been intentionally changed [15, 16].

Proposed Acryloid B-72/Diethylbenzene Consolidant

The outstanding properties of Acryloid B-72 and polyvinyl acetate singled them out as the best candidates for the consolidation of powdery, matte paint. The problem remained to find a solvent and an application technique which could be used with one of the resins and which would provide effective consolidation of the paint while altering the object's appearance as little as possible.

Many of the objects in need of consolidation are made of wood and bark and are painted with weakly-bound pigments. To stimulate the conditions presented by these objects, pigments without binder were painted on thin plywood panels for tests. A variety of consolidants were applied to panels that had been painted with water slurries of kaolin, calcium carbonate, red iron oxide, yellow ochre, terre verte, and raw umber. It was found that the two pigments most affected visually by consolidants were yellow ochre and terre verte, so those two pigments were used for subsequent test panels.

A test was devised to compare the effect of different solvents on consolidants based on Acryloid B-72 and Bakelite polyvinyl acetate AYAA (Union Carbide Corp. tradename). A plywood panel was painted thickly yellow ochre pigment in water, and another was painted with terre verte pigment in water. One, two, and three coats of the following resin and solvent combinations were sprayed on the panels. Each coat consisted of three separate passes with the sprayer. Diethylbenzene was chosen as one of the solvents for Acryloid B-72 because of its very slow evaporation rate [19, 20 p. 37].


                         Solutions Used

4% Acryloid B-72 (w/w) in:         diethylbenzene
                                   1:2 diethylbenzene:toluene
                                   1:10 diethylbenzene:toluene
                                   toluene
                                   1:1 toluene:acetone
                                   xylene
                                   acetone

4% Bakelite PVA AYAA (w/w) in:     ethanol
                                   toluene
                                   1:1 toluene:acetone


                           Test Panel

<graphic status=omitted>

(Graphic shows a rectangular plywood panel painted overall with
pigment slurry. Ten separate rectangular test areas are shown
within the painted panel. The ten test areas correspond to the
ten resin-solvent combinations. Each test area has three zones:
1-coat, 2-coat, and 3-coat.)


The test panels showed decisively that the use of diethylbenzene made a great difference in the appearance of the consolidation [for related findings, see 32, 38]. None of the other solutions tested approached the appearance quality of Acryloid B-72 in diethylbenzene. In fact, three coats of the 4% solution were nearly invisible. It appeared that the use of the slow evaporator had two beneficial effects: it prevented the formation of a skin, which occurs when a fast-evaporating solvent is used and the consolidant solution begins to dry before penetrating the paint. It also avoided the poultice-like effect of a fast-evaporating solvent drawing the resin to the surface during drying.

The panels dried for five months, and they were then tested for the quality of consolidation. Various styluses were drawn across the panels, and the extent to which the paint crumbled in each consolidated area was compared. The Acryloid B-72 in diethylbenzene was at least as good as the other consolidants in this rough test, and distinctly superior to several of the combinations.

A second test was made by placing adhesive tape across each test rectangle; each tape strip crossed the 1-, 2-, and 3-coat zones of its rectangle. The tapes were pulled up and the results examined. The Acryloid B-72/diethylbenzene combination exhibited the best consolidation in this rough test method. It consolidated the pigment into a film, and it attached the film to the plywood substrate.

Because the appearance of some of the consolidants mentioned in past literature was said to be good, a test panel was made to compare 4% Calaton CB in methanol, 4% Bedacryl 122x in xylene, 0.75% Methocel A15C (Dow Chemical Company tradename) methylcellulose in water with a small drop of surfactant, and 4% Acryloid B-72 in diethylbenzene. The four were applied in 1, 2, and 3 coats on a panel painted with terre verte pigment. A visual comparison showed that Acryloid B-72 had a superior appearance. Scratch tests, like those described above, showed that the quality of the consolidation of Calaton CB was about as good as that of the Acryloid B-72, Bedacryl 122x was slightly less good, and Methocel A15C consolidation was poor compared to the other three.

Application Technique

The consolidant may be applied either by spraying or with a brush, but the rate of application is more easily controlled by spraying. We have found successful the use of a general-purpose airbrush and a small portable air compressor operated at about 10-12 psi. Use of high air pressure while spraying may displace pigment particles or blow off flakes. Areas which must be avoided may be masked with blotter paper.

The solution may also be applied using a paint sprayer with a disposable cartridge of propellant, although the air pressure is not adjustable and it is less comfortable to use than an airbrush.

We usually spray a 4% solution, and apply two or three coats with an hour or so between applications.

Diethylbenzene

Diethylbenzene is an aromatic hydrocarbon in the same family as toluene (methylbenzene) and xylene (dimethylbenzene). Like xylene, it has three isomers.

The boiling point of diethylbenzene is 180-184 deg. C (356-363 deg. F), and it evaporates very slowly. For comparison with two other slow evaporators used in conservation, the boiling point of diacetone alcohol is 168 deg. C, and that of Cellosolve is 135 deg. C.

Although diethylbenzene has a strong odor, studies show that toluene and xylene are both more toxic in vapor form [26 p.52]. An atmosphere of 2600-3200 ppm toluene and 2300-4600 ppm xylene led to prostration in mice, while 5500 ppm of diethylbenzene was required for the same effect. The low evaporation rate of diethylbenzene adds to its safety in this respect. Diethylbenzene is considered moderately irritating to skin [53], but it is believed that absorption through the skin is slow and very unlikely to cause systemic intoxication [26 p.45, p.53]. It does tend to remove fats from skin and cause chapping [26 p.45]. No corneal injury was found when diethylbenzene was put in test animals' eyes [53].

Application, and drying after application, should be done in a ventilated place. An organic vapor-absorbing mask should be worn while spraying the solution. Hands are protected best with "nitrile" gloves; diethylbenzene seeps through disposable polyvinyl chloride and polyethylene gloves.

Diethylbenzene is a slightly weaker solvent than toluene and xylene [20 p.232; 22]. Some substances were placed in diethylbenzene to test their solubilities in it.

                         Dissolved
               poly n-butyl methacrylate
               poly iso-butyl methacrylate
               Ketone Resin N
               dried Magna paint

                       Hazy Solution
               Victory 155 microcrystalline wax
               beeswax

                     Slightly Softened
               gum copal
               PVA AYAC and AYAF
               dried Liquitex paint
               dried Jade 403 PVA emulsion

                       Not Dissolved
               isinglass
               gelatin
               rabbitskin glue
               dried egg white and yolk
               gum kauri
               gum sandarac
               gum benzoes
               Indian gum
               shellac
               dried nitrocellulose
               carnauba wax flakes
               poly methyl methacrylate

Miscibility of diethylbenzene with several common solvents was checked. It was found to mix with Stoddard Solvent, hexane, ethanol, 1-butanol, acetone, xylene, and toluene. It did not mix with methanol or water.

Conclusion

Two conclusions were reached during this study. First, Acryloid B-72 in diethylbenzene is an effective consolidant for the treatment of powdery matte paint. It has been used with success on many wooden objects from New Guinea with paints that had weak or minimal binder, or no binder at all.

The second part of the conclusion is that solvent evaporation rate seems to have a major effect on the appearance and penetration of a consolidating solution for matte paint. Additional slow- evaporation-rate solvents should be found for use with Acryloid B-72 and polyvinyl acetate resins to increase their usefulness as consolidants.

Note

I would like to acknowledge the enthusiastic participation, advice, and encouragement of Pete Dandridge through this project.

Postscript (May 28, 1993)

This project was undertaken in the Objects Conservation Department of the Metropolitan Museum of Art in New York, New York in 1979 and 1980.

It was initiated to address a particularly difficult conservation problem:

A group of painted palm spathes (each about 3 ft. by 1.5 ft. in height and width, each varying from 0.25 inch to 2 inches in thickness) had been commissioned in New Guinea to be installed in a replica of a ceremonial structure in the Michael C. Rockefeller Wing of the museum. The paint on these spathes was very weakly bound, matte in appearance, often powdery on the surface, and in some cases separating from the smooth substrate. The replica ceremonial structure, a permanent part of the Oceanic exhibition, was to be out in the open, in an area with no relative humidity controls. The painted spathes were to be tied to the interior ceiling and walls of the ceremonial structure, paint-side-down.

When this paper was delivered at the 1980 AIC Annual Meeting, it was accompanied by a slide presentation.

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