The Use Of Consolidants In Mural Conservation

by Nathan Zakheim

(Published originally in MCLA Newsletter, v. 5, ns. 2 & 4 and v. 6, n. 1, 1993/4)

Today's murals are almost always painted in acrylic. Although this medium is usually designed for outdoors application, artists are often dismayed to find the colors eventually fade, or that the mural begins to "alligator," or develop cracks, bubbles or loose areas that can be peeled of by curious passers-by. Sometimes one layer of paint will flake away from the layer below, and in some cases the mural paint fails to stick to a wall at all. The solutions I recommend here are best suited to a dry surface. Retaining walls, stone walls that wick moisture up from wet ground, walls that are soaked with water during normal rainfall require extensive additional preparation and treatment--a subject we will examine in a future column. As a matter of ethics, non-reversible applications should not be used on any mural unless done by the artist, or with the artist's clear permission. Reversible consolidation should be done with care to ensure that no damage to the original paint will occur. Fading is common on acrylic murals. We have all seen examples of murals whose original vivid hues have become pastel or even close to white. Since most acrylic paint is reasonably colorfast, the color loss is generally not due to fading of the pigment itself. The real cause is the crystallization of the acrylic medium that binds or coats the pigments. Usually the medium has broken down in the ultra-violet rays of the sun, becoming so granular that, even though brightly pigmented, light is diffused off of numerous minute surfaces. The result is the appearance of color fading. The paint can be re-formed. This means that the acrylic granules can be wetted down with solvents like toluene or xylene. These cause the paint to revert to a strong plastic film. At the same time, with the restoration of the refractive index, all of the colors will return as well. Since toluene or xylene will dissolve acrylic paint if applied in sufficient quantity, treatment must be done with spray equipment only. It is essential, in terms of safety, that protective clothing and OSHA-NIOSH masks be worn when working with this material. Acrylic medium may be lost dunng the ultra-violet breakdown of the paint or varnish film, thus it is a good idea to mix acryloid B-72 into the toluene (10-15% solids), as that will mix with the existing powdered acrylic medium and create a very solid, strong, flexible, water-tight paint film. This process is not reversible, but it will dramatically enhance and increase the vividness of any acrylic mural. Colors that appear to be faded altogether will emerge as if by magic.

I have seen murals that were almost totally destroyed brought to such a well-restored condition that they could be expected to last for decades without further fading problems. Bear in mind that this treatment makes the paint water- and air-tight. As mentioned at the top of the article, I repeat: It should only be used on walls that do not transmit moisture from the reverse side.

Next I will consider consolidants that will re-attach loose areas, strengthen areas that are "crazed" or alligatored," flaked, or in other damaged conditions. The most common type of mural damage is localized in small areas: delamination of the paint, paint and ground layers cracking, chalking, peeling, etc. The following questions and answers touch on the seven most common problems that I have encountered in my experience with outdoor murals:

What can I do when I find that an area of paint has become loosened over a friable wall surface?

Sometinss only a few areas of a wall are friable, that is, prone to disintegration, crumbling and/or powdering. In such cases, if it is the paint that has loosened, then a Rhoplex gel such as Nova or Liquitex can be applied under the loose area. The material can be injected with a culinary piston syringe, or a small slit can be made in the loose paint, and the gel then introduced with a long palette knife. If it is the surface of the wall under the paint that is loose, diluted Rhoplex can be injected with a syringe until it is thoroughly soaked, then the gel can be applied. If the paint layer is composed of multiple coats of dissimilar paint media, then consolidation must be achieved before reattaching it to the wall. If the wall area is very dry and powdery, then diluted Rhoplex should be applied and allowed to dry. If multiple layers are delaminating firom one another a solvent base consolidant, such as Acryloid B66, B67, or B72 should be applied. Once dried, gel can be used to reattach the loose paint.

Sometimes I find that there is a thin layer of acrylic over deteriorating layers of oil-based paint such as house paint, and that those layers may not be well attached to the wall. How can I secure the painting?

The best solution is to soak the mural surface with a very dilute mixture of Acryloid B67 or B72 in V.M. & P. Naptha. This encourages the thin consolidant to soak to the fine cracks of the mural surface, thus laminating the crumbling paint layers together. If B72 is used, it must be thinned with Toluene or Xylene and applied ONLY by spray. It will chemically re-form the paint. Important: if you use a brush or roller it will NOT work. If B67 is used, it can be thoroughly applied using a large brush. If it becomes tacky, dilute the mixture further. Don't be shy about the quantity you use to cover the surface. The excess on the surface will act as a protective varnish when it's dry, thus stregthening the paint film. Once consolidated, the paint layers may lift off a damaged wall surface, as they will have formed an integrated film that is more cohesive than adhesive. Should this occur, the loose portion(s) can be reattached with Rhoplex gel applied by injection or with a spatula.

My wall seeps moisture, causing my mural to develop wet bulges, and even causing some areas of paint to be dislodged. What should I do?

This is a serious problem! If there is dirt behind it, the difficulty is even worse. With a retaining wall the best solution is to dig out all the dirt behind the wall, sand-blast and treat it with muriatic acid. Apply several wet coats of fiberglass imbedded in polyester resin. Drill drain holes along the bottom, and backfill behind the wall with dirt. Add a layer of at least four to six inches of gravel against the back of the wall. If the wall is drawing water from the ground up, the problem is yet more difficult. Full drainage should be arranged to eliminate the accumulation of water. The extreme answer is to isolate the wall from the ground with a layer of neoprene (Yes, this has been done!). The powdery efflourescence that sometimes occurs when there is the presence of water-activating salts in concrete can be reversed with Barium Hydroxide mixed in de-ionized water. Inject this mixture into the void areas behind the paint bubbles. When this dries, the wall can be consolidated with Rhoplex, or the paint can be reattached with gel if the wall is sound. You can always accelerate drying time and produce uniform, smooth results by using an iron and silicon paper to lay down the loose areas. A brayer and plastic scraper are also essential tools for this work. The gel must be washed off the surface of the mural, as it will become very brown and unsightly with time if it is allowed to remain. Clear Rhoplex products react to ultraviolet (UV) conditions much differently than the same media when serving as the binder for paint pigments (i.e., in the form of paint). Sunlight bounces back and forth between the twin surfaces of a Rhoplex layer, increasing the effects of UV--and making it turn brown. In addition, the polymer Rhoplex, unlike monomers such as Acrylloid B72, absorbs smog and other outdoor grime, holding them in a sponge-like fashion.

My paint layer is very strong, but it appears to be lifting from the wall in some areas. Can this be remedied?

Cut carefully along the bottom and sides of the affected area. It may be wise to take some liberty with the actual weakened area and cut along design elements of the artwork. Lift or roll up the loose area. If the wall surface beneath is crumbling, soak it with diluted Rhoplex varnish until it will absorb no more. Allow it to dry for a few hours, then replace the damaged area with Rhoplex gel. If the loose paint has shown a tendency to shrink, it can be relaxed using a heat gun, or an iron set on low over a sheet of silicon treated paper. The cut seams can be sealed with a treatment of thick Acrylloid Fs72 applied over seams softened with Xylene. When it dries, it will have fully bonded into a strong sealed surface that can be then varnished for aesthetic uniformity.

My mural looks glossy in some areas, and very chalky in others. The chalky areas even seem to rub off like chalk. Why is this?

The most likely problem is underbound pigment. This means that in some areas there is too much pigment in relation to the medium. When this occurs, the pigment actually aids in the disintegration of the acrylic binder. It tends to break down in sunlight and weather conditions. The cure is to apply a thin spray mixture of Xylene and Acryloid B72. This will re-form the paint layer, redissolving the acrylic polymer and mixing it with the monomer of the Acryloid B72. The colors, which would have appeared "faded" will come back vividly. Their refractive index made the pigments appear pastel because of a lack of continuity of the paint surface. The Xylene/Acryloid B72 spray restores this.

Many areas of my mural have become whitish and clouded, like an eye cataract. Can this condition be corrected?

The clouding is caused by trapped moisture in the crystalline structure of the polymer of monomer varnish coating. The goal is to get the moisture out. This calls for skill and patience, and a certain degree of nsk. Nonetheless, it is most probably the best solution to address this problem. The first goal is to thoroughly clean the affected areas. When the cleaning water has fully evaporated, spray a very dilute application of Acryloid B72. The solvent should be Xylene, and may be mixed with diethyl-benzine (for slow drying). When the surface has begun to soften, and has absorbed a lot of the solvent, and you have avoided sags, runs, and other disasters, then take a stiff bristled brush wet in the Acryloid B72 solution, and stab away at the white areas. I recommend a stencil brush of manageable size for this. If the area becomes dry, add more solution with your spray gun, continuing the agitate the varnish coat until the white goes away. Care must be taken not to rearrange or otherwise damage the mural paint layer. When the area has become clear, spray a uniforrn varnish on the whole painting, carefully blending in the treated areas and allow it to dry. The problem is unlikely to recur. In some cases, the varnish is so thick and white that it is best to pare it down or pull it away from the mural, as it's thickness will make it a liability even if treated. The original colors wait patiently under this thick and discolored "skin" to be retrieved.

I have seen a mural's paint peel off in sheets from some previously painted surfaces. Can this be halted?

Only with difficulty. The basic problem is that the substratum was of a type that resists water-miscible emulsions sticking to it. For this reason, the mural paint can be pulled off in variously sized pieces. If the problems are localized, and not comprising the majority of the surface area, then the best thing to do is to carefully peel back the paint, and apply a bonding medium to the paint below. In the case of a thin paint layer, the mural paint can be built up with varnishes so as not to cause it to break into small pieces. A heat gun will aid reapplication.