Anna Perkins

Retouching Materials

Big Picture

Some of the most important and outward-facing work that a paintings conservator does is to strategically retouch, or “inpaint,” over areas of loss. After the loss, or “lacuna,” has been filled (see "Filling Materials"), a conservator needs to paint over it such that both the damage and the restoration are imperceptible (or minimally perceptible). They strive to only paint over losses, leaving as much of the original artist’s work visible as possible (see "Code of Ethics").

The choice of inpainting materials is important for the longevity of the restoration. The AIC Wiki states that “Inpainting should only be done using chemically stable, easily reversible pigments so that the intervention will not result in future damage to the work of art.”¹ Non-reactive pigments that have good lightfastness, meaning they resist fading and are considered “permanent,” should be used instead of fugitive dyes or pigments, meaning colorants that are expected to fade. Choice of binder is also important for lightfastness, as pigments can behave differently in different binder chemistries and at different pigment to binder ratios.

Matching color frequently requires the use of visible spectroscopy, x-ray fluorescence, or other analytical techniques to ensure the inpainting looks seamless with the surrounding paint across lighting conditions. Different pigments of different chemistries have different absorbance and reflectance properties that can change how they appear under daylight as opposed to gallery-lit conditions (and even different indoor lighting conditions can produce this effect). This phenomenon is known as metamerism.

In their article, “Multiple Pigment Selection for Inpainting Using Visible Reflectance Spectrophotometry,” conservators Berns, Krueger, and Swicklik write, “Choosing the proper colors for this process can be difficult, due to the inevitable variations in lighting conditions, visitor perception, and aging between old and new materials.”² When conservators identify pigments using techniques such as x-ray fluorescence or visible spectroscopy, they can then use the same pigments as the artist when color-matching to the adjacent paint, which makes for a more harmonious restoration. Berns, Krueger, and Swicklik also acknowledge the additional complication that the pigments used for restoration may age differently (i.e. fading or yellowing) than the pigments used in the original work – one reason that conservation work must be reversible, as a work of art is likely to need another intervention in the future.

To best match the effect of the surrounding paint, conservators can mimic brush textures with various filling materials or use retouching paints with similar impasto or leveling properties. Depending on the artist’s medium, conservators can use water-based or solvent-based paints; they choose paints that are chemically dissimilar enough that they can be removed without removing the underlying original paint, but that are not so incompatible that their adhesion to the substrate is poor or that they do not imitate the look of the painting.³

Conservators can choose to make their own paints, which is sometimes necessary if an obscure pigment must be used, or buy commercially available conservation paints. Commonly, they will mix pigments into a commercially available removable varnish, such as Gamvar (which contains Regalrez 1094).⁴ Sometimes, a pigment has fallen out of favor and is no longer manufactured or is not preferred for health and safety reasons, like lead white. In such a case, conservators can mix alternative pigments together to match the target color.

Inpainting Approaches

In the mid-twentieth century, there was much debate about the “deception” of imperceptible inpainting, and some art historians and conservators argued against inpainting at all, considering material authenticity to be more important than the legibility, or “unity,” of the artwork. This perspective led some, like 1960s curator of the Yale University Art Museum Charles Seymour, to enact cleaning campaigns that revealed to the public the extent of the damage the art collection had suffered without any attempt at inpainting to restore the original composition.⁵ Since it is unlikely that the artist’s intent would be for the painting to be damaged, since the aesthetic and narrative meanings are less legible to the viewer, many artists, conservators, and art historians were unsatisfied with this method of cleaning without retouching. Recognition of the artwork as both an aesthetic and a historical object led other mid-century conservators like Cesare Brandi, Helmut Ruhemann, and Caroline & Sheldon Keck to a more balanced approach.

In their article “The Ethics of Inpainting,” Albertson and Murray summarize Brandi’s views: “[S]acrificing unity for authenticity does not serve the painting or the viewer.”⁶ Brandi viewed paint loss as an “interruption” that introduced new, unintended forms into the composition. Brandi wrote extensively on the topic of ethical inpainting, and based on his ideas, he and his successors developed the visible inpainting method of tratteggio, where the inpainting was convincingly imitative from afar, but close inspection would reveal a series of vertical lines, almost like a pixelated image. The authenticity of the painting was not compromised, in this view, since the hand of the conservator was distinct from that of the original artist.

Similarly, Ruhemann spoke of the ethics of inpainting, as Getty conservator Laura Rivers describes: “[C]omplete reconstruction was better suited to the goal of restoring coherence to a damaged work and that any ethical objections to matched retouching had given way with the introduction of UV light as a means of differentiating new from old paint [Jessell 1976, 1; Jessell, personal communication, 2000].”⁷ Since UV light, photographic documentation, or other techniques can readily show where inpainting is located in a painting, Ruhemann asserted that conservators can ethically use a range of materials for “invisible” or “imitative” retouching.

One of Ruhemann’s conservation projects, however, was the controversial and extensive cleaning of over seventy paintings at the National Gallery, London during WWII.⁸, ⁹ The resulting exhibit prompted accusations that he had “ruined” the paintings through overcleaning, since their appearance changed so drastically, but Ruhemann, through careful documentation, was able to demonstrate that he had not removed original paint.⁸, ⁹ Rather, he had revealed the colors under discolored varnishes, removed past restorations, and inpainted only sparingly. While he believed that retouching was ethical, he used it as minimally as possible.

Sheldon and Caroline Keck, a married conservation duo and founder of two conservation graduate programs in the United States, asserted the value of imitative retouching. Albertson and Murray summarize Sheldon Keck’s views, highlighting that a painting serves several functions:

Though the history of a painting bears significance, a deteriorating condition should not be allowed to detract from the formal qualities that allow the painting to convey an aesthetic meaning to the viewer. The Kecks, and others, viewed inpainting as crucial to conveying the artist’s intention, which they regarded higher than material authenticity.

Contemporary views on inpainting in the United States align much with Ruhemann’s and the Kecks’ perspectives, a less-is-more approach to conservation with invisible inpainting where required to bring “unity” to the painting. Brandi’s ideas about visible inpainting can be applied to larger areas of loss so that conservators are “inventing” as little of the composition as possible. While Albertson and Murray say, “no definitive approach is capable of delivering an ethical justification for inpainting in every case,” they provide that minimal intervention, or a “staying within-the-lines approach[,] is widely accepted as an ideal ethical practice.”⁶

Read more about historical approaches to conservation in “Historical Practices”.

Commercial Conservation Paints

While their most apparent advantage is convenience, commercial paints tend also to be smoother, brighter, and higher tinting than hand-ground paints. Due to the use of industrial mixing and milling equipment, paint manufacturers provide superior pigment grinding and dispersing than what a conservator can achieve in their studio. These companies also have access to premium pigment supplies that may not be available to conservators in the small quantities that inpainting would demand.

Commercial paints are often thickened to a heavy bodied viscosity that can be used for impasto, and these paints can also be thinned with the solvents recommended by the manufacturer. Since the paints are designed for use in conservation, paint companies, such as Golden and Gamblin, are often willing to share chemical information and conduct testing, such as lightfastness and removability, at the request of conservators. Golden and Gamblin both worked with conservators to develop their conservation paints, and Golden has a custom lab where custom colors can be matched to a paint sample or spectrophotometer measurement.

Common Commercial Conservation Paints

Polyvinyl Acetate

Because of their perpetual resolubility in alcohol and other gentle solvents, polyvinyl acetate (PVAc) resins have been a staple option for the retouching of oil paintings. PVAc binders, primarily used as a wood or paper glue, can also be mixed with pigments and various solvents to use for inpainting. Conservators Kristin deGhetaldi et al. discuss the history of PVAc resins in conservation: “By the 1950s, PVAC resin was being used routinely as a retouching medium, both as a water-miscible emulsion as well as a simple solution binder that could be thinned with organic solvents.”¹⁰ As the coatings industry continued to innovate, several PVAc resins that conservators considered tried-and-true were discontinued in the early 21st century. Feeling the loss of PVAc as a retouching tool, conservators worked with Golden Artist Colors to evaluate other PVAc resins for conservation purposes, and based on this research, Golden developed a line of PVAc conservation paints that are still commercially available.⁸

PVAc binders, while excellently reversible, are not as UV stable as acrylics, nor as durable or water resistant. Polymer scientists Kaboorani and Riedl write, “PVA[c] has three main drawbacks: low resistance to water and humidity, poor performance at elevated temperatures, and high susceptibility to creep [flow/deformation over time under pressure].”¹¹ The best quality of PVAc binders is their reversibility with water, but this makes them particularly susceptible to humidity. Their UV resistance is much better than traditional animal glues or natural varnishes but not as long-lived as acrylics. Yellowing or cracking may not be noticeable for the lifespan of a restoration treatment under gallery conditions, especially if the restoration is finished off with a synthetic varnish containing UV blockers.

Another drawback, depending on the particular PVAc resin chosen, may be fungal susceptibility. Biologists who examined 29 synthetic binders used in paints found that “Almost all the acrylic resins studied proved to be resistant to microbial attack, while all alkyd resins and some poly(vinyl acetates) turned out to be biodegradable.”¹² While biodegradability is good for the environment, it is not preferred for conservation, where coatings are meant to protect or, at least not bring harm, to the artwork. Thus, fungal and bacterial testing would be important in evaluating a PVAc binder for use in conservation. Or, dry film preservatives could be added to the paint mixture.

Regalrez 1094

A popular varnish resin in conservation, Regalrez 1094 is a low molecular weight synthetic polymer binder that is highly resoluble in many solvents, notably odorless mineral spirits, which is gentler on paintings and less hazardous to the health of conservators than solvents containing aromatics, which conservators often have to employ to clean off other types of varnishes. The resin is supplied as pellets that can be left to dissolve in solvent.

Gamblin’s Gamvar Picture Varnish, as developed with conservators, is based on Regalrez 1094.¹³ Thus, a conservator could mix pigments into a Regalrez solution or into Gamvar to make a conservation paint. This varnish is very high gloss, making it a good alternative to damar, which is very glossy but darkens significantly over time.

Because of its low molecular weight, Regalrez 1094 does not form a very durable or flexible coating, so it is not the most protective varnish. Its low molecular weight helps greatly, on the other hand, with removability since it readily redissolves in various solvents. Accelerated aging testing shows that in combination with UV blockers, it remains easily removable over time, but without, it embrittles and cracks after significant light exposure.¹⁴ When pigments are mixed with the resin, as is the case with inpainting, the pigments can also act as UV inhibitors.

Laropal A-81

Laropal A-81 is a low molecular weight urea-aldehyde resin soluble in alcohols and mineral spirits, among other solvents.¹⁵ Conservators worked with Gamblin to develop their conservation colors based on this resin. Leonard et al. write, “A urea-aldehyde resin produced by BASF was chosen as a medium because of its photochemical stability, excellent pigment wetting, and working properties similar to those of a natural resin medium. The paints are soluble in relatively low-aromatic hydrocarbon solvents and remain so after extensive accelerated aging.”¹⁶

Gamblin states that their “Conservation Colors are made from Laropal A-81, mineral spirits, and lightfast pigments. Alumina hydrate is added to the modern organic colors to adjust tinting strength. No additives are used.”¹⁷

Acrylic

Acrylics have superior lighfastness and UV resistance along with durability. Unlike “normal” artist’s acrylic paints that are water-based, there are acrylics that are soluble in organic solvents. Whereas water-based acrylic emulsion artists’ paints are designed to be permanent, there are some solvent-based acrylic resin systems that are reversible. Since acrylics tend to be higher molecular weight, the gloss is usually lower than a paint made with damar, Regalrez 1094, or Laropal A-81. The higher molecular weight brings with it desirable durability, flexibility, and chemical resistance.

Conservators could dissolve an acrylic resin in their preferred solvent and mix in pigments themselves, or they could purchase Golden Mineral Spirit Acrylic paints, which are designed for use in conservation.¹⁸

Conservation researchers have found some of the acrylic resins used in conservation to crosslink over time and become harder to resolubilize with the same solvents upon ageing. One such resin that becomes less soluble is Dow Paraloid B-67, though the use of UV inhibitors helps significantly to maintain removability. Whitten et al. write of B-67-type acrylic resins, “PiBMA is known to crosslink on exposure to heat and ultraviolet radiation.”¹⁹

Conservators often use Dow Paraloid B-72, which they have found to be resistant to yellowing and reversible after aging, but which requires 100% aromatics to dissolve (stinky). Elvacite 2044 (the same polymer at a higher molecular weight to the discontinued Paraloid F-10) similarly is reversible and resistant to yellowing. Its lower Tg (glass transition temperature) makes it too soft to use as a final varnish (attracts dust) but makes it suitable for an inpainting medium that will not embrittle.¹⁷

Gum Arabic

Gum arabic, a natural resin derived from tree sap, is the binder used in traditional watercolor and gouache paints. There are many commercial manufacturers of traditional watercolor paints (Daniel Smith, Winsor & Newton, Holbein, Schmincke, etc.), but it also is straightforward for a conservator to dissolve gum arabic powder into water to create an aqueous binder to combine with pigments. Since traditional watercolor can crack if used too thickly, it is best suited for transparent glazing applications. Gum arabic crosslinks over time, which can cause shrinking and cracking, and this chemical change in the binder diminishes its long-term ability to be removed with water, but isolation coats and/or final varnishing may mitigate this effect. Depending on the fill or varnish type, watercolors and gouache may have poor adhesion and be less suitable as an inpainting medium.¹⁸

Aquazol

Unlike gum arabic, the synthetic binder poly(2-ethyl-2-oxazoline), known as Aquazol, is perpetually resoluble in water. Like PVAc, Aquazol lacks water resistance and is susceptible to creep in high humidity, so the painting would need to be stored or displayed under climate-controlled conditions, or the Aquazol inpainting would need to be followed by an isolation coat or varnish layer to protect it from the elements.

The AIC Wiki provides a brief history of the material and its use in conservation. As an adhesive, Aquazol is also a useful binder for filling materials and for gluing down flaking paint layers.²⁰ Another advantage of Aquazol for conservation applications is its clarity, demonstrated by its refractive index close to that of glass.¹⁸

This binder is supplied as solid resin pellets that dissolve in water, among other solvents. Aquazol is offered at several different molecular weights that result in different viscosities and substrate penetration properties – the smallest molecular weight grade of Aquazol will tend to soak into the filling layer much more than the largest molecular weight, which will sit more on top of the filling layer. The AIC Wiki notes that “the lower molecular weight grades are more flexible and water soluble compared to the higher molecular weight products, which have more ‘strength.’”¹⁸ Though it is more resistant to cracking than gum arabic when applied thickly, it can remain sticky at mid to high levels of humidity or at higher temperatures if under-bound or used too thickly, so it would require isolation layers.

Golden manufactures QoR Watercolors using Aquazol as the binder, but a conservator could also buy Aquazol pellets, dissolve them, and mix in pigments. By making their own Aquazol inpainting material, a conservator could choose the grade of Aquazol most suited to the wetting and sheen of the color match. AIC conservators have successfully used QoR for inpainting.²¹

References

Filling Materials Code of Ethics