ARTICLES > Conservation of Paintings8050

Choosing Varnishes.
In between the concept and the reality falls the practicing conservator…
by Christabel Blackman

 

Abstract

This article explains the importance of the step between new technological advances and their incorporation in the  conservator’s studio. Sharing information is vital between hands-on conservators and investigators. It particularly concentrates on varnishes and how new available materials have proved to be useful. After their application in many  different paintings, certain reflections have been made and conclusions have been drawn about their usefulness for conservators.

 

 

It is important for practicing conservators to be able to digest and assimilate new findings and technological advancements into the reality of the studio. For this reason, it is essential that helpful information is transmitted to conservators in a simple and understandable way.

Very often the hands-on conservators are so busy working that it is difficult to keep up to date with advances that may or may not be useful for us or beneficial for the works of art that are being treated. New techniques or materials are frequently heard about by word of mouth, or alternatively by working on projects with fellow colleagues and interchanging knowledge, skills and knowhow. Sometimes the realistic possibilities of the uses of new materials and techniques are not fully understood until they are applied over and over again in real-life conservation practices. The benefits or  shortcomings of their use for hands-on conservators surges forth from the many paintings treated and the observations about their different positive or negative aspects and results.

Varnish-making Decisions

As practicing conservators it is easy to become self indulgent in carrying out interventions that have been previously carefully chosen for a given work of art. Sometimes it is difficult to take a step back and look at the overall result of the work, until the job has been completed, often against the clock or in less than adequate conditions and with the couriers poised with masking tape in hand.

This is not only important in the moment of making our conservation decisions but also in the choices that we make about the “finish” that the conservation object will have.  In the field of easel painting conservation, the in-painting styles and materials are considered to be vital choices. However, often the way we re-varnish paintings does not take on as much importance as it should.
The original varnish of a painting was applied in a deliberate manner, it was intentional. The choice of varnish, its drying qualities, thickness, colour, mixture with other substances, its shininess or its sheen, spot varnishing, over glazing and so forth were all deliberate choices. We should take into consideration these different qualities of the original varnish when we think about the alternatives that are available to us on the product shelf of our studios.
Varnish is a protective layer which filters oxygen and light, it also serves to saturate colours and form an overall sheen. Aging and yellowing of varnish (oxidization) occurs because of the breakage of the double bonds in the molecular structure, resulting in free radicals that for new cross-linking. These changes at a molecular level alter the state of the original varnish, forming a more complex structure and for the practicing conservator that means that the polarity of the material changes and the varnish is no longer soluble in its original solvent. The stability of a varnish is an important quality because it ensures a greater reversibility for future interventions.

So which varnish should I use?

Traditional natural varnishes are akin to the desirable “finish” that we wish to achieve in an old painting, however they age and yellow quite rapidly – their inevitable oxidation will render them more difficult to remove in the future and the strong solvents required to eliminate them may be detrimental to the underlying painting. Low molecular weight hydrogenated hydrocarbon varnishes have similar qualities to natural varnishes, but they are more stable and may be adapted for better use. The most frequently used varnishes of this type are Regalrez 1094 or Laropal A81 (which should not be confused with the ketone varnish Laropal A80).
Acrylic polymer varnishes (e.g. Paraloid – also known as Acryloid) form a very uniform film and create a homogeneous brilliant layer. However, it is difficult to achieve a natural finish as they tend to sit very much on the surface because of their complex molecular structure. Low molecular weight synthetic varnishes have a more fluid viscosity which allows them to be better extended over the paint surface. One important difference between polymer varnishes and natural resin varnishes is that polymer varnishes work very well when applied on flat surfaces, but tend to mix the reflected white light with the paint colour when applied over more textured surfaces, thus reducing the saturation of the colours. On the other hand, natural resin varnishes form a glossier and smoother film and produce less scattering of surface light and more colour  saturation.

We must remember that varnishes in a dissolved state have a greater capacity to degrade than when they are stored in a solid state. For this reason, conservators mix varnishes when they are needed and according to the individual requirements of each work of art. Custom-made varnishes are preferable to industrially prepared varnishes, because even though their content (rarely their proportions) is marked on the bottle, we can never fully ensure to what extent degradation has occurred during its previous shelf-life.

More about low molecular weight synthetic resins

Low molecular weight synthetic varnishes are physically and chemically stable. They are easily soluble in aliphatic  hydrocarbon solvents like white spirit, and thus the application solvent does not affect the painting in the evaporation process as other solvents may (this effect is always dependent on the original materials used in the painting, their state of degradation and the nature of the varnish solvent).

For a greater understanding of how the varnish solvent may affect the underlying paint structure it is worth commenting on a particular physical property of these varnishes. The crystallization temperature is the temperature at which a substance changes from a liquid to a solid state (i.e. for Regalrez 1094 is 33ºC, Paraloid B44 is 60ºC while Paraloid B72 is 40ºC) and determines the rigidity of a varnish coating. Reaching the crystallization temperature results in the lessening of viscosity at a specific moment of the application of the varnish which is its “no flow” point. This is easy to perceive as it becomes no longer possible to manipulate the varnish and difficult to move the brush. From this moment on, the drying process may seem to have stopped, however it still continues; the solvents seep out through the underlying layers. For this very reason, the solvent that is chosen for the fabrication of the varnish may very well affect the picture.

Can I improve or adapt these varnishes?

To achieve maximum stability with varnishes, it is recommended to incorporate a HALS additive (hindered amine light stabilizer) like Tinuvin 292. This substance acts as a free radical scavenger: it inhibits the formation of free radicals and thus avoids the process of oxidization while maintaining an optimal level of reversibility. It guarantees the stability of the varnish and of the underlying layers (it should not be confused with a UV filter).
Polymer additives (e.g. Kraton) are useful to help increase viscosity and flexibility and to modify flow properties if required. Also, in order to reduce surface shine some conservators add a small amount of wax (e.g. Cosmoloid).

How can I use this in the conservation studio?

In pictures that have been over cleaned and have suffered the loss of some of their original binder, by excessively strong solvents and an inappropriately over zealous restorer, what Raymond White has referred to as “leached out”¹, the smaller molecular size of these varnishes facilitates penetration into the emptied inner areas of these poor paint layers, enabling the colours to become better saturated. Sometimes there are particular colours that do need a more specific treatment. It may be difficult to achieve an evenly varnished surface especially with differing porosities and application thicknesses of the paint layers. Usually pigment and binder proportions differ between colours, causing a patchy visual effect of contrasting surface sheen which was never intentional.
Conservators who have worked on old panel paintings will know that certain areas like the Virgin’s blue cloak almost always present difficulties, because the varnish tends to sink deeply into the surface, due to the nature of the pigments employed (it may be a granular Lapis lazuli or a brownish oxidized azurite). To solve this problem, it is necessary to spot varnish with Paraloid, thus creating an isolating layer which sits purposefully more on the surface.

The big advantage of these varnishes is that they can be eliminated in the future without affecting the underlying layers due to the differentiation that is achievable because of their distinctive polarity parameters of solubility. As a consequence, it is possible to apply low weight molecular varnishes on top of other varnishes with a distinctive polarity, thus isolating different layers and facilitating future removal. They are particularly useful when applied over historical varnishes (and coloured glazes) to saturate and protect. They are an excellent choice to regenerate visual harmony and eliminate optical confusion when applied over blooming paint or varnishes.

Painting conservators may work for months on tedious and very complicated retouching on pictures that have suffered extensive damage to the paint surface, be they large lacunae or numerous diminutive blister spots. In cases like these it can be beneficial to apply a final coat of sacrificial varnish, so that in some future moment, the upper varnish may be eliminated without the laborious underlying restorations being touched.
Accordingly, a sacrificial varnish may also serve to separate out different layers that may in the future need to be  removed. It may be applied and then later removed during various stages of the conservation process if necessary. In large public works like modern murals that are at the continual mercy of problems like graffiti or pollution, it may also be used so that future conservators have an easier job. In this way we are not only thinking of future generations of observers, but also of future colleagues.

Stability and distinctive solubility is the great advantage of low molecular weight varnishes. However, it may have a downside, too. Their ability to remain soluble in the same original solvent means that only the first application may be done with a brush, other successive layers need to be applied by spraying. For this same reason, sheen problems can not be solved by brush manipulation techniques; instead, they must be controlled by spraying methods, or wax additives.

Finishing Off

The choice of a painting’s final finish is usually the choice of the conservator-restorer technician unless otherwise decided or specified (which is rare).
Knowledge, experience, wisdom and know-how along with a certain “sympathy and understanding” of the intrinsic nature of the painting is all part of that decision making process. Obviously the greater the knowledge we can count on the better. However, direct and useful information is needed to avoid overdose extremes which can cause a retracting effect on the conservator towards the good-old, always-on-the-studio-shelf products. It is important that hands on conservator-restorers share their wealth of experience with their fellow colleagues as it can be as equally useful as scientific findings, if not, more so.

Notes
¹Interview with Raymond White, National Gallery of London, 2001.

 

Bibliographical references

1. Roy S. Berns and René de la Rie, “The effect of a varnish’s refractive index on the appearance of oil paintings,” Studies in Conservation 48 (2003) pp. 251-262

2. Marl Leonard, Jill Whitten, Robert Gamblin and René de la Rie, “Development of a new material for retouching”, Tradition and Innovation: Advances in Conservation, IIC 2000 Melbourne Congress, International Institute for Conservation of Historic
& Artistic Works, London (2000) pp. 29-33

3. René de la Rie, Conservation Science Unvarnished, Oration delivered on the assumption of the special chair for the chemistry of conservation and restoration at the University of Amsterdam, 30 October 1997, Stichting Bijondere Leerstoel voor de Chemie van Conserfvaering en Restauratie, Amsterdam (1999)

4. René de la Rie, “Polymer additives for synthetic low-molecular-weight varnishes,” Preprints of the 10th Triennial Meeting of the ICOM Committee for Conservation, Washington, DC., Paris (1993) pp. 566-573

5. René de la Rie, New Varnishes for Old Masters,

http://www.okhra.com/@fr/5/17/85031/articlepopup.asp
(accessed on 29th July 2007)

6. Jill Whitten, “Regalrez 1094,” “Measuring Tinuvin 292,” “Varnish Solutions,” “Characteristics of Low Molecular Weight Resins,” and “Characteristics of Polymers,” in Painting Conservation Catalog, Volume 1: Varnishes and Surface Coatings,  Paintings Speciality Group, The American Institute for Conservation of Historic & Artistic Works, Washington, DC. (1998)

7. Jill Whitten, “Low-Molecular-Weight Resins for Picture Varnishes,” Paintings Specialty Group Postprints, The American Institute for Conservation of Historic & Artistic Works, Washington, DC. (1995) pp. 124

8. Salvador Muñoz Viñas, Contemporary Theory of Conservation, Butterworth-Heinemann (2005)

9. Rafael Romero, et al., “Una reseña sobre el análisis y caracterización de barnices originales en pintura de caballete del siglo XVII”, Actas, I Congreso del GEIIC, Valencia (2002)

 

About the author:

 

Christabel Blackman
contact: christabelblackman@gmail.com

The author is a freelance practicing conservator-restorer in easel painting and specializes in Gothic and Renaissance Mediterranean panels and 17th, 18th and 19th century canvases. She has a Diploma in Restoration of Easel Paintings on Wood and Canvas, and is currently completing the Official European Masters Degree in Conservation and Restoration in Valencia.
She works as freelancer from her own private studio for public institutions, ecclesiastical and private collectors, etc.
She has written several monographic publications for the Generalitat Valenciana, catalogue chapters, congress papers, newspaper and magazine articles, etc. She was born in Melbourne, Australia in 1959, has lived and worked in Valencia, Spain for the last twenty years and holds dual nationality.

 

Download Issue 1

Download this article

 

Permanent link to this resource: http://www.e-conservationline.com/content/view/568/100/