Mastering Varnishing Techniques: Insights for Artists from Scientific Research

Varnishing is a critical step in preserving and enhancing a painting’s appearance. The selection and application of varnish can profoundly affect both the visual and physical integrity of a work of art. This article explores key findings from the study Strip ‘Teas’: Solubility Data for the Removal (and Application) of Low Molecular Weight Synthetic Resins Used as Inpainting Media and Picture Varnishes, providing artists with actionable insights into varnishing techniques, materials, and best practices.

Why Understanding Varnishing Techniques Matters

The study addresses a significant challenge in art conservation: ensuring varnishes perform effectively without damaging underlying paint layers. It focuses on modern synthetic resins, such as Laropal A-81 and Regalrez 1094, which are valued for their clarity, resistance to yellowing, and solubility in mild solvents. The research offers practical guidance for varnishing techniques, including application and removal, by investigating these materials’ solubility properties through Teas plots.

Understanding these findings is essential for artists because varnishes are more than just protective layers. They can enhance the depth and sheen of a painting while safeguarding it for future generations. However, varnishes may disrupt underlying paint layers without careful selection and application or produce uneven finishes.

How the Solubility of Varnish Resins Was Tested

To ensure accurate predictions of varnish behavior, the study employed a systematic method to test the solubility of low molecular weight resins. Each resin was dissolved in 50 different solvents, and their behavior was observed. This included classifying the resin-solvent interactions into three categories: solvents (which fully dissolve the resin), borderline solvents (which leave some gelled residue), and non-solvents (which do not dissolve the resin or cause only swelling).

Samples were prepared by combining three grams of resin with ten milliliters of solvent in sealed jars. These mixtures were agitated and observed over several hours to determine solubility. Results were plotted on Teas charts, enabling conservators to visualize solubility parameters and identify safe solvent combinations for resin application or removal. The study provided valuable insights into refining varnishing techniques by systematically mapping resin behavior in various solvents.

The Role of Teas Plots in Refining Varnishing Techniques

Teas Plots are triangular charts that visually represent a material’s solubility behavior. They divide solubility into three fractional parameters:

  • Dispersion Forces (fd): Non-polar, Van der Waals interactions.
  • Polar Forces (fp): Dipole-dipole interactions.
  • Hydrogen Bonding Forces (fh): Attraction due to hydrogen bonds.

These parameters help identify solvent combinations that effectively dissolve specific resins while minimizing unintended interactions with other materials. For example, Laropal A-81 and Regalrez 1094 have distinct solubility windows, meaning they interact differently with various solvents. This understanding is critical for artists refining their varnishing techniques, as it helps predict how these products might behave on a painting surface.

Appendix 1: Solubility Data for Laropal A-81 and Teas Plot

The Teas Plot in the Appendices visually represents solvent-resin interactions based on solubility parameters. Each solvent in the table is assigned a number corresponding to its position on the Teas Plot. These coordinates on the Teas plot are derived from the solvent's dispersion, polar, and hydrogen bonding properties. The plot's key indicates whether the resin is soluble, borderline soluble, or insoluble in a given solvent based on its position relative to these coordinates. This Teas plot helps to inform decisions for selecting the optimal varnish for the resin.

Appendix 1: Solubility Data for Laropal A-81

# Solvent fd fp fh
1 Hexanes 100 0 0
2 n-Heptane 100 0 0
3 Odorless mineral spirits 98 1 1
4 ShellSol 340HT 96 2 2
5 Cyclohexane 94 2 4
6 Mineral spirits 90 4 6
7 Ethylbenzene 87 3 10
8 Turpentine 77 18 5
9 Benzene 78 8 14
10 Toluene 80 7 13
11 Xylenes 83 5 12

Insights into Low Molecular Weight Resins

Low molecular weight (LMW) resins, such as Laropal A-81 and Regalrez 1094, have become indispensable in art conservation due to their favorable properties. These resins, classified as synthetic oligomers, are defined by their molecular weights being less than 1,000. In contrast, high molecular weight (HMW) resins, like Paraloid B-72, an ethyl methacrylate copolymer, have molecular weights typically exceeding 20,000. LMW resins offer clarity, low viscosity at high concentrations, and excellent optical characteristics. They dissolve in weak solvents, minimizing risks to underlying layers during application or removal, while HMW resins provide greater durability but require stronger solvents for dissolution, which can pose a greater risk to delicate surfaces.

LMW resins also resist yellowing and maintain their solubility even after extended aging. For artists refining their varnishing techniques, these resins provide a practical and reliable option for achieving uniform finishes while preserving the integrity of the artwork. However, the specific solubility characteristics of each resin demand careful solvent selection to avoid adverse interactions with paint layers.

Key Findings for Artists Refining Varnishing Techniques

Ensuring Compatibility of Varnishes with Paint Layers

One of the study’s critical insights is the potential overlap in solubility between varnishes and underlying paint layers. For instance, Laropal A-81 dissolves in solvents with moderate polarity, while Regalrez 1094 is soluble in non-polar and slightly polar solvents. If a varnish solvent’s solubility range overlaps with that of the paint, the varnish may dissolve or disrupt the underlying layers. This emphasizes the importance of testing varnishes on a small, inconspicuous area before full application.

Testing and Brush Application Techniques for Varnishing

To apply Regalrez 1094 effectively by brush over Laropal A-81, it is recommended to use primarily aliphatic solvent blends with an aromatic content below 25%. A safer margin would be 20% aromatic content. Aliphatic solvents, which are less aggressive, help minimize the risk of disrupting the underlying paint layers. However, if the aliphatic solvent evaporates faster than the aromatic solvent, or if the aromatic solvent is absorbed into the underlying paint layer, it may lead to bleeding or smearing of restoration colors. Artists should also consider the specific composition of solvent blends. For instance, petroleum distillates with higher cycloparaffinic hydrocarbons, such as ShellSol 340HT, can shift the solubility behavior due to the higher affinity of these solvents for certain resins. This variability makes careful solvent selection crucial.

Additionally, gentle and controlled application techniques are essential to avoid disruption. Artists should test commercially-prepared varnishes on sample panels to check their compatibility with underlying paint and their suitability for multilayer varnish applications. Begin with thin, even layers, allowing each to dry thoroughly before applying subsequent coats. These tests help ensure that the varnish does not disrupt previous layers or cause smearing during brushing. By focusing on practical testing and precise application methods, artists can achieve smooth, durable finishes while protecting the integrity of the underlying paint.

Appendix 2: Solubility Data for Regalrez 1094 and Teas Plot

Appendix 2: Solubility Data for Regalrez 1094

# Solvent fd fp fh
1 Hexanes 100 0 0
2 n-Heptane 100 0 0
3 Odorless mineral spirits 98 1 1
4 ShellSol 340HT 96 2 2
5 Cyclohexane 94 2 4
6 Mineral spirits 90 4 6
7 Ethylbenzene 87 3 10
8 Turpentine 77 18 5
9 Benzene 78 8 14
10 Toluene 80 7 13
11 Xylenes 83 5 12

Understanding the Solubility of Dried Oil Paint and Its Overlap with Varnish Resins

The solubility parameters of dried oil paint are critical for selecting compatible solvents for varnishing and conservation. These parameters help identify solvents that can dissolve or swell a paint layer, which is crucial to avoid damage during varnish application or removal.

Dried oil paints, particularly those based on linseed oil, exhibit maximum swelling in solvents with solubility parameters ranging from approximately 8.8 to 10 (cal/cm³)¹/² or 18 to 20.5 MPa¹/² Hansen Solubility Parameters (HSP), dried oil paints can be described in terms of their components:

  • Dispersion Forces (fd): Around 70%, indicating a significant contribution from non-polar interactions.
  • Polar Forces (fp): Approximately 15%, reflecting moderate dipole-dipole interactions.
  • Hydrogen Bonding Forces (fh): Roughly 15%, indicating a minor but present contribution from hydrogen bonding.

These parameters align closely with certain solvents used for varnishing and cleaning, which can lead to potential interactions between the varnish solvent and the paint layer. When plotted on a Teas Plot, these solubility characteristics overlap partially with those of Laropal A-81 and Regalrez 1094.

For example:

  • Laropal A-81: Exhibits higher polar forces, making it more soluble in moderately polar solvents, which could overlap with the solubility range of dried oil paint.
  • Regalrez 1094: Dominated by dispersion forces, its solubility overlaps minimally, reducing the risk of paint disturbance during application.

The Role of Solvents in Mitigating Overlaps in Solubility

While the solubility parameters of varnish resins like Laropal A-81 and Regalrez 1094 may overlap partially with those of dried oil paint, the key factor in avoiding damage lies in the choice of solvents used to dissolve these resins. Solvents directly influence the interaction between varnishes and underlying paint layers, making their selection a critical component of varnishing techniques.

How Solvents Influence Solubility Interactions

Each solvent possesses its own solubility parameters, defined by its dispersion forces (fd), polar forces (fp), and hydrogen bonding forces (fh). These parameters dictate the solvent’s strength and compatibility with both the varnish and the paint layer. Solvents that fall within the overlapping solubility window of dried oil paint are more likely to cause swelling or softening, leading to potential damage.

For example:

  • Non-polar solvents like aliphatic hydrocarbons (e.g., hexanes, heptane) have low polar and hydrogen bonding forces. These solvents are less likely to interact with dried oil paint, making them safer for dissolving Regalrez 1094.
  • Moderately polar solvents, such as mineral spirits, strike a balance by dissolving Laropal A-81 effectively while minimizing the risk of disrupting the paint layer. However, the aromatic content in such solvents must be limited to below 20-25% to reduce the risk further.

Practical Guidelines for Solvent Use

  • Select Solvents Outside the Paint’s Solubility Range: Use solvent blends with solubility parameters that minimize overlap with dried oil paint.
  • Control Aromatic Content: For a safer application, maintain the aromatic solvent content below 20% when using Laropal A-81.
  • Test Before Application: Always test the solvent-varnish combination on a small, inconspicuous area to confirm compatibility and safety.

Artists and conservators can mitigate the risks of overlapping solubility windows by focusing on solvent selection as a crucial parameter. This strategic approach ensures that varnish layers can be safely applied and removed without compromising the integrity of the underlying artwork.

Applying a Multilayered Varnishing Technique Using Conservar Varnishes

Artists seeking to use multilayered varnishing techniques can turn to Conservar varnishes for isolating and finishing layers. Conservar Isolating Varnish, based on Laropal A-81, provides an excellent foundation layer. It minimizes absorption into matte or porous surfaces, ensuring that subsequent varnish applications achieve an even sheen. For the finishing layer, Conservar Finishing Varnish, formulated with Regalrez 1094, offers a high-gloss, protective topcoat that enhances the artwork’s visual depth.

Steps for Application:

Isolating Layer:

  • Apply a thin layer of Conservar Isolating/Finishing Varnish using a soft, wide brush.
  • Allow the layer to dry completely, ensuring a uniform protective barrier before the finishing layer.
  • Test the isolating varnish on a sample piece to ensure compatibility with the paint layer.

Finishing Layer:

  • Use Conservar Finishing Varnish as the topcoat for added protection and aesthetics.
  • Brush on thin, even layers to achieve a smooth and polished finish.
  • Allow each layer to dry thoroughly before applying additional coats if needed.

By combining these Conservar varnishes, artists can effectively implement multilayer varnishing techniques that enhance the appearance of their artwork while ensuring its preservation.

Mastering Varnishing Techniques: Final Thoughts

The practice of applying two varnish layers is increasingly popular due to its advantages:

  • Isolating Layer: A preliminary coat, often with Laropal A-81, creates a barrier that protects the painting when the upper varnish layer is cleaned or removed.
  • Surface Uniformity: Laropal A-81 reduces issues like “sinking in,” where the varnish is unevenly absorbed by a matte or porous paint surface.
  • Enhanced Aesthetics: This layer corrects uneven sheen better than Regalrez 1094 alone, ensuring a more polished appearance

For artists, these varnishing techniques offer both aesthetic and practical benefits. They enhance the painting’s visual appeal and ensure long-term preservation.

Recommendations for Artists

Selecting and Testing Varnishes

Artists often rely on commercial varnishes and cannot control their solvent compositions. However, understanding the principles of solubility can guide better choices. Look for varnishes designed for compatibility with specific media and test products on sample panels or inconspicuous areas.

Refining Varnishing Techniques with Layering

Apply an isolating layer before the final varnish to improve protection and aesthetics. Products like Laropal A-81 are excellent for this purpose, but alternatives with similar properties may be available commercially.

Additional Application Tips for Success

  • Avoid vigorous brushing, which can mechanically disrupt underlying paint layers.
  • Use thin, even layers for initial applications to minimize solvent penetration.
  • Allow adequate drying time between coats to ensure proper adhesion and prevent bleeding.

Mastering Varnishing Techniques: Final Thoughts

This study underscores the importance of understanding varnishing techniques for successful application. By considering solubility properties, testing products, and employing thoughtful techniques, artists can achieve beautiful, durable finishes that protect their work for years to come. The insights gained from this research inform current practices and empower artists to make more informed choices about their materials and methods.


Bibliography

Smith, Gregory Dale, and Ronald Johnson. “Strip ‘Teas’: Solubility Data for the Removal (and Application) of Low Molecular Weight Synthetic Resins Used as Inpainting Media and Picture Varnishes.” WAAC Newsletter, vol. 30, no. 1, January 2008, pp. 11-19. Download the Article.

Abstract: This article investigates the solubility properties of three low molecular weight synthetic resins commonly used in art conservation. By generating and analyzing Teas solubility charts, the study provides practical guidance for effectively applying and removing these materials.


The feature image of this arfticle is by Eric Johnson varnishing his painting with Conservar Varnish and using a Conservar Varnish Brush.


The Benefits of an Isolation and Final Picture Varnish

In varnishing paintings, using both an isolation and a final picture varnish has become a recommended practice. This technique offers critical benefits for both the preservation and aesthetic enhancement of paintings. By applying different varnish resins for the isolation and final coats, artists and conservators can achieve greater control over the appearance and longevity of their works. Here, we explore the evidence supporting this approach.

1. Protection During Varnish Removal

One of the most significant advantages of applying an isolation coat is its added protection during future maintenance. Over time, picture varnishes yellow and accumulate surface grime, even those made with high-quality varnishes. Removing the aged varnish becomes necessary to restore the artwork's original appearance.

When an isolation coat, such as one made with Laropal A-81, is applied beneath the final varnish, it acts as a barrier, protecting the paint film from direct contact with the solvent used to remove the top layer. This layer prevents damage or alteration to the underlying paint during cleaning or varnish removal, preserving the integrity of the artwork for future generations.

2. Correction of Matte or Uneven Gloss Surfaces

Oil paintings with matte or uneven gloss surfaces often absorb low molecular weight (LMW) varnish resins unevenly, resulting in a finish that mirrors these imperfections. This issue is particularly problematic for paintings with absorbent and matte surfaces, as the varnish tends to sink into the surface, creating a dull or blotchy appearance.

An isolation coat can correct this fault by providing a smooth, even layer over the painting surface. This initial coat of high molecular weight (HMW) or a polydisperse resin minimizes the varnish absorption into the paint layer, ensuring the final varnish coat achieves the desired level of gloss and uniformity. This step is especially valuable when working with materials like Laropal A-81, which are designed to seal the surface without altering its underlying texture.

3. Versatility in Varnish Choices

The final coat of varnish always determines the final appearance of a painting. By applying an isolation coat, artists can separate the functional and aesthetic roles of the varnish layers, allowing for more versatility in varnish selection. For example:

Isolation Coat: A resin like Laropal A-81 can be chosen for its sealing properties and compatibility with the painting surface.

Final Picture Varnish: A different resin, such as Regalrez 1094, can then be used to provide the desired aesthetic finish, whether it be high gloss, satin, or matte.

This two-layer system empowers artists to achieve the perfect combination of protection and appearance. The isolation coat provides a stable base that safeguards the artwork. In contrast, the final coat allows for customization of the visual impact, ensuring the artwork meets both preservation and aesthetic goals.

Benefits of Two Varnish Coats

The combination of an isolation and a final varnish is a practical and effective strategy for oil paintings. It protects the paint film during future varnish removal, resolves issues of uneven gloss, and allows greater flexibility in varnish selection. By incorporating these techniques, artists can create visually stunning works that are prepared to withstand the test of time.


The Varnisher's AtelierThe Varnisher's Atelier
The Varnisher's Atelier

Conservar Varnishes

Made with solvents with the best balance of solubility for varnish resin and dried oil paint. Conservar Varnishes are available as ready-made varnishes in amber glass bottles, as well as as kits.

The Varnisher's Atelier

This professional set equips artists with the tools to preserve and enhance their masterpieces. Featuring an isolating varnish for sealing layers, a finishing varnish for gloss or matte protection, a precision varnish brush, Picture CleanGel for meticulous cleaning, and a premium solvent for perfect dilution, this kit ensures a flawless finish every time.