Picture Varnish Aging: Insights from a 15-Year Study

Understanding the long-term behavior of picture varnish aging is critical for artists and conservators alike. For instance, knowing how natural resins like dammar yellow over time versus the stability of synthetic resins such as Regalrez 1094 helps make informed decisions about preserving artwork. The 1994 Canadian Conservation Institute (CCI) workshop titled “Varnishes: Authenticity and Permanence” provided an opportunity to explore both traditional and modern picture varnish formulations. Fifteen years later, an extensive review of the aging characteristics of these picture varnishes was conducted, providing invaluable insights into discoloration, gloss retention, and solubility. This essay summarizes key findings from the study, offering guidance on best practices for artists and conservators while considering broader implications for material selection and application techniques.

Preparation of Samples for the Study

During the 1994 CCI workshop, participants prepared multiple test panels to evaluate the behavior of different varnishes. The panels consisted of pre-primed artists’ canvas boards, measuring 35.5 cm x 45.7 cm, coated with various paint media. Most were primed with Winsor & Newton lead white oil paint, though some used alternative background colors, including black, brown, red, and blue. The paint layers were applied either as flat coats or textured with brushwork to simulate low impasto. Vertical strips of varnishes, including both natural and synthetic formulations, were applied in thin layers, with a narrow, unvarnished strip left as a control sample.

The varnish formulations varied, including proprietary commercial varnishes, newer synthetic varnishes, and 19th-century recipes recreated from artists’ manuals. Each panel was labeled and mounted in the paintings lab adjacent to a north-facing window at the Centre de Conservation du Québec (CCQ). For 15 years, the panels were subjected to natural aging under controlled environmental conditions that simulated museum settings. These rigorous testing environments ensured that the aging characteristics of each varnish type could be studied under consistent and realistic conditions, making the findings highly applicable to real-world artwork preservation.

Natural and Synthetic Resins: A Comparison of Picture Varnishes

Discoloration and Stability

The study revealed significant differences in discoloration between natural and synthetic picture varnishes, emphasizing the impact of material choice on both aesthetic outcomes and preservation. For artists, this distinction highlights how the yellowing tendency of natural resins such as dammar may compromise the appearance of their work over time, while synthetic options like Regalrez 1094 provide a more stable alternative for long-term durability (O’Malley 2010, p. 4). Conservators, too, benefit from these insights by being better equipped to select varnishes that align with the specific needs of an artwork.

Natural resins such as dammar and mastic demonstrated varying degrees of yellowing over the 15-year aging period (O’Malley 2010, p. 3). Interestingly, mastic and dammar varnishes are generally indistinguishable when used at equivalent resin concentrations. This is likely because varnishes made with relatively low molecular weight resins and similar refractive indices tend to resemble one another in appearance and behavior. Notably, formulations that included the stabilizer Tinuvin 292 exhibited far less discoloration, underscoring its importance in preventing premature yellowing in natural resins. For example, dammar varnish stabilized with Tinuvin 292 showed minimal yellowing compared to its unstabilized counterpart, which displayed moderate to severe discoloration depending on the solvent used. This finding underscores the practical necessity of incorporating UV stabilizers into varnish formulations to maintain aesthetic integrity over time.

Synthetic resins, on the other hand, vastly outperformed natural ones in terms of color stability. Regalrez 1094, a hydrocarbon resin, emerged as a standout performer (O’Malley 2010, p. 4). Even unstabilized samples showed little to no yellowing, with only one sample exhibiting slight discoloration due to the presence of a viscosity modifier, Kraton rubber. This highlights Regalrez 1094 as a strong candidate for varnishing both traditional and modern artworks. Its superior performance makes it a preferred option, particularly for applications where long-term clarity is essential.

Varnish Discoloration on White Test Panels

Excerpted from Table II in the paper Review of Samples for the 1994 CCI Workshop “Varnishes: Authenticity and Permanence” after 15 Years of Natural Ageing

Varnish Discoloration on White Test Panels

Excerpted from Table II in the paper Review of Samples for the 1994 CCI Workshop “Varnishes: Authenticity and Permanence” after 15 Years of Natural Ageing

Insights from Table II of Picture Varnish Discoloration

Table II highlights the degrees of discoloration across various varnishes after 15 years of aging. Natural varnishes like dammar and mastic demonstrated varying levels of yellowing, particularly when unstabilized. For instance, unstabilized dammar varnish prepared with turpentine exhibited moderate yellowing, while stabilized variants using Tinuvin 292 showed no visible yellowing. Similarly, synthetic varnishes such as Paraloid B-72 and Regalrez 1094 consistently performed well, with no or minimal discoloration. These findings underscore the critical role of stabilizers in both natural and synthetic formulations, especially in maintaining visual clarity over time.

Beyond discoloration, the study also observed changes in varnish surface texture and sheen. For example, older natural varnishes often became more brittle and developed micro-cracking over time. This was particularly evident in varnishes prepared using historical recipes that lacked modern stabilizing agents. These physical changes not only altered the visual appearance of the artwork but also posed additional challenges for conservators aiming to remove or replace the aged varnish without damaging the underlying paint layers.

Gloss and Saturation: Aesthetic Implications of Picture Varnishes

Assessing Gloss Across Different Picture Varnishes

The study highlighted the variability in gloss retention across varnish types (O'Malley 2010, p. 5). Natural resins such as mastic and dammar generally exhibited higher gloss and saturation levels when compared to synthetic resins. On colored paint layers, these natural resins significantly enhanced the depth of darker hues, a quality prized by many artists for its aesthetic appeal. This ability to enhance visual depth plays a pivotal role in achieving desired finishes, particularly for historical recreations or highly detailed works.

Interestingly, the gloss performance of synthetic varnishes was more variable, which impacts both the consistency of aesthetic outcomes and the predictability of their application. For example, variability in gloss might result in uneven finishes on larger works, requiring additional adjustments or reapplications by the artist or conservator. While Regalrez 1094 showed relatively low gloss when applied in a single layer, its performance improved when applied in multiple layers. Conversely, MS2A maintained a consistent, glossy appearance regardless of the application method, making it an attractive option for artworks requiring high gloss. These observations highlight the need for artists to experiment with application techniques to optimize results.

Saturation Effects on Different Paint Media

The ability of varnishes to saturate underlying paint layers varied significantly. Natural varnishes like dammar and mastic outperformed synthetic alternatives in enhancing the richness of colors on oil-based paints. However, the differences were less pronounced on acrylic paints, with all varnishes appearing uniformly glossy. These findings suggest that the choice of varnish should be tailored to the specific medium and aesthetic goals of the artwork. Understanding these nuances can inform material choices and application methods for artists working across media to achieve cohesive visual outcomes.

The Importance of Picture Varnish Layering

The study found that the order of varnish layers significantly impacts both saturation and gloss. For instance, red paint appeared slightly more saturated when a low-molecular-weight resin, such as dammar, was applied as the uppermost varnish layer. In contrast, applying a higher-molecular-weight resin, like Paraloid B-72, over dammar varnish resulted in a less glossy and saturated surface. This insight highlights the importance of carefully considering the sequence of varnish applications to achieve desired visual effects.

Moreover, layering different varnish types provides additional benefits, such as enhancing the mechanical stability of the surface. For example, a harder varnish like Paraloid B-72 can provide a robust base layer, while a softer, glossier varnish like dammar can create a visually appealing topcoat. This approach allows artists and conservators to balance durability with aesthetic considerations, ensuring the artwork remains visually compelling and well-protected against environmental factors.

Solubility and Reversibility

Solubility is crucial for any varnish, particularly for conservators tasked with removing or replacing varnish layers. A varnish's solubility determines the ease with which it can be reversed, a vital aspect for preserving underlying paint layers during conservation efforts. For example, hydrocarbon-based varnishes like Regalrez 1094 can be dissolved using non-polar solvents such as isooctane, ensuring minimal risk to the artwork. This property allows conservators to perform necessary interventions with precision and safety, maintaining the integrity of the original artwork.

The study demonstrated that all tested natural and synthetic varnishes remained soluble in mild solvents after 15 years. Hydrocarbon-based varnishes like Regalrez 1094 and Arkon P-90 were especially easy to dissolve, even with non-polar solvents such as isooctane (O'Malley 2010, p. 6). This level of solubility ensures that these varnishes can be safely removed without damaging the underlying paint layers. By maintaining reversibility, these varnishes support ongoing preservation efforts while accommodating future changes in conservation techniques.

Implications for Artists

Choosing the Right Picture Varnish

The findings underscore the importance of selecting varnishes based on both aesthetic and practical considerations. While natural resins like dammar and mastic offer superior gloss and saturation, they require stabilization with additives like Tinuvin 292 to mitigate yellowing. Synthetic varnishes such as Regalrez 1094 provide exceptional color stability and ease of reversibility, making them suitable for contemporary artworks and sensitive surfaces. These insights guide both artists and conservators in balancing aesthetic preferences with preservation needs, ensuring that varnish choices enhance rather than detract from the artwork.

Best Practices for Applying Picture Varnish

To achieve optimal results, artists should carefully consider application techniques and layer thickness. Multiple thin layers generally enhance gloss and uniformity, as evidenced by the performance of Regalrez 1094 and MS2A in the study. Additionally, tailoring varnish selection to the paint medium—whether oil or acrylic—ensures compatibility and maximizes aesthetic appeal. Artists and conservators benefit from meticulous planning and testing before committing to a particular varnish, ultimately achieving finishes that meet visual and preservation goals.

The Conclusion on Picture Varnish Aging

This 15-year study provides a comprehensive view of the aging characteristics of natural and synthetic picture varnishes. Artists and conservators can glean valuable insights into how varnish choices impact their works' visual and physical longevity. By understanding the differing aging patterns—from discoloration in natural resins to the stability of synthetic options like Regalrez 1094—they can make informed decisions tailored to their specific needs. These findings reinforce the importance of stabilizers such as Tinuvin 292 and underscore best practices in varnish application to achieve desired aesthetic and preservation outcomes.

Moreover, this study offers a roadmap for future research and development in varnish formulations. By building on these findings, the art and conservation communities can continue to refine their approaches, ensuring that materials evolve in step with the demands of contemporary and historical practices. Understanding the nuances of varnish behavior empowers practitioners to strike a balance between artistic vision and long-term preservation, safeguarding cultural heritage for generations to come.

Bibliography

Carlyle, Leslie, and James Bourdeau. Varnishes: Authenticity and Permanence Workshop Handbook. Canadian Conservation Institute, 1994.

De la Rie, E. R., and C. W. McGlinchey. “New Synthetic Resins for Picture Varnishes.” In Cleaning, Retouching, and Coatings, IIC Brussels Congress, edited by J. S. Mills and P. Smith, 171. London: International Institute for Conservation of Historic and Artistic Works, 1990.

O'Malley, Michael. “Review of Samples from the 1994 CCI Workshop 'Varnishes: Authenticity and Permanence' after 15 Years of Natural Ageing.” Journal of the Canadian Association for Conservation 35 (2010): 3-8. Download the Article.

Conservar Varnishes comprises synthetic and natural resins tested in the 15-year natural aging study. They all contain TInuvin 292, the stabilizer that minimizes the effects of varnish discoloration. Learn the differences between each varnish by visiting the individual product page.

Frequently Asked Questions (FAQ) About Picture Varnishes