At Atelier CROMA, the restoration of paintings is a science that requires a deep understanding of the differences between modern and ancient artworks. Cleaning methods vary significantly based on the materials and techniques used by artists. Adapting cleaning techniques according to the age and composition of the paintings is crucial to preserving their integrity and aesthetics.
Cleaning of a modern oil painting with a buffer (Anaelle Duault‘s picture)
Old Oil Paintings :
Ancient artworks, primarily created before 1860, are often covered with varnish that protects the underlying paint. This varnish prevents dirt from penetrating the paint layer, making cleaning easier.
Natural resin varnishes, commonly used, were appreciated for their quick drying properties and long-lasting protection, although they yellow over time, sometimes altering the perception of the original colors.
Modern Oil Paintings:
Modern paintings, produced after 1860, use industrial materials and are rarely varnished. The absence of varnish makes them more vulnerable to dirt and degradation.
Dirt adheres directly to the paint layer, complicating cleaning and increasing the risk of damage. Additionally, modern materials exhibit different chemical behaviors compared to traditional materials, necessitating a tailored cleaning approach.
Cleaning Challenges
Materials Compatibility :
With industrialization and the invention of tube paints, oils are treated and clarified, often with activated clay, leading to an increased pH. Alkaline oil behaves differently with pigments and becomes less compatible, requiring industrial additives to improve pigment dispersion. Additives like aluminum stearate and zinc stearate, used especially after 1920, facilitate dispersion but can “affect the sensitivity of the dry oil paint films to water” [1] (Burnstock et al., 2006). Their degradation over time can lead to the formation of metal salts and soaps, making the paint more vulnerable to moisture and cleaning processes.
The hypothesis (Burnstock et al., 2006) suggests that aluminum stearate-based soaps may hydrolyze over time, forming hydrated aluminas [Al(OH)3] and free fatty acids or soluble fatty acid soaps. If sufficiently present, the hydrolyzed additive could prevent the oil from drying, making it vulnerable to moisture. The hygroscopic nature of hydrated alumina amplifies the harmful effects of water during cleaning. [2]
Although this theory has not yet been confirmed with certainty, previous studies link the use of solvent-soaked cotton swabs with changes in the appearance of the paint layer. [3]
Thus, the technique and materials used by the restorer must take this into account.
Dirt and Medium Skin:
Modern oil paintings, due to treatments applied, are biphasic, with an organic layer, the medium skin, and a paint layer. This medium skin results from the migration to the surface of additives, surfactants, and free fatty acids present in modern oils (Hendriks et al., 2017). These elements trap dirt, making cleaning more complex as the dirt is imbibed directly in the paint layer and difficult to remove without risking damage.
Dirt is differentiated into imbibed dirt and imbedded dirt in the paint layer (Hendriks et al., 2017). Dirt can penetrate the layer (in powdery/porous, water-sensitive, or brittle paints) or become imbibed in the medium skin or paint exudates.
Cleaning Methods
Recent discoveries highlight the importance of using appropriate and carefully controlled cleaning methods to preserve modern artworks.
Dry cleaning with polyurethane makeup sponge.
Dry Cleaning :
This method uses specific sponges to remove dirt without using solvents. However, dry cleaning may not be sufficient for embedded dirt and risks damaging the medium skin.
Makeup sponges, made of polyurethane,[4] are preferred for their softness and effectiveness. Some dry cleaning methods can leave residues, smooth, flatten, or polish the surface. It is recommended to rinse the sponges in demineralized water before use to remove any potential residues that could interact with the paint. The goal is to achieve clear water with a neutral pH during rinsing.
Buffer Solutions :
A buffer solution contains a weak acid and its conjugate base in nearly equal amounts. These solutions can maintain pH despite the addition of small amounts of acid or base, or dilution. Buffer solutions allow safer cleaning compared to demineralized water, as they prevent excessive acidification or alkalinization of the paint, which could lead to chemical degradation of the surface.
Installation and use of buffer solution (Anaelle Duault‘s picture)
However, even better than water, the buffer solution can still remove more than just dirt, potentially degrading part of the paint layer. According to the Cleaning Modern Oil Painting website:
“Adjusting the pH and conductivity (free ion content) of water-based cleaning systems, as well as the use of silicone-based emulsifiers […] showed some promise by offering the ability to remove the soiling while causing minimal change to the underlying paint film properties.” [5]
Thus, beyond pH, conductivity can also be adjusted. This helps control the interaction between the cleaning solution and the paint surface, ensuring even distribution of ions and avoiding undesirable effects due to inappropriate conductivity.
Gels :
Using gels, such as agar gel, Pemulen® gel, and silicone gel, allows controlled solvent penetration, minimizing the risk of mechanical and chemical damage to the paint surface.
Agar gel forms a solid gel when cooled, limiting solvent pene-tration into the paint (Van der Berg & Volk, 2014). When heated above 90-95°C, agar powder dissolves in water, forming a colloidal solution.
“When cooling down to 30–40°C -depending on the type of agar- a network of double helixes develops through hydrogen bonding, resulting in a stiff, rigid gel” [6]
This method is particularly useful for sensitive surfaces and allows surface dirt to be solubilized without requiring rinsing. Application varies: it can be applied as a rigid gel or hot solution, depending on the surface to be cleaned.
Pemulen® gels are polymers that create gel systems containing organic solvents and water, offering precise control over solvent application and limiting absorption by the paint (Cremonesi, 2003). They can be adjusted to modify their viscosity and compatibility with various solvents. Mainly used to stabilize oil-water emulsions, this minimizes hygroscopicity, respecting the artwork’s surface.
Silicone gels are used for their ability to encapsulate solvents and release them in a controlled manner without penetrating deeply into the paint layer, reducing the risk of deterioration and offering a gentler, more effective cleaning method. They are compatible with a wide range of organic solvents, making them suitable for different types of dirt and residues. They can also be used in combination with other gels or buffer solutions for specific treatments, offering great flexibility in cleaning methods.
The soft texture of the gel allows for cleaning delicate surfaces without abrasion. The main drawback of these gels is their cost.
Conclusion
Cleaning modern and ancient paintings requires a differentiated approach due to their distinct compositions and techniques. At Atelier CROMA, we apply precise and customized cleaning methods, ensuring optimal conservation and restoration of artworks while respecting their historical and material integrity. Our expertise allows us to choose the most appropriate techniques and materials for each artwork, ensuring its preservation for future generations.
For more information or to discuss a restoration project, please contact Atelier CROMA. We are dedicated to the preservation and restoration of artworks with expertise and passion.
[1] Burnstock et al., 2006. p.179.
[2] « Partially hydrolyzed aluminum stearate has surfactant-like functionalities and could potentially form micelles with external hydroxyl polar functional groups that may facilitate water wetting during cleaning. ». Idem. p.186.
[3] Burnstock et al., janvier 2008. p.657.
[4] « Most makeup sponges are produced from isoprene, styrene butadiene rubber (SBR), and mixtures of both on an industrial scale. The analyzed SBR-isoprene sponges all contain antioxidants and accelerators used in the rubber-making process. A minor part of the sponges consists of polyurethane, which contains hardly any additives. » Daudin-Schotte et al., 2013.
[5] Lee & Ormsby, 2017.
[6] Van der Berg, & Volk, 2014. p.392.
Sources
Burnstock, A., Jan Van Den Berg, K., De Groot, S., & Wilnberg, L. (2006). “An investigation of water-sensitive oil paints in twentieth-century paintings.” Modern Paint Uncovered Symposium, Getty Conservation Institute.
Burnstock, A., Megens, L., Van Keulen, H., & Jan Van Den Berg, K. (Janvier 2008). “Water sensitivity of modern artists’ oil paints.” ICOM Committee for Conservation, Painting Vol II.
Cremonesi, P. (2003). “The Use of Solvents and Solvent Gels in the Cleaning of Paintings.” Archetype Publications.
Daudin-Schotte, M., Bisschoff, M., Joosten, I., Van Keulen, H., & Jan Van Den Berg, K. (Janvier 2013). “Dry Cleaning Approaches for Unvarnished Paint Surfaces.” Smithsonian Contributions to Museum Conservation, Vol III.
Hendriks, E., Jan Van Den Berg, K., Steyn, L., & Stols-Witlox, M. (Septembre 2017). “Cleaning modern oil paints: The removal of imbibed surface dirt. Towards an integrated conservation methodology for the assessment, contextualization and treatment of imbibed surface dirt on unvarnished modern oil paintings.” ICOM-CC 18th Triennial Conference, Copenhagen.
Lee, J., & Ormsby, B., (March 2017). The trouble with modern oils: Recent CMOP progress at Tate. Dans Tate [consult the 12/06/24] URL: https://www.tate.org.uk/about-us/projects/cleaning-modern-oil-paints-0/trouble-modern-oils.
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