Conservation of art using electrochemistry

2024年6月3日

文章

Preserving and protecting art from harm can be a complicated affair. Conservators strive to find a balance between protecting the artifact and retaining the original artistic intent. Fortunately, a large scientific tool kit exists to help conservators analyze everything from pigment composition to the age of artifacts, guiding preservation methods. Finding non-destructive techniques can be challenging, but surprising solutions like electrochemistry offer vital contributions to art conservation. Here, we highlight three cases showcasing electrochemistry's role in protecting art for future generations.

The following topics will be covered (click to jump directly to each):

Controlling the corrosion of coatings using electrochemical techniques
Cleaning and restoration of artwork with electrochemistry
Electrochemical surface-enhanced Raman spectroscopy (EC-SERS) for non-destructive pigment analysis

Controlling the corrosion of coatings using electrochemical techniques

The application of electrochemistry to corrosion science is well-known. Among other things, it covers the application and study of different coatings on metals to improve their corrosion resistance.

A similar approach using the techniques of polarization resistance (PR) and electrochemical impedance spectroscopy (EIS) can be applied to historically and culturally significant metal artifacts prone to corrosion. A high polarization resistance indicates better protection from corrosion. The major difference between this application and that of more typical industrial corrosion studies is the fact that the coating must retain a pleasing aesthetic appearance (i.e., be transparent as much as possible) [1].

An electrochemical study conducted in Bologna, Italy checked the efficacy of various coatings on a sample of fire-gilded bronze [2]. Using electrochemical analysis, the corrosion rate under different laboratory conditions was determined. This test was scaled up to real-world conditions to determine how the various coatings applied might impact corrosion of a real artifact. In this case, the analyzed sample served as a replica for the Gates of Paradise (Figure 1) [2]. Also shown in Figure 1 is a typical equivalent circuit for a metal in corroding environment and a Nyquist plot which can be used together to estimate the polarization resistance.

Left: Gates of Paradise by the sculptor Lorenzo Ghiberti, a pair of gilded bronze doors installed in the Florence Baptistery. Right: fitting of EIS data with the right equivalent circuit allows estimation of Rp, the polarization resistance.

Figure 1. Left: Gates of Paradise by the sculptor Lorenzo Ghiberti, a pair of gilded bronze doors installed in the Florence Baptistery. Right: fitting of EIS data with the right equivalent circuit allows estimation of Rp, the polarization resistance.

EIS has also been used on bronze statues to test the electrochemical stability (corrosion resistance) of both the underlying bronze and the patina which is often formed in corrosive urban environments [3]. In this case, Raman spectroscopy and hyphenated spectroelectrochemistry techniques (read more below) have become invaluable for determining the chemical makeup of the patina. Both techniques (EIS and Raman) have also been applied to study the composition of patinas formed on ancient bronze coins [4].

Cleaning and restoration of artwork with electrochemistry

Part of a conservator’s job involves restoring damaged artifacts to their original condition, or as close to it as possible. This often involves abrasive cleaning or immersion in chemical cleaning solutions. However, this is not always possible, especially when the artifact has intricate details.

The Rijksmuseum in Amsterdam encountered just such a problem. To preserve their unique artifacts, the museum employs a team of conservators specializing in various materials, including metals like silver. Among this team is Joosje van Bennekom, a senior metal conservator who faced the challenge of restoring a delicate silver table ornament which was made in 1549 by Wenzel Jamnitzer (Figure 2).

Tarnishing, a common issue with silver, occurs when silver reacts with sulfur compounds in the air, forming silver sulfide (Ag₂S) and creating the typical black color associated with tarnish. Traditional tarnish removal methods risked damaging the intricate artwork, prompting the development of an innovative solution: an electrolytic pencil. This tool, refined through collaboration with researchers and engineers, allows precise, localized cleaning of tarnished silver surfaces without risking damage.

From left to right: The silver table ornament that needed to be cleaned, a replica of the detailing made to test the electrolytic pencil prototype, and cleaning of one of the pieces from the Saint-Maurice d’Agaune abbey’s treasury.

Figure 2. From left to right: The silver table ornament that needed to be cleaned, a replica of the detailing made to test the electrolytic pencil prototype, and cleaning of one of the pieces from the Saint-Maurice d’Agaune abbey’s treasury.

The electrolytic pencil addresses this challenge by offering a controlled, localized cleaning process. It utilizes electrolysis to selectively reduce the silver sulfide, restoring the surface without damaging the artwork's delicate structures. Despite facing initial technical challenges, including issues with stability and leakage, the pencil has proven its efficacy in restoring medieval silverware at Switzerland's Saint-Maurice d'Agaune abbey (Figure 2). With its success documented online and its design freely available, the electrochemical pencil has since been applied to a variety of other artifacts [5,6].

Electrochemical surface-enhanced Raman spectroscopy (EC-SERS) for non-destructive pigment analysis

Raman spectroscopy has emerged as a powerful technique in art conservation [7]. Traditional Raman spectroscopy involves shining a laser onto a sample and analyzing the scattered light to identify molecular vibrations characteristic of specific materials. This technique has been widely employed to analyze pigments, dyes, varnishes, and other organic and inorganic materials used in artworks. As long as the laser power is tunable, its non-destructive nature makes it particularly valuable for examining delicate or irreplaceable objects.

Learn more about Raman spectroscopy in our blog series.

Frequently Asked Questions (FAQ) about Raman spectroscopy: Theory and usage

The inherently weak Raman signal often makes detection of certain compounds difficult. One area of advancement within Raman spectroscopy is the development of surface-enhanced Raman spectroscopy (SERS) and also hyphenated electrochemical surface-enhanced Raman spectroscopy (EC-SERS). EC-SERS combines the principles of Raman spectroscopy with electrochemistry, to enhance the signal strength and sensitivity of the Raman spectrum.

Read our related blog articles for more information about SERS and EC-SERS.

Raman vs SERS… What’s the Difference?

Raman spectroelectrochemistry from India to Spain: History and applications

A study published by researchers in North America focuses on the identification of polyphenolic components in yellow lake pigments, which are commonly used in paintings and other artworks [8]. Traditional methods for analyzing these pigments often require complex separation steps. EC-SERS, however, allows for the direct analysis of these pigments without the need for separation, bringing added value as a tool for art conservation.

The researchers demonstrate the effectiveness of EC-SERS by analyzing individual polyphenolic compounds, a model dye mixture, and two real yellow lake pigments: reseda lake and stil de grain (Figure 3). By applying a voltage to the SERS substrate, they are able to selectively detect different dye components in the pigments. This allows them to identify the presence of multiple polyphenols in each pigment, which would be difficult or impossible to achieve using traditional methods.

The stil de grain color and the polyphenolic compounds that make up many of the yellow pigment and dyes used in artworks from classical artists such as Rembrandt. Bottom left: a hyphenated EC-Raman system from Metrohm Autolab.

Figure 3. The stil de grain color and the polyphenolic compounds that make up many of the yellow pigment and dyes used in artworks from classical artists such as Rembrandt. Bottom left: a hyphenated EC-Raman system from Metrohm Autolab.

The results of the study highlight the potential of EC-SERS as a powerful tool for art conservation. By providing a sensitive, selective, and non-destructive method for analyzing natural pigments, EC-SERS can help conservators to better understand and preserve artwork.

Conclusion

Electrochemistry has proven to be a valuable part of the analytical tool kit for art conservators/restorers. Several other application examples support this point. These include VIMP (voltammetry of immobilized particles) which identifies paint, oils, and primers via their redox signatures at the ng scale [9]. The electrochemical signature of component materials has also been used as a method for both accurately dating artifacts and as well as authenticating them as originals [10].

Widespread adoption of these techniques will require collaboration, and as interest rises the techniques will surely become more accessible to non-experts.