Copper Supports

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The information presented on the Paintings Conservation Wiki is the opinion of the contributors and does not imply endorsement or approval, or recommendation of any treatments, methods, or techniques described.
Author: Josh Summer
Editors: Claire Winfield, Christine Gostowski
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=Introduction=
Elemental copper can serve as a primary support for oil paintings. Hammered or rolled metal provides a non-absorbent surface that can increase color saturation. Depending on the plate’s preparation, the reddish-orange metal can also provide a warm tonality to the finished painting, much like other supports including wood, or other paintings with reddish or pink ground layers. Moderate fluctuations in relative humidity and temperature cause negligible dimensional changes to copper supports compared to the typical hygroscopic counterparts, canvas or wood. Copper and oil bound paint films also share similar thermal coefficients of expansion (Wadum and Streeton 2012). These properties minimize stress-induced cracking and crack propagation in the paint and ground layers.
=History=
During the early 16th century, Italian artists began experimenting with painting on hard, rigid supports that included stone, marble, and copper. By the early 17th century, the practice of using copper supports had spread north, west and east to the Low Countries and Eastern Europe (Horowitz 2017). Cities like Antwerp helped to increase the popularity of the support through trade and other artistic production methods like intaglio printmaking (fig. 1). Soon after, the practice spread even further to the Iberian Peninsula and New Spain (Wadum 2017).
=Manufacture and Preparation=
Copper is either rolled or hammered to make a flat support suitable for painting. Early plates may not have the same thickness throughout, and the majority were hammered prior to 1770, when the use of rolling machines became more widespread. The production method of the copper plate can often be discerned under raking light. Hammered plates may exhibit concentric rings or shallow dents throughout the surface. Early rolled plates may exhibit lateral waves or undulations that are parallel. Sometimes the plates were hammered and rolled, a technique that could produce both waves and shallow dents (Horowitz 1986). 
To prepare the plate for painting, the surface was usually roughened. Sometimes the plates were rubbed with garlic or oil before the preparatory layer was applied. The intended effect of a garlic rub is not well understood, however it has been hypothesized that the material was used to clean the plate, create a more even surface, and/or preserve the color of the metal before painting (Horowitz 1986, Boers 2017, Stock 2017). 
If a ground layer was applied, it was often a thin layer of lead white, chalk, umber, carbon-based black or a mixture thereof in linseed oil. Treatises from the 17th and 18th centuries included instructions for applying this layer with fingertips instead of a brush (Boers 2017, Horowitz 2017, Stock 2017).
=Interactions Between Materials=
The research of Broers, Mecklenburg, Fuster-López, and Pavlopolous each indicate the presence of strong physiochemical interactions between drying oils and elemental copper. Copper ions are highly mobile and are easily solubilized in drying oils, leading to faster oxidation and polymerization, depending on the type and preparation of the oil (Fuster-López and Mecklenburg 2017). Adhesion between the plate and oil film is likely increased by the formation of copper carboxylates (Pavlopopous 2017). This layer may be green in color and should not be confused with corrosion of the plate.
=Mechanisms of Deterioration= 
Like other paintings on wood, canvas, stone, or other metals, paintings on copper are composite objects with numerous, complicated mechanisms of deterioration. However, when compared to other supports, paintings on copper are durable and stable if kept under dry, environmentally consistent conditions and handled with care.
Unlike wood and canvas, copper does not change shape with fluctuations in relative humidity (RH). It is also uncommon for paintings on copper to include a glue sizing layer, offering more stability against changes in RH. However, extreme conditions such as a sustained and high RH can lead to corrosion (Fuster-López and Mecklenburg 2017).
Copper is a soft, malleable metal and easily misshaped. This can lead to cracking in the organic layers of the paint film, effectively breaking the protective organic barrier between the metal and the environment. If not kept below 65% RH, moisture can access the copper surface, leading to corrosion and delamination of the ground and/or paint layers (Horowitz 2017). Figure 2 shows an example of this. Larger and/or unframed plates may be more susceptible to damage, especially on the edges and corners.
=Conservation Treatments=
Elemental copper is sensitive to water; therefore, improper use of aqueous cleaning methods, consolidants, and filling materials can lead to corrosion of the support. Use of solvents containing nitrogen and sulfur can also pose problems (Horowitz 1981). While synthetic materials dissolved in some organic solvents can fulfill the needs of treatment and re-treatability, the impermeable nature of the metal can make adhesion problematic. Highly specific tools and delivery methods may be necessary to consolidate flaking or detached layers. Water-soluble filling materials can be used, provided there is an impermeable barrier between it and the metal (Horowitz 2017). 
=References=
Broers, N. 2017. “Preparation techniques and their impact on the conservation of copper paintings.” In Proceedings from the symposium, Paintings on copper: and other metal plates: production, degradation and conservation issues. València, Spain. 71-82.
Horovitz, I. 2017. “Paintings on Copper: A brief overview of their conception, creation and conservation.” In Proceedings from the symposium, Paintings on copper: and other metal plates: production, degradation and conservation issues. València, Spain. 17-26.
Horowitz, I. 1986. “Paintings on Copper: Techniques, deterioration, and conservation.” The Conservator 10: 44-48.
Pavlopoulou, L. “Chemical, physical and mechanical decay processes in oil painted copper objects.” In Proceedings from the symposium, Paintings on copper: and other metal plates: production, degradation and conservation issues. València, Spain. 43-54.
Stock, A. 2017. “Reconstructing a painting on a copper support: Hendrick van Balen’s ‘Adoration of the Shepherds’ from the Fitzwilliam Museum.” In Proceedings from the symposium, Paintings on copper: and other metal plates: production, degradation and conservation issues. València, Spain. 43-54.
Wadum, J. 2017. “The Spanish connection: the making and trade of Antwerp paintings on copper in the seventeenth century.” In Proceedings from the symposium, Paintings on copper: and other metal plates: production, degradation and conservation issues. València, Spain. 27-42.
Wadum, J. and Streeton, N. 2012. “History and use of panels or other rigid supports for easel paintings.” In Conservation of Easel Paintings: Principles and Practices. Stoner, J.H. and Rushfield, R. eds. P. 105. Routledge 2012, NY.
=Further Reading=
Costaras, Nicola, Erma Hermens, and Annemiek Ouwerkerk. 1998. Looking through paintings: the study of painting techniques and materials in support of art historical research. Baarn: de Prom.
Oliveira, M. 2015. "A technical investigation of an oil painting on copper support, including a study on consolidants for treatment." Graduate Thesis. Faculdade de Ciências e Tecnologia e a Universidade Nova de Lisboa, Lisbon, Portugal.
Phoenix Art Museum, Nelson-Atkins Museum of Art, and Mauritshuis (Hague, Netherlands). 1999. Copper as canvas: two centuries of masterpiece paintings on copper, 1575-1775. New York: Oxford University Press.