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MAY 2011
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In the Studio:
Conservation of Historic Metals
Private collections in New York City include stellar examples of decorative art metalwork, including the above precious metal examples we had the privilege to treat recently. The image on the left is part of a sterling silver coffee and tea service, c.1925-30, by Georg Jensen with horn handles, smooth metal, and a brilliant finish. On the right is a detail of exquisite ormolu mounts on a finely veneered eighteenth-century commode. The smooth, burnished areas contrast with more matte textures on different areas of the mounts. The clients brought these objects to the studio because they were concerned about their deteriorated appearances due to normal wear and tear. These treatments gave us an opportunity to illuminate some aspects of historic metals and their conservation.
An Overview
The tea service is made of sterling silver, a precious metal alloy composed of at least 925/1000 parts pure silver and the remainder is usually copper added for hardness. Britain has a long history of setting a legal standard for the percentage of silver required in a silver object, and the sterling standard has been consistent since 1720. Although the set was made in Denmark, Jensen has used this alloy since 1915. The silversmith made the individual items in the coffee and tea service from flat sheets of silver that were hand hammered over steel anvils and stakes and slowly manipulated into shape. The other photograph is of the drawer handle made of ormolu, which is bronze covered in a very thin layer or "plate" of gold. The bronze was cast into shape and then steel tools were punched or chased onto it to add texture. The gold was applied to embellish the bronze through a technique called "fire gilding," in which mercury and gold were mixed together to form a liquid amalgam that was brushed onto the bronze. The amalgam was then heated until the mercury evaporated, and the remaining gold was burnished to make a highly reflective surface such as on the handles, while the textured surface was left matte. The craftsmen who made these objects had a clear understanding of their materials and their working procedures. As conservators, we must understand their fabrication techniques, how the objects once appeared, and how their current aged condition should be addressed.
Conservation Issues
Silver tarnish is a type of corrosion formed when the metal reacts with pollutants in the environment and the topmost surface of the metal chemically changes or corrodes. Conservators desire treatments to be as minimal as possible and reversible; however, to restore the brilliance of metal generally requires the removal of its topmost layer. This is one of the central issues of metals conservation. Gold, on the other hand, has the advantage in that it is chemically resistant to most pollutants, but unfortunately pure gold would be too soft and not much use to pull out heavy drawers. The thin top layer of gold protects the bronze from corrosion, although natural wear from use and cleaning slowly removes some of the gold, and the bronze below is exposed to the environment and forms a patina.
Treatment
The sterling tea set was severely tarnished, had accretions in the recesses from previous cleaning residues, and the horn handles were blanched from repeated washing. The ormolu on the commode had lost gold from the high points such as the handles and round berries, and the bronze was very dark brown. To address these issues, we carefully researched and tested abrasive and chemical compounds that would remove only the minimal amount of tarnished metal to achieve the desired brilliant finish for both these objects. While the cleaning procedures were different for the sterling and ormolu, on each we used specific chemicals and highly refined fine abrasives that were removed from the surface immediately after. This took them from dull, dark surfaces to the bright, lustrous sheen desirable in a precious metal. The horn handles were later toned with a pigmented coating that contrasted with the highly reflective silver. The discolored bronze of the ormolu mounts was polished and made bright and visually integrated with the surrounding gold.
A Conservator's Ethic
While there are many commercial metal cleaners available in the local store, most achieve quick results that create long-term harm, such as removing far too much metal from the gold plate, or leaving residue in the recesses that traps moisture and pollutants that begins a more corrosive process over time. Our clients are concerned about the long-term preservation of their furniture and objects. The conservators at Period Furniture Conservation would be pleased to examine any historic metals in your collection to review condition and explore treatment options.
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Furniture Conservator Yuri Yanchyshyn has worked with wooden objects for over 25 years, from a cabinet-maker's shop to the laboratories of The Metropolitan Museum of Art. Private collectors, museums, architects and designers have all entrusted precious items from the 14th through the 20th centuries to Yuri's care, and institutions such as Christie's invite him to lecture. Yuri holds degrees from the University of Michigan and the California Institute of the Arts and received advanced training from the Amsterdam Academy for Restoration and the Smithsonian Center for Materials Research and Education. © 2011-2012 Period Furniture Conservation LLC. All rights reserved.
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