Atomic layer deposition and art conservancy – a marriage made in Maryland
Silver is a remarkable medium for artistic expression. It takes well to engraving, sculpting, casting, and fine detail while also having sufficient strength (especially in alloys) to insure the durability of art objects formed from this metal. Unfortunately, silver tarnishes when exposed to moist air, and removing layers of tarnish can damage the fine detail of artistic treasures. A new method for preventing the ravages of silver tarnish is now being developed by researchers in Maryland.
In the early days of art conservancy, efforts to preserve objects of great historical and/or artistic value were mainly aimed at restoring damage that occurred during storage or display. When such objects were made of silver, the primary concern was the removal of tarnish.
Tarnish is a thin surface layer of silver sulfide (Ag2S) that is formed when the surface of silver is exposed to sulfur in the presence of moisture. Silver and sulfur atoms are held together by covalent bonds, so the tarnish is very strongly attached to the silver surface. Tarnish is traditionally removed by abrasive or chemical attack, either of which is likely to damage the silver object by removing fine details over time.
This threat of damage began to change the focus of silver conservancy from restoration to preservation. Once a silver artifact is rendered free of tarnish, it can be stored or displayed in an air-tight box having a low-humidity atmosphere, but such boxes often fail in prolonged use. Alternately, methods such as the inclusion of activated charcoal in the display help to remove any sulfur-containing gases that might find their way into the display.
While these stopgaps are still utilized, the main technique to prevent or deter tarnishing is to apply a very thin coating of lacquer to the surface of the object to be protected. This will protect against the worst ravages of tarnish, so that over-polishing is not a serious danger.
The lacquers used range from nitrocellulose to several soluble plastic resins. In use, lacquers are dissolved in a suitable solvent (often acetone), and are brushed onto the silver surface in as uniform a layer as possible. Thickness of the dried lacquer is typically a micron or two. The delicate task of applying such a thin uniform layer usually requires the attention of an artist trained in conservation. When protected in this manner, tarnish can be held back for 10-20 years. However, the lacquer can give freshly protected silver a slight bluish cast, and may turn yellow with time.
The principles of art conservancy require that nothing be done to an art object that cannot be reversed, and that nothing be done that alters the natural appearance of an object. While these principles are often observed in the breach, they nevertheless represent goals that guide the efforts of conservators.
Nanometer-thick, metal oxide films offer new hope
A new approach to prevent the tarnishing of silver surfaces for up to a century per application is being developed by scientists from the University of Maryland and conservators from the Walters Art Museum in Baltimore.
Atomic layer deposition (ALD) is a method for growing thin films on a surface one atom at a time. Essentially, a surface is alternately exposed to precursor gases, one of which deposits a monolayer of aluminum, and the next converts the aluminum layer into a monolayer of aluminum oxide (though the actual process is a bit more complicated).
Because the deposition process is carried out a monolayer at a time, the deposited thin film of alumina has a consistent thickness everywhere on the silver surface. In addition, ALD is a highly conformal deposition technique that evenly coats the surface regardless of the depth and/or narrowness of creases, cracks, or holes. As a result, the silver surface is completely protected by the ALD-deposited alumina.
Simply covering the surface is not the only property the anti-tarnish ALD layer must exhibit.
"First, its appearance must be acceptable for display in a museum context," explained Walters Art Museum Conservation Scientist Glenn Gates. "It has to be tough enough to endure transport and handling, but not so tough that it can't be removed. It needs to be completely removable so an object can be re-treated to meet future standards of conservation and aesthetics. And finally, it should not cause any harm to a piece, even if it breaks down."
The alumina layer is completely transparent to light, but like all transparent thin layers, light impinging on the layer from certain angles will interfere, causing the appearance of false colors. Alumina is strongly bound to an ALD silver surface, and a removal technique that completely removes such a layer but does no damage to the newly exposed silver surface has not yet been established. This is particularly difficult considering the ability of the ALD deposition to coat nearly inaccessible areas.
Although it is clear that ALD-protected surfaces have great potential for use in the conservancy of silver surfaces, much more work is required before an overall set of methods is established that may be risked on irreplaceable art works. Further development of ALD-deposited thin films as anti-tarnish protection will advance by slow steps combined with a great deal of testing.
An overview of the new approach appears in the National Science Foundation video below.