Tarnish is a discoloration or dulling of a metal surface due to a chemical reaction, such as oxidation. Typically, silver will react with atmospheric pollutants, such as sulfur oxides, to form a black silver sulfide tarnish. Copper, brass, aluminum, and lead are also susceptible to tarnishing (Mayer 1969).
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Prevention[edit | edit source]
Lacquer coatings can be used on silver objects to protect it from the corrosive gases in the atmosphere, such as sulfur dioxide, that cause it to tarnish. In the 1980s, The Winterthur Museum began testing lacquer coatings and chose Agateen Lacquer #27, a cellulose nitrate polymer, to care for their silver collection. This coating provides the best visual results and is compatible with the silver surface, resulting in a durable bond. Agateen provides minimal visual intrusion, stability, and tarnish protection. However, there is a limited life expectancy for lacquer coatings - approximately 20-30 years dependent on environmental conditions.
The surface of the silver needs to be completely dry and free of tarnish before the coatings can be applied. Two coats of Agateen are then applied using brush or by spraying. This is the most difficult part of the process. Achieving an even, continuous coat is necessary to properly protect the silver from tarnish. Once the coating is applied, it is allowed to dry for 2-3 weeks. Agateen coatings are soluble in acetone and become yellow and brittle with age (MFA Boston CAMEO).
Removal[edit | edit source]
One of the most common abrasives used to polish silver is calcium carbonate. It can be mixed with either water or ethanol to create a slurry that can be applied to the surface using cotton cloths or cotton swabs. Once the tarnish has be reduced or removed, the surface should be wiped clean using water or ethanol and buffed with a soft, clean cloth. Calcium carbonate is often used in museums because it can not scratch the surface of the silver and does not leave unwanted residues.
References[edit | edit source]
Mayer, Ralph. A Dictionary of Art Terms and Techniques'. New York: Thomas Y. Crowell Company, 1969.
Museum of Fine Arts Boston. Agateen Lacquer. CAMEO. http://cameo.mfa.org/wiki/Agateen_Lacquer (accessed March 24, 2014).
Ohio Historical Society. Making it Shine. Ohio Historical Society Collections Blog. http://ohiohistory.wordpress.com/2011/06/02/making-it-shine/ (accessed March 24, 2014).
Winterthur Museum. Preserving the Sheen of Winterthur's Silver. http://www.winterthur.org/pdfs/Silver_Coating.pdf