Volume 11, Number 1, Jan. 1989, pp.4-5
Editor's Note: I asked Glenn to prepare a pared down version of the lecture on silver he presented at the WAAC Conference in Yosemite.
The following is a brief summary of the procedures that are used to clean and lacquer silver. Please remember that each object has its own story to tell, and this summary does not attempt to cover the wide variety of problems one faces in conserving silver artifacts.
In general, cleaning with acidified thiourea is preferred over abrasive cleaning. Thiourea chemically dissolves silver sulfide (tarnish). Tarnish on gilt surfaces should always be removed chemically, since abrasives will wear away the thin layer of gold over time. The intensive washing which follows the thiourea treatment may prevent its use on silver with associated organic materials such as ivory or wood.
After careful inspection to insure that no lacquer is present on the object, the surface should be mechanically cleaned of surface grime and old polish residue. This can be achieved with wood skewers, stencil brushes or ultrasonics. Next, the tarnish can be removed with acidified thiourea, using either cotton swabs or stencil brushes. The solution takes approximately one minute to dissolve the tarnish. After several minutes of light rubbing or brushing, rinse the silver to avoid redeposition of tarnish. Redeposited tarnish can be very difficult to remove. Swabs must be tossed and brushes must be rinsed before cleaning the next area to prevent contaminating the thiourea solution.
After tarnish removal, the object must be intensively washed. Following a thorough sudsing with deionized water and neutral detergent (e.g. Triton X-100), rinse the object for one hour in a tub with slowly running deionized water. After a final sudsing and rinsing, the object should be dried with filtered compressed air, clean diapers, and reagent grade ethanol. The surface of the silver will have a characteristic micro-roughness after thiourea cleaning, which can be removed by light buffing with a clean diaper.
Thiourea is a suspected carcinogen, and should be handled with extreme care. Protective gloves and toxic vapor masks must be worn at all times. Conservators should consult the manufacturers and distributors of thiourea for proper safety precautions. [Editor's note: Thiourea is on the Governor's list of Chemicals known to the State of California to Cause Cancer and is therefore covered by the Safe Drinking Water and Toxic Enforcement Act of 1986 (proposition 65). Warning signs are required for thiourea as of January 1989 and the disposal restrictions will go into effect in August 1990, see WAAC Newsletter, Vol. 10, no. 2 (May 1988), for further information.]
Because of the health hazards associated with thiourea, abrasive cleaning is sometimes preferred. This can be achieved with precipitated calcium carbonate in a slurry of deionized water to which several drops of Triton X-100 have been added. Cotton diapers, cotton swabs, and stencil brushes are used to rub the abrasive slurry on the silver. If precipitated calcium carbonate fails to remove the tarnish then gamma alumina (0.05 microns) may be tried. For stubborn tarnish, alpha alumina (0.3 microns) may be used. The aluminas are more abrasive however, and will remove more silver during the cleaning process. They also impart a higher reflectance to the surface, which may be inappropriate for historical silver.
After complete drying, the object should be thoroughly degreased with reagent grade ethanol, acetone or methyl ethyl ketone. Gloves should be worn at all times when handling silver. Grease from handling will prevent lacquer from adhering to the surface. There is a current debate among conservators whether cellulose nitrate or acrylic lacquers are preferable. Among the products commonly used are Agateen No. 27 (cellulose nitrate) and Acryloid B-72. The lacquer may be applied either by brushing or spraying. The application should always consist of two layers, to insure complete coating.
Acidified Thiourea Solution:
Phosphoric Acid 3%-5%
Triton X-100 0.5%
Deionized Water 88.5%-86.5%
Thiourea; Phosphoric Acid
Chemical Supply House
Triton X-100; Acryloid B-72
1165 Marietta Way, P.O. Box 2884
Sparks, NV 89431
Precipitated Calcium Carbonate
21st & Clearfield Streets
Philadelphia, PA 19132
Gamma and Alpha Aluminas
41 Waukegan Rd.
Lake Bluff, IL 60044
Agateen No. 27; Thinner No. 1
Agate Lacquer Co.
11-13 Forty-Third Rd.
Long Island City, NY 11101