Historical silver can be maintained for years of use and enjoyment provided that some basic care and attention is given to its preservation. The conservation staff at The Henry Ford has compiled this information to help people care for their objects and collections. The first step in the care of collections is to understand and minimize, or eliminate, conditions that can cause damage. The second step is to follow basic guidelines for care, handling and cleaning.
IDENTIFYING SILVER OBJECTS
Most people know that silver is a white, lustrous metal. Pure or “fine” silver is called “Sterling” if it is not less than 925 parts silver to 75 parts alloy. Silver objects, especially coins and jewelry, contain copper as an alloying metal for added hardness. The copper may corrode to form dark brown or green deposits on the surface of the metal. Silver is usually easy to differentiate from lead or pewter which are generally dark gray and not very shiny. If your object forms a white powdery substance anywhere on the surface, it indicates lead corrosion. This proves that the object is not silver, or that it is silver-plated.
Silver is often plated (deposited) onto other metallic alloys, almost always with an intermediate layer of copper in between. The earliest plating process, “Sheffield Plate” was developed in England in 1742. By the mid-19th century, the process was largely replaced by electroplating (which used less silver). The base metal in plated artifacts may consist of any of the following metals or alloys: copper, brass, “German silver” or “nickel silver” (50% copper, 30% nickel, 20% zinc), “Brittania metal” (97% tin, 7% antimony, 2% copper), or a “base” silver containing a high percentage of copper. Hallmarks or other stamped marks on the underside can usually aid in determining the composition of silver or silver-plated artifacts.
The most common cause of silver damage is tarnish, abrasion and denting.
Tarnish (silver sulfide) is a form of corrosion characterized as a dense, thin, black layer that disfigures the surface of an object. Silver will tarnish on exposure to air containing sulfide gases. Humidity in the air is also required for the corrosion to progress. Since the Metro-Detroit area has heavy industry and elevated pollution levels, as well as hot, humid summers, both the criteria for tarnish are met in this area.
Tarnish does not itself pose a threat to artifacts. Most damage to silver occurs as a result of the required polishing to remove the tarnish. Over-polishing results in a loss of detail definition in raised areas of design over time. On plated objects, frequent polishing can actually remove the silver plating, leaving dull areas of exposed base metal that may be mistaken for stubborn areas of tarnish.
In rare cases where the silver object has been exposed to high airborne salt concentrations, “horn silver” may develop on the surface. This corrosion, silver chloride, is characterized as dirty purple or slate gray. It is dense, compact and usually quite difficult to polish off.
Old lacquers, applied in the past to protect the piece, may wear or peel off in some areas. This leaves the exposed silver to tarnish, while the rest may remain bright.
Rough handling could damage objects made of silver, a relatively soft metal. Raised areas and handles are especially susceptible to denting and joint failure. Display pieces should be handled with care, lifting from the center of gravity, never by the handle or lip. If historic silver serving pieces are being used, their owners should accept a certain amount of wear and tear from handling and more frequent cleaning.
Repairs to valuable silver, which may involve soldering or raising and reshaping dents, should be done by a qualified metalsmith familiar with historical techniques or an art conservator. In some cases, jewelers may be willing to do small repairs on silver artifacts.
Old lacquers must be removed prior to cleaning. This is best done with acetone, preferably by immersion. Acetone is a volatile solvent that should never be used in poorly ventilated conditions. (Please consult the manufacturer or Material Safety Data Sheet for complete safety requirements.)
Polishing with a mild abrasive is the only safe cleaning method conservators can recommend for most historic silver artifacts. Commercially available “silver dips” may contain undesirable components such as hydrochloric or sulfuric acid that act too quickly and remove more metal than simple polishing does. Conservators do resort to special dips in certain, extreme cases, but for most tarnished silver, this method is too aggressive. Silver which has been dipped usually requires further burnishing to restore luster to the surface. Some commercial paste polishes are quite abrasive and may scratch your fine silver. Use caution and always do your homework when deciding on a commercial polish.
For most polishing, we use fine calcium carbonate, CHALK (“whiting”), worked into a slurry or runny paste with equal amounts of ethanol (denatured alcohol or ethyl alcohol) and distilled water. The paste is rubbed across the surface working a small area at a time with cotton balls or clean, cotton rags. Detailed areas may be polished with Q-tips or with cotton wadding on the end of a sharpened bamboo skewer. Depending on the design of your object, it may not be desirable to over-clean every crevice, as this decreases the overall contrast of the detailing. It is important to remove all residual polish with distilled water. Drying may be accelerated by adding ethanol to the rinse water, or by giving the object a final wipe with ethanol.
Polishing exposes fresh, reactive metal to the atmosphere and, therefore, to further tarnishing. Objects that will not be used can be lacquered for protection. This process involves the use of solvents to clean the metal properly (acetone or trichloroethane). It also requires spraying on the lacquer. In general, spray lacquering is a task best left to qualified individuals with the background and equipment necessary to do a good job. Poorly applied lacquers can actually cause more severe corrosion if small areas are left exposed.
It is not advisable to wax polished silver because the effect is too variable. It is difficult to achieve a continuous, even coat of wax. Most people who do not have access to professional services must accept the fact that they will have to polish their silver as often as is needed.
Polished silver should not be handled with bare hands. Salts and oils from your skin can etch into any polished metal and may even cause permanent damage. Soft, cotton gloves, or any clean glove or rag may be employed for this purpose.
A simple way to preserve fine silver, and to reduce the necessity for polishing, is to store silver properly. Maintaining an even, low humidity where metal objects are kept (ideally below 50% relative humidity) will help. In most homes, this is difficult to ensure, but generally speaking, basements are often damp in the summer and, therefore, should not be used for silver storage. Humidity sensors are available through various suppliers for those who wish to check conditions near their collections.
Silver Tarnish Inhibiting Cloth (not the polishing kind) is available from better fabric stores for storing silver. It should be wrapped around the silver piece; it protects the object by absorbing tarnishing pollutants. The wrapped silver may then be placed in a clear bag, preferably made of “Mylar” (turkey baking bags are good) or “polyethylene” clear plastic. Never use “polyvinyl chloride” plastic bags to store artifacts. Silver kept wrapped and stored properly can be taken out and enjoyed as often as you like with the minimum amount of polishing and trouble
The Henry Ford (also known as the Henry Ford Museum of American Innovation and Greenfield Village, and as the Edison Institute) is the largest indoor-outdoor museum complex in the U.S., located in the Detroit suburb of Dearborn, Michigan. For more information: www.thehenryford.org; or call 313-982-6001 or 800-835-5537
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