Metals

Silver

Silver's lustrous appearance and relatively low natural abundance, its corrosion resistance and ability to be easily worked have made it a prized metal. Silver is often used for coins, jewellery and cutlery.

Most silver items found in Australia will be either sterling silver or plated silver.

Sterling silver

Sterling silver is the standard alloy used in jewellery and cutlery. It is made up of 92.5% silver and 7.5% copper. The addition of copper to silver increases hardness of the alloy, without any significant loss of lustre or colour.

Plated silver

The two common forms of plated silver are Sheffield plate and silver plate/electroplate.

Sheffield plate is made by fusion-bonding—sweating sterling silver to both sides of a copper or brass sheet; it is then worked to produce the desired object.

Silver plate or electroplate is formed when a thin layer of pure or sterling silver is deposited electrolytically on the surface of a base metal. Common base metals include copper, brass, nickel silver—an alloy of copper, zinc and nickel—and Britannia metal-a tin alloy with 5–10% antimony.

Electroplated materials are often stamped EPNS for electroplated nickel or silver, or EPBM for electroplated Britannia metal.

As commercial electroplating was developed in the 1840s, it is likely that a lot of the materials in the collections of local museums in Australia will be made of silver plate.

Signs of corrosion on silver

Silver artefacts tarnish if they are not kept polished. The silver surface changes through a faint purplish hue to a deep black. The tarnish is a layer of silver sulphide.

Artefacts excavated from underground or from the sea may be coated with pin, silver chloride and copper corrosion products. If these materials have been exposed to sunlight, the surface will have a purplish tinge.

Cleaning silver

Clean silver only when absolutely necessary, and not as a routine treatment. This is because any cleaning removes minute amounts of silver.

Take great care to differentiate between tarnish and decorative treatments which are an inherent part of the object and which would be destroyed by cleaning. For example, a decorative treatment which should not be removed is niello, a black silver sulphide/silver copper sulphide that is used deliberately to highlight engraved sections of silver jewellery.

Avoid abrasive cleaners. They can cause fine scratching of the surface and remove small amounts of silver.

There are a number of proprietary brands of silver dip solutions available, which readily remove tarnish. These are usually made up of thiourea and acid mixtures. Silver dips should be used only when the object is badly tarnished.

If you decide to use a silver dip, apply the solution, or dip the object, only for as long as it takes to remove the tarnish.

Silver tray after treatment. Photograph courtesy of the Western Australian Museum

Then rinse the object in hot water to remove residues, and dry it with a lint-free cloth. After drying, a final polish with a silver cloth will put a thin layer of corrosion inhibitors on the surface of the silver.

If Sheffield plate is in reasonable condition, it needs no further treatment other than being wiped with a silver cloth and displayed or stored under conditions of low relative humidity and away from wool.

Although there are commercially available solutions which gradually redeposit small amounts of silver on worn areas, these should be used only after considering the history and function of the object.

On no account should the object be replated. This completely devalues the object by removing the technological evidence of its manufacture.

Electroplated or silver plated objects which are in good condition, but on which the silver has been worn away to reveal underlying metal, can be restored by using a solution such as the one described above. This is a better way to rejuvenate the surface than the standard electroplating, which is a lot more costly and not always successful.

If the piece is in good condition, then it is better to maintain it in that condition, rather than continually cleaning it and wearing down the silver coating. Wiping with a silver cloth, followed by storage and display under conditions of low relative humidity and low pollution levels is the simplest method of protection.

Despite the best storage, display and care methods, all electroplated silver will eventually show pin-pricks of corrosion as the plating wears thin. Once the plating has been perforated, the underlying metal is prone to pitting corrosion; and the surface will gradually become covered with blotchy, black and green-blue areas of corrosion.

Click here to see diagram

Simple care techniques don't have much effect on this type of problem. It needs more complex treatment.

Immerse the object in a solution of 10g of thiourea and 50g of citric acid in 1 litre of water. This treatment will clean the object and remove the silver corrosion products.

After cleaning, the object must be placed in a diluted bath of sodium carbonate, 1% by weight in water, and left to soak for an hour or so to remove any citric acid from beneath the electroplate—to make sure that no further corrosion occurs. Follow this with a final wash in fresh, deionised or distilled water.

If the corrosion damage is not too severe, then the underlying metal can be re-touched using one of the silver solutions which redeposit small amounts of silver on worn areas. Finally, polish the object with a silver cloth.

Coating silver

There are lacquers available for coating silver objects, and these can significantly slow down tarnishing processes.

Storing and displaying silver

The general handling, storage and display guidelines apply to silver objects.

If you must place a silver object in a display case which has previously been shown to be corrosive towards silver, add zinc oxide or an acid-free zinc carbonate blotter to the base of the case to absorb the damaging substances. This will help to keep tarnishing to a minimum.

Alternatively, you could use the commercially available sintered zinc oxide pellets, or sachets of multi-metal vapour phase corrosion inhibitors.