Silver

The lustrous appearance, relatively low natural abundance, corrosion resistance and ability to be easily worked has ensured that silver has always been a prized metal, often being used for coinage and jewellery. Most silver objects in collections of former British colonial countries will be sterling silver or plated silver.

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

There are two common forms of plated silver, Sheffield Plate and silver plate or electroplate. 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 or EPBM for electroplated nickel silver or electroplated Britannia metal respectively. As commercial electroplating was developed in the 1840s it is likely that many artefacts in Australian collections will be made of silver plate.

Sheffield plate is a duplex alloy where sterling silver has been fusion bonded (‘sweated’) to both sides of a copper sheet. It is subsequently worked to produce the desired object.

Deterioration

Unless kept polished, silver artefacts will usually tarnish, forming a layer of black silver sulphide. Artefacts excavated from underground or from the sea may be coated with grey silver chloride and blue-green copper corrosion products (Figure 2).

Figure 2: Silver knife handle with high copper content (green corrosion) and evidence of a rusting iron shaft beneath ivory.

With age and polishing, silver may be removed from the high spots of plated silver, exposing the base metal which often looks duller than the silver layer. Even if the gentlest polishing techniques are used, all electroplated silver ultimately will show pinpricks of corrosion as the plating wears thin. Once the plating has been perforated the underlying metal is prone to pitting corrosion and gradually the surface will become covered with a blotchy black and green-blue corrosion matrix.

Preventive Conservation

Apply the guidelines for safely storing and displaying metals described in the general introduction to this chapter.

If there is no option but to place an object in a display case previously shown to be corrosive towards silver, add acid-free blotter impregnated with either zinc oxide or zinc carbonate, to the base of the case. This will minimise tarnishing by absorbing acidic and sulphurous compounds.

Zinc carbonate-impregnated blotting paper can be prepared as follows:

• in a flat tray, prepare a solution which contains a soluble zinc salt such as zinc sulphate, by dissolving 10 g of the salt in 1 L of water;
• immerse a sheet of acid-free blotting paper in the solution;
• once the blotter is wet, pour a solution of sodium carbonate (20 g in 1 L of water) into the bath, producing a white cloudy solution of zinc carbonate; and
• remove the blotter from the bath and allow it to dry under glass to prevent puckering.

When dry, place the blotter underneath textile coverings in the base of a display or storage cabinet, or rolled up and placed in a support underneath a raised platform within a display cabinet. This is a very effective means of minimising corrosion.

Alternatives to blotting paper are commercially available sintered zinc oxide pellets or sachets of multi-metal vapour phase corrosion inhibitors.

Treatments

Cleaning

Only clean silver when absolutely necessary and not as a routine treatment, as any cleaning will remove minute amounts of silver. If a piece is in good condition, 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 low relative humidity conditions will provide the simplest method of protection. If cleaning is necessary, avoid abrasive cleaners as they can cause fine scratching of the surface and will remove small amounts of silver.

Take care to differentiate between tarnish and decorative treatments which are an intricate part of the object and which would be destroyed by cleaning. An example of such a decorative treatment that should not be removed is niello, a black silver sulphide deliberately used to highlight incised sections of silver jewellery and silver ornaments.

Many commercially available ‘silver dip’ solutions readily remove tarnish. These are usually made up of thiourea and acid mixtures. Only use silver dips when the object is badly disfigured and apply them with care. Never use silver dip on objects with decorative treatments such as niello or on composite objects with bronze, stainless steel or organic components. Also avoid the use of silver dips on objects that have hollowed components as there is a danger of the liquid entering any cavities. If using a silver dip, only apply or dip the solution for as long as it takes to remove the tarnish. It is safer to use a cotton bud (or similar) to apply the dip to corroded areas. Following treatment with the dip, rinse the object in hot water to remove residues and then dry it with a lint-free cloth. After drying, use a silver cloth to apply a thin layer of corrosion inhibitors to the surface of the silver.

Any kind of cleaning may expose areas of fire scale (copper oxide) that is sometimes formed during manufacture. These marks can discolour the surface and are very difficult to remove.

If Sheffield plate is in reasonable condition, just wipe it with a silver cloth and display or store it under conditions of low relative humidity. Do not replate the object as it completely devalues it by removing the technological evidence of its manufacture.

Electroplate or silverplate objects in good condition, but on which the silver has been worn away to reveal underlying metal, can be restored by using commercially available solutions which gradually deposit small amounts of silver on worn areas. This is a better way to rejuvenate the surface than standard electroplating. The latter process is a lot more costly and is not always successful.

Electroplate which shows signs of blotchy black and blue-green pitting corrosion will not usually respond to treatment with silver polishing cloths, silver dips and cleaning foams. If confronted with materials of this type, remove the gross corrosion by using a solution made up of thiourea and citric acid. This treatment will remove much of the silver corrosion since thiourea is a very strong complexing agent that also dissolves silver sulphides. To prepare and use this solution:

• dissolve thiourea (10 g) and citric acid (50 g) in water (1 L);
• immerse the object in the solution and gently brush the corrosion areas with a soft brush;
• after cleaning, place the object in a bath which contains sodium carbonate (10 g) in water (1 L). Leave the object to soak for about one hour to neutralise and remove any citric acid from beneath the electroplate; and
• remove any residual sodium carbonate by a final wash in deionised or distilled water.

If the corrosion damage is not too severe then touch up the underlying metal with one of the commercially available silver solutions that gradually redeposit small amounts of silver. Only use this treatment for electroplate or silverplate after carefully considering the history of the object. Polish with a silver cloth to complete the cleaning process.

Coatings

There are commercially available lacquers for coating silver objects that slow tarnishing processes significantly. On the downside, these lacquers can be troublesome to remove if they break down. In addition, unless an even coating is applied, a patchy and blotchy tarnish may develop on the surface.