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Silver and other Polished Metals

By Stephen Ball, Jaine Chandler and Peter Winsor

Many metal objects carry high degree of finish that is intrinsic to their appeal. The more spectacular of these are made or plated with a precious metal, notably gold or silver, but many other metals are used in a polished or burnished condition. Although objects made from non-precious metals are less obviously 'valuable', they may have considerable historical or personal value. Coins and medals are examples of interesting and collectable objects that are not always composed of precious metals.

This fact sheet concentrates on two principal examples: silver, because it is a commonly used precious metal that is at the same time fairly reactive, and thus frequently encountered polished metal objects. The techniques described here can often be adapted for other highly finished metal objects. If you are in any doubt then consult a conservator.

Silver and other Precious Metals

Silver has an attractive colour and takes a high polish. It can be worked in many ways: shaped by hammering and turning, cast into intricate shapes, embellished by engraving, or inset with gems or plated gold. As a decorative item, it often appears in association with polished hardwoods, ivory, and other metals of contrasting colour.

The silver used to manufacture most domestic and display pieces originating from the UK is sterling silver. This is not 'pure silver, as many people suppose, but an alloy consisting of 92.5% silver and 7.5% copper; other qualities are used for special purposes. Many silver objects originating from the Middle of Far East and Central or South America have a very different composition, and usually a much lower silver content in the alloy. In some cases there is no silver at all - German silver is a white alloy made from nickel, zinc and copper, and modern 'silver' coins are based on copper and nickel.

Although highly lustrous when first polished, silver is more reactive than the near-inert gold and is prone to tarnish.
Gold may discolour, but usually because of changes in alloying metals or an associated base metal. Other polished metals may assume various coatings and colour changes with time; unless these represent active corrosion that alters the character and properties of the metal itself, some coatings contribute to highly valued patinas that must not be cleaned off, so always obtain advice before cleaning an unfamiliar metal. However, silver's tarnish is rarely regarded as a valuable patina, and generally detracts from its desirable qualities.

What is tarnish? It occurs as a gradual discolouration and loss of polished finish, the metal surface turning first to pink, then darkening to brown, before ending up as a very dark grey or black with a slight sheen. It is produced when silver reacts chemically with atmospheric sulphur compounds generated by fossil fuel combustion, living organisms (including humans) and decaying organic matter.

Crusty green deposits sometimes appear on silver objects under very harsh conditions or after severe neglect. These are not produced by pure silver metal but are corrosion products from the copper in the silver alloy. There are many causes; common ones are storage in damp conditions and the action of chemicals - especially those in residues from cleaning compounds. If the green corrosion is extensive then you should refer treatment to a specialist conservator.

Cleaning Tarnished Silver

Light Tarnishing

If the tarnish is light, use one of the silver cleaning cloths made by Goddard's and other companies. These are readily available from supermarkets and hardware shops. Provided you follow the instructions on the pack, you can easily remove dust, light sticky or greasy deposits, and the tarnish itself to leave a brightly polished finish.

This method works best for large areas of plain, undecorated silver, and can form part of the regular cleaning programme. Silver polishing cloths deposit a chemical that inhibits tarnishing, but this has only a short effective life. Replace cloths at regular intervals to prevent the build up of grime in the cloth and to maintain an effective level of tarnish-inhibitor. Intricately shaped or decorated surfaces require the use of a liquid cleaner (see below).

Heavier Tarnishing

A number of commercial products are suitable for removing heavier tarnish, including Goddard's Hotel Silver Dip. This works by chemically dissolving the tarnish, and leaves a very clean surface. In fact it works by dissolving the tarnish at a faster rate than the underlying silver, so must be used strictly in accordance with the instructions. Objects can be over-cleaned as a result of excessive immersion times.

When using dips, wear rubber gloves and, for large quantities, goggles. Immerse the object in the silver dip, or apply the solution with cotton wool or a cotton swab or bud. Time immersions carefully, and don't use the method if the object has hollow portions or recesses that could contain solution after withdrawal (use local application instead). Good ventilation is essential because the process gives off hazardous and unpleasant smelling fumes.

Pour a fresh solution every time you use the dip, then rinse with distilled water and dry thoroughly by dabbing with clean cotton cloths. A freshly cleaned silver surface is highly reactive and will quickly tarnish again, so use a silver cleaning cloth to finish the cleaning process and at intervals afterwards to maintain the protection.

Cleaning Methods Unsuitable for Museum Use

There are many other types of silver cleaner on the market, including wadding and creams. These are certainly effective, but are not the first choice of conservators because they contain abrasive compounds that remove a very thin layer of silver along with the tarnish.
Over time and with repeated cleaning, this process wears away the soft silver, thinning it and reducing its strength and slowly removing engraved decoration or inscriptions. These methods may be suitable for removing a very stubborn mark, but it is better to pass the problem to a specialist. To check the abrasive effects of a polish or wadding, try it on a perfectly clear area of acrylic sheeting and examine the resulting pattern of marks.

Creams and wadding also leave residues in crevices or parts of the engraved detail. This is unsightly and acts as a focus for more harmful corrosion, and is very difficult to remove. Where deposits already exist, try washing the object thoroughly using a mild household detergent solution applied with the aid of a natural soft bristle (hogshair) brush. A toothbrush with a plastic handle is far safer than a brush with a metal ferrule, which could scratch the silver.

Never use the much harsher brass or chrome cleaners on silver. Professional silversmiths and restorers use rouge sticks and powder to give silver its highly polished finish. The process demands a high degree of training and experience, and is not recommended for museum staff.

Prevention and Protection

There is little a museum can do about the general atmospheric levels of sulphur compounds, but it is possible to take steps to modify the local environment.

One method that may be feasible in some museum contexts is to fit activated charcoal filters to the air conditioning to absorb pollutant gases (naturally, such filters need periodic replacement and/or reactivation). At a still more local level, you may be able to line display cases with materials that absorb the sulphur compounds, such as Charcoal Cloth, Silver Safe and copper impregnated plastic films, Reduced Relative Humidity (RH) levels can also slow tarnishing, so local dehumidification remedies, such as silica gel in display cases, may help.

In general, avoid using protein based materials such as wool, silk and leather for case linings, padding or mountings, as slow changes in their chemistry will release sulphur compounds and increase the tarnishing of silver objects in the immediate vicinity.

Individual objects destined for storage can be protected to some extent by placing them on their own in polythene bags tightly closed with tape, self-sealing strips or heat seals. Wrap items in unbuffered tissue paper first, and as an additional precaution place small containers of silica gel and activated charcoal in the bag but out of contact with the object.

There is no need to remove ordinary tarnish from an object going into long term storage, on the principle that an object should suffer no ore intervention than is necessary. Normally, tarnish removal is only required for display purposes.

Lacquering

Most modern silver, brass and other polished metals are protected after manufacture by a sprayed lacquer finish. One of the most widely used commercial silver lacquers is Frigilene, which is cellulose nitrate dissolved in organic solvents. This is not available from normal retail outlets, though art shops sometimes sell a similar product that can be applied by brush or spray.
Lacquering is unlikely to be feasible for the protection of existing museum objects. Spraying is a highly skilled job that carries serious health and safety risks associated with atomised solvents and other harmful materials. Brush coating is a simpler and less risky task, but the results are rarely pleasing. The lacquer film will probably be uneven and full of pinholes; the defects may not be evident at the time of application but will show up after a few weeks when patches of tarnish emerge under the faults. When this happens there is no alternative but to remove the lacquer and start again.

Lacquering is best left to a commercial company that offers this specialist service. Insist that they use a lacquer that remains soluble in common organic solvents such as acetone and is thus easily removable. An alternative to lacquer is microcrystalline wax, applied carefully and systematically with due regard to protecting crevices and the prevention of wax build-up; again, this procedure should be carried out in a well-ventilated area.

Handling Silver Objects

Salt and grease deposits from skin accelerate tarnish formation and corrosion, so wear cotton gloves whenever silver is to be touched or carried. Lift objects by placing two hands around the body of a vessel, after first checking that the weight and bulk are manageable. Do not pick up an object by the handles, which are often weakened by age and constant use.

Silver is quite soft and the walls of a vessel may be thin, so take care when you lift or move an item that you do not inadvertently dent or distort it in any way. It may be possible to correct the shape of a vessel by careful manipulation, but dents are usually accompanied by stretching of the metal and will need expert attention.

Cleaning other polished metals

Most other polished metals require less cleaning than silver, either because their finishes do not depart from the required standard as much as tarnished silver, or because polishing may damage a valued surface finish.

Gold should not be polished, whether it occurs as a pure metal or as a plating or coating (as in silver gilt and other forms of gilding), nor should silver plate, most bronze, and any lacquered or painted metals. Cleaning of these materials should be restricted to gentle dusting with a soft pony hair brush.

New brass and copper items are now routinely lacquered, but older pieces may have a history of traditional abrasive polishing. Inspect lacquered pieces at intervals to check the condition of the coating, and if it is sound clean the object using the dusting brush technique described above.

If an object is of unlacquered brass or copper, first check for corrosion, and consider washing away old polish deposits. Then decide whether the object needs to be brought up to a highly polished condition - there should be no automatic presumption in favour of a high polish. If so, use silver or copper cleaning cloths, or if the surface is very dull, a gentle polish such as Peek. As with silver, it is easier for the non-specialist to apply microcrystalline wax than a lacquer coat to protect the polished surface. The wax coating will provide protection for about a year.

An example of polished metal objects: service medals

Service and similar medals struck with a 'metallic coloured surface' provide interesting examples of objects that may be found in even the smallest local museum; coins may raise similar conservation issues. Most medals also come with the complication of an attached textile ribbon, and may have passed into collections after years of polishing by proud owners.

If you have any doubts concerning the nature of a medal or coin or suitable cleaning methods, you should seek further advice from a metals conservator. For example, 'art' or commemorative medals may have special coatings or applied patinas that should never be cleaned.

Recommended cleaning practice

Most recipients of awards have an understandable desire to keep their medals polished, particularly when these are displayed or worn on special occasions. It is therefore likely that for part of its existence a medal has been subjected to some form of polishing technique, and equally likely that a cream or wadding was the chosen method.

However, the earlier strictures against the use of waddings and polishes must apply in the museum context. Any previous damage to detail, inscriptions and finish is irreversible, as is its effect on the medal's aesthetic, historical and monetary value. It is the museum's duty to arrest deterioration and prevent further decay, guided by a policy of minimum necessary intervention using safe cleaning techniques.

Clean the metalwork surfaces as infrequently and gently as possible to preserve the original finishes and surfaces. Wear cotton gloves at all times when handling any part of the medal. Industrial methylated spirits on cotton buds will remove the greasy build-up of dirt and some tarnishing caused by pollutants. For heavier tarnishing of silver medals use cotton buds to apply Goddard's Hotel Silver Dip, following the procedure described earlier in this Fact Sheet. Add a further gentle buffing after using the silver cleaning cloth to bring up a deeper shine.

Medal ribbons are of great significance, so take the greatest possible care that metal cleaning does not compromise the ribbon. Keep solutions and dips well away from any part of the textile. If the ribbon is dirty, gently brush away the dust using a soft brush; where a vacuum cleaner nozzle is placed behind the medal, cover it with muslin or cheesecloth to trap fragments. Never try to wash the ribbon. If brushing does not remove the dirt from a ribbon, seek advice from a specialist conservator.

Storage and display

Metals are often displayed in glazed wooden cases on textile backgrounds, but some of the materials in the case may be producing harmful vapours and gases as they decay, threatening the stability of the medal and its ribbon.

Unless the case is of special significance, the simplest solution may be to move the medals to an environmentally sound replacement built from museum grade materials. A cheaper compromise may be possible if some of the materials - say, the backing or underlying padding - can be replaced. Have the materials in a suspect case tested before condemning them. Treat the old case as a museum object in its own right, and perform the upgrade with conservation objectives in mind. Another simple and cost effective method of improving the case environment is to place scavengers or tarnish inhibitors inside - but this only works if the carbon or other active ingredients are changed at the recommended intervals.

The careful display and mounting of metal objects can help to preserve them. However, any display pins must be coated with polyester to prevent them coming into contact with the object, thus risking electrolytic corrosion. And do not use adhesives to mount metal objects; most create some long-term corrosion risk.

Although otherwise important, the medal's original presentation box may be a mixed blessing from the conservation standpoint. While it keeps the medal away from light and dirt, organic materials in its construction such as leather, wood and fabrics may decay or react with one another to threaten both the metal and textile elements of the medal. Look for clues such as copper corrosion products on the brass hinges of the box. Use the barrier method as a first solution - place a buffered acid free tissue paper or Charcoal Cloth between the medal and the box.

Light is an important factor in the display of medals. Metals - though not lacquers or other common coatings - are relatively unaffected by normal visible light and UV exposure, but this doesn't apply to medal ribbons, which are extremely vulnerable to the decomposition of the textile material and serious colour fading. The normal steps of limiting intensity and exposure time and the fitting of UV filters over windows and display lighting apply here.

Sources of information and advice

Suppliers of tarnish inhibitors: Tarnish Inhibitor Capsules, Plastabs:
Conservation MLAs
Units 1, 2 and 4
Pony Road
Horspath Industrial Estate
Cowley
Oxford OX4 2RD
Tel: 01865 747755
Fax: 01865 747035

Suppliers of tarnish inhibitors: Charcoal Cloth
Charcoal Cloth International
High Tech House
Commerce Way
Arena Business Park
Houghton-le-Spring
Tyne and Wear Dh2 5PP
Tel: 0191 584 6962
Fax: 0191 584 6793

For information on materials testing:
Materials Testing Service
Conservation Research Section
Department of Conservation
British Museum
London WC1B 3DG
Tel: 020 7323 8772
Fax: 020 7323 8636

Sandwith, H. & Stainton, S., The National Trust Manual of Housekeeping, (revised edition), London, Penguin/National Trust, 1993.

The inclusion of a supplier within this Fact Sheet does not imply the approval or endorsement by MLA of the product or service. You are therefore urged, in your own interests, to ensure that any product or service is appropriate to your needs.

For more information about private conservation work please contact:

Conservation Register
Tel: 020 7721 8246

Conservation Register (Scotland)
Tel: 0131 668 8668

Copies of this fact sheet can be provided in alternative formats. Please contact Viola Lewis, Information Officer at MLA for further information.

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