If you’ve visited the shining armouries at the Wallace Collection you would be forgiven for thinking that metals were hard, durable and sometimes deadly materials. However, many metals are vulnerable to all sorts of physical and chemical damage which, once it has occurred, is often irreversible or requires intensive interventive conservation treatment.
Seoyoung Kim, Metals Conservator at the Wallace Collection, gives us an insight into the world of metals conservation with her 10 steps towards the care of these historic collections…
Knowing what they are
Common metals in historic collections include gold, silver, copper, iron, lead, zinc, tin, and nickel. Identifying types of metal is important in deciding the best methods for their care. It is uncommon to see single metals used for an object and most artefacts are made of alloys of multiple metals (e.g. bronze, brass, pewter) for their desired physical and chemical properties (e.g. colour, hardness, resistance to corrosion).
Surface colour of metals, as well as colour of corrosion products, can help identification; for instance, corrosion products of green and/or blue on copper alloys and of orange/red on actively corroded iron. However, the identification of metals can be difficult by visual inspection only, owing to various obscuring elements, such as surface decoration (e.g. patination, surface coatings). Further testing is available for more accurate identification, but would require specialist assistance.
Handling with care
Appropriate gloves should always be worn whenever handling metal objects since even brief direct contact with sweat on the skin, which contains acids and chlorides, could etch the metal surface and leave permanent fingerprints. Objects shouldn’t be lifted by their handles or potentially weak areas. They should, whenever possible, be handled with both hands and supported from beneath. Any stiff elements should not be forced, and mechanical parts, such as firearm mechanisms, should only be operated by experienced persons. Caution should also be taken when handling objects with sharp edges.
Recording the condition of a collection (e.g. damaged surfaces, corrosion, structural weakness, inappropriate previous repairs) is helpful in monitoring any changes and can help to reduce further damage. Visual documentation using digital images is useful as well as written records.
A build-up of dust and dirt on metal surfaces can attract moisture and accelerate surface corrosion of metals. It can also encourage abrasion of susceptible surfaces. When dusting metal surfaces, care should be taken not to scratch the surface. A soft natural hair brush (e.g. hog-bristle, pony hair) is recommended for dusting, in conjunction with a vacuum cleaner to prevent lifted dust re-settling on the surface. The nozzle of the vacuum cleaner (preferably with adjustable suction) can be covered with a gauze material to catch any loose surface material. Conservation grade cleaning cloths (e.g. microfibre cloth) are available for gentle surface cleaning. Tarnish inhibiting cloths (e.g. Goddard’s Long Term Silver Polish Cloth) can be used to clean light tarnish. Removing corrosion products (e.g. heavy tarnish, encrustation) by either mechanical or chemical methods risks potentially removing original material and should only be carried out when necessary by an experienced person.
Protecting the surface
The application of waxes and lacquers on metal surfaces can offer protection from handling and from exposure to unsuitable environments (e.g. high humidity, atmospheric pollutants). Synthetic waxes (e.g. microcrystalline wax) and lacquers (e.g. nitrocellulose and acrylic lacquers) are preferred for use on metal surfaces since natural materials containing fatty acids could affect metals. Waxes can be relatively easily applied with a soft brush or cloth. Waxed surfaces can then be buffed with a clean soft cloth to bring a shine to the surface. Lacquering will provide more thorough protection for longer periods (more than 10 years); however, lacquering should only be done by a professional conservator due to the complexity of application and skills required to achieve satisfactory results. Also, health and safety issues in the use of chemicals should be considered.
Display and storage
Appropriate inert materials should be used during display and storage of metal objects. Organic materials (e.g. oak cabinets, felt linings) should be avoided since they can give off harmful vapours. Adequate support should be provided to objects to reduce physical stresses. Metal (e.g. brass, steel) mounts are often used to display metal objects owing to their versatility and strength. However, direct metal-to-metal contact should be avoided since it can develop corrosion and cause physical damage. Extra care should be taken for the security of certain objects (e.g. weapons, objects on open display). All objects in storage should be kept off the floor at all times as a precaution against unexpected disasters (e.g. flood) and, whenever possible, stored in storage-units (e.g. cabinets, drawers) with contents labelled on the outside. All shelves and drawers should be lined with suitable material (e.g. Plastazote® foam) to minimise shock and abrasion.
A regular collection monitoring programme can help to detect any changes or damage and prevent further damage. The condition of objects on display should be checked ideally on a daily basis. In particular for metal objects, signs of corrosion should be carefully looked for. Any tampering or similar incidents should also be monitored and reported. Objects in storage should also be regularly monitored.
Controlling the environment
Providing and maintaining a suitable environment is important to minimise and to avoid potential degradation and damage to objects. Metals are particularly susceptible to environments with high humidity and volatile chemical compounds (e.g. acids, alkalis, organic vapours). Metals should be kept at consistent low relative humidity (below 60 % RH) to minimise corrosion. However, many objects are made of composite (organic and inorganic) materials and it is often difficult to meet recommended environmental requirements for all constituent elements. Temperature, relative humidity, indoor air quality, pests, and light levels should be kept at suitable consistent levels. Fluctuations in environmental conditions should be avoided, particularly for organic elements.
Good housekeeping will help in the preservation of a collection. All display and storage areas should be kept clean and tidy and free of organic accretions and dust which can pose risks for metals. A regular housekeeping programme should be scheduled. Deep cleaning of a collection or display (e.g. once/twice per year) would also be beneficial.
Call for professional advice
Inappropriate/unsuitable attempts to repair existing damage often risk irreversible damage to metal objects. Remedial conservation of metals, including removing corrosion products and lacquering, benefits from professional and specialist knowledge and skills. Professional conservators should be consulted on such matters. A list of accredited professional conservators can be found at the Institute of Conservation Register website (www.conservationregister.com).
Seoyoung Kim, Metals Conservator, The Wallace Collection