While it is generally acknowledged that relative humidity levels in storage and display areas should be kept below 45 % for most metals and below 35% for most iron objects, the presence of particular corrosion products will affect recommended levels. For example relative humidity levels of 15 % or less must be maintained to prevent corrosion of chloride infested iron and levels of 12 % or less are necessary to prevent corrosion of iron that is in contact with mixtures of the corrosion products Akageneite (β-FeOOH) and hydrated ferrous chloride (FeCl2.4H2O, Watkinson and Lewis 2004).
While high light levels are not damaging to metals, a maximum level of 300 lux is usually recommended so that other more sensitive objects in the vicinity are not adversely affected. Techniques to control the relative humidity and light levels are described earlier (see the chapter Preventive Conservation: Agents of Decay).
Storage, Display and Handling
Since the major agents of deterioration for metals are oxygen, water and airborne pollutants, any storage system which minimises their effects will extend the life of a metal. Simple steps such as storing objects wrapped in acid-free tissue, in acid-free boxes, on painted, preferably baked enamel or powder coated metal shelving will increase their longevity. If more than one metal object is stored in a box, use acid-free padding to separate objects and prevent abrasion. Steps such as these will help avoid both environmental attack and galvanic corrosion.
Never place objects directly on shelves or in drawers. Line these with polyethylene foam (e.g. Ethafoam) to minimise vibrations and abrasion. If metals are stored on open shelving, in addition to wrapping objects, use drapes made from washed cotton, polyethylene or Tyvek to further reduce exposure to dust.
Avoid storage in or near cabinets made of chipboard or wood as these materials give off formaldehyde gas and organic acids which can accelerate corrosion. If there is no option but to store metal objects in cabinets made of these materials, take the following steps:
- coat wood with a water-based polyurethane finish to seal the wood; and
- paint chipboard with a solution of urea (400 g) in water (1 L) and then seal with a water-based polyurethane finish.
Do not seal objects in plastic bags which may trap moisture and increase corrosion. Avoid polyvinyl chloride (PVC) bags in particular as they can give off hydrogen chloride, an acidic gas which will attack most metals.
When handling metal objects wear well-fitting plastic or white cotton gloves. This will prevent the transfer of sweat and fats from the skin to the metal object, minimising potential corrosion problems. Wash cotton gloves regularly to avoid the build-up of salts and organic acids. Do not handle silver objects with latex rubber gloves as there is an increased risk of tarnishing the silver.