The mixture of airborne pollutants around us in the 21st century comes from a variety of sources such as vehicle exhaust fumes (nitrogen dioxide), power stations (sulphur dioxide), industry and even humans (dust). Wood and synthetic products emit organic acids (formic, acetic) which can be produced by materials within the museum as well as from outside. Metals are very susceptible to pollutants; sulphur pollution will result in the tarnishing of silver and acidic gases released from wood used to make storage cupboards will eventually reduce lead objects to powder.
With organic materials such as textiles, bone or leather, pollution can lead to rot, embrittlement, and staining. The effects are aggravated when the humidity rises.
Great care must be taken to prevent pollutants from being introduced into stores or showcases. Some materials, e.g. wood, adhesives, chip board and fabric coatings, continually give off pollutants, notably acetic acid, formaldehyde and sulphur which are particularly harmful to anything coming into contact with them.
Any material being used inside showcases is now tested in the laboratory before being approved for use within the museum. In small areas control is achieved using specially designed 'scavengers', which chemically absorb pollutants. Some showcase fabrics are interlined with charcoal cloth, an effective pollution absorbent, or with mount board that incorporates charcoal. Another new material being used is a foam impregnated with copper particles.
Packing and Storage
Until recently little consideration was given to the importance of packing and storage methods and materials, often resulting in damage to the objects. It is now recognized that proper attention to storage plays a major part in an efficient conservation programme. The materials used now are tested to make sure they are free from pollutants and are approved for archival storage. Tissue paper, mount board and cardboard boxes must be acid-free; polystyrene crystal boxes and polyester films must be inert; and the various grades of synthetic wadding and foam used to support delicate items are also tested and approved for museum use.