Since their introduction in the 1930s, plastics have been incorporated by artists, designers and manufacturing companies into modern and contemporary cultural heritage. Although cheap, widely available and praised for their elastic qualities, different kinds of polymers have unstable chemical structures that make their deterioration rates unpredictable.
The enormous variety of classes of plastics and the form they may take (for example, solid sheets, foams, fabrics and coatings) combined with the relative newness of the material, plastics conservation is a challenging field. Polyethylene, polyester, polyurethane, acrylic, polyvinyl chloride (PVC), and cellulose acetate are just a few of the more familiar classes of plastics that might end up in artworks or objects. The term plastic can refer to a wide variety of synthetic polymers. After adding fillers and colouring materials, the appearance and surface texture of a polymer can be significantly altered, making it more difficult for the conservator to identify how an object should be treated.
Common types of deterioration
Physical deterioration of plastic artworks and objects occur from mishandling, and can result in tears, cracks, and areas of loss. It may be difficult to ascertain the most fragile areas of some plastic objects due to rapid and unexpected deterioration. If a plastic object is functional, for instance, Bakelite cookware, or a child’s doll, then general wear from usage may occur.
Chemical deterioration occurs in plastics due to a variety of factors but commonly involves exposure to light, fluctuations in temperature or relative humidity and introduction to moisture or acids. Other plastics may simply be volatile due to their chemical structure.
Signs of plastic chemical degradation can include a white powder ‘blooming’ on the surface of an object, discolouration, distortion of the object’s shape and strong indicator smells of vinegar, or mothballs.
Stickiness occurs when plasticisers migrate out of the bulk material and onto the surface of an object or artwork. There may also be wet, acidic deposit on the surface. Crazing or cracking can be a sign of both physical and chemical deterioration, resulting in a unique pattern on the surface of an object.
Deterioration may also occur when an object contains other materials than plastic, such as metal parts, that may corrode independently or at the same time as a polymer.
Conservation of plastics involves identification and research in order to ascertain how a plastic will react to conservation treatments. This process involves using a sample from a polymer object and ranges from simple destructive tests to measure the density of the material, to highly specialised instrumental analysis performed in the laboratory.
If a plastic is unable to be identified, a good rule of thumb is to treat it with the same preventive conservation methods as organic collections. That is, with a controlled environment of temperature and relative humidity, low light levels and conservation grade storage packaging.
Plastic objects that show signs of chemical degradation should be immediately separated from other objects and loosely wrapped in acid-free tissue. Cellulose nitrate (old motion picture film) is extremely volatile and must be stored away from other materials with low temperatures (2-5 degrees Celsius) and with good ventilation.
Cleaning of plastics should avoid water and any solvents in order to reduce the risk of beginning chemical reactions. Microfibre clothes, cotton swabs and fine brushes may be used to remove surface dirt, dust and accretions from plastic objects.
AICCM has a Special Interest Group for those interested in the conservation of objects, which includes plastics. Join this group, contribute to its activities, or speak to a specialist conservator.