Norfolk Museums & Archaeology Service

Caring for twentieth century materials

Brenda Keneghan

Plastics and rubbers form a significant part of our cultural heritage. They are found not only in art, historic and technological collections, they are also among the ethnographic materials of the twentieth century. Objects made from these materials range in diversity from radios, clocks, fountain pens, jewellery and hair accessories, kitchen equipment, hair dryers, Tupperware, picnic sets, to plastic robots and dolls, and all are becoming increasingly collectable.

Plastic is a generic term covering all types of synthetic materials. These different types of materials may differ dramatically from each other with respect to their care requirements. It is, therefore, most important to try to identify the type of plastic from which the object is manufactured.

Classification of plastics

Plastics are commonly classified on the basis of their origin:

Natural

These include amber, horn, wax, shellac, rubber

Semi-synthetics

These are chemically modified natural materials like:
Hardened rubber (Ebonite, Vulcanite), casein (Erinoid, Galalith), which is based on milk protein, cellulose nitrate and cellulose acetate, both based on cellulose.

Synthetics

These may be further sub-divided into:

• Early synthetics e.g. phenol formaldehyde (Bakelite), urea formaldehyde (Beatl, Beetle);
• The 'poly' era e.g. polyvinyl chloride (PVC), polymethyl methacrylate (Perspex, Lucite), polyethylene (Polythene), polyamide (Nylon), polyacrylonitrile (Courtelle, Orlon), polyurethane, polycarbonate, polyester, etc.

Identification of plastics

One indication as to what a plastic object may be made from can be obtained from the date of manufacture. This however, is only really applicable to the earlier materials (e.g. pre 1960s), as after this date the 'poly' era production was at its height and very many new materials and combinations of materials were introduced. A list of approximate dates at which many materials were first introduced is given at the end of this fact sheet.

Although the unambiguous identification of a plastic material requires the use of sophisticated equipment, sometimes an 'educated guess' is sufficient. The probable date of the object, the appearance, feel and smell of the material, and the properties necessary for its use are important clues. For example:

• A hard, dark material from the 1940s which must have been used in electrical goods is probably a phenol-formaldehyde or Bakelite;
  A flexible (or once flexible) clear (or yellowed) material from the 1940s is probably poly (vinyl chloride) (PVC);
• A doll from the 1940s or 1950s is possibly made from cellulose acetate;
• A hard, light-coloured material, used decoratively and dated 1912, is quite possibly a casein-based plastic. If the date of manufacture was later (late 1920s, 1930s) it could also very possibly be cellulose acetate;
• Imitation tortoiseshell is likely to be cellulose nitrate, cellulose acetate or casein;
• Polystyrene was used extensively in the 1940s and early 1950s for hard, brightly coloured toys;
• Foam toys are usually natural rubber or polyurethane;
• 'Bendy' (brand name) toys are usually natural rubber.

Appearance and smell

Some appearances are so typical of a particular material that this factor alone may provide sufficient identification. Examples are the pearlised colour effects in casein and cellulose nitrate, the dark mottling of Bakelite. The appearance of stickiness on the surface generally means that a plasticiser has migrated to the surface. This is a common occurrence with PVC.

The smell associated with some plastics is a good aid to identification. It is usually necessary to rub the object with the fingers in order to warm it a little to enhance the smell.

Polymer	Characteristic smell
Phenol formaldehyde	Carbolic (Phenolic)
Hard rubber	Sulphur
Cellulose nitrate	Camphor
Cellulose acetate (degrading)	Acetic acid (vinegar)

Hot pin tests and burning tests are not recommended.

Some typical degradation behaviour associated with various plastics

Cellulose nitrate

Cellulose nitrate is affected by light and moisture and releases oxides of nitrogen and nitric acid during degradation. This is an autocatalytic process i.e. unless the acidic vapours are removed they speed up the degradation process. Storage in plastic bags or other packaging with restricted ventilation is, therefore, not recommended. Early signs of degradation include a smell of camphor, disintegration of wrapping tissue and yellowing, especially with transparent items such as protractors and other drawing instruments. A surface bloom is also an indication of deterioration, as is the detection of acidity.

Cellulose nitrate based film can deteriorate and disintegrate at room temperature. Cinema film in particular, can combust spontaneously. All cellulose nitrate film stock should be isolated and advice on storage sought from the Fire Brigade.

Cellulose acetate

Cellulose acetate degrades in a similar manner to cellulose nitrate, except the acidic product released in this case is acetic acid. A vinegary smell is, therefore, a sign of degradation. Once again this is an autocatalytic process, speeded up by restricted ventilation. A white surface bloom is an early indication of deterioration. The migration of plasticiser to the surface of the object is also a sign of degradation. This results in a tacky surface followed by loss of flexibility, distortion and cracking.

PVC

PVC is produced in both rigid and flexible forms by the incorporation of varying amounts of plasticisers, especially esters. The main degradation problem associated with PVC is the loss of these plasticisers. This results in a sticky deposit on the surface which attracts dirt. The material becomes more rigid and cracks appear. There is often yellowing of the material. Flexible PVC was commonly used in the manufacture of shoes, rainwear, etc. in the 1960s and 70s. It was also the main material used in the manufacture of toys such as dolls from approximately late 1940s onwards.

Casein

Casein is most commonly found in buttons, pens, costume jewellery, dressing table sets, etc. It has also been used in the manufacture of decorative inlays in furniture (Charles Rennie Macintosh), and lamps. Most often the degradation problems are due to stress cracking.

Storage tips

It has long been accepted that storing plastics at low temperatures and relative humidities will slow down the rate of degradation reactions. Plastic objects should also be kept in low light levels. These conditions should be employed for the storage of plastics in general.

Some objects require special care, e.g. those made from cellulose nitrate and acetate. Common objects are dolls, dressing table accessories, combs and generally, decorations from 1930-1950. These should be stored in a cool, dry atmosphere. Never store in humid surroundings such as laundry rooms as this can cause warping. If objects show signs of degradation such as a smell of vinegar or camphor, it is best to separate them from other objects. Either leave the plastics loosely covered with acid-free tissue or even silicone paper to stop dirt getting at them if there is a strong acid smell. If there is any surface acid you could remove it with a dry tissue (it might stick, so be careful), or a very lightly water-moistened one, but dry the surface carefully afterwards.

Cleaning of plastics

As solvents may react with specific plastics causing stress cracking it is best to avoid their usage. Mechanical cleaning is recommended to remove surface dirt. Cotton swabs dampened slightly with water may be used but the surface must be dried thoroughly afterwards.

Chronology of commercial availability of plastics:

1839 Rubber vulcanized - (Ebonite, Vulcanite)
1855 Bois Durci
1862 Cellulose nitrate - (Parkesine)
1869 Cellulose nitrate - (Celluloid)
1900 Casein - (Galalith, Erinoid)
1909 phenol-formaldehyde - (Bakelite, Catalin)
1912 Cellulose acetate based photographic film
1922 Cellulose acetate moulding material
1926 Thiourea and urea formaldehydes - (Beetle resin)
1930 Poly(vinylchloride) - (PVC)
1933 Poly(methyl methacrylate) - (Perspex, Plexiglass, Lucite, Diakon)
1934 Melamine formaldehyde - (Melamine)
1935 Polystyrene
1936 Poly(vinyl acetate)
1938 Nylon
1942 Polyethylene - (Polythene); Polyester
1943 Silicones; Fluorocarbons; Polyurethane
1947 Epoxies; Poly(ethylene terephthalate)
1954 Polypropylene
1955 Polycarbonate - (Makrolon, Lexan)
1959 Polyformaldehyde - (Acetal resin, Delrin)
1965 Polysulphone
1966 Poly(phenylene oxide)
1970 Poly(butylene terephthalate)


Written by:
Dr Brenda Keneghan
First published in May 1997

Brenda Keneghan has been employed as a polymer scientist based within the Conservation Science Group at the Victoria and Albert Museum for three and a half years. During this time she has examined over 8,000 plastic objects as part of an ongoing survey of all the plastic objects within the various collections. She is the London co-ordinator of the Historic Plastics Research Scientists Group. She has an MSc in Organic Chemistry and a PhD in Materials Science.

Contact:
Dr Brenda Keneghan, Polymer Scientist, Conservation Research, Victoria and Albert Museum, South Kensington, London SW7 2RL
Tel: 0171 938 9640 Fax: 0171 938 8661

Further reading/useful contacts:
Classic Plastics
Katz, Sylvia
Thames & Hudson 1984

The Plastics Age
Sparke, Penny
Victoria & Albert Museum 1990

The Plastics Historical Society
c/o Colin Williamson
Mansion House
Ford
Shrewsbury SY5 9LZ

The National Trust Manual of Housekeeping
Sandwith, Hermione & Stainton, Sheila
Penguin in association with The National Trust, revised 1993
ISB0 14 0123344 X

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NB:
This article gives only very general advice on this topic; for your own special requirements, you may wish to seek further professional advice;
The inclusion of a supplier within this handbook does not imply the approval or endorsement by the MGC 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.

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