Plastics 187
objects are collected for the same reasons. In this way, museums act, often
unintentionally, as storage depots for both early plastic materials and the most
recent experimental formulations. Some early plastics are no longer used on a
commercial scale; this may be due either to concern about their flammability,
toxicity or because their performance is considered inferior by today’s stand-
ards. An example of this is cellulose nitrate that is highly flammable, and
therefore no longer permitted for use in public buildings or transport in Europe.
As a result, it is manufactured by only a few companies today. However,
cellulose nitrate can be found in the form of adhesive, spectacle frames,
jewellery and table tennis balls in museum collections.
Today, almost all international museums and galleries possess collections
which contain plastics. Plastics may be identified within building materials,
defence equipment, ethnography, furniture, housewares, information tech-
nology, medical and sports equipment, modern art, photography and toys.
Many combine metals, textiles and wood with plastics in their construction.
In addition to the objects themselves, many of the materials used to store,
transport and display them are also plastics. While museums continue their
policy to collect objects that reflect both everyday life and historical events,
the proportion of plastics in museums will increase.
Once plastic objects are registered in museum collections, the institution
becomes responsible for their long-term preservation, until the end of their
useful lifetime; that point is reached, arguably, when the object ceases to have
a recognisable form or meaning. Most plastics have rather a short lifetime
compared with those of traditional ‘craft’ materials found in museums, such
as ceramics and stone. The definition of useful lifetime as applied to plastics
in museum objects is rather different to that defined by the plastics industry.
In 1954, Quackenbos, an industrial chemist, defined the life of a plasticised
PVC film as the period taken to lose 10% of its original weight. After that
period, the material is considered to have changed so much in character that
it has failed. Calculations based on Quackenbos’ research indicate that, depend-
ing on formulation, lifetimes for PVC films range from 3 months to 1000 years
at 25°C. It is a more complex matter to estimate the life expectancy of a plas-
tic object in a museum. We must first establish how much deterioration is
acceptable before an object shows a reduction in quality. While yellowing
and other changes in appearance are recognised as normal manifestations
of deterioration for objects constructed from natural materials and are
usually left untreated, the same changes in plastic objects are usually deemed
unacceptable.
Since the properties and degradation reactions of plastics are usually influ-
enced by their formulations and manufacturing processes, this chapter will first
discuss those areas. Four plastics have been identified as being more vulnerable
to degradation than others in museum collections; cellulose nitrate, cellulose
acetate, plasticised PVC and polyurethane foam. The most frequently seen