The results of a survey of plastics, carried out as part of a joint venture by PHS and the Conservation Unit of
the Museums & Galleries Commission, were first published in 1994. The report formed the basis of a series of
seminars at the time, but was otherwise not made generally available.
A SURVEY of PLASTICS
in HISTORICAL COLLECTIONS
by John Morgan
Plastics Historical Society and The Conservation Unit of the
Museums & Galleries Commission
1994
FOREWORD
Museums, galleries and private collectors of plastics artefacts will, I believe, find much to
reflect upon in this survey of plastics historical collections. They will also find it to contain
much positive and practical information.
Working under the aegis of The Plastics Historical Society and the Museums and Galleries
Commission, the report's author, John Morgan, has made a marked contribution to a better
understanding of the size and complexity of the problems associated with the degradation of
plastics. His findings are all the more valuable for being based on information supplied by
people with 'hands-on' experience and regular contact with the issues rather than academic
research. The exceptionally high response rate to the questionnaire was, I believe, a measure of
the professional approach.
Finally, I hope many will share my pleasure in a report couched in simple English and free from
the jargon which afflicts so many works of this nature.
Percy Reboul
Chairman of the Plastics Historical Society, August 1993
CONTENTS
1.
Introduction
2.
Objectives of the Survey
3.
Questionnaire
4.
5.
Results from Questionnaire
4.1
Collection size
4.2
Type of objects
4.3
Age of objects
4.4
Reported degradation
Visits to Collections
5.1
Conditions in storage areas
5.2
Conditions in display areas
5.3
Packaging of plastics artefacts
5.4
Plastics in the packaging of other material
5.5
Near neighbours
6.
Deterioration of Plastics Materials
7.
Degradation Problems in Plastics Collections
8.
Labelling Plastics Artefacts
9.
General Findings from the Survey
10.
Recommendations
11.
A Strategy for the Care of Plastics
12.
Future Directions
13.
Concluding Remarks
13.
References
14.
Acknowledgements
List of Museums Visited
1. INTRODUCTION
Appendix
Historical collections nowadays contain increasing amounts of plastics materials, both as
objects in their own right and as component parts in products made by other industries. Indeed,
many modern technologies such as electronics, and some older technologies such as
cinematography, could hardly exist without the contribution of plastics materials. Furthermore,
plastics continue to play an increasingly important role in the design of everyday objects as well
as in art forms such as sculpture. There is, therefore, a need for standards that will actively
promote and encourage the maintenance and long term preservation of plastics artefacts.
The problems, it has to be recognised, are of some complexity. For example, most of us have
experienced the sudden failure of a plastics material through degradation. A typical recent case
experienced by the author was a polypropylene cover which crumbled to pieces on being
removed from a domestic radiator control valve. Similar covers on other radiators fitted at the
same time (about seven years previously) were still satisfactory. The degraded cover was not on
the hottest radiator, nor had it been exposed to more light or used more often. So what
happened? Another pattern of behaviour often observed in plastics collections is where an
article which has hitherto shown no sign of degradation suddenly and rapidly deteriorates while
its neighbours, which may have been showing some signs of deterioration, get no worse. These
illustrations emphasise the need for regular inspection of plastics in collections, especially
because the products of degradation often promote deterioration in plastics, or other materials,
in the vicinity.
For many years, plastics were seen as substitutes rather than new materials in their own right
and this attitude prevented them from receiving the attention they deserved - they were often
stored under poor conditions in the mistaken belief that plastics were almost indestructible - a
belief encouraged by manufacturers' claims about the durability of their products usually in an
attempt to dispel unhappy experiences resulting from earlier use of unsuitable or poorly
stabilised plastics. However, in recent years the contribution of plastics to modern society is
being recognised and historical plastics objects are being avidly collected and are beginning to
command a high price.
With this background, The Conservation Unit of the Museums & Galleries Commission
together with the Plastics Historical Society initiated a programme of work to study problems
associated with the long term retention of plastics materials.
The initial meeting of the two organisations took place in September 1989. It was then agreed
that a pre-requisite to the timely solution of the many problems affecting historical plastics was
a clear definition of those problems. To begin with it would be necessary to identify those
plastics which have been observed to be unstable. An understanding of their degradation
processes could lead to treatment solutions, ie the curative measures. At the same time it was
important to anticipate degradation processes of other plastics now being collected in order to
optimise their storage and display conditions - ie preventive measures.
Following that meeting, a seminar of invited speakers from universities, museums and
industry, all having an interest in polymer degradation was arranged for February 1990. Its
terms of reference were - Under the aegis of The Conservation Unit and the Plastics Historical
Society to determine the causes of degradation in polymers (with emphasis on early materials);
encourage and promote work programmes to meet these problems and collate, classify and
make available relevant information from all parts of the world for the benefit of industry,
conservators , museums, galleries, private collectors and the like.
Following that seminar, it was decided to:
a) prepare an initial document on the conservation of plastics, summarising the
current state of knowledge. It should be made generally available and suitable for
use by non-specialist conservators and collectors.
b) conduct a survey of plastics in historical collections in order to ascertain the
extent of the problem.
c) organise practical seminars for non-specialist curators, conservators and
collectors on problems associated with keeping plastics.
An initial document entitled Conservation of Plastics - An Introduction was published in
October 1991, the first of the practical seminars took place at the Museum of London in May
1992 and this report contains the results from the survey mentioned above.
The survey was carried out under the guidance of a small working party comprising the
following people:
David Leigh
- The Conservation Unit
Percy Reboul
- Plastics Historical Society
Rosemary Ewles
- Museums & Galleries Commission
Anne Moncrieff
- The Science Museum
John Ratcliffe
- Plastics Historical Society
Norman Tennant
- Glasgow University
The timeliness of the initiative is demonstrated by the fact that during the period 1991 to the
present time, three major international conferences have been devoted to the subject of polymer
degradation in museum and archival collections:
Polymers in Conservation Manchester, UK, July 1991
Saving the 20th Century Ottawa, Canada, September 1991
Polymers in Museums ACS National Meeting, Washington DC, August 1992
2. OBJECTIVES OF THE SURVEY
There is growing international concern about the deterioration of plastics in museum and
private collections, especially the early cellulosic materials.
Much research into the degradation of plastics has already been carried out. But a concerted
effort is still needed to collate information and to establish the conditions under which artefacts
made from these materials may be safely stored and displayed. The anticipation of future
degradation problems is necessary so that preventive conservation measures may be taken.
Procedures for arresting deterioration in materials already affected also need to be established
so that restorative techniques may be researched and applied.
As a precursor to this work it was thought necessary to ascertain the nature and extent of the
problem so that appropriate priorities could be established. The principle aim of the survey
would be to pin-point areas of most concern, especially those where further work is required to
develop effective solutions to problems.
The following four aims were considered
·
To provide information on the type of material(s) collected.
To ascertain the present condition of objects and to link this, if possible, to any known
history.
·
To determine the conditions under which artefacts are stored and displayed. This would
include, for example, light, temperature, humidity and the presence in the vicinity of any
material which might be thought to influence (or be influenced by) the degradation of
plastics.
·
To consider the preventive and curative measures already being followed by conservators
and collectors.
The survey was intended to encompass deterioration of plastics and was not concerned with
damage or loss through external influences such as mechanical damage, flood, fire, etc., as it
was considered that these aspects of the safety of collections were adequately dealt with
elsewhere. It should be pointed out, though, that some plastics, especially cellulose nitrate
(Celluloid), may constitute a fire hazard and this factor should be considered whenever storage
of these materials is contemplated.
3. QUESTIONNAIRE
Preliminary notices of the survey were circulated by the Plastics Historical Society and The
Conservation Unit and readers were invited to respond if they felt their collection might provide
useful information. There were twenty responses to these, mainly from private collectors. A
questionnaire was sent to each of these and to a list of 177 museums and museum services
selected on the basis that they were likely to include significant quantities of plastics in their
collections. The questionnaire was also sent to the 8 museums in membership of the Plastics
Historical Society.
An accompanying letter explained the purpose of the survey and included the following
definition of plastics; solid materials made from natural, semi-synthetic or wholly synthetic
polymers. Materials such as Bois Durci, vulcanite and shellac compositions are included along
with Celluloid, casein, Bakelite, and the newer plastics. Rubbers and other materials
possessing rubber-like behaviour are excluded, as are textiles, adhesives and surface coatings.
The survey includes objects which are fashioned by carving, machining or shaping, together
with those produced by casting or moulding under heat and pressure. Objects containing
plastics parts or components are also included .
The questionnaire did not ask for identification of plastics materials as it was thought that
records would be unlikely to hold this type of data, and that conservators and curators would, in
general, not have the necessary experience, facilities and time to provide such details.
Plastics are widely used as the base for information storage systems, ranging from photographic
material, sound recordings and video tapes to computer tapes, disks and more recently CDs and
CD ROMs. They all pose problems associated with degradation of the plastics media, but in
general were not considered in this survey. In these situations, of course, the information
carried is of greater value than the object itself. Such stored information can only be retained in
the long term by copying onto new material before degradation starts to affect its quality.
The questionnaire is reproduced in the appendix to this report. It was kept simple, in order to
encourage its return but at the same time provide sufficient information for the survey. It was
anticipated that the answers would be mostly intelligent guesses as it was considered that
collection records would not necessarily be suited to providing direct answers to the questions
posed.
In the event, the returns were very encouraging, approximately two thirds of the questionnaires
being returned.
4. RESULTS FROM QUESTIONNAIRE
A total of 125 questionnaires were completed, 9 from private collectors and 116 from museums.
4.1 Collection size
A total of 19 collections were reported to contain more than 1000 objects wholly or partly made
from plastics, 6 of these being held by private collectors. Most of the private collections were
devoted almost exclusively to plastics, while some (eg button or toy collections) contained a
significant proportion of these materials and quite often the objects were made entirely of
plastics. In the museum collections, plastics were widely distributed but relatively few in
number and usually formed only part of the objects. For this reason, the reported size of
museum collections may have been underestimated because many objects were not recognised,
or recorded, as containing plastics components. Hardly any museums had displays devoted to
plastics.
4.2 Type of objects
The most popular kinds of object wholly or partly made of plastics are those for the home,
followed closely by personal items and toys. The breakdown is shown more clearly in the chart
below.
4.3 Age of objects
Most objects containing plastics date from the period 1920 to 1960, but perhaps surprisingly,
more than 40% of the collections included in this survey hold some items manufactured after
1980. The popularity of the various periods in plastics collections is shown in the chart below.
Note: the chart shows the percentage of responses to that part of the questionnaire; not the
percentage of objects from any particular period. A single response for two objects from pre1880 carries as much weight as one for 200 objects from 1940-60.
The proportion of post-1960 material is likely to increase as objects from homes, offices and
elsewhere find their way into museum collections and as the world consumption of plastics
continues to rise.
4.4 Reported degradation
The most prevalent type of degradation reported for plastics was cracking or crazing, with
discolouration and fading being almost as common. Again, the figures reflect the number of
responses to that part of the questionnaire and not the proportion of the collection which shows
signs of degradation. The proportion of objects showing serious signs of deterioration would,
presumably, be very small, although slight discolouration or fading would probably be fairly
common in most collections. Information about the degree and extent of deterioration, as well
as about the types of materials involved would be best determined by visiting the collections.
5. VISITS TO COLLECTIONS
Following an initial analysis of results from the questionnaire, a small number of collections
was selected to be visited. The aim of each visit was to provide more detailed information about
the composition of plastics in the collection. The extent and nature of any degradation would
also be assessed and more information obtained about storage, treatment and possible history,
particularly for those items giving cause for concern. A standard assessment form was drawn
up, but it was found to be of little value during the visits because of the diverse range of
museums, materials and conditions, etc. and because plastics cut across many classifications
and collections. In addition, many objects were composed of more than one material and the
plastics element could range from trivial to significant. Because of this, many objects had not
been recognised as containing plastics, and it is probably fair to say that the number of plastics
objects held in museums is, in general, higher than that revealed by the questionnaire.
Most plastics items were not on display in museums, but were kept in store either within the
museum building, or, more often, in another location. As a rough guess, 80% to 90% of plastics
were in storage. Because of this it was generally possible to inspect only a small proportion of
the total plastics collection during the visit. To see more would have been very time consuming,
especially as museum records do not necessarily indicate all of the materials from which an
object is manufactured. Also, because plastics occur over a wide range of different themes, it
was also not possible to survey during a single visit every object composed entirely of plastics.
Nevertheless, the survey as a whole was thought to have encompassed all of the collecting
themes and types and to have revealed the major problems associated with keeping plastics.
The two areas, display and storage are considered separately in this report.
5.1 Conditions in storage areas
Storage conditions vary widely from one location to another. The main reason for the variation
is the diverse range of buildings available to museums for this purpose. Some are poorly
insulated outbuildings with little or no heating, others may be modern factory units with little or
no modification to suit their new use. Some areas are fully temperature and humidity controlled.
Plastics objects are almost always stored with other materials, sometimes on open shelves,
sometimes packaged using a wide range of packaging materials and types (this aspect is
considered later in the report). Wood, chipboard and steel were the main materials used for
shelving. Some areas where wide temperature variations would be expected were equipped with
a dehumidifier, but by no means all had access to such equipment. Sometimes an area at the top
of the building, or a roof area where very high temperatures would be reached in summer was
used for storage. In general, most areas not temperature and humidity controlled were equipped
with recording monitors or were regularly checked.
Ventilation of the storage areas also differed widely, ranging from virtually no air movement to
forced draught situations using large fans mounted in the roof. Dust and air-borne dirt varied
according to the air flow. Air-borne contamination from nearby roads and factories was also a
factor which was not fully under the control of the museum. Concern about this factor was
mentioned at a number of locations. It was found that fumigants were occasionally used, but
only where specific problems had been encountered.
Light in storage areas varied also, from no light (except artificial light during visits) to diffuse
daylight. Windows were not always fitted with UV filters and light levels were sometimes
haphazardly regulated by the use of hand operated blinds. Where light was carefully controlled,
it was often because of concern about other materials with which the plastics were stored, as the
sensitivity of plastics to light was not always fully appreciated.
5.2 Conditions in display areas
Environmental conditions in these areas are primarily concerned with the comfort of visitors.
They are, therefore, not generally subject to wide temperature variations and are consequently
easier to manage. Temperature and humidity in display areas were usually controlled and
almost always monitored regularly, if not continuously. The exceptions were museums which
had outdoor displays in reconstructed shops or workshops, where control is necessarily more
difficult. Within the museum, some areas containing plastics were open displays, but the
majority were in display cases, usually made from wood and glass. Lighting inside display
cabinets was very variable, both in quality and quantity. Some were illuminated by internal
electric lighting, from tungsten to fluorescent whilst others relied on room lighting. Heat
generated by lamps within the cases would have resulted in significant temperature increases,
and also temperature differences where spotlights were used. The best cabinets used fibre optic
lighting, so avoiding these problems.
The level of illumination on displays containing plastics was not often regulated, and in many
cases had never been measured, the exceptions being where plastics were mixed with wellknown light sensitive materials such as fabrics. External windows in many museums were fitted
with filters to remove ultraviolet radiation, but this was certainly not always true. In many
cases, the light levels must be considered too high for plastics materials.
Ventilation of cabinets was generally a factor that had not been measured or considered,
although, rarely, an absorbant material such as activated charcoal was present in the cabinet.
5.3 Packaging of plastics artefacts
Probably about half of the plastics objects encountered during this survey were packaged in
some way. The most commonly used wrapping material was acid-free tissue. Wrapped and/or
unwrapped items were then usually placed together on shelving, in a drawer or, more frequently
in a cardboard box. Other materials used for packing included newspaper, paper bags, plastics
bags and bubblewrap. A few objects were in their original manufacturer's packaging and these
were often found to be in very good condition, but this probably reflects the extent to which the
objects had been used rather than the packaging.
It was not uncommon to find objects packed inside polyethylene bags, or other air-tight
containers, either with or without an intermediate wrap of tissue. This form of packaging is not
recommended for certain plastics, notably cellulose nitrate, cellulose acetate and polyvinyl
chloride (PVC) based materials because these polymers emit acidic degradation products which
can catalyse further degradation in an accelerating spiral of deterioration. Such material should
be given sufficient ventilation to allow the degradation products to diffuse into areas where they
can do minimum harm. Vinyl gramophone records, for example, should not be stored inside
plastics bags, although they were sometimes originally sold in such packaging. Plastics
containing plasticiser (cellulose nitrate, cellulose acetate and some PVC based plastics also
come into this category) should not be allowed to remain in contact with packaging films unless
an intermediate layer of absorbent tissue is present. Plasticiser migrating to the surface of the
object can produce a 'contact mark' and may also attack the material it touches (such as
polystyrene packaging - see also section 5.5).
5.4 Plastics in the packaging of other material
Plastics are used in the manufacture of an enormous range of containers, storage systems,
albums, etc. Many of them have found their way into museums and other collections for the
protection, storage or display of a wide range of material. Occasionally, an example of an
unsuitable use of plastics packaging is encountered and sometimes plastics have found their
way into situations never envisaged by the manufacturer. An example of the former is the use
of transparent, plasticised PVC in albums for storing philatelic material; this presents a risk of
plasticiser migrating into stamps, etc. Acidic degradation products from PVC may also harm
the enclosed material. Similar albums using this polymer have also been manufactured for
holding coins, photographic negatives, prints, transparencies, etc. It is much better to use an
album made with a polyester film, such as Melinex or Mylar, for this kind of application.
Whilst plastics are extremely useful materials for packaging and storage systems it is important
that very careful consideration should be given to the composition of any material before it is
used in archival situations.
5.5 Near neighbours
In general, it was observed that little thought had been given to the different materials packed in
close proximity and examples of potentially 'bad neighbours' were occasionally encountered.
For example, flexible plastics based on PVC were sometimes packed adjacent to polystyrene.
Flexible PVC compounds contain plasticisers which are solvents for (and will therefore attack)
polystyrene and related plastics. Occasionally, these two materials are combined in one object,
for example, cameras based on polystyrene very often have flexible PVC carrying straps.
Where this happens, the PVC should be loosely wrapped in acid-free tissue and prevented from
coming into direct contact with the polystyrene by the use of an inert barrier film such as PTFE.
Another example of antagonistic materials being found packed together was casein and
cellulose nitrate. These materials, although quite different, are easily confused and are often
stored together because they were used for similar purposes, eg buckles, buttons, fashion
accessories and items for the dressing table. Casein plastics contain absorbed moisture;
cellulose nitrate degrades in the presence of moisture to release nitric acid which in turn attacks
casein.
6. DETERIORATION OF PLASTICS MATERIALS
Being organic materials, plastics do interact with their environment through chemical reactions
such as oxidation, hydrolysis, etc. The reactions are usually compounded by physical effects
due to stress or migration of additives (such as plasticiser) resulting in what is generally
referred to as degradation. These effects may eventually result in colour change, distortion
and/or crazing, etc. and are often accompanied by a deposit or bloom on the object. Gaseous
degradation products (often acidic) may be liberated and these sometimes form droplets on the
surface by reaction with atmospheric moisture. The form of degradation depends upon the
plastics material and its history, including such factors as manufacturing process and the
environments to which it has been exposed during its lifetime.
Plastics materials are seldom, if ever, composed solely of polymer and are normally
compounded with a variety of ingredients in amounts varying from fractions of a percent to tens
of percent. Additives are used to assist the manufacturing process, improve the service
performance of the product, alter the appearance of the material or reduce cost. Typical
additives include colourant, opacifier, filler, stabiliser, anti-degradant, uv absorber, plasticiser,
lubricant, or even other polymers. Additionally, the material may contain adventitious
impurities or the surface may be contaminated with polishing residues, applied decoration or
airborn contaminants. An added complication in more recent years is that some short-lived
products (eg some packaging) may contain an additive to promote degradation.
Some of these ingredients may adversely affect the rate and course of degradation of the
plastics material, or be subject to degradation themselves. Degradation of one may adversely
affect another. For example, deterioration of plasticised materials based on cellulose acetate or
PVC may be accelerated by acidic products formed by the hydrolysis (by atmospheric
moisture) of an ester plasticiser. Another example is that an acid sensitive dye may change
colour in the presence of acidic degradation products - an effect sometimes observed in
cellulose nitrate and cellulose acetate plastics when they begin to degrade. Pigments and dyes
are often the ingredient most sensitive to deterioration and, for example, fading may indicate
over exposure to light before significant damage has occurred to the polymer. Although,
equally well, a pigment may sometimes mask early signs of degradation.
Additional information on plastics materials and their deterioration is given in reference 1.
7. DEGRADATION PROBLEMS IN PLASTICS COLLECTIONS
Most museum objects are acquired towards the end of their useful life, and the conditions to
which they have been exposed in the past are unknown factors. It is clear that many objects
would already have shown signs of deterioration when received by the museum and this factor
needed to be taken into account during this investigation.
Ideally, an identification of the material is necessary when assessing the degradation of plastics.
Complete identification including important ingredients is costly and requires one or more
samples to be taken. For the purposes of this survey, therefore, identification was restricted to
polymer type and was carried out using visual inspection and 'feel'. No chemical or other tests
were possible.
Many collections possessed examples of plastics showing signs of degradation typical of the
material. The materials which were observed to produce the majority of problems encountered
during the survey are summarised below:
7.1
Cellulose nitrate plastics, such as Celluloid were observed to cause many problems.
Cellulose nitrate is affected by light and moisture and releases oxides of nitrogen and
nitric acid during degradation. Acidity accelerates the degradation process so that once
initiated the process becomes autocatalytic. Degradation is minimised by conditions
which allow the acidic vapours to escape. Thin sections are, in general, more stable than
thick sections, degradation often spreading from the centre of the material towards the
surface. Acid vapours released during degradation were often observed to have attacked
other materials in the vicinity, such as fabrics and metals.
Early signs of deterioration are yellowing, especially with transparent items such as
protective covers and windows, protractors and other drawing instruments. Colour change
may also occur where an acid sensitive dye has been used to provide colour. For example
some tortoiseshell effects produce a red colouration at the onset of deterioration. A
surface bloom is also an early indication of degradation, as is the detection of acidity.
More severe degradation is indicated by crazing and in extreme cases a 'crystallised'
appearance, leading to cracking and crumbling. With thick sections of material, (approx.
5mm or more) the first sign of trouble is usually the appearance of an internal crack easily mistaken for a mechanical fracture. Severe crazing soon follows. This type of effect
is not visible with opaque material until it has almost reached the surface. This may be the
reason why opaque cellulose nitrate often appears to be more stable than the transparent
variety, but it may also be due to the fact that light does not penetrate below the surface to
initiate this kind of deterioration.
Some objects showed serious degradation and were urgently in need of being isolated to
prevent them from causing harm to other objects. In some cases the degradation may have
been accelerated by being enclosed in a plastics bag or other place of restricted
ventilation. Decorative hair combs are an example of items often stored in plastics bags,
and sometimes showing signs of degradation. Metal parts, such as fasteners or hinges,
attached to the object often showed serious corrosion and other metals and fabrics in the
vicinity sometimes exhibited damage caused by the acidic degradation products.
7.2
Cellulose acetate degrades in a similar manner to cellulose nitrate, except that acetic acid
is released during degradation instead of nitric acid. A white bloom on the surface is often
the first indication of degradation. A smell of acetic acid (vinegar) is also an indication
that the material is degrading. Again, thick sections of material appear to be more
susceptible than thin sections. Cellulose acetate plastics usually contain appreciable
quantities of plasticiser, usually in the form of esters of high boiling point. These also are
subject to hydrolysis and change. With change to polymer and/or plasticiser, the
plasticiser may become incompatible and bleed to the surface. Additionally, plasticiser
may be lost through evaporation or be leached out through washing, etc. Loss of
plasticiser causes stiffening, shrinkage and distortion.
In the course of the survey, cellulose acetate plastics were frequently encountered in doll
and toy collections. Distortion due to loss of plasticiser was the most common form of
deterioration. Some colour changes were observed (colourant probably affected by acid
released during degradation) although fading had also been caused by exposure to strong
light. Spectacle frames and sunglasses made from cellulose acetate sometimes showed
degradation, especially at the hinges which were often corroded. In addition, nickel
reinforcing wire in the arms occasionally showed 'greening'.
Some items, for example hair slides and brooches, had been placed inside polyethylene
bags and on opening these smelled strongly of acetic acid and in some cases showed
degradation in the form of a heavy bloom or crazing.
7.3
PVC (poly(vinyl chloride)) plastics are produced in both rigid and flexible forms.
Flexibility is achieved by the incorporation of relatively large amounts of liquid ester and
other plasticisers. The main problems observed with PVC are usually associated with
plasticiser loss. As with cellulose acetate, plasticiser is lost by evaporation and leaching,
and through loss of compatibility through changes to polymer and/or plasticiser.
Additionally, changes in compatibility of the plasticiser may have been brought about by
adsorption of liquids applied to the surface in the form of cleaners and polishes.
Flexible PVC materials were often encountered in collections of fashion wear, such as
boots, shoes, rainwear and 'hot pants'. Despite being of recent origin (within the last 20
years) many of these showed problems due to exudation of plasticiser causing them to
have a 'tacky' surface. Early flexible PVC materials had often become stiff and sometimes
cracked due to the loss of volatile plasticisers. Additionally, light and/or heat may have
initiated a form of chemical degradation which releases hydrochloric acid fumes and also
causes embrittlement. This kind of deterioration was observed with PVC insulated wiring,
degradation being catalysed by the copper wire. 'Squeeky' toys and dolls moulded from
PVC pastes were often found to have a 'tacky' surface due to exudation of plasticiser, and
to have stiffened considerably.
Plasticiser attacks some plastics such as polystyrene for which it is a solvent and where
PVC is in contact with such materials, marks or more serious damage may occur. Damage
to polystyrene camera bodies by contact with PVC wrist straps was sometimes observed.
For the same reason, PVC should not be packed in polystyrene foam. Manufacturers often
use expanded polystyrene as shock absorbent packaging for electrical and other
equipment - long term contact of PVC, eg electric cables, with such material should be
avoided by wrapping with tissue paper and also putting an impervious film against the
expanded polystyrene.
Many PVC compounds contain lead based stabilisers which can blacken if exposed to
hydrogen sulphide. Some darkening of light coloured material was thought to be due to
this effect, possibly from exposure to sulphurous fumes from nearby vulcanite or rubber.
7.4
Casein plastics in the collections often showed the typical surface crazing associated with
loss of moisture (water is a plasticiser in casein plastics). They were thought mostly to
have been in this condition when accessioned. No other kind of deterioration was noted,
although casein is sometimes liable to attack by mildew and moth larvae. Casein is most
commonly found in collections of pens, propelling pencils, buttons, buckles, costume
jewellery, manicure and dressing table sets.
7.5
Vulcanite (also called Ebonite or hard rubber) degrades through oxidation, mainly
initiated by light. The material is usually black and fades to a brown or greenish brown
colour. A red brown material was also produced and this masks discolouration to some
degree. Oxides of sulphur and sulphuric acid also form during the degradation (the
material has a high sulphur content, including some unreacted sulphur). Care should be
exercised when handling because of the possibility of surface acidity. Copper accelerates
degradation and this effect was often observed with vulcanite objects having brass inserts
or screws, eg brass inserts on a Wimshurst machine had promoted severe degradation
near the inserts, as well as themselves being corroded through the acid produced. Similar
effects were observed with various military apparatus where metals and vulcanite were in
combination. Vulcanite imitation jet jewellery in collections was often faded through
exposure to light.
7.6
Polystyrene yellows on exposure to strong light, and being a brittle material may be
subject to mechanical damage. It is also subject to environmental stress crazing in the
presence of certain liquids or vapours when under stress, including locked-in moulding
stresses. Toughened polystyrenes have been produced by compounding with rubber or by
copolymerisation with one or more monomers such as butadiene and acrylonitrile. These
toughened varieties are not subject to stress crazing.
During the survey, some colourless, transparent polystyrene objects (eg trays imitating
moulded or cut glass) were observed to have been subject to evnironmental stress crazing,
and slight yellowing was also usually apparent. Toughened varieties of polystyrene are
not transparent. This material was often found in toy collections and as moulded boxes or
containers for the kitchen. It becomes brittle with ageing and should be handled with care.
A number of toughened polystyrene objects seen during the survey are thought to have
suffered in this way and to have become quite fragile.
7.7
Bakelite and other materials based on phenol formaldehyde resins are stable materials
which do not appear to become destructively degraded under normal atmospheric
conditions. They do, however, discolour badly when exposed to strong natural or artificial
light. Discolouration is almost always confined to the surface. Many examples of
discoloured phenolic materials were seen during the survey but they were thought to have
been in this condidion before being put into the collection. Some private collectors
remove discolouration, especially from the more colourful cast phenolic resins, by the use
of an abrasive polish. This restores colour, but cannot be considered a conservation
treatment. Discolouration will inevitably recur, abraded surfaces being more readily
discoloured than a moulded, original surface.
7.8
Polyurethane is available as a rubbery material, a flexible foam (often in upholstery) and
as a rigid foam. It was frequently encountered in costume collections as an extensible
fibre in some fabrics, foam padding, waterproof coatings and some types of artificial
suede. Deterioration is often severe in this kind of application, possibly because of the
high surface area to volume ratio. Polyurethane stiffens on ageing and eventually
embrittles while flexible foams and suedes become friable and crumble. These materials
are subject to both oxidation and hydrolysis but, except under very moist conditions,
oxidation predominates and is accelerated by light and/or heat. A brown discolouration
usually accompanies deterioration. Unlike most synthetic polymers, polyurethanes are
also susceptible to bio-degradation by micro-organisms such as fungi, material based on
polyesters being reported to be more susceptible than those based on polyethers.
7.9
Polyethylene & Polypropylene are subject to an autocatalytic form of oxidation initiated
by light and/or heat. Affected material generally develops a yellow-brown discolouration
and becomes weakened and embrittled. Some examples of polyethylene kitchenware such
as canisters and jugs are thought to have stiffened and may have become fragile from this
cause.
8. LABELLING PLASTICS ARTEFACTS
Museum objects generally require to be labelled with a registration number or other
identification. However, the survey revealed that many plastics items had not been so identified
- possibly because of problems associated with marking this type of material. One traditional
method of marking is to apply an insulating layer of lacquer, followed by an ink or adhesive
paper and, finally, a layer of lacquer to protect the ink. This procedure, though, is not
recommended for plastics because components in lacquers, adhesives and inks can cause
damage and may initiate degradation. Pressure sensitive labels, such as price tags, can also give
rise to damage, and may produce stress crazing on acrylic and polystyrene. Labels of this type,
including those applied by the manufacturer, have caused damage when left in place for
prolonged periods. Another point to note is that ingredients in plastics may migrate into the
label so that it becomes illegible. Degradation products from some plastics such as cellulose
nitrate, cellulose acetate and vulcanite can destroy paper labels and thread used for tie-on
labels.
How then should one apply a label? The mark must not cause any harm to the object, yet it
must be sufficiently indelible for reasons of security. Provided that inert, stable dyes or
pigments are selected, an ink mark may prove to be the best choice as minimal quantities of
material are involved. Do not lacquer over; this is likely to be more detrimental than ink
because a larger amount of potentially harmful material is involved. A soft pencil will not cause
damage and may be ideal, especially on matt surfaces, but the mark may not have the necessary
degree of permanence. Some surfaces, such as polyethylene or nylon, do not readily take an ink
mark. It is possible to use a diamond stylus to engrave a small mark on an inconspicuous part of
the object - such marking will have the greatest degree of permanence but the level of damage
may be unacceptable. If the possibility of damage to an object precludes direct marking then the
best solution may be to have a suitably annotated photograph stored with the object.
A recent report (ref. 2) suggests the use of PTFE tape (such as plumber's thread-tape) for wrap
around or tie-on tags. Polyester film, abraded to take a pencil or ink mark, is recommended as a
durable tag. The report also offers advice on making an ink from well known, inert ingredients.
9. GENERAL FINDINGS FROM THE SURVEY
The findings of this report are based upon replies to the questionnaire and on observations made
during visits to collections.
i)
The total range of artefacts seen during the visits was large and wide ranging. However,
artefacts collected by the various museums are often determined by 'local interest', or
what is donated, rather than by reference to the collection policies of other museums. The
consequence is that some areas of plastics may be over represented, while other,
historically important, areas may be neglected.
ii)
Many collections have exceeded the capacity of good quality storage and overspilled into
areas which were not designed for, and not ideally suited to, the proper care of plastics.
iii)
Museums in general do not have staff who are sufficiently familiar with plastics to be able
to identify the various types of material. This is understandable because identification
requires a high degree of familiarity with the subject, and even then it may be necessary
to use sophisticated analytical equipment for proper identification.
iv)
The environmental conditions of light, humidity, temperature and ventilation for storage
and display were very variable; some were very good, others were quite poor.
v)
In some cases, problems had not been recognised because no condition surveys had
previously been carried out.
vi)
Only 37% of museums replying to the questionnaire had access to conservation facilities
and very few of these had any experience with plastics.
10. RECOMMENDATIONS
i)
There is a need for the definition of standards for the care of plastics in collections,
especially as the susceptibility of these materials to deterioration in unsuitable conditions
is not always fully appreciated. Guidance on packaging materials and methods is required
in addition to recommendations for environmental conditions.
ii)
Museums should undertake a survey of plastics throughout their collections in order to
avoid potentially serious problems in the future.
iii)
Curatorial staff need more advice and information on how to look after plastics and
practical seminars like that organised by the Plastics Historical Society at the Museum of
London would go some way towards achieving this aim. Nevertheless, such initiatives
need to be improved and made available to a larger audience. Relevant guidance needs to
be included in the training of curators and conservators, as plastics materials will
inevitably become ever more widespread in collections, and their degradation problems
will increase as the years pass.
iv)
Research into methods of prolonging the life of plastics is also needed if future
conservation problems are to be averted.
v)
A list should be prepared of those with experience and skills in dealing with plastics and
related materials, within the museum service and outside, where advice on such matters
could be obtained.
11. A STRATEGY FOR THE CARE OF PLASTICS
A. Identify materials and objects at risk
It is important that any plastics materials in newly accessioned objects should be identified so
that their vulnerability to degradation is recognised; they can then be given appropriate storage
and display conditions. If the object is currently, or has recently been, in production then
information about its composition should be sought from the manufacturer who may well be
able to offer advice on long-term stability.
For collections already in existence, it is probable that documentation will not adequately
identify all plastics materials, and may not even identify all objects containing a significant
proportion of plastics in their construction. The first requirement in such cases will, therefore,
be to improve documentation so that it permits ready identification of those materials and
objects which are at most risk of deterioration.
Each object should be categorised according to its vulnerability to degradation. It should then
be given care appropriate to its category - rather in the manner in which other items may be
given storage appropriate to, for example, their susceptibility to accidental mechanical damage.
An object may be vulnerable to degradation because of:
·
The polymer from which it is manufactured.
·
The composition of the plastics material, ie nature of additives.
·
Its physical dimensions, eg surface area/volume ratio.
·
Its method of manufacture.
·
It may already be deteriorating, because of age and/or exposure to adverse conditions.
·
Its construction may include other materials that give rise to antagonistic effects.
·
It may be under physical stress or strain.
Clues about the above factors may be ascertained by studying the age, shape and form, intended
use and condition - backed up where possible by scientific tests.
A survey of the collection will be necessary in order to categorise objects according to their
vulnerability. At the same time, objects already showing signs of deterioration must be
identified and appropriate action taken. Look for evidence of degradation, and attempt to assess
and record the extent of damage so that future examinations will show whether deterioration
has stabilised, or is progressive. The most common signs of degradation are:
- a surface bloom, especially on cellulose nitrate and cellulose acetate.
- crazing or cracking, particularly in the thicker sections of transparent cellulose nitrate.
- severe discolouration.
- surface tackiness, especially on PVC or cellulose acetate.
- an oiliness on the surface of vulcanite, PVC, cellulose acetate or cellulose nitrate.
- objects showing signs of stress or distortion.
- surface chalkiness.
If sufficient expertise to undertake this kind of survey is not available, the Plastics Historical
Society, or The Conservation Unit should be contacted for advice and assistance. The Plastics
Historical Society has a network of experienced members who would be willing to identify
materials and advise on other matters pertaining to historical plastics.
B. Regularly inspect the more vulnerable objects
Deterioration of plastics usually follows a pattern of a relatively long induction period showing
little evidence of change followed by a period of rapid deterioration. Regular inspection will
allow degradation to be promptly detected so that stabilising treatment may be started at an
early stage and the object isolated to prevent deterioration from spreading to other objects and
materials.
Early warning of degradation problems, eg by the use of suitable indicators or monitors, would
greatly simplify the task of regular inspection. For example, the author has twice been alerted to
the degradation of cellulose nitrate by the fact that acidic degradation products had affected a
discarded phonecard in the vicinity - this occurred before any damage was apparent to the
cellulose nitrate object itself. Environmental air pollution monitoring devices can determine
small concentrations of particular chemicals and it may be possible to use them to detect the
build-up of degradation products during early stages of deterioration.
C. Control and monitor the museum environment
The following list is by no means exhaustive, but it covers the main environmental factors
likely to affect the longevity of plastics.
·
Always wear clean fabric gloves when handling plastics - disposable gloves may be a
satisfactory alternative.
·
Exposure to light should be kept to a minimum. With those plastics most susceptible to
photo-degradation (eg polyethylene, polypropylene, nylon, cellulose nitrate, cellulose
acetate and perhaps toughened polystyrene) the level of illumination should not exceed 50
lux. With most other plastics a maximum level of 100 lux is considered acceptable,
although the possible effect of light on colourants should also be taken into
consideration.
·
Humidity control is important because most plastics absorb moisture to some extent. A
stable relative humidity of approximately 55% appears satisfactory for most plastics,
although a lower level may be better for plastics susceptible to degradation by hydrolysis
(eg cellulose nitrate, cellulose acetate and polyesters). The emphasis, however, should be
on maintaining a stable RH.
·
Temperature should be kept stable, not least because variations in temperature bring about
changes in humidity. Temperatures in the region of 15 to 20 Celsius are recommended at
present.
·
Ventilation is important for those materials that emit gaseous decomposition products.
Cellulose nitrate, cellulose acetate and PVC, for example, emit acids which, as well as
accelerating degradation, may affect other materials in the vicinity. In almost every
collection, at least one example of degrading cellulose nitrate was observed. Degradation
ranged from slight to severe but in many cases was in danger of spreading to similar
nearby objects. It is important that degrading cellulose nitrate objects should be isolated
where they can do no harm and that surface acidity is not inadvertantly transferred to
other objects through handling.
·
Periodic cleaning should be examined as a method of retarding degradation. There is
increasing evidence that decomposition products accelerate deterioration and that
degradation is often initiated by surface contaminants. However, cleaning procedures
carry with them an associated risk of damage and the methods employed must be
carefully considered and controlled.
·
Be aware that some plastics may have an effect on others. Some examples are mentioned
in this report; materials containing plasticiser should not contact each other or other
plastics; vulcanite or rubber compounds should not be stored near light coloured PVC
compounds that might contain lead stabilisers.
It is most unlikely that the best environmental conditions can be provided for every object in a
collection, and so it will be necessary to be selective about which objects are given the best
care. In addition to susceptibility to degradation, factors (which may be equally difficult to
quantify) such as uniqueness, historical importance and monetary value also need to be taken
into account.
13. FUTURE DIRECTIONS
The cellulosic materials, cellulose acetate and cellulose nitrate, are plastics giving most cause
for concern at the present time. This survey has shown that many collections contain examples
of these materials in various stages of deterioration. Acidic degradation products catalyse
further deterioration in a viscious circle of self-destruction and one way of prolonging the life
of these plasticss is to ensure that the degradation products are eliminated or neutralised. The
use of 'molecular' sieve zeolites for protecting cine films has recently been reported (ref. 3).
Sachets containing these absorbers could also be employed with collections containing
cellulosic materials. The absorber would require periodic regeneration by heating but an
analysis of the products driven off during regeneration might reveal deterioration at an early
stage. Another technique that merits further investigation is the use of wrapping tissue
impregnated with a dry acid neutraliser, such as magnesium oxide.
The above procedures are probably most appropriate for material not showing signs of distress.
However, cellulose nitrate objects already showing severe deterioration have been stabilised by
immersion in an epoxidised oil, an acid neutraliser used in PVC plastics to improve stability
(ref. 2 p49).
Specialised packaging of vulnerable objects to provide a 'micro-environment' is one area which
is currently receiving some attention. The package would be sealed and preferably contain
absorbents to scavenge degradation products and any other potentially harmful agents which
permeate into the package. For this technique it is possible to benefit from developments in the
food industry which also has to counteract degradation, although generally over a much shorter
timescale than would apply to the protection objects in museum collections. Monitoring and
maintenance of 'ideal' conditions over long periods of time may prove troublesome in practice,
especially since the amount of reactant necessary for degradation is very small.
Again, low temperature storage is very widely used for increasing the storage life of food
products. Although little is known at present about long term storage of plastics at reduced
temperatures, this technique is likely to find application in the future for preserving material
most vulnerable to deterioration. However, there would be risks associated with cool down or
warm-up.
Many collectors have their own favourite techniques for protecting vulnerable materials.
Owners of vintage cars, for example, use a petroleum jelly, (eg Vaseline) to protect exposed
rubber parts. One collector uses black shoe polish to protect and enhance the appearance of
black vulcanite mouldings. Wax polish has also been applied to bakelite objects, such as radio
cabinets, over many years and this seems to have maintained them in excellent condition and
protected them from the fadeing and surface dulling to which this material is susceptible. The
use of sacrificial, easily removed protective coatings is one area that probably merits further
investigation to determine usefulness and possible long term effects.
14. CONCLUDING REMARKS
At first, interesting objects were collected and these happened to be made of plastics, but
nowadays many objects are collected just because they are made of plastics. After many years
of being regarded as a cheap substitute, hardly worthy of collection, they are beginning to be
recognised as of historical interest in their own right. Objects once commonplace have become
quite rare and have increased significantly in value, both in monetary terms and as a record of
social history.
Many plastics are, however, inherently unstable in the long term and this brings into question
the advisability of collecting such material unless proper provision is made for its storage.
Objects having increased risk of deterioration may be summarised as:
·
Those manufactured from materials which exhibit autocatalytic degradation mechanisms in particular, cellulose esters, polyurethanes and hydrocarbon polymers such as
polyethylene, polypropylene.
·
Objects comprising more than one material, and where synergistic antagonistic effects
may be anticipated, for example cellulose nitrate or vulcanite with contacting metal parts
- especially those containing copper.
·
Objects which have been exposed to contamination, chemicals or cleaning processes
which may have initiated degradation.
·
Objects which have been exposed to strong light, high temperatures or other adverse
environmental conditions for long periods.
·
Objects constructed in a manner likely to impose mechanical stress on one or more parts,
eg from bolts, rivets, springs, etc. or through physical distortion.
It is likely that other factors which can influence vulnerability to degradation will become
apparent as our experience of these materials in the museum environment continues to expand.
One important outcome of this project was that it increased the awareness of museum staff to
potential, and sometimes actual, problems with collections containing plastics materials. Even
where degradation of plastics was understood, the often contagious nature of the deterioration,
or its possible effects on other materials and objects in the vicinity may not have received due
consideration.
It will not be possible in the short term to provide ideal conditions for storing and displaying all
plastics. Indeed, it is not yet possible to define what is ideal. With the increasing complexity of
objects in both construction and materials, a satisfactory environment for one component may
not be satisfactory for another. Dismantling, if at all possible, might increase longevity, but
would bring other problems, not least because the elaborate construction of modern equipment
often requires special tools and skills. In addition, even in a single material, degradation is
usually due to a combination of several inter-related degradation mechanisms, both chemical
and physical in nature. The safest environment will therefore be a compromise which takes
these various factors into account. Accepted standards for the care of plastics need to be
established soon. These can only be based upon our present understanding of the problems.
Much remains to be evaluated and investigated to provide the definitive answers necessary to
save historical plastics before they are lost forever.
13. REFERENCES
1.
Conservation of Plastics - An Introduction. John Morgan, Plastics Historical Society and
The Conservation Unit (1991).
2.
Saving the Twentieth Century: The Conservation of Modern Materials. Canadian
Conservation Institute (1993) pp 341-349.
14. ACKNOWLEDGEMENTS
The author gratefully acknowledges the guidance and encouragement of all members of the
steering committee. However, this work would not have been possible without the help of the
museum staff and private collectors who took time to complete the questionnaire. Particular
thanks are due to them and, especially, to those who co-operated with the museum visits.
Thanks are also due to many other people who have freely given their time and knowledge in
many helpful discussions relating to the degradation of plastics.
APPENDIX
LIST OF MUSEUMS VISITED
1.
Oxon. County Museum, Woodstock, Oxford.
2.
National Maritime Museum, Greenwich, London.
3.
Royal Albert Memorial Museum, Queen St, Exeter.
4.
Central Museum, Victoria Avenue, Southend-on-Sea.
5.
Prittlewell Park Museum, Southend-on-Sea.
6.
Hereford City Museum, Broad Street, Hereford.
7.
Maidstone Museum & Art Gallery, St. Faith's Street, Maidstone.
8.
Luton Museum & Art Gallery, Wardown Park, Luton.
9.
Imperial War Museum, London.
10.
Watford Museum, High Street, Watford.
11.
Stevenage Museum, St. George's Way, Stevenage.
12.
Bewdley Museum, Load Street, Bewdley.
13.
Museum of Science & Industry, Castlefield, Manchester.
14.
Mill Green Museum, Mill Green, Hatfield.
15.
Derby Museum, Friargate, Derby.
16.
Derby Museum & Art Gallery, The Strand, Derby.
17.
National Army Museum, Camberley.
18.
Fleet Air Arm Museum, Yeovilton.
19.
Cheltenham Art Gallery & Museum, Clarence Street, Cheltenham.
20.
Cusworth Hall Museum, Doncaster.
21.
Horniman Museum, London.
Staff of these museums are warmly thanked for their co-operation in this venture.
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