|
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.
Results
from Questionnaire
4.1 Collection size
4.2 Type of objects
4.3 Age of objects
4.4 Reported degradation
5.
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 Appendix
List of Museums Visited
1. INTRODUCTION
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 pre-1880 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 well-known 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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