Since the 1930's at least, museums and similar organizations have
been concerned with acquiring and displaying articles of social
history to provide a long-term record of our cultural heritage.
Unfortunately this conflicts with the very nature of many modern
articles which are designed to be ephemeral. Artists, always searching
for new materials through which to express themselves, have used
natural rubber latex as a painting medium whilst latex sheets
or latex-coated fabrics have been used as materials on which to
paint or from which to create sculptures. The rapid degradation
of these artworks gives considerable cause for concern.
The conservation of rubber artefacts, and rubber in artefacts,
is thus fundamental to any programme designed to preserve the
cultural heritage, and within this scenario there are three questions
to be asked: what is causing the damage, what can be done to reverse
damage already suffered by older objects, and what can be done
to the more modern items, including those acquired new, to ensure
that they are stored and displayed under optimum conditions and
given any treatment appropriate to prolonging their display and
The answer to the first question is simple: oxygen
and/or ozone (possibly catalysed by light, heat or pro-oxidant
metals) but the next two questions cannot be answered so easily.
Unlike the rubber product manufacturer, who can formulate his
product to optimise its service life - which may not be very long
- the conservator can only take the finished article and consider
his or her options. These are Stabilization - holding the status
quo short-term by limiting existing degradation chemistry - followed
by Conservation - treating the stabilized article so that long-term
degradation is minimised. The latter can entail anything from
Consolidation - very limited (often invisible) reinforcement of
the article through Restoration - treatment which could extend
from consolidation to virtual Replication.
Within these options the conservator is governed by a series
of guidelines which are based on the concepts that between stabilization
and replacement as little as possible is actually done to the
article and that whatever is done should be anachronistic. The
former is intended to preserve as much of the original article
as possible and the latter to enable repairs to be obvious to
the expert but invisible to the layman if conservation records
are lost. An obvious (quite genuine) example of the latter is
the repairing of an 19th Century diving suit with water-soluble
glue. As a final restriction, all conservation should be reversible
in case better techniques are developed in the future. Not surprisingly
these criteria often cannot be met and a compromise must be sought.
An historical lack of awareness of the problems likely to accrue
from the collection of ephemera has now resulted in a vast back-log
of valuable objects which require stabilization and/or conservation
as a matter of extreme urgency whilst in the field of rubber conservation
there is one overriding additional problem. 'Rubber', through
common usage, has become a generic term. It covers a multitude
of different elastomers, natural
and the synthetics,
with different additives included for specific purposes, and with
no obvious way, short of analysis, of distinguishing between either
the additives or elastomers used.
It is revealing to consider the first 'port of call' for a conservator
seeking any information - the 'Conservation Information Network'
- a computerised database operated by the J Paul Getty Trust.
A simple search for 'rubber' and 'conservation', carried out in
late 1993, generated 122 references but, on reading through the
abstracts, it became obvious that most were concerned with the
use of rubber, in various forms, in conservation. Only 15 were
actually related to the conservation of rubber (as well as one
What can be done to protect articles which are socially significant?
Whatever scientific approach is adopted, it must go some way towards
meeting the philosophical requirements of the museum and will
depend on the reason for the conservation and the attitude of
the person responsible for the conservation.
There is no single answer since there is no single material.
The article for which conservation is being sought could be large
and black or light coloured and very thin. Existing historical
artefacts could be unvulcanized whilst modern materials could
consist of a range of polymers, cure and protective systems, and
fillers. However, in all cases there are only the options of leaving
well alone, adding protective agents, coating the product with
an impermeable membrane and/or removing the cause of the trouble
- generally oxygen. For indoor collections ozone, light and temperature
are usually under control although they will not be so for outdoor
exhibits. Stress should also be considered. None of this will
stop the 'sulphur chemistry' continuing
but this will not, by itself, harm the exhibit for several centuries.
It should also be realised that for modern black vulcanizates
of reasonable bulk which are being stored or displayed under ambient
conditions, the ingress of oxygen is limited and, apart from the
surface few millimetres or so, the bulk rubber will remain in
excellent condition for display purposes almost indefinitely.
If the decision is taken to chemically treat the article, then
a choice of protective agent must be
made and the conservator has but two approaches, either to diffuse
the chemical into the rubber or to build up a protective surface
coating. Before considering these alternatives, he or she must
understand that antidegradents can operate by physical (barrier)
means, chemical reactions or both. It is crucially important to
understand which antidegradents work in which ways and what limitations
Para-phenylenediamines (PPD's), added during manufacture, function
both as antiozonants and antioxidants and they operate by reacting
more readily than the rubber double bonds with any ozone present
at the surface of the article. In so doing they build up a protective
film which, as it thickens by continuing migration and further
reaction of the migrated antiozonant, eventually provides an impermeable
barrier to the gas. Any damage to the skin, such as by cracking,
is repaired by further migration. There are many different PPD's
and one of the main reasons for selecting a particular one is
its solubility and rate of migration in the polymer system being
protected, crucial parameters for long-term protection. PPD's
oxidize to blue/purple/black materials and readily stain so their
use must be selective.
Phenolic antidegradents are only antioxidants and have virtually
no antiozonant activity. When oxygen attacks a rubber molecule
various chain reactions occur which may result in both polymer
chain breakage and the insertion of further crosslinks. The phenolics
offer an alternative pathway in the chain reaction sequence and
thus stop it progressing. It is essential to realise that they
do not stop the initiation step so their effect is, at best, to
slow down the oxidative breakdown, perhaps by up to five times.
They do not form a protective 'skin' if oxidized on the rubber
surface and need to be intimately dispersed/dissolved in the rubber
to function. It is, however, possible for a protective skin of
oxidized rubber to form under certain conditions.
Any physical barrier will stop or reduce oxygen and ozone diffusion
into the rubber and might or might not crack depending on its
brittleness and the mode of display or use of the article. Any
subsequent capacity for self-healing will depend on whether there
is more suitable material which can migrate from the bulk, of
the article or whether the barrier material is present only as
an added surface layer.
As long ago as 1931, Semon, Sloan and Craig reviewed proposals,
published over the previous fifty years, to give protection to
objects made without added antidegradents by topical application
of protective agents dissolved in a solvent. Under their accelerated
ageing tests some benefit was found but the protection was not
long-term. Today, some conservators still advocate this approach,
using a solvent for the antidegradent which will swell the rubber
and thus facilitate diffusion into its bulk. This may not damage
a new vulcanizate, although why one should wish to treat it in
this way is not obvious, but, if the object is not vulcanized,
or is degraded so that the surface is structurally weak, the rubber
will either dissolve, or swell to such an extent that the surface
will be severely damaged whilst damage could also be done to the
bulk rubber vulcanizate, particularly since the solvent will be
extracting whatever it can from within the vulcanizate whilst
the antidegradent is diffusing in. If one really wishes to use
this method, then a non-solvent for the rubber must be a better
choice as this will still allow diffusion into the bulk rubber
without appreciably swelling and damaging the surface structure.
It might still, however, extract previously added materials from
the article to its subsequent detriment.
If the choice is not to diffuse protective chemicals into the
unprotected rubber, there is still the option of adding a protective
surface coating, and such a skin can be built up by topical application,
provided the limitations to crack repair and surface finish are
One barrier technique has been well established for many years
and is the coating of the article with an oil lacquer which is
then vulcanized A possible modern equivalent procedure might be
the use of a prevulcanized butyl latex spray-coat since butyl
rubber is relatively impermeable to air.
The ultimate barrier layer must be an enclosed vessel which can
be filled with an inert gas but, even then, the choice of the
material from which to construct the vessel is not always obvious
as many plastics themselves decompose all too rapidly. Simple
plastic bags are readily permeable to air but there are now virtually
impermeable multi-laminate ones which, unfortunately, are not
suitable for display purposes although they could be used for
long-term storage. Perhaps glass is the best material for both
storage and display but nothing will reverse degradation which
has already occurred.
A final point should be noted about the temperature of storage.
Many raw and lightly vulcanized rubbers crystallise at low temperatures
and, particularly with old samples of natural rubber, this phenomenon
can be confused with oxidative brittleness. Although the temperature
for the maximum rate of crystallization of natural rubber is -26°C,
it will crystallize slowly at cool room temperatures (10 - 15°C)
but then has the peculiar property that it can only be melted
by heating to about 30°C above the temperature at which crystallization
took place. An article which has undergone long-term storage in
a freezer will therefore thaw quite quickly when brought to normal
ambient temperature but if a similar article has been stored in
a refrigerator at, say, +5°C it will remain brittle until
it has been warmed to 35 - 40°C. The thawing takes place in
a matter of seconds or minutes for thin articles, depending only
on the transfer of heat through the bulk so any rubber article
which has become brittle after prolonged storage should be warmed
briefly to see if it will regain its elasticity. If the article
has become brittle through oxidation, this will do no further