2.1 The good, the bad ...
A number of lists of "safe or stable materials" have been
published which designers and museum workers can refer to when building display
cases or supports1-5. Examples of these products are polyethylene
sheet, Mylar or Melinex, acid free tissues, etc. In addition, there is often a
list of materials which are not recommended, such as vulcanized rubber,
poly(vinyl chloride), oil based paints and acidic cardboard. By dividing
everything into these two extremes we discourage the use of some materials of
unknown stability which have useful characteristics. Consequently, these
materials are often avoided as display material.
2.2 And the ugly
To minimise these extreme classifications, a grey or "ugly" zone
for materials should be considered. It is not always necessary to utilize the
most stable material. What is important is to prevent damage to an artefact by
using compatible materials.
In this situation, we are defining compatibility as the ability of
exhibition materials and artefacts to exist together in the same environment
without causing damage. Therefore materials with some undesirable properties
can be considered as "compatible" as long as steps are taken to
ensure that the artefact is not harmed.
Although in most cases we are concerned about materials causing damage to
artefacts, compatibility concerns may also be justified in certain cases where
the artefact can cause damage to surrounding display or storage materials. For
example, in natural history collections, preservative fluids may damage the
labels, and then information on the name of the specimens, their origin and
their coordinates will be lost. However, this situation is clearly an
exception; in most cases if an artefact causes damage to a material (e.g.,
acid-free tissue used as an interleaf and becoming acid over time) the damaged
material is considered disposable.
To determine the compatibility between the material and the artefact, the
nature of both must be considered, followed by an examination of the
environmental context. The environmental context is defined as the space and
the micro-climate where the material and the artefact are located. This
includes information such as whether or not they are in contact; what type of
volatile emissions are present; the volume of the space (gallery, display
case); the rate of air exchange; the temperature, and the relative humidity;
and the time spent together. The key to avoiding problems between materials and
artefacts is to consider all parameters and make all necessary corrections to
ensure the compatibility. In this way, many more materials can be used with
museum objets.
This approach of considering the interactions between an artefact and a
material and their context is not limited to exhibition materials but is also
valid for materials used in storage and packing.
3 The nature of things
The nature of the artefact is the first factor to consider. The composition,
the condition and the physical and chemical sensitivity of the artefact must be
determined. What is it made of? Does it need a special support? Is it easily
scratched? With which chemical products or gases will it react? A conservator
should be in a position to provide answers to these questions.
Similarly, information must be gathered on exhibition materials: what is
their composition, their stability and condition, what degradation products and
volatile compounds will they release and at what concentration? Compounds such
as acids, formaldehyde, chlorine, sulphur, peroxide, lignin and plasticizers or
other additives in plastics must be investigated.
An initial step in evaluating the stability of a material can be carried out
by surveying available information such as ingredients listed on the label or
on the Materials Safety Data Sheet and information obtained from the
manufacturer. This information may be insufficient to identify all of compounds
of interest, but it does provide a starting point. Another problem is that
undesirable compounds may be formed during the use of the product or material
and will not be listed as an ingredient. Consultation with a scientist from the
manufacturer, a conservation scientist or a conservator may be warranted. If
necessary, spot tests could be run by the museum staff or the conservation
laboratories to identify specific noxious compounds3,6-9.
After the nature of the artefact and material have been determined, any
potential hazards must be identified and various methods of control must be
explored. Active controls such as avoiding an unstable material altogether, or
blocking it with an interleaf or coating are required. Sometimes controls can
be passive by considering environmental factors such as the relative humidity
or the air exchange rate in the display case.
4.1 Migration of components from material
Contact between an artefact and some material is unavoidable: the artefact
is either placed on a base or support, or suspended on a wire or cord. There is
a possibility that some components of the materials may migrate onto the
artefact once direct contact is made. For example, when vulcanized rubber is in
contact with paper for several months, the plasticizer may transfer into the
paper. This migration can penetrate several sheets of paper. The migration of
products is also particularly common with plastics containing high percentages
of plasticizers such as flexible poly(vinyl chloride). Galvanic corrosion
(migration of ions) may occur if two different metals are in contact.
4.2 Control of migration damage by contact
Contact between an artefact and a material should only be allowed if there
are no components or future degradation products that can be transferred
through the contact point. Unfortunately this is not always possible. For
example, wood is the most common material used for the construction of display
cases. Wood is not considered to be an inert material because it releases
organic acids by hydrolysis and in addition formaldehyde is released by the
adhesive used in some wood products.
4.2.1 Control by blocking
If there is a possibility that harmful compounds may be transferred by
contact an impermeable barrier may be used. The properties of a good isolation
material include high stability (no degradation and inert) and high
impermeability for the compounds of interest. The permeability rate of a
material is mainly determined by the nature of the penetrant and the nature and
structure of the isolation material.
Polyethylene film is frequently used as an isolating barrier. It has a
relatively high permeability to water, but is more effective for other
compounds such as oxygen and carbon dioxide10. However, polyethylene
film, like other plastic films, only reduces the rate of transfer of compounds.
This means that it takes more time to have the same amount of compounds
transferred when using a vapour barrier film, and, in many cases, the transfer
rate is very low. If transfer through an isolating film remains a problem, the
display material or the isolation material should be changed.
5.1 Volatile emissions from materials
The main sources of indoor pollutants (excluding exterior sources), are
combustion, human activities and materials. From materials, there is a large,
complex variety of chemicals that could be emitted. The major volatile
compounds which are known to react with specific artefacts are sulphur
compounds, acid vapours (organic and inorganic), alkaline vapours (ammonia),
aldehydes (mainly formaldehyde and acetaldehyde) and peroxides. Unfortunately,
very little quantitative data exists on the effect of volatiles on artefacts in
museum conditions.
Most organic materials release volatile compounds at different
concentrations and rates depending upon the material emission processes taking
place. They can be simplified in two different categories.
5.1.1 High emission materials
Paints, adhesives and cleaning products release a large concentration of
volatiles when freshly applied, which then decreases exponentially with time.
Some volatile compounds are not necessary dangerous (e.g., water released by
emulsion paints) while others are extremely corrosive (e.g., formic acid
released by alkyd paints). Between these two examples, there is a extended list
of volatiles with an unknown potential for deterioration of artefacts.
Depending on the nature of the material, it may take a few days (e.g. acrylic
adhesives) or a few months (e.g. alkyd paints) before the emissions reach an
acceptable level.
The formation of volatile compounds by chemical reactions can also result in
a high initial emission rate. Some of the vapours released are simply water or
carbon dioxide but others are very corrosive, such as high levels of acetic
acid vapours which are emitted by certain types of silicone sealants. Some
epoxy paints or adhesives, polyurethane foams and some polyurethane paints are
also in this category. Their emission rate and decay are similar to
solvent-release materials.
For the above reasons, it is important that an adequate period of time
should be allowed before placing artefacts in the same environment as these
high emission materials.
5.1.2 Low emission materials
The degradation of organic materials by oxygen, water, ultraviolet radiation
or pollutants is very slow. During this process, degradation products are
formed and gradually change the characteristics of the material.
Discolouration, loss of strength, acidification, cross-linking, migration and
emission of low levels of volatile compounds can be observed. Acidic compounds
are released from wood, poly(vinyl chlorine), poly(vinyl acetate); sulphur
compounds can be given off by wool. Some of these volatiles are dangerous for
artefacts even in low concentrations.
Volatiles released by desorption are also part of this category. Many
materials have the capacity to absorb vapours or volatile compounds of
different natures. A material absorbs volatiles until it reaches an equilibrium
with the environment. If the concentration of the volatile decreases (e.g. the
original source is removed) or if the room temperature increases, the material
may desorb some absorbed volatile compounds until a new equilibrium is reached.
The quantity of volatiles released is probably negligible if the surface of the
material is small compared to the size of the space.
It should be determined if there is a high or low level of volatiles
released by the materials in order to optimize the control of volatiles since
the different control methods have their own action pattern and limitations.
5.2 Control of damage caused by volatiles
It is preferable to avoid all sources of indoor pollutants by using stable
materials. However, if hazardous volatiles are present in the same environment
as sensitive artefacts, then precautions must be considered to avoid
accumulation and to minimize the level of volatiles present. Six ways to
control the effect of volatiles released from materials are described here. For
best results, it may be necessary to use more than one method to control the
volatile concentration levels.
Although it may not be possible to totally eliminate volatile compounds,
damages to artefacts can be minimized by keeping the concentration of volatiles
as low as possible.
5.2.1 Control by blocking
To stop or at least to reduce the volatile emissions, vapour barriers can be
applied to material surfaces. This situation is similar to using a barrier to
control the migrations by contact. The application of a paint film on wood is a
traditional approach to block acid emissions. However, paint is not a complete
barrier (it can reduce emissions by 60%-80%) and the paint film itself can be a
source of volatiles, particularly when freshly applied.
One of the best vapour barriers is a sheet of plastic laminated aluminium
(polyethylene/ aluminium foil/ nylon or polypropylene). One common trade name
for these laminates is Marvelseal and Marvelguard. This vapour barrier sheet is
applied on the material surface with a hot iron. The polyethylene layer is
melted and acts as adhesive. The plastic laminated aluminium is an excellent
vapour barrier if applied correctly with care.
Another way to block the effect of volatiles is to apply a protective
coating directly on artefacts such as applying wax or lacquer on
silver11.
5.2.2 Control by dilution
An interesting way to control the level of pollutants is to modify physical
parameters related to the space such as the volume of air, the material's
surface, and the air exchange rate. By assuming a relatively constant emission
rate from the material in a limited period of time (ignoring sinks), the
concentration of volatiles is then expressed by a simple model:
C=EA/VN
| Where C: |
Chamber* concentration
(mg/m3) |
| E: |
Emission rate (mg/m2h) |
| A: |
Material's surface area (m2) |
| V: |
Chamber volume (m3) |
| N: |
Chamber air exchange rate (h-1) |
*Chamber refer to a display case or a
room.
A large gallery with a ventilation system and
doors has a very high rate of air exchange compared with a fairly well sealed
display case. Volatiles that are present in an exhibit room are diffused
rapidly and thus it can be possible to reduce their concentration to a level
which is harmless to works of art. However, special attention is justified for
display cases because of their confined space and low air exchange rate.
5.2.3 Control using scavengers
Scavengers can be used as a stopgap measure to control problems of harmful
emissions from materials. They can reduce the level of volatiles by two
possible mechanisms: they can absorb a large varieties of volatiles (e.g.,
activated charcoal, porous and fibrous materials), or they can react with some
selective volatiles (e.g., potassium permanganate, calcium carbonate
impregnated in cardboard or finely divided silver particles impregnated in
fabrics (Pacific Silvercloth)). These products are very effective when they can
surround an object, as is possible in storage. Results are less successful,
however, when the artefact is on display, as it cannot be wrapped. A scavenger
will only be effective if it covers a large surface area in a relatively small
volume since there is a competition between the scavenger and the artefact to
absorb or react with the volatiles.
Scavengers have also a limited "lifetime": the activated charcoal
must be regenerated periodically to avoid saturation of the absorption capacity
and the chemical reactive scavengers must be replaced when there is not enough
free reactive sites. The scavenger's lifetime will be optimal if there is a low
air exchange rate, and if the concentration of the compound and its rate of
emission is very low (e.g. source of volatiles from degradation processes or
from very low emission materials inside a limited period of time).
5.2.4 Control by reduction of other reactants and catalysts
Frequently, a deterioration process involves compounds other than the
volatile compounds and the artefact. These compounds may be reactants which are
consumed during the chemical process, or may be catalysts which speed up the
process. The most common reactants and catalysts are water vapour and oxygen.
Low relative humidity can decrease the speed of degradation processes of
artefacts and materials. For example, the tarnishing of silver is strongly
affected by moisture; the tarnishing rate is more effectively decreased by
decreasing the relative humidity than by decreasing the concentration of
hydrogen sulphide12. For a display case, the control of the relative
humidity is relatively simple and can be achieved with low cost technology
(silica gel or a small humidifier/dehumidifier).
Few degradation reactions are possible at low oxygen levels despite having
relatively elevated concentrations of volatile present. The oxygen
concentration can be reduced by using a hermetically sealed case under inert
gas (with positive, atmospheric13,14 or negative
pressure15) or by using oxygen absorbers such as iron oxide
compounds (e.g. Ageless15) in a closed system. Although the control
of oxygen levels is a viable method of control, it is technically difficult to
achieve.
5.2.5 Control by reduction of temperature
Chemical reactions generally proceed more slowly, or may stop entirely, when
the temperature is lowered. Studies on the deterioration of unstable paper have
shown that the rate of the deterioration reaction decreases by a factor of two
for each 5oC drop in temperature16. The effect of
temperature also modifies the emission rate of volatiles such as formaldehyde.
an increase in temperature will result in an increase in both the rate of
diffusion from the material and the rate of hydrolysis17.
Unfortunately, the temperature in exhibition areas is usually dictated by human
comfort levels. Although refrigerated display cases have been
developed18, this is beyond the budget of most exhibitions.
Elevated temperature inside a display case should be avoided by ensuring
that light sources are located outside the case, and by controlling light
levels.
5.2.6 Control by time
The period of exposure required for volatiles or migration products to react
with an artefact and cause damage depends mainly on the concentration of
reactants. A few days can be sufficient to damage some artefacts if there is a
high level of the pollutant; many months or many years will be necessary to
produce the same effect on the same artefact in the presence of a low
concentration of the pollutant.
Time factors are particularly important with materials that emit high
amounts of volatile compounds especially when used in a small, closed system
such as a display case. In order to get complete compatibility a sufficient
delay must be allowed between the application of high emission materials and
the installation of art works in the room or the sealing of the display case.
Another point related to time is that all materials have their
own-"lifetime". A material may not be compatible with an artefact
forever. Many materials have many decades or centuries as a
"lifetime". However, some useful materials like polyurethane foam
progressively lose their initial physical and chemical properties (become
yellow, lose cushioning properties and become brittle) within a period of a few
years and are potentially dangerous to nearby artefacts. Despite its
instability, polyurethane foam may be required for cushioning in certain
circumstances as such as the transportation of extremely fragile artefacts.
This foam is tolerated since it is used for a short period without contact with
the artefact (by using an interleaf). For display or storage applications, the
polyurethane foam may cause problems (without being necessarily in contact with
an artefact) if used for long period of time or if used when already degraded.
6 Before the installation of artefacts for the exhibition
It is important to be cautions and use good judgement in selecting and
utilizing display materials for exhibitions. This specialized subject must be
incorporated in the exhibit management and must be harmonized with other
decision elements.
During the planning of the exhibit, the choice of materials and any
necessary delays (e.g. after painting) must be specified. Ideally, the selected
materials should not cause any potential damage to artefacts, but like all
museum activities, the best solution is not always possible and compromise is
then the realistic alternative. A compromise is acceptable if the compatibility
between materials and artefacts is respected. Once the potential hazards of the
materials have been determined, then it must be ensured that the materials will
be harmless to art works or will be controlled.
When any material is received, its chemical nature should be determined. If
necessary, spot tests can be run to determine if a particular material poses a
threat to the artefact. An evaluation of the exhibition area should be
conducted before the artefacts are installed. The presence of volatiles may be
first noticed by the odour (odour of paints, adhesives or "new
materials"), but then can be measured with special instruments (e.g.
Dräger tubes19). Selective passive pollutant dosimeters like
lead and silver coupons placed inside a display case may be useful for long
term monitoring of specific volatiles.
7 After the installation of artefacts for the exhibition
A survey should be carried out at the beginning of the exhibition and
periodically thereafter to verify the presence of any deterioration on
artefacts. It is easier to detect small amounts of damage using pollutant
dosimeters such as metal coupons, than trying to monitor very subtle changes on
the artefacts themselves. However, remember that a number of agents of
deterioration may be responsible for observed damage. If damage or potential
damage is found to be related to the materials, then the choice of materials or
the methods of control have to be reconsidered. If the situation cannot be
immediately corrected, artefacts should be removed from the harmful
environment. Artefacts suffering from some type of damage may need immediate
treatments to avoid further damage. A conservator should be consulted to
establish the seriousness of the deterioration and to determine if treatment is
required.
8 Conclusion
An exhibition must respect the integrity of the artefact while ensuring its
protection. Museum staff, designers and conservators should be sensitive to the
choice and the use of materials since certain materials are known to have
noxious contaminants or pollutants. Even though not all materials can be safely
used with artefacts, it is possible to deal with different controls that avoid
or minimize their damaging potentials. By extension, most materials could be
considered as "ugly". By evaluation of the context, case by case, and
by making the right decisions, it is possible to take measures in order to
obtain compatibility between materials and artefacts and give enough freedom to
the museum workers to produce an aesthetically pleasing exhibit.
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