DISPLAY MATERIALS:
THE GOOD, THE BAD AND THE UGLY
| From Exhibitions and Conservation. Pre-prints of the Conference held at The Royal College of Physicans, Edinburg. Ed. J. Sage, The Scottish Society for Conservation & Restoration (SSCR), Edinburg, 1994. ISBN 0950-8068-70, pp. 79-87. |
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Abstract
The aim of this paper is to establish an approach for the use of display materials by minimizing the risk of damaging artefacts. This preventive approach is based on the understanding of the nature of artefacts and materials and their possible interactions in the same environment. Appropriate selection of materials and adequate control of their noxious compounds are the keys to reaching compatibility between display materials and artefacts during an exhibition. 1 Introduction It is important to realize that some materials used in the construction of an exhibition gallery, and used in the fabrication of display cases and supports are a potential source of damage to artefacts. Damages can be caused by volatile compound emission or migration of some of the materials components. Typical visual evidence of these reactions are accretions (corrosion on metals or efflorescence on shells), discolouration (stains on paper, discolouration of textiles), tackiness (plasticizer on photographs) or dust (from degradation of polyurethane foam). Also physical considerations must be taken into account poor weight distribution can cause distortion and cracking, and hard or abrasive materials can leave marks on the artefact surface during shock or vibration. In this paper, I will be mainly focussing on the chemical aspects of materials. 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 compound 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 compound 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 volatile compounds 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 volatile compounds 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 volatile compounds 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 volatile compounds are dangerous for artefacts even in low concentrations. Volatile compounds 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 volatile compounds until it reaches an equilibrium with the environment. If the concentration of the volatile compound 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 volatile compounds 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 volatile compounds released by the materials in order to optimize the control of volatile compounds since the different control methods have their own action pattern and limitations. 5.2 Control of damage caused by volatile compounds It is preferable to avoid all sources of indoor pollutants by using stable materials. However, if hazardous volatile compounds are present in the same environment as sensitive artefacts, then precautions must be considered to avoid accumulation and to minimize the level or effect of volatile compounds present. Six ways to control the effect of volatile compounds released from materials are described here. For best results, it may be necessary to use more than one method to control the volatile compound concentration levels. Although it may not be possible to totally eliminate volatile compounds, damages to artefacts can be minimized by keeping the concentration of volatile compounds as low as possible. 5.2.1 Control by blocking To stop or at least to reduce the volatile compound 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 volatile compounds, particulary 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 volatile compounds 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 volatile compounds 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. Volatile compounds 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 volatile compounds by two possible mechanisms: they can absorb a large varieties of volatile compounds (e.g., activated charcoal, porous and fibrous materials), or they can react with some selective volatile compounds (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 volatile compounds. 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 volatile compounds 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 compound 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 compounds (e.g. Ageless®15) 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 5°C drop in temperature16. The effect of temperature also modifies the emission rate of volatile compounds 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 volatile compounds 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 volatile compounds 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 volatile compounds. 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. References [1] Brooke Craddock, A, Construction materials for storage and exhibition, Conservation concerns. A guide for collectors and curators. Cooper-Hewitt Museum and Smithsonian Institution, New York (1992) pp23-28. [2] Donovan, PD and Stringer J, The corrosion of metals by organic acid vapours, Proceeding of the fourth international congress on metallic corrosion, Houston, National Association of Corrosion Engineers, (1972) pp537-543. [3] Green, LR, Selection for materials and methods for display, The British Museum Report 1990/1 (1990) pp1-10. [4] Padfield, T, Erhardt, D and Hopwood, W, Trouble in store, In Science and technology in the service of conservation, IIC Preprint, London (1982) pp24-27. [5] Tétreault, J, Matériaux de construction, matériaux de destruction, Preprint of the conference: La conservation préventive, ARAAFU, Paris (1992) pp163-176. [6] Daniels, VD and Ward, S, A rapid test for the detection of substance which will tarnish silver, Studies in conservation, 27 (1982) pp58-60. [7] Oddy, WA, An unsuspected danger in display, Museum Journal 73 (1973) pp.27-28 [8] Tétreault, J, La mesure de l'acidité des produits volatiles, Journal of IIC-CG, 17 (1992) pp17-25. [9] Williams, SR, The Beilstein test: screening organic and polymeric materials for the presence of chlorine, with examples of products tested CCI notes 17/1 (1993) [10] Yasuda, H and Stannett, V, Permeability coefficients, Polymer Handbook, Brandrup, J and Immergut, EH editors, John Wiley & Sons, 2th ed (1975) pIII-234. [11] Selwyn, LS, Historical Silver: storage, display and tarnish removal, Journal of IIC-CG, 15 (1990) pp12-22. [12] Rogers, G de W, Particular aspects of silver tarnishing, Proceeding of the 1975 annual meeting of IIC-CG, Bulletin 1 (1976) pp5-6. [13] Calmes, A, Chartes of Freedom of the United States, Museum I46 (1985) pp99-101. [14] Lambert, FL, Daniel, V and Preusser, FD, The rate of oxygen by AgelessTM: the utility of an oxygen scavenger in sealed cases, Studies in Conservation 37 (1992) pp267-274. [15] Maltby, SL, Rubber: the problem that becomes a solution, SSCR Preprint, Edinbourg (1988) pp151-157. [16] Michalski, S, Correlation of zero-span strength, fold Endurance, RH and temperature in the ageing of paper: a review of published data, Book and Paper specialty group abstracts, AIC Preprints, Vancouver (1987) pp.228-229. [17] Godish, T, The immediate and long-term effects of formaldehyde, Comments Toxicology, 3 (1988) pp135-153. [18] Padfield, T, Burke, M and Erhardt, D, A cooled display case for Georges Washington's commission, ICOM Preprint, committee for conservation, Copenhagen (1984) pp38-42. [19] Leichnitz, K, Detector tube handbook : air investigations and technical gas analysis with Dräger tubes, 7th ed, Lübeck (1989).
Jean Tétreault graduated in 1989 from the Universite de Montreal with a MSc in analytical chemistry. In the same year,
he joined the staff of the Preventive Conservation Services Section, Canadian Conservation Institute, where he is working with the control of outdoor and indoor pollutants and their effects on artefacts. He had given many seminars on preventive conservation, and on display and storage materials, in Canada and Europe.
Jean can be contacted at Jean_Tetreault@pch.gc.ca or at:
Canadian Conservation Institute, 1030 Innes Rd, Ottawa, Ontario, K1A 0M5, Canada
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IAQ in Museums and Archives, 1999