11 Pollutants
Until relatively recently, the effects of pollutants emitted by the air and materials on collections and cultural property had not been studied in detail. Only some of the complex cause and effect relationships have been examined to date. It has already been recognized however that cultural property is at risk of alteration, contamination and degradation by a large number of pollutants. In most cases, a combination of contaminants contributes to aging and degradation. The impact of pollutants on cultural property is also intensified by certain parameters such as temperature, humidity and light. Preventive measures and a thorough analysis of the properties of materials in storage or on display can help to mitigate the impact of pollution in museums, libraries and archives.
Pollutants in cultural institutions?
The quality of the indoor air in residential settings and at the workplace has been under discussion in Germany for many years already. For instance, employees can be assured that certain limit values are specified to protect their health and it is possible to use low-emission (building) materials at home.
However, only a few people will be aware that both polluted outdoor air as well as concentrations of pollutants in the interior of a museum, library or archive represent a serious hazard for the cultural artifacts they house. In particular the breakdown products of materials containing wood that are conventionally used to build exhibition architectures and display cases contribute to the hazards in these institutions. If they include organic materials, the collections themselves can give off substances harmful to the environment.
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| Industrial emissions | Fingerprints on a dusty surface |
Dust and its components (e.g. mold spores) also affect the quality of indoor air in storage rooms and exhibition spaces. The avoidance of contaminants is consequently a key issue for the protection of cultural property.
Terminology
In relation to the protection of cultural property, the very general term “pollutant” refers primarily to volatile organic compounds that are present in a gaseous state due to their low boiling point. The specialist literature usually follows the conventions of the terminology as used in the English-speaking world which subdivides these substances into:
- volatile organic compounds (VOC)
- semi-volatile organic compounds (SVOC)
- very volatile organic compounds (VVOC)
Dusts on the other hand form an exception. They are grouped by particulate size into small (0.01 to 15-20 micron) and large dust particles (15-20 micron).
A distinction is also drawn between outdoor air pollutants (emissions from industry, traffic, salty air) and indoor air pollutants (vapors emanating from construction materials, storage media and/or packaging).
Sources of pollution
Outdoor air pollutants such as ozone, nitrogen oxide or sulfur dioxide usually enter a building as a result of direct air exchange through building apertures. Large volumes can enter through windows, doors or the ventilation system. But even a small exchange of air, such as a draft through a crack, contributes to the continual concentration of pollutants inside institutions.
The volatile organic compounds emitted by materials used in the structure of the building and for designing the space (floors and walls) are a primary source of pollutants in museums, archives and libraries. These include construction materials such as concrete and wood, in particular insulating materials and sealants as well as adhesives and paint coatings containing resins and solvents. Pollutant emissions in the institutions also stem from the materials used to construct the exhibition architectures and display cases, as well as from substances given off by furnishings and storage media.
The collections themselves can emit pollutant substances to the environment and so also contribute to their concentration within the premises or exhibition spaces. These may be either volatile compounds from organic objects (containing cellulose or proteins) or they may be substances used for conservation and restoration (pesticides, plastics, solvents etc.) which, owing to their lengthy retention times, can continue to give off harmful emissions for a very long time.
Finally, breath and perspiration of visitors also lead to an increased concentration of pollutant substances in exhibition galleries and other spaces open to the public.
Interactions and types of damage
The potential damage caused to cultural property by volatile organic compounds is increased by the interaction of various substances. For instance, the impact of pollutants is greatly dependent on the combination and number of existing compounds, their concentration and the duration of exposure. Environmental parameters such as the intensity of light, high UV values and high humidity and temperatures have a catalytic effect on chemical reactions. To date, little research has been carried out on these interactions between various pollutants and environmental factors.
There is however a greater awareness of the resulting damage, caused above all by sulfurous compounds or organic acids. These often have a characteristic appearance:
- Deposits (corrosion products, efflorescence)
- Color changes (stains, discoloration)
- Stickiness (surface degradation)
- Decomposition and disintegration (as a result of complete decay)
Damage caused by organic acids is characterized by deposits and (usually acicular) efflorescence. For instance, organic acids on calcareous materials (limestone, marble or terracotta) can lead to efflorescence (http://iaq.dk/image/cyprus_vase.htm, retrieved on 27.11.2015) or, if not remediated, to the complete disintegration of the object’s surface.
One very specific type of damage, Byne’s disease, can frequently be found in natural history collections, for instance in molluscs or egg shells stored in oak drawers. In this case the organic compounds emitted from the wood produce a typically white powdery coating on the surface of the exhibit.
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| Example of Byne’s disease on a natural history artifact | White deposits in the interstices of a natural history artifact |
As well as the threat posed by calcareous materials, organic acids frequently react with metals in particular, leading to corrosion on lead, zinc and copper alloys (http://iaq.dk/image/lead.htm, retrieved on 05.09.2016).
Even the preservation of contemporary types of object such as film materials based on cellulose nitrate or photographs and data media containing cellulose acetate is at risk from interaction with pollutants (http://iaq.dk/image/photo_neg.htm, retrieved on 05.09.2016).
For instance, even if they are stored well, films made of cellulose nitrate are subject to decay and give off nitrogen compounds to the environment. These become concentrated in closed film canisters for example, and can attack the film material itself. Early plastics containing cellulose acetates also undergo similar processes. These also decay during storage and give off acid gases (acetic acid) which attack the object. This process, typically characterized by a vinegary smell, leads to the object decomposing or shrinking. This is why this form of damage is also known as “vinegar syndrome”.
Sulfurous compounds, frequently emitted by organic materials and objects themselves (wool, wood), also have a negative impact on the preservation condition of artifacts in collections. For instance, hydrogen sulfide and other sulfide compounds cause the tarnishing of silver objects, the discoloration and blackening of lead-containing pigments in paintings, and promote “red decay” on leather.
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| Tarnished silverware | Red decay on leather-covered furniture |
The damage caused by dust depends on the size of the particles. Small particles of dust adhere to the surface of wax, paper or feather objects and form a greasy layer of dirt. Larger particles are deposited on all exhibits. The layers of dust store moisture and react with the surface of the object to form very hard conglomerated dirt. As dust often contains organic matter that serves as a source of nutrition for many insects and can lead to pest infestations, feeding marks can be a secondary cause of damage.
Measuring, avoiding and reducing
Active and passive methods can be used to measure indoor air pollutants. Testing usually requires fairly extensive technical equipment plus the appropriate laboratory equipment. In addition, simple qualitative analyses can provide an initial idea of pollutant loads.
The units of measurement used to record the pollutants depend on the amount and aggregate state of the pollutants. High concentrations are stated as a volume percentage (Vol %), while lower concentrations are given in parts per million or parts per billion (ppm/ppb). The unit ppm (ppb) corresponds to the concentration of the pollutant in one million (billion) parts of air. Liquid and solid substances are stated as units of mass (μg/m³).
Active pollution sampling is performed under defined and accelerated conditions. A given volume of air is drawn in through a filter material and is then subsequently analyzed in a laboratory using gas chromatography and mass spectrometry. With the aid of various detectors, active pollution sampling can also determine the concentration of the compound. The measurements generally provide a very accurate picture of the type and extent of any pollution.
In the case of passive sampling on the other hand, no qualitative results are obtained and a quantitative calculation is also rather imprecise. Owing to their uncomplicated nature, however, passive sampling methods are frequently used. In the course of passive sampling, the test sample is exposed to the air in the room or display case for up to three months, the filter material absorbs any pollutants present and – in a similar way to active sampling – the sample is then analyzed in a laboratory. Passive sample data collectors include diffusion tubes, glass sensors and metal strips. The latter are used to detect nitrogen oxides or sulfurous compounds.
Glass sensors made of especially sensitive potassium silicate glass, or ones made of “sick” glass fragments as developed by the Fraunhofer Institute for Silicate Research in Würzburg, provide a broad picture of indoor air quality. This type of sensor is also placed in a room or display case for a long period of time. It indicates the pollutant load in comparison to the outdoor air, i.e. the greater or lesser progressive decomposition of the glass as a result of the pollution is analyzed. The analysis is also carried out in the laboratory and is performed using an infrared spectrometer.
In addition to the use of active and passive sampling, it is also possible to determine the level of pollutants using various qualitative methods of chemical analysis. These include:
- Oddy test (sulfur compounds, organic acids)
- Beilstein test (chlorides)
- Azide test (presence of reducible sulfur)
- Iodide-iodate test (volatile acids)
A detailed description of these and other pollution tests relevant to museums and archives can be found on the website of the Indoor Air Quality (IAQ) international working group http://iaq.dk/iap/iaq2003/iaq2003_08.pdf, retrieved on 27.11.2015). Nevertheless two of the most well-known tests, the Oddy test and the Beilstein test, will be described briefly here.
For the Oddy test, strips of lead, copper and silver are placed together with the materials to be tested in sealed test tubes and approximately 1 ml distilled water is added to provide the moisture required. To accelerate the reaction of the lead strip, the respective test tube can be additionally flooded with carbon dioxide. The test tubes are stored for 28 days in a chamber heated to 60 °C. To minimize errors, three to five test tubes should be used for each material to be tested.
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| Oddy test |
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| Different degrees of corrosion on test strips | Drops forming on lead strips |
The results of the Oddy test are evaluated purely visually and require interpretation. While the test does not allow specific substances to be identified, it does however provide an indication. For instance, traces of corrosion on the silver strips indicate sulfurous compounds, while the lead strips are frequently attacked by organic acids, or corroded copper strips indicate the presence of chlorides. For a material to be clearly recognized as safe, no evidence of corrosion should be visible on the test metals.
In general the results of the Oddy test can be interpreted as follows:
- Corrosion-free: the material can be used permanently
- Slight corrosion: the material can be used temporarily (up to six months)
- Strong corrosion: the material should not be used for exhibiting and storing cultural property
The Beilstein test can likewise be carried out with very little technical equipment and provides an initial indication of pollution from halogen compounds, especially chlorides in plastics. To test a substance, it is heated on a well heated copper wire in a gas flame. If the sample contains a chloride, the flame will turn green. This method can be used to test packaging, such as the plastic sleeves that are frequently used in older storage facilities and archives to store coins or various archived materials, for the presence of PVC or plasticizers.
Portable reflectometers are sometimes used to determine the concentration of dust in the room air, and consequently the threat to free-standing cultural artifacts in exhibitions or collections in open storage. These reflectometers determine the gloss of surfaces and calculate the dust content. The device shines light on the measuring area at an angle of 60-85°, measures the reflection and converts it into gloss units (GU).
Since pollutants spread through all areas of museums, archives and libraries, and objects in storage and on display are continually at risk from contaminants, there are increasing calls for limit values to be specified, analogous to those stipulated in the occupational health arena. As a health protection measure, the German Federal Environment Agency (UBA) has at least specified an overall limit for volatile organic compounds (VOCs) in workplaces of 1 to 3 mg/m³ (TVOC value). However recent research findings suggest that a far lower limit (< 0.3 mg/m³) is desirable in the near future.
Until such limits have become established and further research findings are available with respect to the effects of pollutants on cultural property, museums, archives and libraries should avoid or mitigate the threat of pollutants causing damage to their collections.
Avoidance strategies include:
- Multi-stage door system at the main entrance
- Sealing all building apertures not in use
- Using appropriate filters for HVAC systems
The principle of exclusion also applies at the micro level. To exclude air exchange with pollutant gases, glass cabinets, display cases and picture frames for especially sensitive exhibits should have airtight seals.
Above all, avoiding pollutants includes the careful preliminary analysis and selection of emission-free materials. Every material used for construction in exhibition and storage areas must be tested before it is used. Likewise, materials and chemicals used for restoration purposes must be carefully selected with reference to their emissions.
Materials on the following list are considered safe for storing and displaying cultural property:
- Metal (powder-coated, with protective sleeves and without direct contact)
- Glass and acrylic glass
- Ceramics
- Acid-free papers and card
- Textiles (undyed, unbleached and washed)
- PE foams (Ethafoam, Microfoam)
- PE films (Mylar, Melinex)
The following materials can also be used for displaying cultural property for short periods (temporary exhibitions) provided they do not come into direct contact with the object materials:
- Hook and loop fasteners, without direct contact with the object
- Nylon film (Dartek) and cord, without direct contact with the object
- Acrylic paints, without direct contact with the object
Adhesives and sealing materials should likewise only ever be used to a limited extent and after a suitable off-gassing period has elapsed. Soluble acrylates (Acryloid, Paraloid) or in emulsion form as well as clear polyvinyl acetate (PVA) are possible adhesives.
If it is not possible to avoid pollutants, efforts should be made to reduce emissions. This may be necessary for example for older display cases and storage materials that are known to give off pollutants.
In the meantime – and interim solutions should always be aimed at reducing pollutants – emissions can be reduced by:
- Sealing surfaces and putting barriers in place
- Using sorbents (active charcoal/Purafil, Ageless, silver cloth, etc.)
Finally, a word about pollutants given off by wood, a material often preferred in exhibition spaces owing to the ease with which it can be worked. Depending on the species of tree concerned, wood gives off varying amounts of formaldehydes or formic and acetic acids. These are associated with a wide range of damage. To avoid harming cultural property therefore, museums, archives and libraries should completely avoid the use of wooden materials, and should gradually replace existing wooden fixtures and fittings.
Alexandra Jeberien and Matthias Knaut
Photos: Alexandra Jeberien