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The information presented on the Paintings Conservation Wiki is the opinion of the contributors and does not imply endorsement or approval, or recommendation of any treatments, methods, or techniques described.
Author: Elizabeth Wigfield
Editors: Christine Gostowski, Anne Schaffer
- 1 Introduction
- 2 Gaseous Pollutants
- 3 Airborne Particulate Pollutants
- 4 Intrinsic Pollutants
- 5 Determining Safe Air Pollutant Levels
- 6 Testing for Air Pollutants
- 7 Controlling Air Pollutants
- 8 References
- 9 Further Reading
Air pollutants, comprised of airborne gases and particulates, have a detrimental effect on human health and the environment worldwide. In the last century, many measures have been taken to curb pollution. In the United States specifically, the formation of a deadly sulfur dioxide smog over the industrial town of Donora, Pennsylvania in 1948 served as a catalyst for legislative change, leading to the formation of the Environmental Protection Agency. Congress passed the Clean Air Act in 1970, which was revised and greatly expanded in 1990 to curb three major threats to public health and the environment: acid rain, urban air pollution and toxic air emissions. When considering local conditions, one must keep in mind that air pollution is not a regional issue, but a national and worldwide problem. Pollution created in urban and industrial areas easily spreads due to global wind currents.
Air pollution has long been recognized as a threat to cultural collections. In one of the earliest conservation publications on the topic, Thomson (1965) lists sulfur dioxide, hydrogen sulfide, ammonia, ozone, nitrogen dioxide and particulates as the main air pollutant threats to works of art.
This literature review focuses on the effects of gaseous and particulate air pollutants on paintings. Determining the deleterious effects of pollutants is complicated by a number of issues, as paintings are material composites with components that exhibit different ageing properties and react differently in varying environmental conditions. Air pollutants tend to be invisible, and deterioration rates caused by air pollution can be slow and hard to measure. Some air pollutants will damage materials only in combination with other agents of deterioration and some may act as catalysts for the natural ageing process of a painting. Some air pollutants are generated outdoors, from natural or manmade sources, while others can be generated from within a building or be a product of deterioration arising from the painting materials themselves.
Individual pollutants, their sources and effects on paintings are listed in the following section.
Sulfur dioxide, SO2
Sulfur dioxide (SO2) is a product of natural outdoor occurrences, like volcanic activity or forest fires. It is also man-made, produced by the combustion of fossil fuels or the burning of kerosene, coal or wood for cooking and heating.
The hydrolysis and oxidation of SO2 produces sulfurous acid (H2SO3), and sulfuric acid (H2SO4). These contribute to the deterioration of cellulosic materials, such as linen canvas and cotton. Acid-catalyzed hydrolysis of cellulose-based textiles is responsible for weakening fabrics, yellowing and embrittlement. The extent of deterioration of cellulosic materials depends on its exposure to sulfurous products, as well as a number of other factors, such as humidity and ultraviolet radiation (Brysson, Trask, Upham and Booras 1967; Hackney and Hedley 1981; Baer and Banks 1985; Hackney and Ernst 1994; Saunders 2000).
Tests exposing watercolor and drawing media to SO2 have shown that triphenylmethanes such as fuchsin, brilliant green and mauve may be susceptible to fading (Williams, Grosjean, Grosjean 1993). While similar tests have not been undertaken with paintings, these results should nevertheless be useful as a guide when dealing with these artists’ pigments and dyes.
Ultramarine is known to discolor in the presence of SO2 in combination with increased moisture (Saunders 2000).
Exposure of magnesium carbonate paint extender used in 20th century artists’ paints, to a sulfurous environment in high relative humidity (RH), will form magnesium sulfate salts, also called Epsom salts. Their presence renders the paint surface more sensitive to water (Cooper 2014; Silvester 2014).
Nitrogen oxides, NOx
Nitrogen oxides (NOx), produced by internal combustion engine exhausts are, together with SO2, the principal class of acidic pollutant gases in urban areas, contributing to the generation of ozone (see Ozone).
Damage to cellulosic materials in textiles has been associated with the presence of nitrogen oxides, which manifest themselves, among other effects, as a loss of fiber strength (Baer and Banks 1985).
Hydrogen sulfide, H2S
Hydrogen sulfide (H2S), a reduced sulfur compound, is a natural product of volcanoes and geothermal activities as well as a product of fuel and coal combustion. H2S is also generated indoors by humans, display case materials such as silks, wool and felts, animal hide based adhesives, and wool carpets.
Darkening of paintings has been attributed to the reaction of lead pigments and dryers with hydrogen sulfide gas in the air. For example, lead white can react with hydrogen sulfide gas to form lead sulfide, which is brown to black in color (Carlysle and Townsend 1990). Red lead pigment is also susceptible to darkening by reaction with hydrogen sulfide (Grzywacz 2006).
Ozone (O3) is a highly reactive compound produced through a photochemical reaction of hydrocarbons and oxides of nitrogen. Common sources of ozone are automobile exhaust, industrial pollution in sunlight, indoor air purifiers, or other office appliances such as dry process photocopiers.
Ozone is a strong oxidizing pollutant and is hazardous to organic materials at room temperatures. It oxidizes organic compounds, for example destroying chemical double bonds on carbon chains in unstable natural resin varnishes (Thomson 1965; Thomson 1986; Grzywacz 2006).
Ozone also embrittles cellulosic materials by hydrolysis (Thomson 1986; Hatchfield 2002; Grzywacz 2006).
Most organic colorants and pigments exposed to ozone in experimental testing have shown some degree of fading. Curcumin, dragon’s blood, indigo and madder lake were found to be extremely fugitive when exposed to ozone. Synthetic artist’s pigments, on the other hand, were relatively stable (Whitmore, Cass, Druzik 1987). As mentioned previously, these tests are not fully representative of most paintings, since pigments had been applied directly to paper without a binder. Exposure to elevated levels of ozone has been implicated with the accelerated ageing of oil paint films, as well as the surface erosion of oil based house paints (Druzik 1985; U.S. EPA 2006). It should also be taken into consideration that pollutants generally have a deteriorating effect on paintings in conjunction with other agents of deterioration, such a humidity, light, heat, etc., where some act as catalysts for others.
Aldehydes and Organic Acids
Aldehydes and organic acids are organic carbonyl pollutants, which include: formaldehyde (HCHO), acetaldehyde (CH3CHO), formic acid, and acetic acid. These belong to the group of volatile organic compounds (VOCs), hydrocarbons that exist as gases at ambient temperatures (Grzywacz 2006). They are secondary pollutants resulting from atmospheric reactions with industrial vehicle pollution or from off-gassing of construction materials such as laminates, polyvinyl acetate adhesives, oil and alkyd paints, vinyl, linoleum, plywood, urea-formaldehyde containing materials, wood, plywood, fiberboard, cleaning agents, and other household products (Hatchfield 2002).
By oxidation, acetaldehyde and formaldehyde can be transformed into organic acids that cause hydrolysis of cellulose, which reduces the degree of polymerization, resulting in embrittlement (Brysson, Trask, Upham and Booras 1967; Tétreault 2003).
Amines and Ammonium Compounds, R-NH2, R-NH4
Amines are volatile alkaline compounds derived from ammonia. They are commonly contained in amine-based corrosion inhibitors, such as diethylaminoethanol (DEAE), used in building humidification systems and ventilation ducts. Once released into the building environment, they can settle on painted surfaces, leaving an oily film that may appear as a blemish or slight haze (Biddle 1983; Volent and Baer 1985; Hatchfield 2002; Tétreault 2003; Grzywacz 2006).
Ammonium sulphate is one of the byproducts of incomplete combustion of fuels. It accumulates in atmospheric moisture and plays the chief part in the formation of ‘bloom’ on certain chemically unstable varnish surfaces, mainly fresh applications of dammar or mastic (Thomson 1965).
New concrete buildings generate alkaline air, due to minute aerosol particles liberated from the concrete. Linseed oil can be darkened by exposure to alkaline environment in new concrete buildings, with new oil films becoming yellow to brown (Toishi and Kenjo 1975; Hatchfield 2002; Grzywacz 2006). Alkaline pollutants can also discolor certain pigments, such as red lead and massicot (Toishi and Kenjo 1975) and can form copper amine complexes with copper pigments, like malachite (Grzywacz 2006). It has also been noted that hair hygrometers lose their precision sooner in alkaline air (Grzywacz 2006).
Fatty Acids, RCOOH
Fatty acids or carboxylic acids are VOCs that may deposit on the surfaces of paintings, causing blemishes (Tétreault 2003). They are emitted from vehicle exhausts, are used as lubricants in HVAC systems, and are emitted during cooking. Other sources of fatty acids include flooring adhesives, linoleum, objects made from animal parts, and the human metabolism.
Airborne Particulate Pollutants
Particulate matters, such as soot or dust, can enter the building from the outside, but can also be generated by the building itself, either during construction work or as part of the normal use/aging of concrete, stone, marble, plastic, asbestos, furnishings, etc. Indoor fireplaces will create soot. Dust, lint, dead skin cells and other particulates can be brought in by humans and animals. Particulate pollutants are characterized by diameter, from fine (soot, sea salt and tobacco smoke) to coarse (dander, fabric lints, dust from packing crates and cement dust), the latter being visible to the eye (Tétreault 2003). The fine-sized particles are the most harmful, as they can be airborne for days and may easily enter spaces and lodge in small interstices (Tétreault 2003). Particulate pollutants are often hygroscopic in nature and also biologically attractive, and can be food sources for insect pests. Dust has a highly adsorptive capacity for gaseous pollutants and the adsorption of SO2 can produce a corrosive form of dust, that is acidic in nature (Stolow 1987). Cigarette smoke has been a great contributor of particulate matter, such as tar and nicotine, which readily settle on paintings.
Settling of particulates on paintings is not only an aesthetic concern as it is related to the discoloration of surfaces, but is especially critical for those surfaces that are porous in nature and have interstices that entrap dust. Deposits of particulates that are acidic, gritty or hard in nature can result in the necessity for surface cleaning, exposing the painting to potentially harmful abrasive treatments. See also (Tétreault 2003).
Intrinsic pollutants, or more correctly, intrinsic secondary pollutants, are released during the natural degradation of painting components. Fatty acids released during the hydrolytic decomposition of oil films, for example, can evaporate and form ghost images on the glass of glazed paintings (Shilling, Carson and Khanjian 1998).
Metal soaps in paint films, on encountering pollutants or moisture in the environment, can chemically change from soaps to insoluble complex salt mixtures such as lead and zinc carbonate (http://www.conservation-wiki.com/wiki/White_Surface_Hazes; Van Loon, 2012).
Materials inherent in paintings, e.g. wood stretchers, can emit organic carbonyl pollutants that are detrimental to the canvas support.
Determining Safe Air Pollutant Levels
Measuring the rate of deterioration and setting standards for individual pollutants is challenging. Whilst individual painting materials have been shown to be affected by gaseous pollutants in experimental settings, it is far more difficult to quantify the damage to the individual material and to measure the effect pollutants have on the painting as a whole. Deterioration of materials in paintings depends on a variety of contributing factors, such as temperature, humidity, light, pollution, as well as the age of the object.
Grzywacz (2006) has determined the recommended pollutant limits for cultural collections. The gaseous pollutants are listed in volumetric measurements as parts per billion (ppb) which are temperature and pressure dependent. They combine experimental data with theoretical info and exposure time to classify pollutant concentration levels based on the susceptibility of the materials and the length of exposure. The limits are based on the “no observable adverse effect levels” (NOAEL) and the “lowest observable adverse effect level” (LOAEL) standards set by Tétreault (2003).
Testing for Air Pollutants
There are two general methods of sampling gaseous pollution: active and passive systems. Active sampling systems use a pump to pull a sample of air into a monitoring device and can be performed in minutes or hours. Grzywacz (2006) describes passive sampling systems available for museum environments in detail. They are relatively inexpensive and easy to use compared to active sampling systems, as they allow the air to diffuse into the device naturally. In passive sampling systems, gaseous pollutants are trapped on a reactive surface and can be read directly or analyzed in a laboratory.
Monitoring with these sampling systems can be qualitative or quantitative. Qualitative monitoring determines if a dangerous pollutant or class of pollutants is present and evaluates their effect. Quantitative monitoring measures the concentration of a specific gaseous pollutant, and that information is necessary to investigate the most likely source so that remedial action can be evaluated and implemented (Hatchfield 2002; Grzywacz 2006).
The adverse effects of particulate pollutants are measured mainly by the visual impact on paintings, such as loss of saturation and change in surface reflectivity and color.
Controlling Air Pollutants
There are various strategies to reduce the presence and limit exposure to gaseous and particulate pollutants in the air (Tétreault 2003) that also apply to paintings, namely to avoid, block, dilute and filter/sorb pollutants.
The first step would be to exclude indoor generated pollutants by carefully choosing stable and unreactive building and furnishing materials (Hatchfield 2002). Products to be avoided in large quantities are oil or alkyd-based paints and varnishes, wool carpets, and uncoated wood products made with urea formaldehyde-based glue (Tétreault 2003). Materials used in close contact with paintings, such as frames or display cases, should also be chosen according to these guidelines. Where applicable, framing a painting can ensure a simple barrier to block both gaseous and particulate pollutants. Ideally, the frame should include a well-sealed backing board and glazing (Hackney 1990, Hackney and Ernst 1994). Loose linings have also been used to protect against dust build-up on the canvas reverse.
Artists have attempted to protect their paintings against dust and sulfurous gases as well: up to the 19th century, some artists stretched primed canvases behind their paintings with the primed side out, blocking the access of airborne pollutants (Carlysle and Townsend 1990; Hackney 2004).
One treatment method recommended to avoid canvas deterioration by exposure to acids is deacidification by treatment with, for example, methoxy magnesium methyl carbonate, sprayed or brushed onto backs of paintings where material is converted to magnesium carbonate, thus providing a buffer for the acids (Hackney and Ernst 1994).
Varnishes may be applied primarily for aesthetic reasons, but will also serve as protective barriers from particulate matter and gaseous pollutants. 19th century sources, lacking an alternative to lead white, recommended covering the paint with varnish or adding varnish to the media, or adding a covering layer of zinc white over the lead white ground/passages, to avoid lead sulfide darkening (Carlysle and Townsend 1990).
The main measure to control outdoor generated pollutants is to block entry by creating a well-sealed building envelope. A few simple basic measures apply to any kind of painting collection, be it a museum, private collection or historic house. They include closing and sealing windows and doors where possible, keeping a high standard of cleanliness using high efficiency particulate air (HEPA) vacuums to remove particulates, using safe cleaning products (especially avoiding those that are vinegar-based), and controlling visitor traffic and density.
The only effective protection for collections is likely a fully ducted air conditioning system with filtration to remove particulate matter and pressurization to reduce infiltration. Where an HVAC system is not feasible, enclosures may be used to control exposure to pollutants. For recommendations, consult The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) 2015 Handbook for Heating, Ventilating and Air Conditioning (HVAC) Applications. The chapter on Museums, Galleries, Archives and Libraries lists threats to historical collections, how they may be minimized by an effective HVAC system, and describes different types of systems. It is based on recommendations from the Getty Conservation Institute and the Canadian Conservation Institute, and is reviewed every four years.
HVAC systems generally require particulate and pre-filtration to remove coarse-sized particulate pollutants that could foul the system. An additional fine-particulate filtration system will help protect the art collection (ASHRAE 2015). Molecular, or gas-phase filtration will remove acetic acid, formaldehyde, hydrogen sulfide, nitrogen dioxide, ozone and sulfur dioxide. These gaseous pollutants are removed either with activated or treated carbon, or with potassium permanganate beds (Grosjean, Williams II and Hisham 2009; ASHRAE 2015). Portable, mobile stand-alone filter units are also an option (Tétreault 2003).
Six to seven air changes per hour are recommended for the storage of paintings. Electrostatic mats at entrances reduce particulate pollutants being dragged into the room. Generally, local regulations for appropriate workplace pollutant levels are appropriate for long term storage of paintings (Dixon 2012).
The effects of air pollution on paintings is a complex subject matter. Information specific to paintings is gleaned from a wide range of publications. One of the most recent (Hatchfield 2002) defines pollutants in museum environments, listing sources and testing methods for identification and examining construction and display materials to determine their safe use in proximity to works of art. Tétreault (2003) lists pollutants and quantifies their risks, recommending control and preservation strategies according to the ‘no observed adverse effect level’ (NOAEL) and ‘lowest observed adverse effect level’ (LOAEL) standards. Methods of their measurement and monitoring, in particular passive sampling devices, are described in detail in Grzywacz (2006). References are listed in the text above, where applicable, and further reading is noted below.
American Society of Heating, Refrigerating and Air-Conditioning Engineers. 2015. 2015 ASHRAE handbook: heating, ventilating, and air-conditioning applications, Inch - Pound Edition. http://app.knovel.com/hotlink/toc/id:kpASHRAEF2/2015-ashrae-handbook.
Baer, Norbert S., and Paul N. Banks. 1985. “Indoor Air Pollution: Effects on Cultural and Historic Materials.” The International Journal of Museum Management and Curatorship 4: 9-20.
Biddle, W. 1983. “Art in Cornell Museum coated by chemical used in steam lines.” New York Times, July 29. https://www.nytimes.com/1983/07/29/nyregion/art-in-cornell-museum-coated-by-chemical-used-in-steam-lines.html Brysson, R.S., B.J. Trask, J.B. Uham and S.G. Booras. 1967. “The effects of air pollution on exposed cotton fabrics”, in Journal of the Air Pollution Control Association 17 (5): 294-8.
Carlyle, L., and J.H. Townsend. 1990. “An investigation of lead sulphide darkening of nineteenth century painting materials”, In Dirt and Pictures Separated. UKIC: 40-3.
Cooper, A., A. Burnstock, K.J. van den Berg, and B. Ormsby. 2014. “Water Sensitive Oil Paints in the Twentieth Century: A Study of the Distribution of Water-Soluble Degradation Products in Modern Oil Paint Films”. In Issues in Contemporary Oil Paint, edited by van den Berg, K.J., Burnstock, A., de Keijzer, M., Krueger, J., Learner, T., de Tagle, A. Heydenreich, G. Switzerland: 295–310.
Dixon, Tom. 2012. “Storage of Easel Paintings”. In Conservation of Easel Paintings, edited by Joyce Hill Stoner and Rebecca Rushfield. London: Routledge: 672-7.
Druzik, J.R. 1985. “Ozone: The Intractable Problem.” WAAC Newsletter, 7 (3) September 1985: 3-9. https://cool.conservation-us.org/waac/wn/wn07/wn07-3/wn07-302.html
Gridley, Mary, and Bonnie Rimer. AIC Wiki/Paintings/White Surface Hazes. http://www.conservation-wiki.com/wiki/White_Surface_Hazes)
Grzywacz, C. 2006. Monitoring for gaseous pollutants in museum environments (Tools for conservation). Los Angeles, Calif.: Getty Publications.
Hackney, S., and G. Hedley. 1981. “Measurements of the Ageing of Linen Canvas.” Studies in Conservation 26 (1): 1–14.
Hackney, S. and T. Ernst. 1994. “The applicability of alkaline reserves to painting canvases.” in Preventive Conservation: Practice, Theory, and Research, edited by A. Roy and P. Smith. Preprints of the Contributions to the Ottawa Congress, 12-16 September. London: International Institute for Conservation: 223-227.
Hackney, Stephen. 2004. “Paintings on Canvas: Lining and Alternative”, in Tate Papers, 2, (Autumn). https://www.tate.org.uk/research/publications/tate-papers/02/paintings-on-canvas-lining-and-alternatives
Hatchfield, P. 2002. Pollutants in the Museum Environment: Practical strategies for problem solving in design, exhibition, storage. London: Archetype Publications.
Saunders, D. 2000. “Pollution and the National Gallery.” National Gallery Technical Bulletin, 21: 77-94.
Shilling, Michael R., David M. Carson and Herant P. Khanjian. 1998. “Evaporation of Fatty Acids and the Formation of Ghost Images by Framed Oil Paintings”, WAAC Newsletter, September, 21 (1).
Silvester, G., A. Burnstock, L. Megens, T. Learner, G. Chiari, and K.J. van den Berg. 2014. “A cause of water-sensitivity in modern oil-paint films: The formation of magnesium sulphate.” Studies in Conservation 59 (1): 38–51.
Stolow, Nathan. 1987 Conservation and Exhibitions: Packing, transport, storage and environmental considerations. Buttersworths series in conservation and museology, London.
Tétreault, Jean. 2003. Airborne pollutants in museums, galleries and archives: risk assessment, control strategies and preservation management. Canadian Conservation Institute. Thomson, G. 1965. “Air Pollution: A Review for Conservation Chemists”. Studies in Conservation, 10 (4): 147-167.
Thomson, Garry. 1986. The Museum Environment, Second edition, Butterworth Heinemann Ltd, Oxford.
Toishi, K., and T. Kenjo. 1975. “Some Aspects of the Conservation of Works of Art in Buildings of New Concrete.” Studies in Conservation 20 (2): 118-122.
U.S. EPA. 2006. Air Quality Criteria for Ozone and Related Photochemical Oxidants. Environmental Protection Agency, Washington, DC, EPA 600/R-05/004cF, February 2006. Vol. 1. https://cfpub.epa.gov/ncea/isa/recordisplay.cfm?deid=149923
Van Loon, Annelies; Petria Noble and Aviva Burnstock. 2012. “Ageing and Deterioration of Traditional Oil and Tempera Paints.” In Conservation of Easel Paintings, edited by Joyce Hill Stoner and Rebecca Rushfield. London: Routledge: 214-241.
Whitmore, P., G. Cass, and J. Druzik, 1987. “The Ozone Fading of Traditional Natural Organic Colorants on Paper.” Journal of the American Institute for Conservation, 26(1): 45-57.
Williams, E. L., E. Grosjean, and D. Grosjean, D. 1993. ”Exposure of Artists' Colorants to Sulfur Dioxide.” Journal of the American Institute for Conservation 32 (3): 291-310.
Indoor Air Pollution Working group (http://iaq.dk/iap.htm) Bacharach, John et al. 2016. “Museum Collections Environment”, National Park Service Museum Handbook. https://www.nps.gov/museum/publications/MHI/MHI.pdf
Desauziers, V.D. Bourdin, P. Mocho, H. Plaisance. 2015. “Innovative tools and modeling methodology for impact prediction and assessment of the contribution of materials on indoor air quality” Heritage Science 3: 28.
Environmental Guidelines for Paintings, Canadian Conservation Institute (CCI) Notes 10/4. (Last date modified: 10-20-2017) https://www.canada.ca/en/conservation-institute/services/conservation-preservation-publications/canadian-conservation-institute-notes/environmental-display-guidelines-paintings.html
Gibson, L. T., and A. W. Brokerhof. 2001. “A Passive Tube-Type Sampler for the Determination of Formaldehyde Vapours in Museum Enclosures.” Studies in Conservation 46 (4): 289–303.
Grøntoft, T., S López-Aparicio, M. Scharff, M. Ryhl-Svendsen, G. Andrade, M. Obarzanowski, and D. Thickett. 2011.” Impact Loads of Air Pollutants on Paintings: Performance Evaluation by Modeling for Microclimate Frames.” Journal of the American Institute for Conservation 50 (2): 105-122.
Grosjean, Daniel, and Sucha S. Parmar. 1991 “Removal of Air Pollutant Mixtures from Museum Display Cases.” Studies in Conservation 36 (3): 129–141.
Grosjean, D., E.L. Williams II, and M. W. M. Hisham. 2009. “Removal of Air Pollutants by Carbon and Permanganate-Alumina Filtration Systems in Museums: A Case Study.” Final Report to Conservation at the Getty Institute by Daniel Grosjean and Associates, Inc., [4426 Telephone Road, Suite 205, Ventura, California 93003], September.
Hackney, S. 1984. “The Distribution of Gaseous Air Pollution within Museums.” Studies in Conservation, 29(3): 105-116.
Hackney, S. 1990. “Framing for Conservation at the Tate Gallery.” The Conservator 14: 44-52.
Ryder, N. 1986. “Acidity in canvas painting supports: deacidification of two 20th century paintings.” The Conservator 10: 31-36.
Tétreault, Jean. Agent of Deterioration: Pollutants https://www.canada.ca/en/conservation-institute/services/agents-deterioration/pollutants.html Williams, E. L., E Grosjean, and D. Grosjean. 1992. “Exposure of Artists' Colorants to Airborne Formaldehyde.” Studies in Conservation, Vol 37 (3): 201-210.
Volent, P. and N. S. Baer. 1985. “Volatile amines used as corrosion inhibitors in museum humidification systems”. International Journal of Museum Management and Curatorship 4: 359-64.
Yoon, Y., and P. Brimblecombe. 2000. “Contribution of Dust at Floor Level to Particle Deposit within the Sainsbury Centre for Visual Arts.” Studies in Conservation 45 (2): 127-137.