In the late 1980's the Getty Conservation Institute funded a research project carried out at the California Institute of Technology designed to investigate the risks to works of art from indoor airborne particles. At the time very little was known about the concentration and fate of indoor particles in museums and their rates of deposition onto surfaces. Of primary concern was the deposition of fine elemental carbon (soot) particles. These particles, being black, contribute to the slow aesthetic degradation of painted surfaces, and being small, 0.05 to 1.0 µm in diameter, tend to migrate deeply into porous surfaces where they are difficult to remove. Fine carbon particles are also hard to remove by the filters most commonly employed in standard heating, ventilating and air conditioning (HVAC) systems. Therefore, an assessment of indoor/outdoor particle concentrations and their deposition rates on vertical and horizontal surfaces was critical for estimating soiling rates and developing protective strategies for exposed museum collections.
During two-month periods in the summer and winter of 1988, five institutions in Southern California were investigated: the Norton Simon Museum, J. Paul Getty Museum, Scott Gallery of the Huntington Art Gallery and Library, the Southwest Museum, and the Sepulveda House. These institutions ranged from an historical house museum dominated by ventilation through open doors and windows with outdoor air exchanges ranging from 1.6 to 3.6 air changes per hour to museums with fully equipped HVAC systems having low outdoor air exchanges (<1 hr-1) and high air recirculation rates (5-8 hr-1).
In general, measured particle deposition velocities to vertical indoor surfaces ranged from 10-6 to 10-5 m/s depending upon particle size, and regardless of particle type, the fraction of outdoor air particles deposited onto all surfaces ranged from 0.l-0.5% for diameters in the vicinity of 0.15 µm, up to 90% for particles larger than 20 µm. The rate of elemental carbon particle accumulation on vertical walls ranged from 0.08-2.7 µg m-2 day-1. The measured indoor particle characteristics agreed well with predictions made from their mathematical model of indoor aerosol dynamics based upon measured outdoor particle characteristics and building parameters.
To estimate the soiling rates from the deposition characteristics in these building types it was necessary to relate surface deposits to the level where the soiling became "just perceptible" to a standard human observer. Efforts to determine this level had been carried out twice previously, once in the mid-1950's and then again in the 1970's. Both efforts agreed that a coverage of approximately 0.2% of the surface by black particles was just visible. As a result, the estimated soiling times associated with the five Southern California museums ranged from approximately 0.3-18 years. This study was published in two reports from the Getty Conservation Institute in 1992 and 1993. Individual chapters of these reports were subsequently published in the scientific peer-reviewed air pollution literature (Nazaroff et al.).
In the summer of 2000, "A Study on the Human Ability to Detect Soot Deposition onto Works of Art", was published in Environmental Science & Technology by Bellan, Salmon and Cass. Utilizing methods for depositing black particles and characterizing surface coverage that was far superior for modeling soot particles than earlier research, Bellan showed that, even with the ability to view a clean white area edge-to-edge with a soiled one, the onset of the point where a normal observer could just begin to detect that a surface was becoming soiled by microscopic soot particles occurred at approximately 2.4% surface coverage. This was a 12-fold increase over the earlier estimates. This new value expanded the soiling times for Southern California museums from 0.3-18 years to 4-216 years for the institutions previously studied.
In reviewing the original five museums it is clear that the Norton Simon Museum represents a unique class of institutions in its ability to provide a very clean indoor environment. The Simon Museum provides more than twice the protection from soot soiling than the Scott Gallery, yet both seem to be typical, modern air-conditioned museums. Both have fairly similar outdoor air exchange rates and indoor air recirculation rates. The main difference between these two buildings is that the Norton Simon Museum deposition velocity data agrees with predictions for deposition from a forced laminar flow of ventilation air along its walls owing to its use of the entire ceiling as an air plenum for the HVAC system (similar to the air flow system used in clean rooms.) This is not a common feature for museum design although the tendency is toward this in new museum construction projects. At the other extreme, the Sepulveda House with its completely unprotected ventilation strategy and 4 year time to the onset of soiling is so poorly controlled that it is unlikely to be used to house sensitive and extremely valuable art collections; indeed it is used as an historical house museum in which the building and its furnishings are on display. If the premise is true that the Scott Gallery, Getty Museum, and Southwest Museum with their mechanical ventilation systems are representative of the majority of structures that house contemporary art, then the exposure times prior to the onset of perceptible soiling of vertically-oriented surfaces ranges from 24 to 86 years with an average time to perceptible soiling on a white surface (based on an edge-to-edge comparison) of approximately 50 years.
The significance of this new figure is that turning the clock back 50 years puts us near the start of Color Field paintings. Thus the paintings of artists such as Morris Louis, Ellsworth Kelly, Jackson Pollock, Kenneth Noland, Mark Rothko, Helen Frankenthaler, Clifford Still, Franz Kline and Willem de Kooning, and the thousands of square meters of vulnerable canvas they have created, are now entering a stage where soot deposition will become an ever increasing problem for future conservators. That this new estimate may be a clarion of impending events is witnessed by the Tate Gallery in London which has recently begun a program to clean their Mark Rothko paintings.
James R. Druzik *, Senior Scientist, The Getty Conservation Institute, 1200 Getty Center Drive, Suite 700, Los Angeles, California, USA 90049
Dr. Glen R. Cass, Chairman, School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Altanta, Georgia, USA 30332-0340
(*) Author to whom correspondence may be addressed
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