© 1994 by Chicora Foundation, Inc. All rights reserved. No part of this publication may be reproduced or transcribed in any form without permissions of Chicora Foundation, Inc. except for brief quotations used in reviews.
One goal of every museum is to make objects accessible to the public, to researchers, and to other institutions. A second goal is to ensure the long-term safety and preservation of the collections. Objects need one set of conditions while people may need another. Achieving both seems impossible. Managing the museum environment can be difficult since it requires expertise and time. It also requires the efforts of all your staff, as well as the cooperation of the public. But a controlled environment can be achieved, even by small institutions with limited resources.
Heating, Ventilation, and Air Conditioning (HVAC) is a frequently troublesome area of preservation. Architects often fail to understand the importance of a preservation quality HVAC system, instead specifying units that are better suited to commercial construction. Museum curators often don't know how to describe correctly what it is they need, or what the problems are with the current system. Museum directors and their boards are sometimes not convinced that controlling the environment is all that important. And preservationists often fail to allow any latitude in the development of an affordable preservation quality environment.
The importance of the environment in which collections are stored should be clearly realized. For example, for every 14° F rise in temperature the deterioration rate of paper (and likely other organic materials) doubles.
Condensation on the inside of windows suggests a potentially serious problem with the institution's relative humidity control.
Humidity is most often associated with an increased probability of mold growth and other forms of biodeterioration. Levels at 60% RH should be considered the threshold for damage - over that level and the museum will eventually have trouble. But because many collections are also hygroscopic, the humidity levels will also affect dimensional stability. For example, some types of wood board can vary up to one inch in length over a foot between 10% and 90% RH. Variation in relative humidity can loosen furniture joints, cause paint to chip from canvas, and cockle paper. In addition, fluctuating relative humidity can lead to chemical reactions. Metals will corrode, many dyes will fade, and even glass and mineral collections can be damaged.
The quality of the air in the museum will also affect preservation. Particulates are often abrasive and may permanently soil collections. They are also perfect hosts for mold. Particulates increase user discomfort and increase maintenance costs. Gaseous contaminants, such as oxides of nitrogen and sulfur dioxide can attack organic materials by conversion to acids, while ozone is a powerful oxidant, breaking apart every carbon double bond, severely damaging all organic material. Other gaseous pollutants, such as formaldehyde, may be off-gassing from storage cabinets or glues in the museum.
But all of this should not make the creation of a preservation environment sound impossible. Yes, managing the museum environment can be difficult. Environmental control demands expertise, time, and patience. It also requires the coordinated efforts of the entire staff, as well as the informed cooperation of the public, but it can be done.
With some work, and the cooperation of your architect, it is not only possible, but it will also make the library a healthier, and friendlier place for collections, staff and patrons.
When a preservation quality environment is researched the first thing noticed is the lack of agreement between different sources. Some will specify that the correct temperature is 68° F ± 2°, others will suggest a range of 68° to 72°, and so on. Similar problems are encountered when relative humidity, particulate, or gaseous pollution levels are examined.
But there are issues more important than a precise level. Typically, humidity is much more important than temperature and should be controlled first. Further, fluctuations (seasonal and particularly daily) tend to be more damaging than constant levels, whatever they may be.
Ventilation is especially important for libraries, since it not only ensures the health and well-being of staff and patrons (the primary concern of the ASHRAE Standard 62-1981, Ventilation for Acceptable Indoor Air Quality ), but also helps to minimize the potential for mold outbreaks by providing adequate passage of the air through high efficiency filters and maintaining air movement. Pockets of stagnant air are a sure invitation for mold problems and should be carefully avoided by the mechanical engineer.
What all this means to the museum is sometimes hard to understand, but for many institutions it may require:
maintaining a relative humidity between 40% and 55% throughout the year, allowing seasonal fluctuations between the two extremes, but holding daily fluctuations to ± 3%,
maintaining temperature between 65° and 75° F throughout the year, allowing seasonal fluctuations between the two extremes, but holding daily fluctuations to ± 5° F,
designing gaseous filtration to maintain preservation standards throughout the facility, or using area filtration as necessary, and
adequate ventilation to avoid stagnant air pockets, "dead" zones at the ends of ranges and corners of stacks, and similar problems that are conducive to the growth of mold and mildew.
Unfortunately, HVAC or mechanical engineers are most familiar with the American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) guide to human comfort, which is very different from that appropriate for collections. Consequently, when architects and mechanical engineers discuss "comfort" and "design" levels, they are almost always expressing concerns that fail to include the needs of collections.
Given the possible range in seemingly acceptable temperature and humidity, how can an institution select an appropriate level? Sometime the choice comes down to a simple cost analysis or to what the present equipment can accomplish. But recent research by Don Sebera, a consultant to the Commission on Preservation and Access, has developed the concept of "isoperms"--quantitative graphical measures of relative permanence. While currently developed only for paper, efforts are underway to extend the concept to other media such as magnetic tape, textiles, and photographic film. At present, however, the isoperm concept may help us to better understand how different temperature and humidity levels affect the useful life expectancy of organic materials.
In particular, this approach helps us understand otherwise complex questions such as "what is the effect on my collection when the physical plant turns the HVAC system off at night, over weekends, and during holidays?" or "what are the consequences of allowing wider fluctuations in order to contain costs?"
To provide one example, the relative permanence at 95°F and 80% RH is ~ 0.03 (1.00=paper at 68°F and 50% RH for the reference standard). If the paper has an expected useful life of 100 years at the reference standard, under the high temperature and high relative humidity of this example, the expected life is reduced to only 3 years (0.03 x 100). The relative permanence at 50°F and 40% RH is ~ 12 (or 12 times longer than the reference standard). In this example, the expected useful life is increased to about 1200 years!
Without going into great detail, isoperms can also be used to reveal the extent of reduced permanence resulting from cycling collections from high temperatures and relative humidities typical of summer conditions to low temperatures and low humidities typical of winter conditions. Perhaps of even greater importance, isoperms can help minimize the damage from cycling by keeping conditions in relatively high isoperm value levels.
Your institution should become familiar with the use of isoperms and evaluate collection storage conditions not in the context of one "ideal" temperature and relative humidity level, but in the context of long-term permanence.
Without going into detail, a typical large HVAC system will contain a chiller, a cooling tower, a boiler, and one or more air handlers. The chiller provides the cooling, circulating cold water to the various air handler coils. Air passing over these coils is cooled, effectively cooling the museum. The cooling tower is used to dissipate the heat collected by the refrigerants as they are used in the coils. The boiler produces either steam or hot water. Both can be used for heating purposes (used in the coils the same way as the chilled water) or for dehumidification. The air handlers consist of a fan, filtration, and the coil. They are used to distribute the clean cooled or heated air.
Smaller institutions may use direct refrigeration units (also called DX or direct expansion units). These units may contain a packaged cooling system (including the coils, filter, and fan) and a condenser mounted outside the building (serving the purpose of a cooling tower in a larger system).
The decision of which to employ is usually determined by the size of the institution and similar factors. But, both can satisfy the preservation needs of a library, if properly designed. Essential elements include:
The use of a constant air volume system which constantly circulates air at full volume. Often the architect or mechanical engineer will use a variable air volume (VAV) system in its place. The VAV system delivers cooled or heated air in proportion to the heating or cooling needed in a specific zone. While this approach saves energy costs, it does so by placing the maintenance of a stable humidity at risk. Such systems also cannot properly filter the air or maintain sufficient air flow to help prevent stagnant air pockets.
The limitation of outside air makeup to that required by the local codes. Museums should not use air economizers, which introduce large amounts of outside air for "free" cooling and heating. These systems make it virtually impossible to maintain stable relative humidities.
Design based on humidity, rather than temperature, control. It is preferable for temperature to fluctuate, rather than RH, and all controls should be designed with this specification in mind.
The use of electronic, rather than very insensitive pneumatic, controls in the occupied spaces, not in the return ductwork.
The use of reheat coils for dehumidification of cooling air and clean steam humidification. Reheat coils allow the air to be super-cooled by the coils, driving off excess moisture, and then to be reheated to the appropriate level. Dehumidification using desiccant drums is not only a costly alternative, but it is more difficult to maintain and may release a fine dust or powder into the air stream. Clean steam dehumidification is preferred over all other humidification techniques.
The use of prefilters and final high efficiency filters, with each bank monitored by a manometer. The use of electrostatic air cleaners should be avoided since they release ozone into the air.
Constant operation of the HVAC system to ensure adequate environmental controls and eliminate sharp spikes and excess fluctuations of temperature and relative humidity.
These essential design features will help ensure that the museum's system is capable of achieving and maintaining a preservation quality environment.
A first step in all efforts to improve the museum environment should be sealing the structure--using caulk and weatherstripping to make the building weathertight. This step alone will improve the physical condition of the building, reduce air infiltration, reduce pest access, reduce the heating/cooling load, reduce air pollution, and reduce the particulates in the building. Making the building watertight will reduce the sources of water vapor within the structure and may significantly reduce the relative humidity levels.
Another important step is to recognize that the collections need a more stable environment than staff offices, meeting facilities, and similar areas of the building. By reducing the size of the specially designed HVAC system to cover only the collections and exhibit areas, the costs will be appreciably reduced. Consider establishing safe microclimates in display cases and using materials which will help buffer the environment.
Better cleaning, using damp mops (rather than dust mops) and high efficiency (HEPA) vacuums, will reduce the particulates in the library. Hard floors should be damp mopped every 48 hours and regularly waxed to help prevent them from holding dust. If carpets are used (which is not a good idea) they must be vacuumed at least weekly.
The museum should segregate offending activities, such as smoking, photocopying, and use of laser printers, from the collections.
Only preservation quality materials should be used. Avoid materials which will off-gas and cause additional damage. Emphasize the use of pH neutral, alkaline buffered storage containers. Recently preservation supply companies have introduced storage materials which further buffer collections by incorporating activated charcoal and molecular sieves.
Reduce winter heating. The maintenance of the often encountered winter heating temperature of 76° F will cost 75% more than maintaining a temperature of 68° F. In addition, the cooler temperatures (and somewhat higher relative humidities) are better for the collections.
It may be possible to install a central drain through evaporative pad humidifier on forced air systems. While not as effective or reliable as the in-duct steam system, it may still be an acceptable alternative. Such systems, however, do require additional maintenance.
Supplemental filtration, including gaseous filtration, can be obtained by "add-ons" from companies such as Purafil and the Farr Company. Where only a single room requires the additional protection, the cost for room filtration can be relatively low. It may also be possible to install higher efficiency filters, with only minor modifications to existing fans. Better filtration can also be achieved through the replacement of standard filters with either a combination fiberglass/potassium permanganate filter or perhaps pleated filters which do not significantly increase the pressure drop. However, always be sure to check with a mechanical engineer before changing filters to avoid damaging your system.
The levels of temperature and humidity must be quantified. It is not enough to guess that the space is "too dry" or "too humid." It is essential that you measure temperature and humidity at key points throughout the building. To be effective, this monitoring program must be planned and continuous over at least one change of seasons. You want to record both daily and seasonal variations in your collection's environment.
People also have a significant affect on the environment. They raise the temperature and increase the relative humidity in popular exhibition halls. You will want to chart the number of visitors to correlate with the environmental records.
You will also want to keep track of outdoor conditions. It may be that your building and control systems are simply buffering whatever is happening outside, so that during summer thunderstorms the relative humidity inside increases dramatically. Be sure to record any loss of electrical service, so you can determine how long your building can hold constant temperature and humidity levels.
The simplest, and least expensive, method of monitoring humidity is offered by the humidity indicator cards. As the humidity increases or decreases, chemically impregnated spots change from blue (dry) through lavender (about 50% RH) to pink (humid). They have a long shelf or use life, but offer only gross measurements.
Sling psychrometers measure the ambient (dry-bulb) temperature and the temperature of an evaporating water source (wet-bulb; usually a moistened wick surrounding the thermometer bulb). Use of a psychometric chart comparing the two temperatures produces a relative humidity reading. Sling psychrometers require several minutes of vigorous work to maintain the required air flow of 9 to 15 feet per second while the wet bulb temperature is obtained. The readings may be affected by body moisture, local air movement, and even the fitness of the operator. Aspirated psychrometers use a small battery powered fan to provide a rapid, defined air flow over the wet bulb. They can be very good when calibrated and used correctly. Major problem areas tend to be contaminants on the wick, an ill-fitting wick, and inadequate air flow resulting from weak batteries. Often it is necessary to get very close to the thermometers to read the wet- and dry-bulb temperatures, which may alter the readings. Unless psychrometers are used by meticulous staff members, they can provide almost useless results. At least one authority suggests that even aspirated psychrometers are both tedious and prone to such severe operator induced errors that they should not be used for calibration purposes.
Many institutions have used dial hygrometers. While there are a large number of models (and associated prices), these are not necessarily the most accurate, or least expensive method of obtaining quantifiable measurements for either temperature or humidity. The mechanical hygrometer relies on the physical changes of dimension which occur when a material, such as hair, absorbs or desorbs water. They are slow to respond and sensitive to vibration errors. Many of the materials used are subject to aging and often are non-linear, limiting their accurate use to between 25 and 75% RH.
More useful are the recently introduced minimum/maximum digital hygrometers. A built-in memory function automatically records minimum and maximum readings for both temperature and humidity since the last memory check. This allows conditions to be examined for 24-hour periods, or perhaps overnight or even over weekends. They are small, highly accurate, and very inexpensive.
More expensive hand-held, battery-operated portable thermohygrometers are produced by a number of companies. These instruments usually consist of a display unit and a probe at the end of a lead. The sensors usually consist of a hygroscopic material whose electrical impedance varies with relative humidity. These instruments may also be non-linear at low or high relative humidities and tend to drift over time. When calibration corrections are applied, an accuracy of about ± 3% of the reading is possible (for example, a reading of 50% RH would be within the range of 48.5 to 51.5% RH), although ±4% is more likely (placing the 50% RH reading within the range of 48 to 52% RH). In spite of these problems, most electronic hygrometers are very suitable for museum use. They are fast-responding, easy to ready, easy to transport, and not likely to be affected by normal handling.
Hygrothermographs or recording hygrometers incorporate either mechanical or electronic sensors into instruments to record the readings on a moving drum or disk. There are a number of options available and personal preference usually dictates choice. However, electronic sensors are typically better than mechanical sensors, battery operated drives tend to be more functional for most institutions than clock drives, and felt-tip pens tend to be easier and less affected by operator error than inked pens. Some instruments offer remote leads (useful for monitoring closed storage areas or cases without access), high and low alarms, digital readout of current conditions, and AC/DC operation with battery back-up. The resulting charts provide a continuous record of temperature and humidity conditions, offering information on fluctuations which can be correlated with weather and building systems data.
Data loggers are a new system for measuring and storing temperature and humidity data. Unlike hygrothermographs, data loggers do not provide immediate access to the collected information--they require "down loading" to a computer and use of a software program. They can, however, collect data for days, weeks, or months (depending on the collection interval) and then allow the resulting data to be graphed or further analyzed. There is no question that it is much easier to obtain statistical analysis from data loggers than hygrothermographs. But it is also important to realize that data loggers, since they use the same technology as other electronic humidity sensors, will drift and have variable sensitivities. Consequently, statistical analysis must not be more demanding than the data is capable of supporting.
A preventative conservation program must begin with an intensive museum-wide study which examines the collection, the indoor environment, the building, and the current HVAC system. This type of detailed preservation study provides baseline data and allows you to demonstrate the benefits of improvements.
The collection should be examined since different materials require different handling and environmental conditions. Without a knowledge of the condition of the collection and its environmental needs, there is no baseline for any conservation program.
Monitoring allows the temperature and humidity in every museum space to be quantified. To be effective, the monitoring program must be well planned and continuous over at least a change of seasons. You will want to reveal both daily and also seasonal fluctuations.
The building is the first line of collection defense and a survey can establish whether the museum envelope is buffering the outdoor conditions well enough to reduce the need for the most expensive control equipment. The existing equipment--and its operation--should also be examined.
A preservation assessment, coupled with monitoring, can identify problem areas such as this. Leaking condensate, coupled with poor drainage, are creating a serious moisture problem on the exterior of this building.
Data from the environmental preservation assessment can help direct future preservation efforts. It should direct an informed preservation strategy. It should stress that every member of the museum team contributes to the creation and maintenance of a safe environment. And it should press preventative maintenance of the building and the control systems. Such an approach helps ensure the safety of the collection.
If you would like more information about the museum environment these are three sources which may be helpful:
Chicora Foundation also provides free telephone consultation. You may reach us at 803/787-6910. We also provide for-fee consultation on environmental preservation issues, including conservation and preservation assessments.
Chicora began as a small, not-for-profit, public foundation over a decade ago, with the lofty mission of preserving the past for future generations.
Today that means a wealth of innovative programs. All are focused on the realization that museums, libraries, and archives must maximize the benefits of limited preservation and conservation funding. We work with your team to provide practical, cost-effective solutions for your complex problems.
Chicora Foundation is a leader in offering a wide range of preservation services, including on-site consultations, workshops and seminars, and telephone consultations. Our areas of expertise include the care and handling of collections, preservation assessments, preservation planning, integrated pest management, environmental monitoring and controls, fire safety, and disaster planning.
While our telephone consultations are free, more in-depth consulting is offered on a for-fee basis. For more information on our services and the associated costs, call us at 803/787-6910.