Previous sections have discussed best practices for temperature, relative humidity, pollutants, and light levels in collections storage - but how do you actually achieve these goals? Maintaining the recommended values is an ongoing task, even in new structures that are purpose-built for storing collections.
In this section you will learn methods or monitoring environmental conditions, and how to begin evaluating the resulting data to improve storage of your collections.
The preservation environment, with stable temperature and relative humidity, as well as carefully controlled light and pollution levels, is generally agreed to be the most effective factor in maintaining archival collections.
Since our subjective impressions of climate levels tend to be inaccurate, temperature and relative humidity should be systematically documented wherever collections are stored. Monitoring involves consistent measurement of these factors, either constantly or at scheduled intervals.
Recorded data establishes the existing environmental conditions, and can be used to indicate whether systems and controls are operating at optimal levels. Data can also demonstrate the need for additional or improved equipment during certain seasons (such as a portable dehumidifier during the summer). Monitoring should be in practice for a full year to get a sense of seasonal changes, before any alterations are made to your climate control systems.
It is generally recommended that independent monitoring be used even if your HVAC system has a built-in monitor. In-system monitoring may not be accurate enough to provide a thorough account of the actual conditions, and the location of those monitors may not fully reflect the climate throughout your storage spaces,
Monitoring devices vary greatly in cost, complexity, effectiveness, and staff time required to maintain them and to interpret the data. Some monitoring devices can be used to report immediate conditions ("snapshot" monitors), while others are designed to create an ongoing log of environmental conditions.
Examples of "snapshot" monitors include thermometers, dial hygrometers, indicator strips (useful in sealed exhibit cases), and sling or motor-driven psychrometers. "Min/max" digital thermohygrometers provide a middle ground; they can keep track of the highest and lowest temperature and humidity since the device was last reset. Thus, they can give a very general sense of fluctuations in the climate at night or on the weekend.
Recording hygrothermographs were the standard automatic recording device for many years, but in recent years dataloggers (battery operated devices that take readings at intervals specified by the user) have become the monitoring instrument of choice. This is largely due to the ease of downloading data gathered by a datalogger to a computer for graphing and analysis. There are a number of issues that must be considered in choosing a datalogger, including memory capacity, battery life, sensor accuracy across a range of temperature and humidity, and type of display. See the National Park Service's Comparing Temperature and Relative Humidity Dataloggers for Museum Monitoring (PDF) for more information on dataloggers.
It is important to choose the monitoring instrument most appropriate to your institution and situation. See the NEDCC Preservation Leaflet Monitoring Temperature and Relative Humidity for more information on setting up a monitoring program.
Unfortunately, there is no simple and inexpensive way to monitor pollutants in cultural institutions. Because the effect of gaseous pollutants may differ according to the level of relative humidity and/or the presence of other pollutants, most existing methods use indicators (such as silver) that are sensitive to pollutants. The corrosion of these indicators demonstrates the presence of pollutants that may damage collections. These may be useful in institutions with specialized collections that are particularly sensitive to pollutants, such as photographs.
Other options include periodic sampling of air quality over the course of a year by a specialized company, a complicated and expensive option, or the use of instruments designed to measure levels of urban pollution (e.g., devices that pass air through a tube filled with a reactive substance, or electronic monitors that use corrosion detectors).
A light meter is used to measure visible light in units of lux or footcandles. A separate UV meter is required to measure UV energy in microwatts per lumen. Some UV meters can also measure visible light by changing detectors within the device. A 35mm camera with a manual light meter can also be used to measure visible light levels, as described in the NEDCC Preservation Leaflet Protection from Light Damage.
Remember that artificial light does not change significantly, but monitoring natural light can be problematic because it differs according to the time of day, the weather, and the season. Readings should be taken at times when the light is brightest to be sure that the highest light levels are being recorded. A useful tool is the Blue Wool standard, cards containing samples of blue-dyed wool that fade at a known rate relative to each other (the first few samples on the card correspond to light-sensitive materials such as paper). Use of these cards can demonstrate the cumulative degree of fading caused by the intensity of light in a particular location.
The usual approach to evaluating the preservation quality of storage areas is to set specific targets for climate control, monitor the actual climate conditions, and act to correct any divergence from the targets. However, interpretation of climate monitoring data can be difficult, and correcting divergence from target values can be challenging.
Preservation professionals are increasingly arguing for a different approach. Rather than considering the climate in terms of a right or wrong set point, collections managers should determine what the climate is in each storage area (via monitoring), evaluate the effect of those conditions on the collections, and make changes or redistribute collections accordingly.
Evaluating the specific effect on collections of a particular storage area can be difficult. For example, how do you determine the overall effect on collections of a storage area that cycles between 70°F and 80°F every day? How do you decide whether a storage area that is consistently 75°F and 40% RH is better or worse than one that is consistently 65°F and 50% RH?
Research conducted over the last few years by the Image Permanence Institute (IPI) has focused on answering these types of questions, and on evaluating the risks or benefits to collections of individual sets of environmental conditions.
To evaluate the risks associated with specific environmental conditions, IPI has developed two tools: the Preservation Index (PI) and the Time-Weighted Preservation Index (TWPI). The PI provides a comparative measurement of how long vulnerable organic materials (e.g., those with short life spans, such as magnetic tape or newsprint) might survive under various environmental conditions. The baseline for this measurement is 68°F and 45% RH, which results in a PI of 50 years.
As the temperature and humidity are raised and lowered, the PI changes (for example, lowering the temperature to 60°F and the humidity to 35% results in a PI of 110, while raising the temperature to 75°F and the humidity to 65% results in a PI of 18). It is important to remember, however, that this is a comparative measurement, not a literal measurement of years. The numbers are meant to illustrate how different storage environments can slow down or speed up deterioration.
In real storage areas, however, temperature and relative humidity do not remain the same from day to day, or even throughout a single day. Therefore IPI developed the Time-Weighted Preservation Index (TWPI), which takes into account changing conditions over time, as well as the cumulative effect of deterioration, for a particular storage area. The resulting measurement is called the TWPI.
See IPI's Step-By-Step Workbook: Achieving a Preservation Environment for Collections for a more detailed explanation of PI and TWPI.
IPI has also developed a specialized datalogger, the second generation Preservation Environment Monitor (PEM2) and associated software, Climate Notebook, specifically for cultural institutions. These tools record data and calculate PI and TWPI, providing an additional level of data analysis.