Preservation Department: Environmental Control and Monitoring | Yale University Library

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Preservation Department: Environmental Control and Monitoring

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Environmental Control and Monitoring

One of the most important tasks one can do to protect library collections is to store and display collections in a stable environment.  A stable environment is one where the temperature is consistent and as low as possible (and as practical if patrons and staff share space with collections) with a relative humidity that is stable and controlled.  Light levels are kept as low as practical.

How can I get my collections and/or exhibit area monitored/ surveyed?

If you are interested in having an area monitored where your collections are stored or displayed, whether it is for temperature, relative humidity, or light, please contact the Preservation Field Services Librarian, Tara Kennedy.

Why is environmental control important for collections?

Controlling temperature, relative humidity, and light levels will slow many chemical and physical processes that adversely affect collection materials.  High temperatures can result in rapid degradation of materials: every increase of 10°F will double the chemical activity in paper objects.  Fluctuations in relative humidity also increase the deterioration of organic materials, such as paper and leather.  Wide, fast swings in relative humidity cause extreme dimensional change in organics (paper, parchment, and leather), such as distortion and warping.  High relative humidity (65% RH and above) can cause mold to germinate and creates a hospitable environment for insects.  High light levels can fade inks and dyes and discolor paper and leather.

What is Environmental Monitoring/ Surveying?

The Preservation Department conducts an Environmental Survey by placing a device called a datalogger that electronically records temperature and relative humidity in a collections space.  The datalogger is placed in one location for a year, gathering seasonal data for a baseline of the temperature and relative humidity.  The Preservation Field Services Librarian takes the data, analyzes the data, writes a report concerning the findings and makes recommendations to improve conditions in the space. 

What will an Environmental Survey do for my collections?

Having this data available will help identify problems with the HVAC (heat, ventilation and air conditioning) systems in your library or collection area.  The information gives an opportunity to work with Facilities to improve existing conditions.  The data also will assist in future construction and renovation projects, especially those involving renovation of the current HVAC systems, or creation of a new HVAC system.

To keep in mind for future renovation efforts, the temperature target for collection storage should be around 65°F +/- 2°F.  Colder is better, provided the humidity levels can be controlled beyond standard air conditioning.  Humidity control needs to be installed in order to prevent extreme fluctuations and possibility of mold growth, and to be able to keep temperatures lower without raising the humidity. Relative humidity should be between 30% to 50% RH.  It is important to keep the relative humidity out of the "danger zones": above 65% RH can cause mold to germinate and below 30% RH can cause some organic materials to dry out.

What about monitoring light levels?

The Preservation Department has a light meter that measures several types of light that are harmful to collections: visible and ultraviolet.  Light levels may be measured whether the items are on display or in ambient conditions.  For paper-based materials, preservation professionals recommend 50 lux (5 foot candles) as a maximum for visible light levels.  The ultraviolet (UV) light levels should be below 75 microwatts/ lumen.  UV light is the strongest, highest energy part of the light spectrum, and the most harmful to collections.  Since UV light is not required for the eye to discern color, it should be eliminated from all exhibition lighting.