Seasonal Drift

Autumn is here and the falling temperatures that accompany the falling leaves can boost your collection’s preservation. Rather than recommending a year-round temperature target of 70 degrees Fahrenheit, preservation experts are now acknowledging the need for, and even benefits of, seasonal drift. Environmental conservators suggest gradually scaling the thermostat downward for collections storage areas and historic houses during the cooler months. In many cases, temperatures can drop to just below 60 degrees safely. Managing seasonal drift, then, can be an economical solution to many climate control problems.

Of course, human comfort should always be a concern when determining appropriate temperatures. But in storage areas that do not need frequent or prolonged access, lower temperatures may be reasonable to achieve during the cooler months. Visitors coming to historic houses are usually already bundled up on cold days and cooler indoor temperatures can be comfortable for them. Some smaller museums and historic sites are not climate controlled at all. For these buildings, staff should try to seal cracks and gaps around doors and windows to keep temperature and relative humidity as stable as possible, while allowing for gradual seasonal changes.

Changing relative humidity (RH) can be a more immediate threat to artifact collections than changing temperature. Seasonal temperature adjustments are an important way to help control RH—keeping it fairly stable at levels below 65%. (Higher RH levels for more than 48 hours or so encourage mold growth.) Cooler air can hold less moisture and consequently the dew point (temperature at which air is saturated with moisture) is crucial to avoid. Dropping temperatures below the dew point will lead to condensation, metal corrosion, mold, and swollen wood.

There is now a wonderfully handy new online tool from the Image Permanence Institute. Its dew point calculator is a great way to become familiar with the interrelationship between temperature, relative humidity, dew point, and the preservation prospects for your artifacts. This is a web-based application, so no downloading is necessary. As you are monitoring temperature and RH changes at your institution, simply move the sliders up or down and watch as the gauges, the dew point, and the preservation index also change. Regular monitoring and checking the dew point calculator can help alert you to when your artifacts are in the danger zone. Add IPI’s dew point calculator to your favorites and stay cool (for awhile anyway).


About collectionsconversations

This blog will contain posts from the C2C project staff on a variety of topics related to collections care and disaster preparedness. Enjoy the posts and let us know if you would like additional information or have a topic you would like for us to address.

Posted on October 4, 2012, in collections care, historic houses, historic sites, storage and tagged , , , . Bookmark the permalink. Leave a comment.

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