Keys to Preservation: Temperature and Humidity and Paper

            This morning it was 46 degrees here in Texas.  Less than 36 hours ago it was 98 degrees.  Now that’s a change that gets your attention and that kind of rapid change is bad for objects like paper and photographs.  If you remember nothing else about temperature and humidity remember that that abrupt fluctuations are bad.  Even if you or your archives have an HVAC system, the outside world can still negatively affect your collections if the fluctuations are not well controlled.

            Temperature: What is important to know is that paper likes it cooler than people.  Most objects are happier with the temperature at or below 68 degrees.  The International Museum Association states that 70 degrees is acceptable for museum objects including paper.  That temperature represents a balance between the comfort of people and promoting preservation of collections. The rule of thumb is not to let temperature fluctuate more than 2 degrees in a 24-hour period.  (Tough in West Texas even with an HVAC system.)  By the way, HVAC is the acronym for heating, ventilation and air conditioning.  These systems are designed to control the internal environment of a building.  Some work well at that task while others do not.  To tell if your system is working you need to monitor the temperature and relative humidity (RH) for a year to see if you have a fluctuation problem.  Numerous devices are designed to monitor buildings.  Simple and inexpensive ones, like the one pictured here, require the manual recording and graphing of the information at set times each day

Digital Hygrometer
Grainger Industrial Supply (http://www.grainger.com)
Cost around $14.00

The more old fashioned, but fancier ones graph the data throughout the day.  Graphs are collected and studied at set intervals.  Sometimes you will see these in the corner of a museum exhibit.  

Other computer compatible measuring devices known as data loggers save the information for download to a computer through a USB port. The data can be collected monthly or as needed.  There are million types (well probably not quite that many) pictured on line.  Data logger Hobo by Onset is the only one I know.  They cost $100 and up and consist of a small device that attaches to a shelf or whatever and a collector like a thumbdrive.  Whatever type you choose will monitor your environment and indicate when and if you have temperature and humidity problems.  Once you determine that you have problems and when those problems are occurring your archive can get the appropriate help to adjust your HVAC system or take other measures.

            Relative humidity: The other ingredient in all of this is humidity, relative humidity to be exact.  Relative humidity is the percent of water vapor in the air at a certain temperature.  To express this mathematically, relative humidity equals actual vapor density over the saturation vapor density times 100.  Saturation density is the dewpoint when the air has enough moisture to cause condensation.  The relationship between temperature and relative humidity is an inverse one.  In other words, the hotter it is the more water molecules it will take to saturate the air. Another way of saying that is that in a closed system when the temperature rises, the relative humidity goes down because the air can "hold" more water.  In winter when the temperature goes down, it takes less water molecules in the air to cause dew or condensation.   RH is different from absolute humidity, which is the actual amount of water vapor in a set amount of air regardless of temperature. Since that is all probably clear as mud, all you really need to know is that the relative humidity should be kept between 40-60% with a fluctuation of no more than 6% in a 24-hour period.

            What you need to take away from all of the above is what I said in the beginning – The goal is to keep your environment as stable as is possible.  More on how to achieve that next time.