Thursday, December 8, 2011

Environmental Concerns

     When considering museum fields, most people tend to think of art and history.  Would it surprise you to learn that when working with artifacts a background in science is also necessary? 

     I posted earlier about monitoring for insects at the museum to prevent damage to the artifacts.  The temperature and relative humidity (RH) of the environment must also be monitored.  Temperature and RH are considered together since they are directly related; as the temperature increases the RH decreases, and as the temperature decreases the RH increases.

     Why is this important in protecting artifacts, you ask?  Changes in temperature and RH can affect materials in several different ways.  Most materials expand when they are heated and contract when they are cooled.  Heating or cooling an object to extreme temperatures, or subjecting an object to constantly changing temperatures can cause mechanical damage, like cracking, splitting, or warping.  In addition, organic materials like wood, bone, and paper expand in high RH and contract in low RH, which can cause similar mechanical damages.  The issues can be compounded if an artifact is composed of more than one material, since different materials can expand and contract at different rates.

This medical chest lid is made of wood which is reinforced with metal edges. The crack in the wood could be the result of the wood becoming too dry, or of being in an environment which fluctuated. Wood can absorb more moisture than metal, so the crack could be the result of the wood trying to expand further than the metal would allow.   

     Changes in temperature and RH can also affect the chemical reactions which cause materials to degrade.  If you ever took a class in chemistry, you should remember that heat speeds the rate of chemical reactions.  Or if you cook, you know that heating a liquid allows substances to be dissolved in it much more quickly!  Water (or a high RH) is also necessary for certain reactions to take place, as in the corrosion of metal.  Other chemical reactions can cause paper to yellow, glass to cloud, and dyes to fade.

This iron pestle exhibits some pitting and spots of rust, resulting from being in an environment with a too high RH.  

     Mold is another danger to artifacts.  Mold spores are in the air all around us, and can grow quickly in a warm, moist environment.  It’s nearly impossible to eliminate the spores from the air, but mold growth can be prevented by keeping the RH below 65%.

This is a small piece of chamois leather with two surgical needles. Notice that the metal needles are rusty and that there is evidence of mold on the corners of the leather. It is a good illustration of the effects of a high RH on an inorganic material (needles) and an organic one (leather).

     At the NMCWM we have small digital thermo-hygrometers for monitoring our exhibit cases.  They measure the temperature and RH and record the high and low points during a set time period.  They also have a display so that the temperature and RH can be checked at a glance. 

The thermo-hygrometer is on the side wall of this exhibit. It is easily visible to me when I need to check the environmental conditions, yet it doesn’t interfere with the display. Most visitors don’t even notice it.

     We also have data loggers placed in the galleries.  They measure the temperature and RH every hour and save the data.  I download them to my computer on a monthly basis, and review them to identify any problem areas.

The data loggers are mounted inside small locked, ventilated Plexiglas cases. Ideally, they should be located on an interior wall near the center of the room, and away from any vents.

Here’s a printout from one of the data loggers (click on photo for larger version). You can clearly see which day our air conditioning unit broke! Notice that as the temperature rose, the RH dropped.

It’s one more way I help to preserve and protect the artifacts, and earn my Guardian title!

Photos courtesy of the National Museum of Civil War Medicine.

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