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.
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%.
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.
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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