Uninvited Guests

Contributed by John Bergman-McCool

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Meet Mold and Salt

In early February we wrapped up inventorying and rehousing collections that originated in Missouri. The work was followed by two weeks of cleaning, and as a result we’ve completed one of the regions held in the collection. So you may be wondering what we were cleaning. During the inventory, we encountered bone and antler objects covered with salt crystals and patchy dormant mold. The objects were cleaned before they were stored inside their new boxes to remove salt and minimize the risk of mold spreading to unaffected objects.

Where did the mold and salt come from and why are they a problem?

Mold spores are found everywhere in the environment. When the humidity is high, those spores germinate resulting in mold. Because it is a living organism, mold is classified in the museum world as an agent of biological deterioration. It eats organic matter, in this case the dust resting on the outer surface of bone or the bone itself, and secretes waste that can stain or damage the surface that it is growing on. Mold can appear to be dormant, but when the conditions are right mold will generate spores that are easily borne on the wind, allowing it to spread quickly.

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Agents of deterioration

Salts on the other hand are considered chemical agents of deterioration. They are brought into museum collections on the objects themselves.  Dissolved salts present in groundwater can be absorbed by porous objects, such as bone, while they are buried underground. After the water evaporates the salts are left behind. Excessively humid conditions can dissolve soluble salts, allowing them to move through porous objects. When they arrive at the surface they form crystals. If the crystals form below the surface they can exert enough force to cause damaging cracks and spalling.

Mold and salt thrive in damp environments where the Relative Humidity (RH) is above 65%. The institute’s storage area is located in the basement, which does not have a controlled climate, so mold and salt are an unfortunate reality. Currently, the RH in the basement is somewhere below 15% (15% is as low as our monitoring equipment can read). RH levels this low shouldn’t support active mold and salt growth, so what we found is inactive, but it is hard to say when and for how long the growth of mold and salt was last active.

At the Peabody we are committed to maintaining the longevity of this very important research collection and salt and mold pose a risk to its scientific viability. Damage to the surface of artifacts caused by mold and salt can negatively impact the research value of the collection.

How do we prevent mold and salt?

While we don’t have an HVAC system maintaining the environment in basement storage, we have adopted a few practices that will mitigate and prevent future growth of mold and salt. First, we use dehumidifiers during times of high humidity that typically arise during the summer. Second, we are moving the artifacts currently stored in open drawers and rehousing them in closed archival boxes. The enclosed space of a box helps create a buffer that protects the contents from rapid shifts in RH that lead to mold and salt growth. The boxes will also keep future mold growth from spreading; something that was not possible with open-drawer storage.

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Vacuum defeats Mold and Salt

In addition to the rehousing, we have implemented a cleaning program. When mold and salt are identified we isolate the effected object to keep spores from spreading. We clean the affected artifacts with a dry brush and vacuum. After we clean we use sachets of silica gel to absorb excess moisture, thereby providing another buffer against cycles of RH increases and decreases. In six months we’ll check on the status of the bone to see if the mold and salt are staying away.

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