Mold in museum collections is the environmental "canary in a coal mine"

A common issue found in collection storage is the presence of mold. Mold can unfortunately be found on collections, but also at times on the layers that are to protect collections. Museum professionals strive to keep their environment stable, their storage areas clean with good housekeeping practices, and surround their collections with archival materials. Yet mold out-breaks still can occur. So why does this happen, particularly if the protective layers are archival, and the environment is being monitored?

ABOVE: Mold growth on a military jacket. The jacket
was displayed on a mannequin form under glass bonnet.

It turns out that mold is a tricky organism, and it wants to live. And given a chance, live it will. And of course, museums have in their collections the perfect materials for those tenacious spores to live and set-up house.

"Microscopic molds are both very beautiful and absorbingly interesting. The rapid growth of their spores, the way they live on each other, the manner in which the different forms come and go, is so amazing and varied that I believe a man could spend his life and not exhaust the forms or problems. 

— David Fairchild

The World Was My Garden (1938, 1941), 55.

All organic materials (and even some inorganic materials) will support mold, with natural fibers being the most susceptible. Mold is a microorganism that produces enzymes that convert the cellulose in fibers to soluble sugar that is metabolized as food. Proteins are generally less susceptible, but keratinophilic fungi will feed on, and damage, these fibers as well. Mold is found first on soiled areas, but also on materials that are starched, sized, have brightens, or have in the past been treated or exposed to some substance that is still present. And sadly, all mold growth creates a permanent, irreversible stain. (See below).

ABOVE: Before treatment

ABOVE: After treatment

The photos on the left and right show mold on paper. While the mold has been reduced substantially, the permanent stains it has left behind are now part of the object.

RH = Relative humidity is a measure of the capacity of air to hold water. This amount varies as temperatures increase or decrease

Mold is omni-present and if mold is not actively growing, its spores are always in the air waiting for the ideal conditions so that they may grow. Ideal conditions for mold growth are relative humidity (above 65% with a temperature of 75 F or above (25 C). Humidity is by far the most important factor in facilitating mold growth, and if the you have an 80% RH you can be certain that mold is actively growing and it is spreading.  However, keep your temperature below 65 degrees F and your relative humidity below 50% and your collection will be quite safe.

ABOVE: this printout shows a 7 day record of temperature (in red) and the RH (in blue).

However, even at moderate conditions, an outbreak can start in a surprisingly short time. Ideal conditions for mold growth are slightly different for each mold species but mainly within this "sweet spot" of above 65% RH/75 degrees F. But remember that mold is tricky, in fact, mold growth has been noted as low as 50% RH. How can this be? Mold shouldn't grow at 50% RH!  Well, because mold is everywhere and it is a survivor, and the typical museum can not create/afford a "clean room" like those used in hospitals or high tech industry. Therefore, museums rely on creating an environment that is not conducive to growth. Key to this is keeping the relative humidity down. However, it must be noted that if a collection or an artifact has already been affected by active mold in the past, there is an increased chance of a breakout at a lower relative humidity. Think of the later mold moving into an already furnished apartment, everything they need is already there, they just need to move in where their "roots" had successfully taken hold in the past.

ABOVE: Sneaky mold. Here are three of the same types of artifacts, same material
(wood), same time period, same storage area. Yet, the artifact in the middle is almost
entirely untouched by mold while its sandwiching neighbors are nearly covered.   

Determining an actual set point for ones storage environment can be difficult as that there are many factors that are in play. One might even read conflicting recommendations. One of the issues is the amount of ventilation that an area receives. As that storage rooms are broken up by all sorts of cabinetry and shelving units, both open and closed, micro-climates can easily be created. This can especially be the case in historic structures with older HVAC systems of any kind.

The other issue, is how dirty artifacts are in the collection. The artifacts, especially in historic collections, have had an earlier life that includes the acculmative soiling and embedding dirt. All of which, mold spores love! So, even with good and regular housekeeping mold can still appear. 

As stated earlier, mold is tricky and sneaky. Perhaps you have done all of the right things and there is still a persistent out break. Well, there could be inherent issues of moisture that are beyond the specific room. A roof leak nearby, a damp basement, a leaky pipe, etc. Do not over look these seemingly small or large problems that are outside of the immediate vicinity of a mold outbreak. I have frequently come to an institution because of a mold out-break, only to find that it is the canary telling them that something else is going on.

Historical structures come with historical foundations. This
particular historical property experienced water in their basement
causing the RH to rise, causing…you guessed it, a mold outbreak in
the rooms and floors above. Did we mention that mold is sneaky?

Therefore, take any presence of mold seriously, keep your relative humidity down as low as possible, learn how the air moves in your storage space, be diligent in housekeeping, and know the "food" mold likes to eat.

Food sources for mold:

• soiling and dirt on the surface of the artifact.
• starch or other finishes that have not been washed out. Pre-washing muslin and even the twill tape has been found to be critical to remove these finishes.

ABOVE (top and bottom photos) Muslin wrapped, rolled textiles with mold on the surface of the wrapping.

LEFT: Image of detection of mold on unwashed muslin. UV light shows the mold is quite pervasive. So while the museum has done a great job of housekeeping and their storage is thoughtfully organized, the muslin coverings of their carpet/rug collection was at risk for a pervasive mold outbreak simply because the muslin was not washed prior to being used.

RIGHT: Another image of UV light to detect mold growth. This mold is growing on the twill tape straps. The twill tape was not washed prior to being used to tie the ends of the items in rolled storage.

Several years ago, the National Park Service produced a conserve-o-gram that focused on mold. Read it here. And the Smithsonian talks about mold here, which is also good reading to know more about the fungus among us. Lastly, Alaska State Museum's (ASM) experience with mold, or what they affectionately term, "white stuff", has been well documented and researched. ASM has experience and know-how about mold and how sneaky it can be - read about their battle with mold and the vigilance with which they maintain their collection here.

Do you have a mold success story? Do you have a mold challenge that seems unmeetable? The options for treatment and the factors to consider when determining how best to treat objects, textiles, or paper are numerous and often case specific. If in doubt about how a moldy artifact should be treated, call a conservator. We are always here to help!
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Gwen Spicer is a textile conservator in private practice.  Spicer Art Conservation specializes in textile conservation, object conservation, and the conservation of works on paper.  Gwen's innovative treatment and mounting of flags and textiles is unrivaled.   To contact her, please visit her website.