October 29, 2022 | Last updated May 1, 2023

Cool It! Using Relative Humidity to Protect Collection Materials

Preserving the life of collections objects is not easy. One of the best ways to make sure they last as long as possible is to manage the temperature and relative humidity of their storage environment.

The conservators at our Ford Conservation Center are experts in preserving precious items

The storage and exhibition environment best suited for the long-term survival of art materials is one in which the relative humidity is as stable as possible and the temperature is as low as practically possible.  Temperatures that fall in a range below 72° F and above freezing are acceptable provided that relative humidity is controlled.  Each degree the temperature is lowered, under stable relative humidity conditions, will slow the rate of deterioration of collection materials. Above 72° F some materials begin to soften and flow and below 32° F any materials containing water will begin to freeze and expand.  Lower room temperatures with stable relative humidity will prolong the life of the collections, but may interfere with human comfort.

celluloid protractor that has shrunk on one side, turned yellow, cracked, leaking acidic byproducts

This protractor has succumbed to inherent vice.  The degradation cannot be stopped or treated, but it can be slowed if stored at cold temperatures. Read more about the protractor here.

For Nebraska collections, relative humidity levels in the range of 50% to 30% are thought to be best for general collections; however, it is actually the stability of the relative humidity that is paramount rather than the actual value.  This is certainly true in an arid environment like that in western Nebraska where 50% Relative Humidity (RH) may be difficult to maintain particularly in the cold winter months.  Fifty percent relative humidity is not essential; it is the stability that is most important.  Therefore, if the relative humidity is more stable at 45%, then all efforts should be focused on maintaining that relative humidity.  Above 50% RH mold and mildew can form and below 30% RH serious dehydration of organic materials can occur.

ivory handle on knife that has cracked due to low relative humidity

The handle on this knife is made of ivory.  Due to low relative humidity, it has cracked.

Mold is a growth of various types of fungi that produce a furry mass on the surface of materials.  Mold spores are everywhere and invisible to the naked eye.  Mold reproduces by sending out a large number of spores into the air which then travel to new locations and germinate under specific conditions.  When the spores germinate, they produce fuzzy hair-like stalks called hyphae, which in turn produce more spores.  Mildew is an early stage of mold that appears as a thin film.  Mold can be found in a multitude of colors and will attach to a variety of surfaces and organic materials.  Molds will attack the starches found in adhesives, sizings and textiles, proteins found in leather, parchment and animal glues, and cellulose, which is the main component in paper.

Mold damages the objects it settles on by excreting digestive enzymes.  These enzymes alter, weaken, and stain the objects.  Once an object is weakened by mold, it becomes more susceptible to future damage as it becomes more fragile and porous.

The easiest way to preserve objects is to prevent the mold from forming in the first place.  In order to germinate, specific environmental conditions need to be in place.  Molds love high humidity, still air, darkness, and a food source (i.e. adhesives, cellulose, leather).

extensive white mold growth on a leather saddle

Due to high relative humidity, mold has grown over the surface of this leather saddle.

The best way to prevent mold growth is by maintaining stable and moderate temperature and relative humidity.  Do not store collections in spaces that are known to be damp or prone to leaks and flooding.  Avoid attics, basements, and spaces along exterior walls.  Air-conditioners and/or dehumidifiers should be used to lower the RH.  Be sure air-conditioners or dehumidifiers are properly draining and drip pans are emptied regularly.

Still air allows mold spores to settle and can increase the moisture content of collections.  Maintain good air circulation in storage areas.  Do not aim fans directly at collection items.

Keep storage areas and objects as clean as possible.  Dust and dirt house spores and can also act as food for active mold.  Protective enclosures keep any dust and dirt off the object and provide an extra layer of protection.  Keep windows closed to prevent dust and spores from entering the storage space.  Vacuum storage areas and objects regularly.

Change HVAC filters according to the manufacturer’s recommendations.  Have the HVAC system regularly maintained to prevent the system itself from growing mold and affecting the entire building.

Make sure pipes and exterior walls are well insulated to prevent condensation from forming and creating pockets of moisture where mold can form.  Regularly check pipes for leaks and make sure gutters are cleared.

Create a separate receiving space for incoming collection items.  Check all incoming items for mold or other pests.  This prevents contamination of the rest of the collections.  If a new acquisition is found to contain mold, follow the steps found in our Mold Remediation Guide.

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