Preventive Conservation: Relative Humidity
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Mounted head of a roan antelope, Hippotragus equinus, severely damaged by low relative humidity in mammal storeroom.

 

Conservators measure relative humidity (RH) to determine whether environmental conditions are appropriate for specimens. RH is the amount of water in a given volume of air relative to the maximum amount of water that air can hold at a given temperature, expressed as a percentage. For example, when the RH is 100% the air is saturated, holding all the water it can; at 50% RH, air holds half the water it can.

Controlling RH levels is crucial to the long-term preservation of specimens. Organic materials such as wood, leather, skin, hair, feathers, ivory, fibers and paper are most susceptible to RH changes. Because these materials give off moisture when the RH is low and absorb moisture when the RH is high, they expand and contract when RH fluctuates. Repeated cycles of these changes eventually lead to warping, cracking and possible disintegration. Consistently low RH over a long time also causes damage because organic materials give off moisture, including water that is part of their chemical make-up, and can become dry, brittle, shrunken and warped. A combination of fluctuating and low RH levels over time have caused the skin on the neck of the antelope above to shrink and crack.

A ceramic figurine from the Central Andes in Peru (YPM 257145). The face and arm of this archaeological figurine were damaged by water-soluble salts.

 

Inorganic materials can also be affected by changes in RH. While buried in the ground, porous materials such as some ceramics and stone become permeated with groundwater containing water-soluble salts. When the artifact is removed from the ground these salts remain and go in and out of solution in response to changes in the RH. Because the salts take up a larger volume in their solid state than when in solution, the constant cycling in and out of solution causes the salts to exert mechanical pressure beneath the surface of the artifact, literally pushing it off, as seen here on the face and arm of this ceramic figurine.

Cleveland shale containing invertebrate fossil specimens. The shale is delaminating and breaking apart from exposure to inappropriately high and continuously fluctuating RH in storage.

 

Shales are fine-grained argillaceous sedimentary rocks that are laminated in a direction parallel to the bedding plane. They contain various amounts of clay minerals. Many types of shale will swell when exposed to very damp air as moisture is absorbed by these minerals. When exposed to low RH, these shales shrink, split and become delaminated. Repeated cycles of fluctuating RH lead to severe deterioration, as seen in the specimen above.

Fossilized wood fragments in black pyritic shale from Brooklyn Heights, Ohio. High RH levels have entirely destroyed this specimen, which was collected for the Peabody Museum in 1981.

 

Some fossils contain pyrite, an iron sulfide. Pyritic fossils are notoriously unstable and sensitive to moisture. When exposed to high RH levels, the iron oxidizes, causing cracking and the build-up of efflorescence on the fossils. This condition is called “pyrite disease.” If unchecked, it will eventually lead to the total disintegration of the fossil.

Over the years, conservators have identified appropriate RH levels for various materials and spend considerable time trying to ensure RH levels in all areas where collections are housed, exhibited and studied are kept at these levels.