Go back to article: A sustainable storage solution for the Science Museum Group

Deterioration caused by relative humidity

For each material, there is a level of environmental moisture content, related to the associated relative humidity, which is consistent with its physical, chemical or biological stability. (ASHRAE, 2007)

Relative Humidity is the ratio of current absolute humidity in the air relative to the maximum absolute humidity at a particular temperature. As warm air can hold more moisture than cold air, RH drops as temperature rises and conversely rises as temperature drops. At 100 per cent RH, the air is saturated and the moisture can condense into water droplets. Changes in RH can cause physical, chemical and biological damage to the materials out of which objects are made.

In materials derived from plant and animal matter (organic materials), which naturally contain water, moisture content adjusts to equilibrate with the RH of the environment, and there can be subsequent changes in dimension or physical characteristics which can result in physical damage. Damage from moisture gain can comprise swelling, cockling, warping, plasticising, leaching, dissolving, fading, staining, weathering, cracking and weakening. Conversely damage from moisture loss can result in shrinking, splitting, cracking, breaking, crumbling, delaminating, stiffening and weakening. The extent of damage will depend on the internal physical structure of a material, any external constrictions, the environmental RH values and rates of change, and overall time of exposure.

Chemical damage can also occur to both organic and inorganic materials at high humidity levels. Organic materials may lose strength, colour, shape and flexibility when chemical bonds are broken by the addition of a water molecule (hydrolysis). With low RH levels, salt contaminated ceramics, metals and stone may develop white crystals on their surface (efflorescence) or even disintegrate, and unstable glass may turn opaque (crizzle). In high RH levels salts from deteriorating glass can attract moisture, with droplets forming on the glass surface, and metals can corrode; a layer of dust will accelerate corrosion rates when the RH rises above 75 per cent. Biological deterioration is mostly caused by micro fungal growth (mould); some moulds will germinate around 65 per cent RH with a sufficiently warm temperature. Mould can grow on both organic and inorganic materials if there are suitable nutrients available.

Figure 9

Colour photograph of an old bicycle showing mould groth on the leather seat

Mould growth on a leather bicycle seat

For mixed material museum collection preservation, the museum group RH guidelines agree with the wider guidelines, set out in the recent joint declaration from the International Institute for Conservation and the International Council of Museums – Conservation Committee, advocating a moderate range of 40–60 per cent ±5–10 per cent. While RH levels in the A1 store at Wroughton remain within the set points due to consistent low-level heating, RH in the hangars rarely drops to the highest end point. Even the most extensively renovated hangar has higher than acceptable levels of humidity for much of the year. Replacing the old hangars with suitable buildings for the storage of the group’s collection is the ultimate solution. Designing sustainable storage buildings to museum preventive conservation policy guidelines is the challenge. 

Component DOI: http://dx.doi.org/10.15180/150405/004