Fluctuations and extremes of relative humidity may be commonly found in gallery areas near local heat and light sources, e.g. radiators and windows, near catering facilities and where visitors congregate e.g. cafeterias.

Humidity in the museum environment is measured as relative humidity using a hydrometer. Monitoring of gallery areas and objects is recommended to establish records to indicate daily, months, seasonally and yearly fluctuations of conditions.

RH fluctuations have a greater effect on objects than temperature fluctuations. To protect against these effects a constant RH should be maintained, materials will not absorb or expel moisture and so are less likely to expand or contract.

To protect against the effects of RH fluctuation it is necessary to install humidification equipment to control the RH. This can be plant equipment installed in the building or stand alone unit[s] in the gallery the object will be displayed in. If this cannot be achieved, a showcase can be fitted with buffering materials to help cushion the effects within the showcase.

Silica gel is a useful buffering material to stabilise the micro environment inside a sealed case. This must first be dried by heating to between 110ºC and 250ºC. Once placed induced the case, it should be checked and removed regularly to be reconditioned. Therefore the design of display cases must accommodate enough silica gel to condition the internal space [ 20 kg per cubic metre for year round protection in a well sealed case ] and allow access for it to be removed regularly. This should be placed to provide circulation into the case but be sealed from external RH. A hydrometer should also be placed inside the case to permit monitoring without opening the case.

Recommendations