Art galleries and museums around the world include a broad range of objects with widely differing ages: dinosaur bones, stone-age flutes made of mammoth tusks, sensitive photographs, paintings with thick layers of paint, contemporary sculptures. In all cases, the building has to maintain and protect the objects displayed and stored inside.
Excessively dry air
Dry air absorbs humidity from objects, their weight is reduced and they contract. In the case of humid air, it is the other way round. Climatic fluctuations thus keep the objects in permanent movement and sooner or later a crack appears on the canvas or the color gilding chips off the baroque sculpture. Stabilization of the relative air humidity helps avoid tension in the material texture of the exhibits, the Building Climate Institute emphasizes.
The preservation of enshrined cultural artifacts generally requires a constant indoor climate which is defined within relatively tight limits . This climate has to be technically created. The air requires humidification — at least periodically. The values reached are measured using measuring systems. Nowadays, due mainly to lease agreements, international indoor climate values of between 50 and 55 ±5% RH and 20°C are required . The American ASHRAE standard formulates corridors for the indoor climate in even greater detail — from the narrowest AA to D. The narrowest climate corridor specifies RH = ±5% and T = ±5 K as long-term tolerance with seasonal adjustment. Positive from a conservational viewpoint is that there is a slow, seasonal adjustment of the indoor climate to the outdoor climate which lies within these limits.
The external climate and the relative air humidity show significant seasonal fluctuations. In winter, the RH is sometimes extremely low. In summer during rainfall, 100%. The external space and inner areas are more or less closely related at all times.
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This is expressed by warping of parchment or paper, or by tears or bubbles on panel and canvas paintings or on color-gilded sculptures. On papers, humidity fluctuations lead to a displacement of soluble components such as the ink. Specialist terms here are ink corrosion and copper corrosion.Fabrics, photographs, metal and stone
In textile objects, excessively low air humidity advances the fragility of the tissue. In photographic objects, substrates and binding agents become fragile and brittle in environments with low RH. Comfortable in a climate of between 20 and 60% RH, stone and ceramic can tolerate a low air humidity.