Special Collections
Cool Storage
Set point
10°C; 30–50% RH
(even if achieved only during winter setback, cool storage is a net advantage to collections as long as damp is not incurred)
Collection risks and benefits
Chemically unstable objects will remain usable for a century or more. Books and papers tend to have a low mechanical vulnerability to fluctuations.
Comments
No specific fluctuation specifications are offered in the ASHRAE table. One can apply the classes of control AA, A, and B, if desired. A much more detailed discussion of climate control specifications for such material is available in Technical Bulletin N° 23 (Guidelines for Humidity and Temperature for Canadian Archives).
Cold Storage
Set point
-20°C; 40% RH
Maximum fluctuations
±2°C; ±10% RH
Collection risks and benefits
Chemically unstable objects will remain usable for millennia. RH fluctuations under one month do not affect most properly packaged objects at these temperatures. (Time out of storage becomes the lifetime determinant).
Comments
Cold storage is used in archives and libraries, as well as for 20th-century collections of plastics and rubbers. This level of control allows chemically unstable objects such a photographic film to remain usable for millennia. Properly packaged objects are insensitive to monthly fluctuations in RH. The lifetime of objects in cold storage is determined by the time spent out of storage. A much more detailed discussion of climate control specifications for such material is available in Technical Bulletin N° 23 (Guidelines for Humidity and Temperature for Canadian Archives).
Dry Rooms
Set point
0–30% RH
Fluctuations
The RH should not exceed a critical value. This value is often 30% RH.
Comments
There is no specific note of risks and benefits in the current ASHRAE table; however, the benefit can be presumed to be the prevention of corrosion, especially on metal objects with contaminants such as chlorides. Fluctuations are not an issue per se — it is simply a question of not exceeding a particular RH.
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