Classes of Control
AA (precision control, minimal seasonal changes to temperature only)
Permissible short-term fluctuations of ±5% RH and ±2°C, with a seasonal set point change in temperature of up 5°C and down 5°C. No seasonal change in RH set point is allowed.
The images below show the worst case scenario with maximal permissible fluctuation.
Collection risks and benefits
No risk of mechanical damage to most artifacts and paintings and there is minimal risk even to objects of very high vulnerability. Some metals and minerals may degrade if 50% exceeds the value that is critical for a particular element. Chemically unstable objects will become unusable within decades.
Comments
This is the traditional, very narrow specification for collection climate control, with a modest amount of seasonal temperature adjustment. However, it must not be understood as the "perfect" specification. It does not address the large problem of chemically unstable materials in 20th-century collections, and it may be unnecessary for much of the collection. Also, even systems that keep fluctuations within this range 97% of the time will usually fall outside it a few days a year. (Currently, the ASHRAE specifications are ambiguous about the strict design definition of these classes of control.) In spite of these limitations, this is a plausible target for an institution with the mandate and the resources to provide the best, as long as the resources allocated to the system and its operation do not diminish resources needed to address much larger risks to the collection.
A (good control, some gradients or seasonal changes, but not both)
This degree of control has two subcategories:
- short-term fluctuations of ±5% RH and ±2°C, with a seasonal temperature change of up 5°C and down 10°C, and a seasonal humidity change of up 10% RH and down 10% RH
- short-term fluctuations of ±10% RH and ±2°C, with a seasonal temperature change of up 5°C and down 10°C
The images below show the worst case scenario with maximal permissible fluctuation.
Collection risks and benefits
Small risk of mechanical damage to high vulnerability artifacts; no mechanical risk to most artifacts, paintings, photographs, and books. Chemically unstable objects will become unusable within decades.
Comments
The letter "A" was assigned to this specification because it was felt to be the most cost-effective degree of control for most collections, given the ability and mandate to provide a climate-controlled building. Such systems will probably operate within AA levels most of the time. To stay reliably within A conditions year-round is a resource-intensive task in most climates. The larger seasonal adjustments in set points are a recognition that even major museums must face energy and sustainability constraints, and that these suggested temperature changes are not a significant risk.
B (control, some gradients plus winter temperature setback)
This degree of control allows short-term fluctuations of ±10% RH and ±5°C, with a seasonal temperature change of up to 10oC. Note that the temperature can not be allowed to rise above 30oC but can fall as low as necessary to maintain RH control.
The image below shows the worst case scenario with maximal permissible fluctuation.
Collection risks and benefits
Moderate risk of mechanical damage to high vulnerability artifacts; Little risk of damage to objects of medium vulnerability. Tiny risk to most paintings, most photographs, some artifacts, and some books; and no risk to many artifacts and most books. Chemically unstable objects will become unusable within decades and sooner if collections are routinely maintained at 30°C. Cold winter conditions may retard the rate of deterioration considerably.
Comments
High vulnerability artifacts in most historic collections have already experienced large fluctuations, and have formed the cracks that are possible. Further fluctuations of the same size have a very low risk of further damage. This is known as the "proofed fluctuation". Thus the risk to high vulnerability objects stated above must be recognized as applying only to objects that have come from better conditions, or have been repaired. B class of control is suggested as a reasonable target for collections in historic buildings that can tolerate the intervention of some mechanical systems but must not be damaged by the humidity conditions.
C (prevent all high risk extremes)
Within 25–75% RH year-round. Temperature rarely over 30°C, usually below 25°C.
Collection risks and benefits
High risk of mechanical damage to high vulnerability artifacts; moderate risk to most paintings, most photographs, some artifacts, and some books; and tiny risk to many artifacts and most books. Chemically unstable objects will become unusable within decades. Cold winter conditions may retard the rate of deterioration considerably.
Comments
High vulnerability artifacts in most historic collections have already experienced large fluctuations, and have formed the cracks that are possible. Further fluctuations of the same size have a very low risk of further damage. This is known as the "proofed fluctuation". Thus the statement of risk to high vulnerability objects stated above must be recognized as applying only to objects that have come from better conditions, or have been repaired. C class of control recognizes that most of the damage potential of uncontrolled climate can be prevented simply by avoiding extremes in humidity. Between 25% RH and 75% RH, hygroscopic response is only one-half to one-third that seen below 25% RH or above 75% RH. Mould and rapid corrosion are avoided. C class of control is suggested as a target for open display collections in historic buildings that are as important as the collection.
If parts of the collection are still at risk from these conditions, then provide climate control for these objects through the use of cases, cabinets, packaging (in storage), and special rooms or zones.
D (prevent dampness)
This degree of control dictates only that humidity is "reliably below 75% RH".
Collection risks and benefits
High risk of sudden or cumulative mechanical damage to most artifacts and paintings (including objects of low vulnerability) due to low humidity fracture; however, high humidity delamination and deformations, especially in veneers, paintings, paper, and photographs, will be avoided. Mould growth and rapid corrosion will be avoided. Chemically unstable objects will become unusable within decades. Cold winter conditions may retard the rate of deterioration considerably.
Comments
Avoiding mould and rapid corrosion due to damp is probably the single most important aspect of climate control for the smaller (and larger) museum. Although such a target is not normally considered an engineering design condition, it is when applied to basic historic buildings with poor quality building envelopes. It is also a legitimate target for any collection in a simple building in a humid climate, such as in the Maritimes. Well engineered systems rely more on careful design than the sheer power or ultimate capacity of the HVAC system.
If parts of the collection are still at risk from these conditions, then provide climate control for these objects through the use of cases, cabinets, packaging (in storage), and special rooms or zones.
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