BS EN 15757-2010 pdf download.Conservation of Cultural Property — Specifications for temperature and relative humidity to limit climate- induced mechanical damage in organic hygroscopic materials.
When dealing with composite objects or when objects made of different materials occupy the same location, an evaluation of potential interaction between materials or parts of objects made of similar materials Is necessary. as this situation is more complex. If there is synergy in the behaviour of the materials, the object as a whole may be more vulnerable than the most vulnerable of its component parts. However, the specifications concerning stability of temperature and RH and the importance of a careful consideration of the historical climate generally remain valid.
5 Approach to specifying temperature and RH for organic hygroscopic materials
5.1 DeterminatIon of priorities
Changes in EMC of organic hygroscopic materials induce dimensional changes, with possible internal stress cycles and related risk of deformation or cracking depending on issues of construction and materials. The risk is higher for anisotropic materials than for isotropic ones.
If the EMC and the resulting dimensional change of hydroscopic materials is affected more by changes in RH than in temperature, then RH should be maintained at historical climatic levels as closely as possible.
If, however, objects can be significantly affected by temperature change, the requirements for stability of both RH and temperature should be taken into account and adjusted accordingly.
5.2 Maintaining stable environmental conditions
When a stable RH is the pciority, variations in RH should be avoided while taking into account reasonable parameters of cost, risk and benefits to the object. and spatially consistent; sharp variations in gradients and areas or devices that absorb or dissipate heat and moisture should be avoided.
Stabilising RH within a target range around average values or seasonal cycles typical of the climate history of the room will reduce the risk of physical damage. The most appropriate target range should not exceed the historical variability to which the artef act has become acclimatized, and should be based on all available past climate records covering a period of one or more calendar years. Only entire multiples of a calendar year should be used. and not fractions of it. to avoid bias due to an unbalanced number of different seasons. The method for determining the target range for RH is given in informative Annex A.
The objective of the preservation/preventive measures against physical damage is to avoid short-term fluctuations and cycles and to reduce steep and/or frequent variations in both temperature and RH which lead to physical damage of objects. Appropriate monitoring protocols should ensure that daily cycles be avoided or mitigated i.e. RH should remain constant during both day and night.
An analysis of data on the external, ambient conditions should be conducted in parallel with the regular monitoring of the internal environment. These conditions are influenced by the seasons, diurnal changes and weather extremes. The range of seasonal cycles should be narrowed and balanced the need for sustainable control.
if the moisture content in air is variable, combine the two above methods (if temperature changes have not relevant impact on objects).
5.3 Priority of historical climate
When the condition of objects made of organic hygroscopic materials that have been kept in a specific microclimate for a prolonged period (at least one year) has been found satisfactory by qualified specialists in indoor climate and conservation: the historical climate in the room, including average RH levels, the range of variability of the natural cycles (seasonal or daily) and the rates of change should be kept unchanged. The only acceptable changes are improvements that reduce fluctuations in the climatic conditions.
When an object is moved for restoration or exhibition, the historical climate should be maintained as closely as possible. This holds also for transport or storage.
5.4 Further specifications
If. for whatever reason, the indoor microclimate is to be changed from the historical conditions, or if an object made of organic hygroscopic materials is to be moved into a new microclimate, research should be undertaken to evaluate the adaptation of the object to the new temperature and RH conditions, taking into account the storage history and the response of the object. Unfortunately, artefacts may appear to be adapting to microclimate changes without damage, but then fail suddenly and catastrophically (e.g. by cracking) In order to release accumulated stresses.
If for whatever reason, the historical climate is to be modified; the temperature and RH should be changed at a rate which is sufficiently low to allow a gradual adaptation. A slow rate of RH change reduces both gradients of moisture contents inside materials and related differential dimensional response leading to stress. Furthermore, stress relaxation occurs if the time period of the RH change is equivalent to the relaxation time constants of an object. A thorough investigation of the situation and the material should be carried out in order to determine this rate. It is also necessary to continually monitor the object during the period of change. Provided the object has been carefully assessed and the RH and temperature variations appropriately controlled, no change should be distinguishable in an object. Needless to say, if any deterioration is observed, the ongoing changes in RH and temperature should be halted immediately and the situation re-evaluated immediately.BS EN 15757-2010 pdf download.