BS 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 priority, 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 artefact 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. The insulating capacity of the building and the materials from which it is constructed are important factors for the maintenance of internal environmental stability.
If RH and temperature have the same priority, the above specifications hold also for temperature. Stable RH can be obtained in one of the following ways: if the moisture content in air is constant, maintaining the temperature as constant as possible;
if the moisture content in air is variable, vary the temperature in order to maintain a constant RH (when changes in temperature have no relevant impact on objects);
if the moisture content in air is variable, add or remove moisture to the air, without altering temperature (if changes in temperature have relevant impact on objects);