Review paper Allowable microclimatic variations for painted wood Lukasz Bratasz Jerzy Haber Institute of Catalysis and Surface Chemistry, Polish Academy of Science, PL-30239 Kraków, Poland Environmental standards for cultural heritage collections have been much debated in recent years. The interest in the issue has been driven by the growing movement towards green museums, that is, managing indoor museum environments in a responsible and efficient manner, especially in terms of reducing energy consumption and carbon emissions but at the same time maintaining high standards of collection care. Painted wood is among the category of heritage objects most vulnerable to relative humidity and temperature fluctuations. Therefore, scientific understanding of how changes in environmental conditions ultimately affect painted wood is crucial to the development of rational guidelines for the control of climate in museums and historic buildings. This review provides a systematic progression through two fundamental approaches to establish the allowable ranges of climatic variations – an analysis of the mechanical response of painted wood as a complex, multilayer system to climate variations, and an analysis of the historic climate to which the objects have acclimatized. The climate specifications and standards based on both these approaches are reviewed. Keywords: Allowable microclimate variations, Specifications and standards, Painted wood, Physical damage, Cracking, Green museums, Energy consumption Introduction Environmental standards for cultural heritage collec- tions on display, in storage, or in transit have been much debated in recent years. The transcriptions of two roundtables of the International Institute for Conservation: ‘Climate Change and Museum Collections’ in 2008 and ‘The Plus/Minus Dilemma: The Way Forward in Environmental Guidelines’ in 2010 (IIC, 2008, 2010) illustrate the problems dis- cussed. The interest in the issue has been driven by the growing movement towards green museums, that is, managing indoor museum environments in a respon- sible and efficient manner, especially in terms of redu- cing energy consumption and carbon emissions but at the same time maintaining high standards of collection care. Painted wood is among the category of heritage objects most vulnerable to relative humidity (RH) and temperature fluctuations and therefore scientific under- standing of how changes in environmental conditions ultimately affect painted wood is crucial to the develop- ment of rational guidelines for the control of climate in museums and historic buildings. Painted wooden objects are complex multi-layer struc- tures composed of humidity-sensitive materials – wood, animal glue, gesso, and paints which respond dimen- sionally to variations in RH and temperature in their environment. Different dimensional changes of the indi- vidual layers in the structure induce stresses, which can cause cracking and flaking of the ground and paint layers. Further, the constraint of wood from free move- ment can cause deformation and cracking of the wood, and subsequent cracking and flaking of the pictorial layer. The concept that a stable climate offers long- term stability for painted wood has, for a long time, been derived from practical observations; a much quoted example was the observation that the wartime storage of the collection of the National Gallery in London in a slate quarry reduced flaking previously found to occur while the collection was on exhibition at the Gallery (Davies & Rawlins, 1946). However, only relatively recently were two key issues – the dimen- sional response of the objects to changes in temperature and RH, and the critical levels of strain at which materials begin to deform plastically or fail physically – systematically examined (Mecklenburg & Tumosa, 1991a, 1991b; Mecklenburg et al., 1998). The structural analysis of painted wood has allowed maps of allowable RH variations to be produced which take into account their amplitude, duration, and starting RH level (Mecklenburg et al., 1998; Jakiela et al., 2008a) as well as proposing environmental specifications for collections Correspondence to: Lukasz Bratasz, Jerzy Haber Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, ul. Niezapominajek 8, 30-239 Kraków, Poland. Email: ncbratas@cyf-kr.edu.pl © The International Institute for Conservation of Historic and Artistic Works 2013 DOI 10.1179/2047058412Y.0000000061 Studies in Conservation 2013 VOL. 58 NO. 2 65