12Microcracking induced by wide fluctuations in temperature affects granite quality and durability, making the stone more vulnerable to decay. 13Determining the extent of that effect is not always straightforward, however, given the excellent durability of these materials. 14 Four types of construction granite quarried in the region of Madrid, Spain, and frequently used in both the built heritage and in de novo 15 construction (Alpedrete, Cadalso de los Vidrios, Colmenar Viejo and Zarzalejo) were exposed to 42 thermal cycles (105-20º C; UNE-EN, 14066, 16 2003). Petrographic and petrophysical properties were analysed using both destructive and non-destructive techniques. Microcracking generated in 17 the granite stones by 42 thermal cycles had barely any impact on their petrophysical properties, which are the parameters normally assessed to 18 establish material quality and durability. Their petrographic properties, which are not generally assessed in this type of studies, were affected, 19 however. This study contends that petrographic analysis is needed to objectively quantify the actual quality and durability of the most highly 20 resistant materials when petrophysical studies are inconclusive. Petrographic and fluorescence microscopy, along with fractography, are among the 21 most prominent techniques for petrographic exploration. Thanks to the deployment of these techniques, mineral microcracking could be monitored 22 throughout the present tests conducted. 23The microscopic findings revealed substantial micro-textural and microstructural change in and around the granite minerals, which play a 24 prominent role in decay. The findings showed that pre-existing microcracks coalesced and generated further microcracking as decay progressed. 25Microcracking was most intense in Zarzalejo granite due to its textural characteristics determined by its high feldspar content. Microscopic 26 observation revealed that the microstructure of feldspar minerals, with their crystallographic anisotropies and secondary mineral phases, favoured 27 microcrack development. Zarzalejo granite exhibited lower quality and durability than Colmenar Viejo and Cadalso de los Vidrios granites, which 28 were more resistant to heat treatment. 29 30

The indoor environment and its natural dynamics in small Spanish historical churches such as the studied here depend on the variations of outdoor climate and the moisture dynamics of walls, built with different materials. Such indoor environments are impacted by local factors, which may put at risk the conservation of a church's cultural assets. Natural ventilation in spring, the presence of people and especially the wintertime use of ageing heating system induce substantial fluctuations in indoor environments primarily affecting the stability of relative humidity (RH). RH is the physical parameter that can induce efflorescence as well as plaster blistering and detachment in its inside walls, drying and cracking in the timber and efflorescence and disgregation in the carved dolostone.Where the RH inside building is not high, as in the present case, natural and induced fluctuations may lower it considerably (<25 %), which is detrimental to conservation and human well-being both. Human presence partially counters the steep declines in RH attributable to heating in winter and warm, dry summer weather, although the trade-off is a rise in CO 2 levels inside the church.Heating induces substantial changes in the T and RH on the high altar and in the upper areas of the nave, while natural ventilation affects the RH at the base of the church and favours the elimination of CO 2 . The results obtained have allowed us to develop a series of recommendations that might be useful for the preventive conservation of such historic buildings, without compromising human comfort.

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