1 Causes of scaling on bush-hammered heritage ashlars. A case study: Plaza Mayor of 1 Madrid (Spain). 2 D.M. Freire-Lista (1, 2) and R. Fort (1, 2) 3 (1) Instituto de Geociencias IGEO (CSIC, UCM) Spanish Research Council CSIC - Complutense 4 University of Madrid UCM. Madrid, Spain (d.freire@igeo.ucm-csic.es, rafort@csic.es) Phone: +34 5 913944903 6 (2) CEI Campus Moncloa, UCM-UPM and CSIC, Madrid, Spain 7 8 Masons have traditionally used granite anisotropy to cut and lay the stone. Scaling, a common 9 type of granite decay, is observed worldwide. 10 This study explored the relationship between weathering and cut planes in heritage ashlars, 11 specifically in the stone on Madrid’s Plaza Mayor, whose construction dates back to 1590. The 12 71 rectangular granite columns that support its porticoes are oriented toward the four cardinal 13 points. All 71 have one exposed side that faces the square, one protected side facing inward and 14 two semi-protected sides perpendicular to the other two. The sides of the columns are also 15 oriented to the points of the compass. 16 This study aimed to identify the prevailing orientation of scaling, if any, in the granite ashlars 17 and to determine how this process is affected by climate, microclimate (orientation), use, 18 hewing and exfoliation microcracks. 19 All four sides of the 71 columns were mapped (284 in all) to analyse scaling height, distribution 20 and orientation. The findings showed that the microcracks are vertically oriented and decline in 21 density and length with depth from the surface. Scaling was observed on the lower ashlars in the 22 columns to a maximum depth of 3 mm. 23 Determining the direction of exfoliation microcracks is imperative to understanding decay 24 mechanisms in granite ashlars and sculptures and that information must be taken into 25 consideration when applying conservation treatments. 26 Keywords: decay, granite, exfoliation microcracks, bush-hammering. 27 28