Causes of scaling on bush-hammered heritage ashlars: a case study—Plaza Mayor of Madrid (Spain)

Abstract

Masons have traditionally used granite anisotropy to cut and lay the stone. Scaling, a common type of granite decay, is observed worldwide. This study explored the relationship between weathering and cut planes in heritage ashlars, specifically in the stone on Madrid’s Plaza Mayor, whose construction dates back to 1590. The 71 rectangular granite columns that support its porticoes are oriented towards the four cardinal points. All 71 have one exposed side that faces the square, one protected side facing inward and two semi-protected sides perpendicular to the other two. The sides of the columns are also oriented to the points of the compass. This study aimed to identify the prevailing orientation of scaling, if any, in the granite ashlars and to determine how this process is affected by climate, microclimate (orientation), use, hewing and exfoliation microcracks. All four sides of the 71 columns were mapped (284 in all) to analyse scaling height, distribution and orientation. The findings showed that the microcracks are vertically oriented and decline in density and length with depth from the surface. Scaling was observed on the lower ashlars in the columns to a maximum depth of 3 mm. Determining the direction of exfoliation microcracks is imperative to understanding decay mechanisms in granite ashlars and sculptures and that information must be taken into consideration when applying conservation treatments.