(a)
(b)
Figure 2: SEM-EDX analysis of a black crust over limestones from façades of XIII-XV centuries from Seville (Spain). (a) The blue EDX spectrum is calcium, and the red EDX spectrum is sulphur; (b) the purple EDX spectrum is calcium, and the turquoise EDX spectrum is sulphur. EDX shows the presence of a compound of sulphur and calcium on the surface (gypsum) and the matrix of calcium from limestone.