Orvieto Cathedral was built in bichrome sty le using two different building materials: travertine and a volcanic rock named "basaltina". Two different lithologies of "basaltina", fine grained (FGB) and coarse grained (CGB), were employed in the construction of the Cathedral. In the apse, the circular windows and the sustaining pillars of the vault are made of alternating FGB and travertine blocks, whereas the inner walls are made of CGB blocks only. In the SSE wall, severe exfoliation of CGB in the upper gallery and of FGB from the window can be observed, along with intense load fracturing of FGB and travertine in the upper side of two opposite pillars. Detailed characterization of sound and deteriorated material shows that the exfoliation of FGB cannot be explained by chemical causes, whereas the exfoliation of CGB can be related to analcimization. A mechanical model of the SSE wall and pillars with subsidence of part of the foundation is in very good accordance with the observed fracturing of the pillars. The stress pattern can also explain the exfoliation of CGB in the gallery and the exfoliation of FGB in the circular window.
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