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Gypsum is the only secondary phase recurring in crusts or efflorescences on the cave surfaces of all the three studied sites (Fig. 3). The best examples of gypsum crusts, thick and compact, were observed in Taya Caves, where they may jeopardize the readability of the carved decorations; they have a composite stratigraphy, characterized by the presence of an intermediate layer of calcite between the surface gypsum and the host rock. As 224for the gypsum efflorescences, the most extensive were observed in the WWII tunnels of Yoshimi Hundred Caves, where salt weathering is actually particularly severe, accounting for the crystallization of other sulfates, of Na, Al, Fe, and Mg – jarosite, alunogen, halotrichite, alum-Na, tamarugite, epsomite, and thenardite (Horiguchi et al. 2000; Oguchi et al. 2010). Oya stone also suffers from sulfate-rich efflorescences, constituted of gypsum, mirabilite, and thenardite, each phase preferentially crystallizing in different microenvironments. Low-crystallinity efflorescences were finally found in Taya Caves, composed of chlorides (sylvite, in particular), phosphates, and sulfates.

Figure 3: Examples of gypsum crusts and efflorescences with the relevant analytical data.

In addition, we conducted a complementary study of the rock-water interaction in Taya Caves, considering the lack of previous researches and the constant presence of percolating rainfall, rising damp, and extremely high humidity in that environment. We measured an extremely rapid water absorption (~25 %), a more contained yet significant adsorption of hygroscopic water (~5 %), and a very fast decay during the jar slake test (Santi 1998). These findings point out a high susceptibility to clay mineral-promoted swelling and slaking deterioration, which may produce decay patterns like erosion, rounding, scaling, peeling and, in the long term, lead to structural decay and collapses.