9 8. DISCUSSION

From the above examples it is clear that salts may not always be the original cause of deterioration, but it is certain that their presence increases the deterioration rate significantly. The key factor for the effectiveness of salts as deteriorating agents is the presence of water and/or moisture. While moisture may also substantially diminish the mechanical resistance of some stones, its distribution within the porous material determines where deterioration occurs. This area corresponds to the zone of maximum moisture content (Snethlage and Wendler 1997) and refers not only to the damage at the base of a wall with rising damp, as described by Arnold (1982) but also to the formation of gypsum veils at the top of the rising damp area, as discussed by Zehnder (1993, 1996).

As Rodriguez-Navarro and Doehne (1999) point out, crystallization will depend on both the physical properties of the salt solution, such as water vapor pressure, and its interfacial tension with the pore system of the stone. Although changes of crystal habit through the presence of substances that affect the contact angle between substrate and water have long been observed and studied, the implication this phenomenon may have in the conservation of porous materials has been discussed in relatively few papers. P�hringer has repeatedly mentioned this effect and its possible application for salt extraction or prevention of salt growth (P�hringer and Engstr�m 1985; P�hringer 1986). The changes in morphology of gypsum crystals as a result of the application of consolidating and/or water repellent treatments was pointed out by Tucci et al. (1985), while G. Grassegger and S. Adam (1994) discussed the spiral growth of sodium nitrate in the presence of silicate ester gels, presumably due to the presence of heavy metals, such as the tin-bearing catalyst of the consolidating product. These observations were made in laboratory studies, but field observations, such as the detailed study by J. Gisbert et al. (1996), have not reported any influence of the presence of extraneous materials—for example, fly ash, micro-organisms, or biofilms—on the shape and size of the gypsum crystals present in the surface crust of a calcareous stone.