## 1 Introduction  
This paper contributes to the Special Issue \"Results of the II National Research project of AIAr: archaeometric study of the frescoes by [PERSON] and workshop at the church of San Panfilo in Tornimparte (AQ, Italy)\" in which the scientific results of II National Research Project conducted by members of the Italian Association of Archaeometry (AIAr) are discussed and collected. For in-depth details on the aims of the project, see the introduction of the Special Issue [1]. One of the most serious forms of damage affecting porous stone materials used in monuments and other construction materials is directly linked to the physical action exerted by the crystallization of soluble salts [2; 3; 4; 5; 6]. This process can cause several degradation phenomena, including flaking, cracks, detachment or staining of the pigment layer, efflorescence, sub-efflorescence and formation of crusts, all of which can lead to irreversible damage of the original material [7; 8]. As documented in numerous studies [7; 9; 10], walls and frescoes suffer greatly from decay induced by salt crystallization, and conservation strategies often need to be put in place in order to salvage the artworks. However, these efforts may in turn exacerbate the problem by introducing additional salts or solutions, as explained in the following paragraph, which may cause the artifacts to decay more rapidly instead of preventing this process. The salts responsible for this damage can originate from the building material itself or derive from other sources [11]. For instance, they can be channeled from the groundwater and penetrate the wall via capillary rise, with an efficiency that depends on the distribution of the pore size and the wetting behavior of the material [12]. Alternately, they can derive from polluted atmospheres rich in particulate matter, marine aerosols and other polluting agents which are deposited and eventually penetrate the walls [13]. Additionally, acidic and alkaline products used in cleaning and restoration treatments usually contain salts and, if not applied correctly, can give rise to the problems associated with salt crystallization mentioned previously. Finally, they can derive from the metabolic activity of microbes [14]. When the saline solutions that permeate the material, following the evaporation of water, reach suitable thermodynamic conditions, nucleation and growth of crystals can take place. As the water evaporates, the concentration increases and the salt begins to precipitate. The degree of damage is directly proportional to the concentration of the salt and the size of the crystal, and inversely proportional to the size of the pores [12]. Degradation from salt formation derives from the pressure exerted by the crystallization process on the pores of the material. This can occur via different mechanisms: hydrostatic pressure, hydration pressure and linear pressure. Hydrostatic pressure develops when the solution occupies a volume which is smaller than the one of the precipitating crystals [15]. In some cases, this may only constitute a minor effect, since the increase in pressure is associated with an increase in solubility, limiting the impact of salt crystallization. Hydration pressure occurs following the increase in volume generated by the hydration of the salt [16]. This effect is strictly related to the nature and physical-chemical characteristics of the salt; therefore, the impact may be different from case to case. Finally, linear pressure develops following the crystallization of a salt in the pores of the material, and the exerted pressure will be inversely proportional to the pore size [17]. Many calcarenites with pore sizes smaller than 0.1 micron suffer greatly from this type of degradation [15; 16]. Often, these three mechanisms have a combined effect, leading to an increase in pore size and eventually to cracks, efflorescence and sub-efflorescence [16].