Effect of evaporative surface area on salt efflorescence and subflorescence formation in a given porous material

Salt crystallization in composite layered porous materials such as wall paintings, frescoes, tiles, and ceramics can lead to cracks or delamination of the upper layers (paintings and glaze). The resulting open surface areas induce asymmetrical flow of liquid and ions from the heart of the material towards the delaminated evaporative region. By performing drying experiments in truncated cone--shaped sandstone samples as a model system for such flow, we show that for a given volume and a given salt concentration, the size of the evaporative surface (i.e., the large base and the small base) leads to different drying kinetics and salt-crystallization patterns of $\mathrm{Na}\mathrm{Cl}$. By characterizing the temporal evolution of the ion-concentration profiles by NMR in the liquid during evaporation and by determining the P\'eclet number over time, we explain this different behavior and the resulting adhesive or nonadhesive efflorescence and subflorescence when the truncated cone--shaped sandstones are dried with either the large base or the small base as the evaporative surface. Fluid transport from a larger region beneath the surface to a smaller, localized evaporative area occurs faster as the surface is approached. This, in turn, combined with the evaporation rate induces greater ion advection, resulting in salt precipitation as nonadhesive efflorescence at the surface. As the damaged area increases, subsurface crystallization interlaced with the efflorescence leads to an adhesive salt crust at the surface. The latter can increase subsequently the chance of mechanical damaging with time. These findings bring new insight into why with aging and the progress of delamination areas the risk of accelerating the deterioration of artworks will also increase.