Salt crystal formation, or efflorescence, on cement or masonry walls comes from groundwater seeping into a building. A report provides new insight into why the crystals form in bunches rather than spreading out evenly over a surface (Phys. Rev. Lett., DOI: 10.1103/physrevlett.108.054502). Efflorescence can cause structural damage to walls and is a particular concern for conservationists trying to preserve old buildings and artwork. A group led by Marc Prat of the University of Toulouse, in France, used computational simulations and carried out experiments to study how a sodium chloride solution flows through and evaporates from a porous cylinder. The researchers found that crystals form where channels promote the best fluid flow and surface evaporation is greatest. Crystals that arise in a particular spot then reinforce crystallization in that area by providing additional surface area for evaporation. The results should help researchers develop models to predict efflorescence and point to ways to prevent it, such as by using humid air to suppress evaporation, Prat says.