In chemistry, efflorescence (which means "to flower out" in French) is the migration of a salt to the surface of a porous material, where it forms a coating. The essential process involves the dissolving of an internally held salt in water, or occasionally in another solvent. The water, with the salt now held in solution, migrates to the surface, then evaporates, leaving a coating of the salt. In what has been described as "primary efflorescence", the water is the invader and the salt was already present internally, and a reverse process, where the salt is originally present externally and is then carried inside in solution, is referred to as "secondary efflorescence". Efflorescences can occur in natural and built environments. On porous construction materials it may present a cosmetic outer problem only (primary efflorescence causing staining), but can sometimes indicate internal structural weakness (migration/degradation of component materials). Efflorescence may clog the pores of porous materials, resulting in the destruction of those materials by internal water pressure, as seen in the spalling of brick. Examples A 5 molar concentration aqueous droplet of NaCl will spontaneously crystallize at 45% relative humidity (298 K) to form an NaCl cube by the mechanism of homogeneous nucleation. The original water is released to the gas phase. Gypsum (CaSO4.2H2O) is a hydrate solid that, in a sufficiently dry environment, will give up its water to the gas phase and form anhydrite (CaSO4). Copper(II) sulfate (bluestone) (CuSO4.5H2O) is a blue crystalline solid that when exposed to air, slowly loses water of crystallization from its surface to form a white layer of anhydrous copper(II) sulfate. Sodium carbonate decahydrate (Na2CO3.10H2O) will lose water when exposed to air. Masonry Primary efflorescence Primary efflorescence is named such, as it typically occurs during the initial cure of a cementitious product. It often occurs on masonry construction, particularly brick, as well as some firestop mortars, when water moving through a wall or other structure, or water being driven out as a result of the heat of hydration as cement stone is being formed, brings salts to the surface that are not commonly bound as part of the cement stone. As the water evaporates, it leaves the salt behind, which forms a white, fluffy deposit, that can normally be brushed off. The resulting white deposits are referred to as "efflorescence" in this instance. In this context efflorescence is sometimes referred to as "saltpetering." Since primary