A composition is provided comprising a substance of interest encapsulated by a continuous layer of a plurality of salt crystals which layer is impermeable to the substance of interest. Also provided is a method of producing a continuous layer of a plurality of salt crystals, the salt comprising a cation and an anion, which method comprises (i) providing a substrate which comprises a plurality of negatively charged binding sites for the cation and/or a plurality of positively charged binding sites for the anion, and (ii) contacting the substrate with a solution of the salt under conditions which cause nucleation of salt crystals comprising the cation and the anion at a plurality of said binding sites, the plurality of charged binding sites being present on the surface of the substrate at a sufficient density so that the resulting plurality of salt crystals form a continuous layer.