2.(Chem.) To change on the surface, or throughout, to a whitish, mealy, or crystalline powder, from a gradual decomposition, esp. from the loss of water, on simple exposure to the air; as, Glauber's salts, and many others, effloresce.
3.To become covered with a whitish crust or light crystallization, from a slow chemical change between some of the ingredients of the matter covered and an acid proceeding commonly from an external source; as, the walls of limestone caverns sometimes effloresce with nitrate of calcium in consequence of the action of nitric acid formed in the atmosphere.
...
