Kettle limescale. Coloured scanning electron micrograph (SEM) of fur or limescale (calcium carbonate and calcium sulphate crystals) from a domestic kettle. The fur is the result of repeated use of the kettle in a hard water area. Water hardness is a qualitative measure of dissolved calcium salts. When this water is boiled or evaporated the calcium salts precipitate to form crystals. Magnification: x550 at 5x7cm size.