A soft, porous white mineral composed of fine-grained limestone. Also called whiting, pure varieties of chalk contain up to 99 percent calcium carbonate as the mineral calcite. Chalk was formed in the Cretaceous period and occurs naturally in thick beds in many parts of the world, such as the chalk cliffs along the English Channel. Chalk beds are collections of the shells of such tiny marine organisms as foraminifera, coccoliths, and rhabdoliths. Ground chalk has been used as a pigment since ancient times. When mixed with glue, it was the most common ground for northern European paintings from medieval times well into the 18th century. Chalk was also commonly used for making lime, Portland cement, putty and polishing powders. Chalk is stable and inert. It is made synthetically by precipitating fine particles of calcium carbonate. It is listed among the inorganic additives used for stucco marble preparation according to literature. Synonyms: whiting; English white; Paris white; gilder's whiting; Champagne chalk; calcite; limestone; calcium carbonate; marble white; Kreide (Ger.); craie (Fr.); creta (It.); greda (Sp.). Wittenburg (1999); Museum of Fine Arts, Boston (2000). [J. Cassar, R. de Angelis]