A hard, white ground layer primarily used for tempera painting and water gilding. As the Italian word for gypsum, the term gesso was applied to ground layers prepared from gypsum and animal glue. Gesso grounds were used since medieval times in Europe. They were often applied in multiple layers with the initial gesso layer containing coarse gypsum particles (gesso grosso) and the final layer containing fine gypsum particles (gesso sottile). The dried surface was polished to an ivory-like finish. Because it was hard and inflexible, gesso was usually applied to a rigid support, such as a panel, picture frame or sculpture. From Renaissance times, a hard gesso prepared from calcium carbonate (chalk) in glue was used in northern Europe for oil and casein paintings. More recent formulations for gesso use synthetic binders and with one or more white pigments such as chalk, zinc oxide and titanium dioxide (about 10% for opacity). Synonyms: gesso grosso; gesso sottile Museum of Fine Arts, Boston (2000). [J. Cassar, R. de Angelis]