plaster

A mixture of lime as the binder and sand or crushed marble (marmorino) as the aggregate. It protects the wall, levels it, and serves as the base for the mural. It can be applied in one or more layers. Various inorganic or organic substances – such as crushed bricks, Puzzolan earth, marble, gypsum, straw, animal hair, casein, sugar, oil, or blood – were sometimes added to improve the quality of the basic mix.