Calcium Sulphate Minerals in the Grounds of Italian Paintings

Microscopic and x-ray diffraction studies revealed that the CaSO 4 ground material, which serves as the undercoat on wood panel paintings, especially those from the Tuscan region, contain mostly fibrous, highly bire-fracting particles identified as anhydrite. In many of the 40 specimens examd. the ratio of anhydrite to gypsum is 4:1. In 10 specimens from paintings of the Venetian school, however, the material is wholly gypsum. Specimens of anhydrite prepd. from gypsum taken in modern times from Tuscan quarries show the same fibrous structure. One complete analysis of a specimen of CaSO 4 grounds from a 14th-century painting by Giuliano da Rimini shows a water content of about 5% and an animal glue content of about 10%.