Re-interpretation of the Old Masters' practices through optical and rheological investigation: The presence of calcite

Lead white, composed of a mixture of cerussite and hydrocerussite (respectively PbCO₃ and Pb₃(CO₃)₂(OH)₂), is often associated in paintings with other white pigments, especially calcite. By combining in-situ analyses with paint reconstructions, we attempt to get a better understanding of the role of this addition of calcite and to investigate how artists may have used it to modify the properties of their paints. Lead-based white pigments of a Dutch 17th-century nuancier have been analysed as well as historical paintings. Two examples are given: one from a painting by Roger van der Weyden, in the 15th century, and one by Nicolas Poussin, in the 17th century, from specific zones that indicate the addition of calcite. The presence of calcite and pigments enhances both the optical and the rheological properties. Visible reflectance spectroscopy was carried out on pure paints as well as mixtures and indicated an increase in the transparency of the paint mixture in comparison to pure lead white. Rheological measurements also indicated an increase in the elastic and viscous moduli, as well as of the yield stress again in comparison to pure lead white. Calcite could thus have been used to assist in the creation of impasto effects in lead white paints.