Thermal analysis of carbonate pigments and linseed oil to optimize CO2 extraction for radiocarbon dating of lead white paintings

Abstract

Lead white was a white pigment extensively used in paintings from Antiquity until the beginning of the 20th century. It is composed of hydrocerussite (Pb3(CO3)2(OH)2) and cerussite (PbCO3). Historical recipes report metallic lead, vinegar and horse manure as ingredients to produce the two lead carbonates. The incorporation of organic CO2 carrying 14C representative of the time of manufacture of lead white allows radiocarbon dating of this pigment. One possible protocol for lead white paint preparation for dating is based on the extraction of carbon from lead carbonates by thermal decomposition. Since lead white paints are mixed with a binder and other pigments, a thermogravimetric study was implemented to study CO2 extraction from each component. The measurements were carried out on samples of modern paints prepared with lead white (lead carbonate), Meudon white (calcium carbonate) and linseed oil. The results show that it is possible to separate the carbon of interest (carrying the 14C signature: lead white and linseed oil) from the dead carbon coming from geological carbonates (containing no radiocarbon: Meudon white) by heating the paint sample at 400 °C. This study provides a better understanding of the thermal behavior of these three paint constituents and confirms the validity of the thermal protocol for the preparation of lead white paints for radiocarbon dating.