Thermal Decomposition of Lead White for Radiocarbon Dating of Paintings

Abstract

Lead carbonates were used as cosmetic and pigment since Antiquity. The pigment, known as lead white, was generally composed of cerussite and hydrocerussite. Unlike most ancient pigments, lead white was obtained by a synthetic route involving metallic lead, vinegar and organic matter. Fermentation of organic matter produces heat and CO2 emission, leading to the formation of carbonates. As lead white is formed by trapping CO2, radiocarbon (14C) dating can thus be considered. We have developed a protocol to prepare lead white. We selected modern pigments for the experiment implementation and ancient cosmetic and paintings for dating. After characterization of the samples by XRD, thermal decomposition of cerussite at various temperatures was explored in order to select the appropriate conditions for painting samples. CO2 extraction yield, SEM and XPS were used to characterize the process. Thermal decomposition at 400°C was successfully applied to mixtures of lead white with other paint components (oil as binder, calcite as filler/extender) and to historical samples. We obtained radiocarbon measurements in agreement with the expected dates, demonstrating that thermal decomposition at 400°C is efficient for a selective decomposition of lead white and that paintings can be directly 14C-dated by dating lead white pigment.

Radiocarbon dating

“…--removal of exogenous carbon Physical (washing, ultrasonic baths, sieving, manual selection, removal of surface) 61,62 Chemical (Acid, Base, solvents, oxidation, leaching with acid, bleach) 61,62,397 --selection and extraction of endogenous carbon if a particular compound or compound class is required for measurement Compound specific (cellulose [398][399][400][401] , amino acids bones 120 , biomarkers 44,63-66,402-406 ) DNA 246 Thermal decomposition (lead white 407,408 , mortar 409 )…”

Compound Specific Radiocarbon (14C) Dating of Our Colorful Past: from Theory to Practice

“…The application potential was thereafter highlighted in a forgery case, where the collected paint sample weighed no more than 160 μg and sufficed to reveal a forger's strategy in recycling an older canvas support [20] . Additionally, the possibility to date specific pigments, such as one of the most common artistic pigments lead white, a basic lead carbonate composed mainly of the lead carbonates hydrocerussite (2PbCO 3 ⋅ Pb(OH) 2 ) and cerussite (PbCO 3 ), was demonstrated [28–37] . This development represents another breakthrough with massive implications as lead white has been the most extensively used white pigment by artists worldwide since Antiquity up to the 20th century.…”

“…[20] Additionally, the possibility to date specific pigments, such as one of the most common artistic pigments lead white, a basic lead carbonate composed mainly of the lead carbonates hydrocerussite (2PbCO 3 • Pb(OH) 2 ) and cerussite (PbCO 3 ), was demonstrated. [28][29][30][31][32][33][34][35][36][37] This development represents another breakthrough with massive implications as lead white has been the most extensively used white pigment by artists worldwide since Antiquity up to the 20th century. With the continuous decrease in sample size, sampling requirements for 14 C dating now align with those routinely taken for material analysis and cross section sampling, substantially increasing the number of cultural objects that can be ethically sampled.…”

PRETREATMENT PROTOCOLS PERFORMED AT THE ROYAL INSTITUTE FOR CULTURAL HERITAGE (RICH) PRIOR TO AMS 14C MEASUREMENTS

“…For the moment, the lead carbonate and mortar samples are not subjected to chemical pretreatment; the CO 2 is directly extracted from selected part of the samples. For lead carbonates, the protocol follows the method developed by Beck et al (2018Beck et al ( , 2019 consisting of chemical decomposition in a quartz tube (without copper oxide) at 400°C for 2 hr.…”