Abstract
Recent developments in chemical imaging can force a breakthrough in one of the long-standing, key challenges for painting conservation: the treatment of hybrid artworks, presenting heritage professionals with a puzzling amalgam of original materials and non-original layers, accumulated during various interventions. In particular, the recent introduction of Macro X-Ray Powder Diffraction imaging (MA-XRPD) in the heritage field now allows to differentiate sub-types of pigments with unprecedented chemical specificity, without the need for sampling or moving the artwork. The aptitude to identify and quantify crystal phases in artists' materials allows for the first time to reliably discriminate pigments employed by Old Masters from their industrial-age equivalents, used by the (hyper-)restorers and forgers of the 19th-20th-C. Within the artist's palette, lead white holds the highest potential to serve as a marker for signaling 'pigment anachronism' due to its omnipresence, occurring in almost all pre-WWII paint layers. In addition, preliminary MA-XRPD experiments on historical paintings indicated a clear variation in the mass ratio of the constituting compounds of lead-white: cerussite (PbCO3), hydrocerussite (2PbCO2.Pb(OH)2) and plumbonacrite (PbO.3PbCO3.Pb(OH)2). These variations were tentatively linked to the gradual improvement of production methods over time. Although the drastic switch to large-scale industrial processes at the end of the 18th-C is expected to have a fundamental impact on the chemical composition of lead white, the production processes and crystallographic composition of industrial-age lead white were never studied. As a result, the proposed discrimination of original brushwork from younger paint strokes, based on the determination of the relative crystal phase composition of lead white, is currently prevented by the lack of reliable information on industrial-age lead white compositions (1750-1940). In the context of lead white, the discovery and production of CO2 in 1750 is considered as starting date for the industrial age, whereas WWII marked the gradual replacement of lead white by titanium white. Therefore, we propose defining the chemical signature of (modern) lead white by analyzing pigment powders, accurately reproduced in a chemical lab environment according to the production processes described in historical textual sources. In this way, the chemical fingerprint of the reproduced sub-types of lead white will serve as a ground truth for the anticipated variation in historical paintings. In the next step, the representativity of this fingerprint is benchmarked by analyzing a relevant group of late 18th to early 20th-C paintings in museum collections. Next, the ability of MA-XRPD to chemically contrast industrial-age lead white from traditional lead white on actual, complex paintings will be assessed by in situ experiments on well-studied 'hyper-restorations'. Examples of the latter are 15th-C paintings with well-documented areas of excessive overpaint. In a final step, the added value of this research will be valorized during the third and last phase of the conservation treatment of Van Eyck's Ghent Altarpiece, which confronts conservators with a particularly convoluted, hybrid and heterogenous paint system.
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