Darkening of lead‐ and iron‐based pigments on late Gothic Italian wall paintings: Energy dispersive X‐ray fluorescence, μ‐Raman, and powder X‐ray diffraction analyses for diagnosis: Presence of β‐PbO₂(plattnerite) and α‐PbO₂(scrutinyite)

Abstract

This work reports the results of an extensive study carried out on the wall paintings preserved inside the Saint Stephen's chapel in Montani (Val Venosta, Bozen, Italy), by means of μ‐Raman spectroscopy, portable X‐ray fluorescence (XRF) spectrometry, and powder X‐ray diffraction, in order to characterize materials and their alteration products that cause the blackening of paintings. In situ XRF analysis allowed identifying the areas of interest where the blackening appeared. The complementary analytical techniques allowed reconstructing a chromatic palette used by the artists that includes expensive pigments, such as lapis lazuli and cinnabar, indicating a wealthy client. Raman spectra at very low power recorded on blackened degraded samples showed the presence of a mixture of the two polymorphs of lead (IV) dioxide, plattnerite (β‐PbO2), and scrutinyte (α‐PbO2), as degradation products of lead‐based pigments. On these samples, no evidence of white lead was found, although a white lead conversion treatment had been applied on the paintings. The degradation of red lead (Pb3O4) into a mixture of plattnerite, scrutinyte, and anglesite (PbSO4) was demonstrated in some darkened samples. Furthermore, the degradation of haematite (Fe2O3) with the formation of magnetite (Fe3O4) and coquimbite (Fe2(SO4)3·9H2O) was also identified as responsible for the blackening of the paintings. The influence of original materials and environmental and anthropogenic factors such as the valley's orography and the presence of contaminating agents (SOx) is discussed to explain the decay phenomena.

Use of Temperature Controlled Stage Confocal Raman Microscopy to Study Phase Transition of Lead Dioxide (Plattnerite)

“…However, keeping the laser power low enough, it is possible to obtain from the Raman spectrum of β-PbO 2 that it is characterised by three bands at 653, 515 and 424 cm −1 , identified as deriving from the B 2g , A 1g and E g modes, by analogy with the corresponding isostructural SnO 2 modes located at 776, 634 and 475 cm −1 . Recently, in one of our works the presence of another band at 159 cm −1 was proposed as belonging to β-PbO 2 , since it has been identified both in commercial and in mineral plattnerite using Raman lasers at different excitation wavelengths (632 and 532 nm) [23].…”

Glass-based pigments in painting: smalt blue and lead–tin yellow type II

“…The presence of plattnerite correlated with lead-tin yellow type II deserves more accurate research. In another case study (Costantini et al 2020), the darkening associated with this glass pigment was explained as the total transformation of white lead mixed with lead-tin yellow type II even if no traces of the original white pigment have been detected. Microstructural and microchemical analysis are still in progress to understand more in depth the role that the production technology of this pigment can play in the darkening process of the painted surfaces.…”

Pigments—Lead-based whites, reds, yellows and oranges and their alteration phases

“…associated with both lead white and red lead (9 cases), red lead (6), plattnerite (4), lead-tin yellow type I (3), scrutinyite (3), anglesite (2), galena (2), lead white (2), litharge (2), PbMg(CO 3 ) 2 (2) and chrome orange and yellow (1): found in paintings ranging from the 1 st to the 19 th century, in Europe, India and Iran: present on all types of common supports, such as wood ceilings, glass icon as (fatty tempera), illuminated manuscripts and wall paintings (fresco and secco), as well as in powdered raw pigments and wood cupboards (Burgio et al 1999;Bruni et al 2002;Smith and Clark 2002a;Castro et al 2008;Kotulanová et al 2009;Aliatis et al 2010;Ravindran et al 2011;Hradil et al 2013;Lukačević et al 2013;Duran et al 2014;Bratu et al 2015;Holakooei and Karimy 2015;Felix et al 2018;Kantoglu et al 2018;Vanmeert et al 2018;Arjonilla et al 2019a;Khramchenkova et al 2019;Costantini et al 2020b;Fioretti et al 2020).…”

“…Overall, these phases are fundamental in the study of paintings, as they represent alteration indicators. However, while plattnerite has been partially studied and frequently observed in wall paintings (Daniilia et al 2008;Daniilia and Minopoulou 2009;Kotulanová et al 2009;Hradil et al 2013Hradil et al , 2014Gutman et al 2014;Liu et al 2016;de Ferri et al 2019;Costantini et al 2020b), scrutinyite has rarely received attention (Kotulanová et al 2009;Lytle et al 2009;Costantini et al 2020a;Vagnini et al 2020).…”