Pigment Degradation in Oil Paint Induced by Indoor Climate: Comparison of Visual and Computational Backscattered Electron Images

Abstract

For the first time the degradation of lead white pigment in mature oil paint has been used as an internal marker for the degree of saponification and hence chemical degradation of oil paint. Computational image analysis of the backscattered electron images quantified the degree of the intact lead white pigment versus the nonpigmented and lead-rich areas (degraded lead white) in the paint layers. This new methodology was applied to a series of paint samples taken from four painted wall hangings (dated 1778), which makes it possible to study the influence of indoor climate on chemical degradation of aged oil paintings. The visual interpretation and computational image analysis of the backscattered electron images revealed clear trends. The highest degree of lead white degradation in the room was found in samples from the north wall close to the windows, whereas degradation diminished further away from the window. Lead white from the south wall was less degraded, but showed a similar trend as in the paintings on the north wall. These results imply a strong relationship between chemical degradation of paint and location of the paint in the room.

Insight into the effects of moisture and layer build-up on the formation of lead soaps using micro-ATR-FTIR spectroscopic imaging of complex painted stratigraphies

“…areas of the same painting showing heavy age craquelure alongside areas with minimal age craquelure) depends on the pigments (composition of the primary pigments and their reactivity when used in mixture to form new colours) and the organic materials which encompass a specific paint layer and the surrounding environment. These features have been extensively investigated on single-layer model samples [17] and on real-case stratigraphies [4,19] having a relatively simple build-up of layers and composition.…”

“…It is important to seek advanced knowledge in understanding the mechanisms governing the aging processes in order to develop the best conservation approaches targeting the formation of metal soaps [13][14][15]. In particular, great attention has been devoted to characterize the causes of the formation of metal soaps, including temperature [16,17], pH [17], composition [16][17][18] and thickness of the paint layer [2], presence of water [16,17], and the nature of the ground [4] and the conservation environment [4,19].…”

Computational modelling of metal soap formation in historical oil paintings: the influence of fatty acid concentration and nucleus geometry on the induced chemo-mechanical damage

“…As a first modelling step, the dependency of the metal soap reaction rate on these factors is therefore neglected in the present analyses. However, environmental conditions can generally influence both the mechanical properties [10] and the chemical characteristics [11,12] of paint layers, and may affect the choice of conservation treatments [13]. The sensitivity of metal soap formation to temperature and relative humidity variations still needs to be better quantified in experiments in order to adequately account for this effect in the modelling of chemo-mechanical degradation of paint systems.…”

Elemental and Molecular Segregation in Oil Paintings due to Lead Soap Degradation

“…Soap formation has been identified in hundreds of works of art dating from the 15 th century to the present, and in numerous cases it has been linked to environmental factors such as light exposure, changes in relative humidity, and/or elevated temperatures 10 . However what factors trigger the process, what the mechanisms are, and how it can be arrested or prevented is not well understood 5 .…”

“…A wide range of methods have been used to characterize soap deterioration in paintings and to study the reactions in model paint samples. Spatially resolved techniques such as Raman spectroscopy, Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy - energy dispersive X-Ray spectroscopy (SEM-EDS), secondary ion mass spectrometry (SIMS), and XRF have been employed to analyze the soaps localized in paint cross-sections and, along with Gas chromatography–mass spectrometry (GC-MS) and direct temperature-resolved mass spectrometry (DTMS), in sample scrapings and model paint samples 1 – 6 , 10 – 21 . Raman spectroscopy and FTIR instrumentation used in laboratories generally allow one to obtain molecular information with spatial resolutions on the order of a few microns; SEM-EDS gives elemental mapping with a spatial resolution of ~ a micron and SIMS has a spatial resolution in the order of 100 nanometers 15 .…”