Electron Microscopy Imaging of Zinc Soaps Nucleation in Oil Paint

Abstract

Using the recently developed techniques of electron tomography, we have explored the first stages of disfiguring formation of zinc soaps in modern oil paintings. The formation of complexes of zinc ions with fatty acids in paint layers is a major threat to the stability and appearance of many late 19th and early 20th century oil paintings. Moreover, the occurrence of zinc soaps in oil paintings leading to defects is disturbingly common, but the chemical reactions and migration mechanisms leading to large zinc soap aggregates or zones remain poorly understood. State-of-the-art scanning (SEM) and transmission (TEM) electron microscopy techniques, primarily developed for biological specimens, have enabled us to visualize the earliest stages of crystalline zinc soap growth in a reconstructed zinc white (ZnO) oil paint sample. In situ sectioning techniques and sequential imaging within the SEM allowed three-dimensional tomographic reconstruction of sample morphology. Improvements in the detection and discrimination of backscattered electrons enabled us to identify local precipitation processes with small atomic number contrast. The SEM images were correlated to low-dose and high-sensitivity TEM images, with high-resolution tomography providing unprecedented insight into the structure of nucleating zinc soaps at the molecular level. The correlative approach applied here to study phase separation, and crystallization processes specific to a problem in art conservation creates possibilities for visualization of phase formation in a wide range of soft materials.

Formation of zinc oxalate from zinc white in various oil binding media: the influence of atmospheric carbon dioxide by reaction with 13CO2

“…Based on these IR investigations it appears that the formation of zinc oxalates follows the formation of zinc soaps which recently has been shown to be present as ionomeric clusters [34][35][36][37][38]. We believe that these clusters could be the precursors for the formation of zinc oxalates by bond breaking and oxidation of the long chain fatty acids through a radical mechanism, the radicals possibly being caused by light absorption/photoactivity of ZnO.…”

The Identification of Multiple Crystalline Zinc Soap Structures Using Infrared Spectroscopy

“…In support of this idea, transmission electron microscopy experiments have shown that zinc soap material formed in ZnO model paint is indeed very irregular, with crystalline domain sizes in the order of tens of nanometers. 12 Moreover, FT-IR spectra of zinc palmitate recrystallized from linseed oil have been reported that showed some signs of νa COO band splitting. 34…”

A Critical Review on the Analysis of Metal Soaps in Oil Paintings

“…In fact, the characterization of model paint samples prepared under controlled conditions with varying paint compositions (including different pigments, pigment‐to‐oil ratios and additives) allows the controlled study of the effect of paint ingredients on metal soap formation. [ 35 , 49 , 51 , 55 , 58 , 63 , 64 , 65 ]…”