The bright, vivid palettes used by artists, such as Van Gogh and Matisse, were available thanks to a revolution in materials synthesis in the 19th century. Novel pigments, such as cadmium (Cd) yellow, had a high tinting strength and thus became popular for use in paintings. Recent research has shown that Cd yellow can undergo photodegradation, causing significant visible changes to a painting. For example, Cd yellow pigment degradation in Matisse’s The Joy of Life resulted in darkening of the yellow foliage at the upper left of the painting (Figure 1). It is therefore crucial to understand how a pigment’s structure and chemical composition (including the impurities present) will affect its reactivity.
Cd was first discovered in 1817, and by the mid-1840s, Cd yellow became commercially available as an artists’ pigment. It was widely used throughout the 19th and 20th centuries. Cd yellow, or cadmium sulfide (CdS), is synthesized one of two ways: either by a “dry” process, where Cd metal or Cd oxide/carbonate is calcined with sulfur at 300-500°C, producing hexagonal CdS (h-CdS, Figure 2, left) or by a “wet” process, where Cd sulfide is precipitated by reacting Cd salt with a soluble sulfide, favoring the production of cubic CdS (c-CdS, Figure 2, right). Other non-crystalline, or amorphous, phases can also be formed during either synthesis.
Recent work by Ghirardello et al. studied the chemical and photophysical properties of nine historical and two modern Cd yellow pigments. Using a combination of X-ray diffraction, X-ray fluorescence, and Raman spectroscopy, they found three unique chemical compositions of the historical samples: 1) a mixture of either h-CdS and c-CdS, 2) coexistence of both phases of CdS, or 3) hexagonal ZnxCd1-xS. Minor components detected include barium sulfate (a common extender material), calcium sulfate, cadmium carbonate, and lead sulfate. The two modern pigments were composed of only h-CdS or ZnxCd1-xS, but both pigments contained barium sulfate.
The photophysical properties of the historical Cd yellow pigments were analyzed by time-resolved photoluminescence (PL) microscopy. PL occurs when an absorbed photon is re-emitted from a material at a different wavelength. In this study, two types of PL emission pathways for Cd yellow are studied: near band edge emission (nanosecond lifetime) and deep trap state emission (microsecond lifetime). Time-resolved PL spectral maps were taken of both the NBE and TS PL emissions at two different spectral regions as shown in Figure 3. Both maps show heterogeneity indicated by green luminescent spots in the composite images. This heterogeneity indicates that there are imperfections (defects and/or substitutions) in the crystal structure of CdS and the presence of other impurities. Similar maps were obtained for the other historical pigment samples.
Based on the collected data (crystal structure and size, additives present, and PL mapping of impurities and crystal defects), the nine historical pigments studied were grouped into five different categories. The first three categories contained pigments synthesized via a wet process method. The second two categories were classified as a dry process method. Not only were the authors able to distinguish between an “imperfect” versus a refined wet synthesis method, they also were able to distinguish between early and later dry process methods. This study is an excellent foundation for the physical understanding of the relationship between pigment composition and observed degradation processes in a paint film. However, most of these pigments are found within a complex matrix of binders and/or additives. Therefore, future studies of model paint films can further our understanding of cadmium yellow degradation pathways within real paint samples.
All figures reproduced/adapted with permission. Figure 2 adapted from http://www.vias.org/genchem/chem_cds.html. Figure 3 adapted with permission from Ghirardello et al.; copyright (2018) American Chemical Society.