The Effects of Moisture on Cadmium Sulfide Based Oil-Paint Degradation

Katie Skinner

Abstract: Cadmium Sulfide based oil-paint, which appears in shades of yellows, oranges, and even reds, is prone to degradation over extended periods of time. Degradation of cadmium sulfide paint can lead to major effects on famous works of art. This article will go over the effects that environments and moisture have on the degradation of Cadmium Sulfide based oil-paint.

In the late 19th and 20th century, Cadmium Sulfide (CdS) based oil-paint was used by many artists in their paintings due to its vibrant yellow and orange colors. Cadmium orange and cadmium yellow can be found in famous paintings such as The Scream by Edvard Munch1 , Femme (Époque des !Demoiselles d "Avignon”) by Pablo Picasso2 , Flowers in a blue vase by Vincent Van Gogh, Joy of Life by Henri Matisse3 , and others. While CdS is now also used in a variety of different electronics such as lasers, light-emitting diodes, solar cells, and photodetectors as it is a key semiconductor because it is both chemically and thermally stable, it is mainly used as a pigment.2 Pure CdS based oil-paints tended to remain vibrant for a time, while light exposure caused poorer quality CdS paint to lose its color rapidly enough that paint producers and artists would notice right away. These were some of the early signs of CdS degradation. Other signs of degradation included areas pained with cadmium yellow becoming brittle, the formation of off-white and opaque crusts, chalking, discoloration, flaking, or the color darkening to a brownish-yellow. The quality of the CdS paint, the humidity of the area the artwork is in, and light exposure are all factors that play into how fast the CdS paint degrades.

There are two processes used to produce CdS paints, the wet and dry methods. For the wet method, a soluble sulfide compound is combined with one or multiple types of cadmium salts in a precipitation reaction.1 For the dry method, cadmium carbonate, metallic cadmium, or cadmium oxide are calcined without oxygen with pure sulfur to form cadmium yellow pigment. Due to the various different combinations of cadmium salts and sulfide compounds and because neither reactants are ‘pure’, the wet method produces a much larger variety of cadmium yellow paints, but the paints produced through the wet method are more susceptible to chemical change than paints made through the dry method.1 Because of the large number of different types of CdS produced, each has a different rate of degradation, so it is difficult to pin down exactly how fast certain paintings that include CdS based oil-paints will discolor or degrade.2 Another factor that plays into the degradation process of CdS based oil-paints is their crystalline structures and the existence of “trap states.”2 CdS can have a crystalline structure that ranges from amorphous to the more common hexagonal and cubic. The shape of the structure not only influences what color the pigment is but also how chemically stable it is and therefore how it reacts with oxygen and water to cause degradation.4

Moisture in the air greatly affects CdS based paints as it heightens the possibility of oxidation to occur. The exposed CdS crystalline structure is vulnerable to H2O and

O2. The larger amount of CdS atoms on the surface create “trap states” which are dangling CdS bonds that trap excited electrons. Because of different concentrations of electrons, vacancies of cadmium or sulfur atoms appear along the exposed ends of the structure (surface level paint).2 A vacancy of a cadmium atom in the structure causes water molecules to dissociate and fill the vacancy while a vacant sulfur atom on the outer ends of the structure is filled by one oxygen molecule (O2).4 This can be seen in Figure 1. This causes the discoloration of the CdS based paints, including the degradation into a

brownish-yellow color. Figure 1: hexagonal crystalline structure of hex-CdS affected by oxidation a) red oxygen molecule absorbed onto a cadmium atom vacancy creating an S-O2 bond; b) blue area is where the oxygen molecule has become charged and green areas are where the sulfur ion’s charge has been depleted4

The hexagonal structure is largely more stable, while the cubic structure allows for a higher density of dangling CdS bonds in one area and therefore is more vulnerable to degradation.2 While the difference between hexagonal and cubic structure degradation is a small factor, the quality of the CdS paint is a much greater factor in CdS degradation. Higher quality CdS paint produced using the dry method show little to no signs of degradation over time whereas low quality CdS paint produced using the wet method, whether the CdS structure is hexagonal or cubic, is much more prone to degradation.1

There are several ways that CdS paint degradation is studied, one of the ways is through an artificial aging process with mockup painting. Additionally, some of the same studies also performed macroscale spectroscopy studies using synchrotronradiation x-rays. In the mock-up aging studies, it was found that moisture in the air caused oxidation reactions changing CdS to the white CdSO4 and altering the paint. The oxidation reactions produce products such as sulfites and sulfates, even when not exposed to light. Different cadmium chloride compounds were also found to oxidize CdS while CdS pigment migration, recrystallization of watersoluble CdS phases, and dissolution were all secondary reactions caused by humidity.1 Moisture levels of around 95% relative humidity (RH) triggered CdS yellow to oxidize into CdSO4 white but RH levels of around 45%, which is considered to be ‘normal’ levels of humidity, did not oxidize. This data can be seen in Figure 2. It was also found that electron-hole recombination and trap state repair happen when humidity decreases.3 Therefore, for the importance of maintaining art and art conservation, low or ‘normal’ humidity is the best way to store artworks to prevent CdS based oil-paint from degradation.

Fig. 2: results of 4 tests of artificial mock-up aging of hex-CdS and cub-CdS. Most aging appears when artificially aged with thermal 95% relative humidity (RH)3

1 Monico, Letizia, et al. "Probing the Chemistry of CdS Paints in The Scream by in Situ Noninvasive Spectroscopies and Synchrotron Radiation X-ray Techniques." Science Advances, vol. 6, no. 20, 15 May 2020. Science, https://doi.org/10.1126/sciadv.aay3514. 2 Comelli, Daniela, et al. "Degradation of Cadmium Yellow Paint: New Evidence from Photoluminescence Studies of Trap States in Picasso's Femme (Époque Des 'Demoiselles D'Avignon')." Analytical Chemistry, vol. 91, no. 5, 1 Feb. 2019, pp. 3421-28, https://doi.org/10.1021/acs.analchem.8b04914. 3 Monico, Letizia, et al. “Role of the Relative Humidity and the Cd/Zn Stoichiometry in the Photooxidation Process of Cadmium Yellows (CdS/Cd1−xZnxS) in Oil Paintings." Chemistry – a European Journal, vol. 24, no. 45, 25 July 2018, pp. 11584-93, https://doi.org/10.1002/chem.201801503. 4 Giacopetti, Laura, and Alessandra Satta. "Reactivity of Cd-yellow Pigments: Role of Surface Defects." Microchemical Journal, vol. 137, Mar. 2018, pp. 502-08, https://doi.org/10.1016/J.MICROC.2017.12.013.