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. Introduction 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 22
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. Creation of Cadmium Sulfide Paint and its Structures 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 Crystalline Structures and and How Moisture Affects CdS Degradation 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
