Cadmium pigments
Encyclopedia
Cadmium pigments are a class of pigment
s that have cadmium
as one of the chemical components. They are highly toxic and can produce cadmium poisoning
. Most of cadmium
produced worldwide is used in the production of nickel-cadmium batteries, but about half the remaining consumption, which is about 2,000 tons annually, is used to produce colored cadmium pigments. The principal pigments are a family of yellow/orange/red cadmium sulfide
s and sulfoselenides. Cadmium yellow is cadmium sulfide
(CdS); by adding increasing amounts of selenium
, colors ranging from orange to nearly black (the color of cadmium selenide
) can be produced. Cadmium yellow is sometimes mixed with viridian
to give a bright, pale green mixture called cadmium green.
s. Their greatest use is in the coloring of plastic
s and specialty paints which must resist processing or service temperatures up to 3000°C. The color-fastness or permanence of cadmium requires protection from a tendency to slowly form carbonate salts with exposure to air. Most paint vehicles accomplish this, but cadmium colors will fade in fresco or mural painting.
Cadmium sulfide and a mixture of cadmium sulfide with cadmium selenide are commonly used as pigments in artists' paints. They have an excellent reputation for color permanence although this is partially based on two reasons which are not necessarily directly related to their properties:
. These are similar in lightfastness to the cadmium colors and have the advantage of both being cheaper and non-toxic. (Cadmium is a known carcinogen
as well as acutely toxic.) With respect to lightfastness the lemon yellow cadmium pigment is an exception: the azo-variety is highly superior in light-fastness.
In some countries, such as Australia
, consumer activists such as Michael Vernon
were successful in banning the use of cadmium pigments in plastics that could be used for toy manufacture, owing to the toxicity of cadmium.
Pigment
A pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption. This physical process differs from fluorescence, phosphorescence, and other forms of luminescence, in which a material emits light.Many materials selectively absorb...
s that have cadmium
Cadmium
Cadmium is a chemical element with the symbol Cd and atomic number 48. This soft, bluish-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Similar to zinc, it prefers oxidation state +2 in most of its compounds and similar to mercury it shows a low...
as one of the chemical components. They are highly toxic and can produce cadmium poisoning
Cadmium poisoning
Cadmium is an extremely toxic metal commonly found in industrial workplaces. Due to its low permissible exposure limit, overexposures may occur even in situations where trace quantities of cadmium are found. Cadmium is used extensively in electroplating, although the nature of the operation does...
. Most of cadmium
Cadmium
Cadmium is a chemical element with the symbol Cd and atomic number 48. This soft, bluish-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Similar to zinc, it prefers oxidation state +2 in most of its compounds and similar to mercury it shows a low...
produced worldwide is used in the production of nickel-cadmium batteries, but about half the remaining consumption, which is about 2,000 tons annually, is used to produce colored cadmium pigments. The principal pigments are a family of yellow/orange/red cadmium sulfide
Sulfide
A sulfide is an anion of sulfur in its lowest oxidation state of 2-. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.- Properties :...
s and sulfoselenides. Cadmium yellow is cadmium sulfide
Cadmium sulfide
Cadmium sulfide is the inorganic compound with the formula CdS. Cadmium sulfide is a yellow solid. It occurs in nature with two different crystal structures as the rare minerals greenockite and hawleyite, but is more prevalent as an impurity substituent in the similarly structured zinc ores...
(CdS); by adding increasing amounts of selenium
Selenium
Selenium is a chemical element with atomic number 34, chemical symbol Se, and an atomic mass of 78.96. It is a nonmetal, whose properties are intermediate between those of adjacent chalcogen elements sulfur and tellurium...
, colors ranging from orange to nearly black (the color of cadmium selenide
Cadmium selenide
Cadmium selenide is a solid, binary compound of cadmium and selenium. Common names for this compound are cadmium selenide, cadmium selenide, and cadmoselite ....
) can be produced. Cadmium yellow is sometimes mixed with viridian
Viridian
Viridian is a blue-green pigment, a hydrated chromium oxide, of medium saturation and relatively dark in value. It is composed more of green than blue. Specifically, it is a dark shade of spring green, the color between green and cyan on the color wheel...
to give a bright, pale green mixture called cadmium green.
Artists paints
Brilliantly colored, with good permanence and tinting power, Cadmium Yellow, Cadmium Orange, and Cadmium Red are familiar artist colors, but of little use in architectural paintPaint
Paint is any liquid, liquefiable, or mastic composition which after application to a substrate in a thin layer is converted to an opaque solid film. One may also consider the digital mimicry thereof...
s. Their greatest use is in the coloring of plastic
Plastic
A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs...
s and specialty paints which must resist processing or service temperatures up to 3000°C. The color-fastness or permanence of cadmium requires protection from a tendency to slowly form carbonate salts with exposure to air. Most paint vehicles accomplish this, but cadmium colors will fade in fresco or mural painting.
Cadmium sulfide and a mixture of cadmium sulfide with cadmium selenide are commonly used as pigments in artists' paints. They have an excellent reputation for color permanence although this is partially based on two reasons which are not necessarily directly related to their properties:
- when introduced, there were hardly any stable pigments in the yellow to red range, especially orange and bright red was very troublesome, when the cadmium pigments replaced e.g. mercury sulfide (the original vermilionVermilionVermilion is an opaque orangish red pigment, similar to scarlet. As a naturally occurring mineral pigment, it is known as cinnabar, and was in use around the world before the Common Era began. Most naturally produced vermilion comes from cinnabar mined in China, and vermilion is nowadays commonly...
), the light-fastness was greatly improved, - companies sell the cadmium-containing paints at premium price. Although the pigments are certainly more expensive, the premium price is often not fully justifiable, with reasons more in the marketing area than in the actual raw material cost.
Light fastness
The cadmium pigments have been partially replaced by azo pigmentsAzo compound
Azo compounds are compounds bearing the functional group R-N=N-R', in which R and R' can be either aryl or alkyl. IUPAC defines azo compounds as: "Derivatives of diazene , HN=NH, wherein both hydrogens are substituted by hydrocarbyl groups, e.g. PhN=NPh azobenzene or diphenyldiazene." The more...
. These are similar in lightfastness to the cadmium colors and have the advantage of both being cheaper and non-toxic. (Cadmium is a known carcinogen
Carcinogen
A carcinogen is any substance, radionuclide, or radiation that is an agent directly involved in causing cancer. This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes...
as well as acutely toxic.) With respect to lightfastness the lemon yellow cadmium pigment is an exception: the azo-variety is highly superior in light-fastness.
In some countries, such as Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
, consumer activists such as Michael Vernon
Michael Vernon
Michael 'Mike' Vernon A.M. was a prominent Australian consumer activist. Vernon was born in Portsmouth, United Kingdom in 1932 to John Ernest Vernon and Caroline Vernon . He emigrated to Australia in 1955 and settled in Canberra, Australia...
were successful in banning the use of cadmium pigments in plastics that could be used for toy manufacture, owing to the toxicity of cadmium.