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Alizarin, Azo, Cadmium, Quinacridone, Phthalocyanine: The fascinating world of Pigment Families


Pigments
Pigments - Image by Alena Darmel (pexel.com)

One of the things that has always fascinated me by looking at acrylic paints are the pigments and their particular names, sometimes very difficult to be pronounced.


In this post I will talk about the most important groups or families of absorption pigments in terms of their history and manufacturing way.


As I wrote in my previous posts about acrylic paints and pigments, absorption pigments are one of the components of acrylic paints (together with an acrylic binder and water) and are the ones responsible for the colour of the paints, due to the fact that they can absorb a particular wavelenght of light and reflect the one we perceive as their hue. Absorption pigments can be either organic or inorganic and they can be produced naturally or synthetically.


Throughout history colours where obtained by pigments that sometimes resulted to be too volatile or, in the worst cases, even deathly toxic for human health (for example the pigment used to produce the colour green contained arsenic).

Today dangerous pigments have been mostly substituted by synthetical (and sometimes even better for artistic purposes) versions and, according to the way pigments are obtained or which raw material is used, absorption pigments can be divided into five different pigment families:


  • Alizarin pigments

  • Azo pigments

  • Cadmium pigments

  • Quinacridone pigments

  • Phthalocyanine pigments


Pigments Families

Alizarin pigments

Alizarin is an organic compound used as a fabric dye and obtained from the madder plant (the same plant used to produced lake pigments such as rose madder and alizarin crimson). Even though it is often used to indicate a very deep red, its colour range can vary from pink through purple to brown, according to the mordant employed to create the dye. In the past it was also known under the name “Turkey red” due to the most common manufacture process coming from that country.

Until the 18th century Alizarin was produced naturally from the madder root, which contained two red components purpurin and alizarin crimson. Then, in 1868 the German chemists Carl Graebe and Carl Liebermann after a lot of experimentation discovered that the alizarin could be obtained synthetically using anthracene. This new method, which was less expensive than the one using madder root, became overnight the standard one used to manufacture the alizarin dye and pigment until 1958, when it started to be replaced by the more lightfast quinacridone pigments developed by the chemist DuPont.


Azo Pigments

Azo pigments are organic insoluble pigments that contain at least a nitrogen group. They are similar to azo dyes in the composition, are lightfast and their colour range can vary from yellow, orange, red to brown, even though basically all colours were possible (the colour blue and green are substituted by the better and cheaper phthalo pigments).



Examples of AZO-pigments based acrylics


Cadmium based pigments: Yellow, Orange, Red

As the name suggests, the cadmium pigment family includes inorganic pigments containing cadmium, a rare soft metal, similar to zinc, whose use as a commercial pigment started when it was discovered as a by-product of the zinc refining process.

The term “cadmium” comes originally from both: the Latin word “Cadmia” used by many naturalists to indicate various earths and oxides and the Greek “kadmeia”. Cadmium is metal used for the production of both batteries and coloured pigments. Even by small amounts and if breathed, it is a very toxic metal for both human and animals. For this reason it started to be replaced in many art supplies by other pigments.

There are three categories of cadmium based pigments, according to the substance added to the cadmium sulphide: yellow (by adding zinc sulphide), orange and red (by adding selenium sulphide). They have a high brilliance, a strong colour tinting a good lightfast but they fade very fast when exposed to light.


Cadmium Yellow

The most important cadmium sulphide based yellow pigment is PY37 Cadmium Yellow.

Cadmium Yellow pigment varies from Light, Medium and Deep. The light shade is usually called Cadmium Yellow Lemon; the darkest Cadmium Yellow Deep.

The pigment Cadmium Yellow was first discovered in 1817 by the German chemist Friedrich Stromeyer, who observed that the metal cadmium combined with zinc sulphur created a yellow compound. Before then the main yellow pigment used by artists was “Orpiment” (PY39), from the Latin word “auripigmentum” meaning “gold pigment”. This was substituted by cadmium yellow due to its high toxicity (it contains arsenic).


Cadmium Orange

The second colour from the cadmium family is orange, whose production started after 1820. Cadmium Orange (PO20) is a bright, warm and opaque colour and a variation of cadmium yellow, where the elements used to produce the yellow are combined in a different way to get the orange.


Cadmium Red

Cadmium red (PR108) is the third colour from the colour family. It was developed around 1919 and obtained by adding selenium sulphide to cadmium sulphide. It is a very strong, warm and opaque red and, starting from the 20th century it replaced the distinctive but toxic vermilion. It can have light, medium and deep variations.

At the beginning Cadmium red was produced by heating the cadmium yellow together with selenium. Nowadays the production of the red pigment is very expensive and the pigment itself is the final results of several controlled chemical reactions involving ingredients such as mineral acids, sodium sulphide flakes, water and selenium.


Cadmium Free-Colours

Due to possible toxicity problems, there is a new tendency to substitute cadmium based colours with cadmium free alternatives, which give back the same hue by using different combinations of other pigments. Examples of cadmium free acrylics can be found the heavy body series by Liquitex.


Quinacridone Pigments

Quinacridone is an industrial organic compound and the base for quinacridone pigments. Quinacridone Pigments, discovered in 1935 and standard for the manufacture of paint from 1958, are very much appreciated by artists due to their colour intensity, their stability and their resistance to light, heat and solvents. Its colour palette can vary from purple through pink, red, maroon to gold.


The most important Quinacridone Pigments are:

  • Quinacridone Magenta (PR122). The colour Quinacridone Magenta, a deep pink with a purple bias, is produced by using either the pigment PR122 Quinacridone Magenta or the pigment PR202 Quinacridone Crimson. Developed as a substitute to alizarin colours, it is a high transparent colour, with no wavelength and a bridge between the colours red and violet.

  • Quinacridone Rose/Quinacridone Violet (PV19). The pigment PV19 can vary from a pinkish shade to a violet shade. In the first case (Quinacridone Rose) it is usually used as a substitute for rose madder and goes by the name Permanent Rose. In the second case (Quinacridone Violet) the pigment is used as more red-purplish magenta.

  • Quinacridone Red (PR209). The colour quinacridone red can vary from a red-orange (PR209) to a more pinkish Quinacridone Scarlet (PR207).

  • Quinacridone Maroon (Quinacridone Burnt Scarlet PR206)

  • Quinacridone Pink (PV42)

  • Quinacridone Purple (PV55)

  • Quinacridone Gold (PO48)


Quinacridone Pink, Magenta and Violet are wonderful to create purples when mixed to Ultramarine Blue or Phthalo Blue (RS). Here more ways to use quinacridone pigments in colour mixing.


Phthalocyanine Pigments

The last pigment family I will talk about is the one of Phthalo Pigments.


Phthalo Pigments are copper phthalocyanyne based organic compound, obtained by the reaction of phthalic anhydride, urea, copper and ammonia, in the colours blue and green.

  • Phthalo Blue (PB15) is available in a red and a green shade. A red shade is more similar to ultramarine blue and is good to get warmer tones or earth greens. The green shade, on the contrary, is good to get brighter greens.

  • Phthalo Green, a derivate from phthalo blue, is available in a blue shade (PG7) and a yellow shade (PG36). The blue shade has a very deep blue undertone and, when mixed with white, it creates wonderful turquoise shades. The blue shade mixed to alizarin crimson creates a chromatic black. Pththalo Green (YS), obtained by treating Phthalo Blue with bromine, is a more expensive variation of Phthalo Green (BS). When mixed with red it can create other interesting chromatic blacks.



Phthalo Blue (Green Shade) and Phthalo Green (Blue Shade)


I hope you enjoy this little overview about pigment families and that you have learnt a little more about colour mixing.


Thank you for reading. If you like the post, leave a like, a comment and don't forget to follow my blog, Instagram and Threads and share the content on your social media.


Happy painting!


Laura


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