"When Cadmium Red was introduced in 1919 vermilion had been the favourite of artists for hundreds of years. It was generally permanent if good quality and was considered the perfect red for making flesh tints in portraits and figures. Red earth colours were the only reds that could match its lightfastness but it did suffer from 3 great flaws. If the pigment was of poor quality it could turn black over time. It was rare but it happened. More importantly it couldn???t be used in watercolours reliably and couldn???t be used in pastels at all. Most important of all is that it was pure mercuric sulfide. It was a very poisonous pigment dangerous for workers to make and potentially dangerous for artists to use. Cuts in the skin and ingesting the pigment by eating in the studio or by smoking while painting meant that vermilion usage was very unwise.
Although we now know that cadmium is also toxic, it is much less so than traditional vermilion, and is reliable in watercolours. Consequently it steadily grew in popularity and passed vermilion usage by mid century. Today the usage of traditional vermilion pigments is rare and Cadmium Red is regarded as the king of the permanent reds on the palette.
Cadmium Yellow can be considered the basic form of cadmium pigments. It consists of cadmium sulfide. Orange and red cadmium pigments are made by co-precipitating the cadmium sulfide with selenium sulfide which is a closely related compound. between 5% and 14% of selenium sulfide makes the various shades of orange and 15% to 25% produces the reds. As the selenium portion reaches its maximum the reds loose their vibrancy and produce dull maroon reds, The sweet spot for the deepest red that still retains purity of colour is the Cadmium Red Medium used by Matisse.
The mid red shade of cadmium is the favourite among artists for many reasons. It has a bright blood red hue that is very attractive and it is used as a general purpose red on the palette. It has very high lightfastness, although it is not as permanent as Cadmium Yellow it was the most permanent red available to the artist prior to the introduction of the pyrrole reds. it is possible that in the long run the pyrroles may usurp the crown of king of the reds but for now Cadmium Red is holding its own because it has a number of advantages. It is very opaque. There are times when transparency is desired, but even more times when opacity and covering power are important. No other bright red pigment is as opaque as Cadmium Red. It is also seen as a good value pigment when compared to some of the newer reds which can be comparatively expensive.