Dioxazine Purple

What color is dioxazine purple?

The story of dioxazine purple

Dioxazine purple is what happened when chemists stopped imitating snails and simply built the strongest purple they could.

Purple spent most of history being expensive. The ancient world got its famous purple from murex sea snails. Medieval scribes stained parchment purple for chant books written in gold and silver ink, and the purple faded anyway; the leaf below has gone a deep pink in twelve centuries. Painters had no proper purple pigment of their own until cobalt violet in 1859, and that one was weak and pricey, so violet went on being mixed from red and blue, the way it had been since the Middle Ages.

The fix came out of coal tar. Dyes built on a five-ring structure called triphendioxazine had been known since 1928, mostly as blue dyes for cotton. In 1952 chemists found that if you started with carbazole, a coal-tar chemical, and condensed it with chloranil, you got a dark purple solid that melts at 385 degrees Celsius, dissolves in nothing and tints everything. The Colour Index filed it as Pigment Violet 23; its makers called it carbazole violet. Within a few years it was in car paint, plastics, printing ink, artists' colors and purple candles, and it is still the most important pigment of its family.

Textbooks drew its molecule wrong for decades. Everyone assumed the rings ran in a straight line; an X-ray study in 1987 showed the real molecule is bent into an S. The straight version can be made in a lab but never was in a factory, because it costs more and does the same job. I find that oddly comforting. The standard violet of the pigment trade was sold for thirty-five years by people who did not know what shape it was.

Then the arguments. Dioxazine has a reputation in some circles for fading, and the reputation is half right. At full strength it is very lightfast. Thin it to a pale tint and it holds less well, which is true of every pigment but shows more in one that is so often used pale. And it comes in two grades: the red shade is much more lightfast than the blue shade, and the tube rarely tells you which you have bought. That explains a lot of the arguments.

What I actually think: buy it, and treat it like hot sauce. A pea of dioxazine will tint a whole palette, and used straight it looks like a bruise. Used in drops, in the shadow under a lemon or the dark heart of an iris, it does what no mixed purple can, which is stay transparent while going nearly black. Cobalt violet is prettier and quinacridone is cleaner. Dioxazine is the one you reach for when a shadow needs to mean it.

Pigment facts

These describe the pigment in general. Every brand's tube behaves a little differently, so test yours on scrap paper.

How to mix dioxazine purple

You can mix a purple without buying one: ultramarine and a quinacridone rose, about equal, gives a clean violet that is lighter and softer than dioxazine and lifts better. What you cannot mix is the depth. Nothing made from two pigments goes as dark as dioxazine while staying transparent, which is why the tube earns its place.

Yellow, its opposite, is the pairing that matters. One speck of Lemon Yellow in a puddle of dioxazine kills the glow and leaves a warm gray; a little more and you have a shadow brown that no earth color can match for transparency. That is how you tame it, with its complement rather than with water.

Using dioxazine purple in watercolor

Use dioxazine purple where a shadow has to be dark and still see-through: under the eaves of a white house, inside a bunch of grapes, the core of a purple iris. It is also the fastest way to deepen a blue sky at the top of the sheet without turning it gray.

Glazing is the technique. A whisper of dioxazine over a dry yellow wash makes a shadow that reads as the same object in shade, which a gray glaze never does. Over dry orange it makes a deep brown-red; over a green it makes the dark side of a hedge. Keep the glaze thin and judge it dry; it dries a step lighter and warmer than it looks wet.

It pairs with the yellows. Lemon Yellow beside it looks brighter, and Green Gold next to a dioxazine shadow reads as sunlight on leaves.

One thing I learned the hard way: never dip a brush wet with dioxazine into a clean pan. I once tinted an entire yellow half-pan purple-gray with one careless touch, and yellow is not a color you can rescue. Rinse twice, and mix it on the far side of the palette.

Dioxazine purple vs cobalt violet (and ultramarine violet)

Cobalt violet is the opposite kind of purple: a mineral, granulating, semi-opaque, weak and expensive, a reddish violet that sits on the paper in little grains and lifts off with a damp brush. Dioxazine is bluer, transparent, staining and cheap, and a teaspoon of it would tint a bucket of cobalt violet. Cobalt is the one for delicate lilac light; dioxazine is the one for shadow.

Ultramarine violet, PV15, is ultramarine's shy cousin: the same sulfur-silicate chemistry pushed toward violet, soft, granulating and pale, with none of dioxazine's punch. It is lovely for hazy distances and hopeless for a dark.

If dioxazine purple is a bruise, cobalt violet is a lilac, and ultramarine violet is the lilac's shadow on a wall.

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Mel, founder of Tobio'sI make portable watercolor kits and I named the company after my cat. I write these pages for the beginner I used to be, one pigment at a time.