Cadmium Red

What color is cadmium red?

The story of cadmium red

Cadmium red began as a fault in the zinc sold by a German pharmacy.

Friedrich Stromeyer taught chemistry at Göttingen and had a side job inspecting the town's apothecaries. In 1817 he noticed that a batch of zinc carbonate, sold as a medicine, changed color when heated, which pure zinc carbonate does not do. He chased the impurity down and found a new metal. A chemist called Karl Hermann found the same thing the same year in an off-color zinc oxide, and for the next hundred years Germany was the only country that produced the stuff in any quantity.

The name is older than the metal. Stromeyer called it after kadmeia, the Greek word for the zinc ore it hid in, and kadmeia is named for Cadmus, the prince in Greek myth who followed a cow, killed a dragon, sowed its teeth and founded Thebes. So a plate painted in 1526 with a cow and a dragon is, at two removes, a picture of where cadmium red got its name. Chemists saw at once that cadmium sulfide made a strong yellow, but the metal was so scarce that cadmium yellow did not reach painters until the 1840s. Monet used it, and so did Van Gogh.

Red took another seventy years. Cadmium sulfide on its own is yellow; to push it toward red you swap some of the sulfur for selenium, and cadmium selenide on its own is a black-red solid. Blend the two and you get cadmium orange, then cadmium red, with fifteen to thirty percent selenium in the red. The pigment reached the market shortly after 1910, and it arrived at the right moment, because the bright red painters had leaned on for two thousand years, vermilion, was mercury sulfide, expensive, toxic, and had a habit of turning black. Cadmium red matched its color and its covering power and stayed put. Within a generation vermilion was nearly gone from the paint rack.

Now the part everyone asks about. Cadmium is toxic, and its compounds are classed as carcinogenic to humans; in Japan, cadmium that got into rice caused the disease called itai-itai. But the pigment is not the metal. Cadmium sulfide is nearly insoluble, and the cadmium in a pigment particle is locked up, so the real risk is dust: dry pigment, pastel chalk, sanding, spraying. A wet pan of cadmium red is not going to hurt you unless you eat it. Europe restricts cadmium under its REACH rules; in 2013 Sweden asked the European Chemicals Agency to ban it from artists' paints too, arguing that paint washed down the drain ends up in crops, and the proposal was turned down. The tubes are legal. They are not for children.

Then there is the cadmium-free shelf. A family of organic pigments called pyrrole reds, PR254 and PR255 on the label, matches cadmium's hue and most of its opacity without the metal; they were developed for car paint. In tints they are a little brighter and a little less deep, and they stain more. I own both. I reach for the cadmium when I want a red that sits on the paper like a painted door, and the pyrrole when I am painting with kids around. What would be wrong is buying the cadmium for its danger, as if the risk were the point. The point is a red that has not faded in a hundred years.

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 cadmium red

You cannot mix a cadmium red, because every mixture is duller and more transparent than the pigment itself, but you can mix the color. A warm red with a touch of orange gets you cadmium red light; a warm red with a speck of crimson gets you the deep. What you will not get is the opacity.

Green is the partner that matters. Red and green sit opposite each other, so a little phthalo green or Forest Green knocks cadmium red back to brick, then to a warm brown, then to a near-black with more life in it than anything from a tube. With blue it makes dull, brownish purples, and they are not even interesting ones.

Using cadmium red in watercolor

Use cadmium red for the lit side of red things: a barn wall in sun, a poppy at noon, a child's coat in a gray street. It comes forward, so one small patch will pull a whole painting's attention. Put it where you want the eye to land and nowhere else.

It is the red for dry brush and flat shapes. Because it is opaque, you can drag a fairly dry brush across rough paper and get solid, broken red like old paint on a boat. It looks completely opaque while wet and dries a step more transparent, so judge it dry.

Grays and greens are its friends. Next to Forest Green both look more like themselves; next to Payne's gray it glows. Next to a crimson it makes the crimson look tired.

One thing I learned the hard way is to respect the dust, not the paint. I used to scrape dried cadmium off a palette with a knife and blow the flakes away. Do not do that. Wipe it up damp, and never tip cadmium rinse water into a pot plant.

Cadmium red vs vermilion (and pyrrole red)

Vermilion is the color cadmium red was built to replace: mercury sulfide, warm, opaque, and prone to darkening toward black. Cadmium red light matches it almost exactly in hue and covering power and does not go black. Almost nothing sold as vermilion today is genuine anyway.

Pyrrole red is the cadmium-free answer. PR254 and PR255 give a very similar warm red, nearly as opaque, with excellent lightfastness, no metal and no dust warning. In tints they are a shade brighter and a shade less deep, and they stain the paper more. If you paint with children, buy the pyrrole and do not look back.

If cadmium red is a fire engine parked in the sun, vermilion is the same engine in a museum, and pyrrole red is the model on the shelf: identical from across the room, lighter in the hand.

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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.