Chromium Oxide Green

What color is chromium oxide green?

The story of chromium oxide green

Chromium oxide green is the color of the corridor outside a school nurse's office, and it will outlast the school.

A red rock started it. In 1761 a German mineralogist, Johann Gottlob Lehmann, found an orange-red mineral in the Beryozovskoye mines in the Urals and called it Siberian red lead. Samples reached the Paris pharmacist and chemist Louis Nicolas Vauquelin in 1794, and in 1797 he pulled a new metal out of it by heating its oxide in a charcoal furnace. The metal was named for the Greek word for color, chroma, because almost everything it made came out colored: the red of the ore, the yellow that became chrome yellow, the green of the oxide.

The green is the simplest thing chromium makes: two atoms of chromium to three of oxygen, Cr2O3, cooked hot enough to drive off everything else. It has the crystal structure of corundum, the mineral rubies are made of, and is nearly as hard, which is why the same powder is sold for polishing knives and razors as green rouge. It melts at 2,435 degrees Celsius. The pigment trade calls it PG17; the paint industry calls it chrome green and uses it for institutional green, which is where the school corridor comes in. By 1996 the world was making 28,000 tons of it a year, most of it for things that are not paintings.

Artists were slow to take to it, and I understand why. The dry oxide was known as a pigment from about 1809 and in paint boxes by around 1840, but next to the greens of the day it looked like a wet afternoon. In 1838 two Parisians, Pannetier and Binet, found a way to make a hydrated form of the same oxide, with water locked into the crystal, and kept the recipe secret; that transparent, glowing blue-green is viridian, PG18. Viridian cost three or four times as much, though, so by mid-century the cheap chrome green was one of the alternatives painters bought instead. It also finished off two ancient greens, malachite and verdigris, which both gave way to chrome green during the 1800s.

Then the armies noticed something the painters had missed. Chromium oxide green reflects near-infrared light the way living leaves do, so a vehicle painted with it looks, to an infrared sensor, like the hedge it is parked beside. That made it the main ingredient in the infrared-reflecting paints armed forces use on vehicles, and the principal pigment in military camouflage paint. A color painters found too dull turned out to be the exact dullness of nature, measured by a machine.

My take is that chromium oxide green is the most underrated tube in the green family. In the pan it looks like something scraped off a pond. Its gift is what it does to other greens: a little of it knocks a shrieking phthalo down to a hedge, and mixed with a yellow it makes the only tube foliage green I know that needs no correcting. It is the green of leaves in shadow, of moss, of the far side of a hill, and it will still be that green when the paper has gone brown.

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 chromium oxide green

You can get near the hue of chromium oxide green by dulling any green with its complement, but you cannot get the opacity, and the opacity is the point. The pairing that matters is chromium oxide green with a yellow: a little Lemon Yellow wakes it into a believable sunlit leaf, a lot turns it into a lime that still has some earth in it. The other pairing is with phthalo green. A touch of chromium oxide tames phthalo faster than any red does, and without turning it gray.

Add it to the mix last and in small amounts. It has more tinting strength than its looks suggest, and it grays whatever it meets.

Using chromium oxide green in watercolor

Use it where the green is supposed to be quiet: foliage in shadow, moss, the dark mass of a pine, a hill two fields away. Chromium oxide green never shouts, so it never steals the picture.

Its best trick is dry brush. Because it is opaque, a fairly dry brush dragged over rough paper leaves broken, solid marks that read instantly as lichen, bark or old paint on a door. Thin washes dry to a soft, slightly cloudy gray-green that is perfect for the far side of a valley.

It pairs with the yellows and the earths. Lemon Yellow lifts it into daylight; Venetian Red turns it to tree bark; a little Titanium White makes an opaque sage for a pale leaf against a dark background.

One thing I learned the hard way: it dries darker and duller than it looks on the brush. I once painted a whole hedge in what looked like a lively olive and came back an hour later to a strip of army blanket. Mix it a little brighter than you want and let the drying do the rest.

Chromium oxide green vs viridian (and olive green)

Viridian is the same chromium oxide with water bound into the crystal, and that single difference changes everything. Viridian, PG18, is transparent, cool and a little glowing, and it granulates gently. Chromium oxide green, PG17, is opaque, dull and flat. Same chemistry, two moods: one is a glass of sea water, the other is the moss on the pier.

Olive green is the color chromium oxide green is most often mistaken for, and the two are close in hue. The difference is body. Olive is a mixed color, usually a yellow dulled with black or blue, and as transparent as its ingredients. Chromium oxide green is a single heavy pigment that covers. Paint both over a pencil line and the olive shows the line; the chromium oxide hides it.

If viridian is a stained-glass window and olive green is a linen shirt, chromium oxide green is a painted iron gate: dull, solid, and still there in a hundred years.

More color stories

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.