Phthalo Green

What color is phthalo green?

The story of phthalo green

Every tube of phthalo green starts life as a tube of blue.

The blue was an accident, twice over. In 1927 two Swiss chemists, Henri de Diesbach and Edmond von der Weid, were trying to turn one compound into another and got an intensely blue by-product instead. Around the same time, workers at Scottish Dyes in Grangemouth kept finding traces of a blue dye when they made phthalimide in the presence of iron. It took until 1934 for Patrick Linstead to work out what the molecule was, and in 1935 ICI put the copper version on sale as Monastral blue. DuPont followed in the United States in 1937. It is still the highest-volume pigment made anywhere.

Chlorine turns the blue into the green. Copper phthalocyanine has sixteen hydrogen atoms around its rim. Treat it with chlorine, with aluminum chloride as a helper, and those hydrogens are swapped for chlorine one after another. Thirteen to fifteen of them in practice; the last one or two are stubborn. The color slides from blue to a deep blue-green, and that is PG7. Use bromine for part of the job instead and you get PG36, which comes out of the drum a yellower green. Commercial production of the green started in 1938.

Think about what a green was before that. For most of history a serious green meant copper. Malachite, a copper mineral dug from the ground and crushed to powder, went into paint from antiquity until about 1800; it was only moderately lightfast and it hated acids. Verdigris, the crust on old copper, was the other option. The 1800s added emerald green, a copper and arsenic compound that poisoned the people who made it, and viridian, which was safe but expensive. Phthalo green is a copper compound too. It is just the first one that did everything right.

What you buy today is PG7 or PG36, and it is sold under a ridiculous number of names. Some labels say phthalo green. Some use a house name the company invented. Some borrow an older name: viridian hue is phthalo, and most tubes labeled emerald green are phthalo with a yellow stirred in, because the real thing was arsenic (I told that story on the emerald green page). Read the small print for the pigment code.

Here is my problem with it. Phthalo green is so strong that a pinhead colors a whole wash, and it is a green that does not exist outdoors. Nothing that grows is that blue or that clean. Beginners love it for about a week, paint a lawn with it, and wonder why the lawn looks like a sports drink. Then they learn the trick, which is the rest of this page.

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 phthalo green

You cannot mix phthalo green from other colors, because nothing else on the palette is that clean. The pairing that matters is phthalo green with a red. A cool red like quinacridone rose (PV19), about half and half, gives a neutral gray that goes to a deep black with less water, and it is the best black I know. An earth red does something gentler: a speck of burnt sienna turns the neon into a believable leaf.

Rule of thumb: put the other color on the palette first and add phthalo to it, never the other way round. A brush that has touched phthalo will color the next three things you rinse it in.

Using phthalo green in watercolor

Use it where nature actually goes that dark and that cool: the depths of a lake, the shadow side of a pine, the glass of a bottle, a tropical sea in the last light. A tiny amount in a shadow mix cools it without turning it gray.

Glazing is where it earns its keep. Because it is transparent and does not granulate, a thin phthalo wash over a dried yellow gives a glowing green you cannot get by mixing the two on the palette. Let the yellow dry completely and do not go back in.

It pairs with its opposites. Poppy Bloom or Venetian Red next to phthalo green makes both look more expensive, and a touch of either mixed in makes the green believable.

One thing I learned the hard way: phthalo travels. A pan of it colors the water jar, the brush, the rag and the pans next to it. Keep it at the far end of the palette, rinse twice before you touch another color, and never let your favorite brush for pale washes anywhere near it.

Phthalo green vs viridian (and emerald green)

Viridian (PG18) is the green phthalo is most often mistaken for, and the hue is close. Everything else is different. Viridian is hydrated chromium oxide; it granulates gently, lifts off the paper, and has modest tinting strength. Phthalo is a copper compound that dries flat, stains, and takes over any mix it is invited into. A tube labeled viridian hue is phthalo in a costume.

Emerald green is the other confusion. The historical pigment was a copper arsenic compound and is no longer made; the modern tube is a mixture built on phthalo, with yellow and sometimes white added. So emerald is phthalo with the brightness already stirred in.

If viridian is a cello, phthalo green is the same note through a stadium speaker, and emerald green is the pop remix.

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.