Hey, everyone, wanna paint with crushed glass?

Lots of Renaissance painters, like Vermeer and Rembrandt, did just that, turning to smalt as a cheaper alternative to azurite and ultramarine for their underpaintings.

Smalt was made from ground cobalt glass, specifically. The particles had to be coarse because they would lose their color if ground too finely. In linseed oil, it would turn olive green. It faded and streaked.

I can’t even imagine painting with that. I cringe just thinking about what it would do to my brushes.

swatch of Cobalt blue, PB28

Louis-Jacques Thénard invents Cobalt blue

Finally, the French government called upon a chemist named Louis-Jacques Thénard to produce the blue of overseas. In 1802 (some sources say 1799) he began some experiments with cobalt phosphate and alumina and came up with what he called Thénard’s blue. He published his results in 1803-4, and in 1807 France began producing Cobalt blue. Also known as Egyptian blue and Delft blue, this color quickly became a favorite among painters.

Modern Cobalt blue is made from a calcined (roasted) mixture of oxides from cobalt and aluminum. It’s known as a neutral blue, meaning it has no green or red bias. With excellent chemical stability, lightfastness and better hiding power, it’s no wonder artists preferred it to smalt despite its high cost.

Working properties of Cobalt blue

You may notice that Cobalt blue dries faster than others. Cobalt is a desiccant — it absorbs water and keeps things around it dry. (Like those silica gel packets you’re not supposed to eat.) Finely ground smalt was often added to slow-drying substances for that reason.

This may be problematic for acrylic painters who like to paint wet-in-wet. Retarder gel, an additive you can use to slow drying time, may be a solution. but misting the paint with water works too if you don’t want to use a retarder.

Mixing blacks with Cobalt blue

Here I have tried to mix black using Cobalt blue and a handful of warm colors. From left to right: Cadmium orange (PO20), Perinone orange (PO43), Burnt Sienna (PBr7), and Burnt Umber (PBr7). Perinone orange has a surprising cool bias, as you can see by how purple the mixture looks.

mixing blacks and browns with Cobalt blue

Cerulean blue comes from cobalt

In 1805, Andreas Hopfner began tinkering with Cobalt blue, developing a process in which cobalt and tin oxides were calcined together. The result was Cerulean blue (PB35), a greenish blue that was just as stable and lightfast. It was not introduced until the 1860s, when Rowney and Company first offered it as an artists’ pigment.

Then came Cerulean blue chromium (PB36), made from oxides of cobalt and chromium. This is an intense greenish blue, more opaque with a higher tint strength than its cobalt tin oxide predecessor. All three cobalt siblings have the same stability and lightfastness, but they do look very different from each other.


Here are the same mixes I showed with Cobalt, but this time with Cerulean blue chromium.

mixing blacks with Cerulean blue chromium

See the difference in how it mixes?

These are just a few of the efforts I’ve made into trying to mix blacks and one thing that amazed me was how much darker some of the mixtures are than either of their parent colors. Some of these blues are dark, but they’re still not as dark as black.

This comparison between Cobalt and Cerulean blue chromium is great for demonstrating just how much chemistry impacts pigment processing. Tweak one little thing in the process, like roasting cobalt with chromium instead of tin, and the change in the finished product can be significant.

Give Cobalt blue your own road test

I highly recommend you try out your own mixes to see for yourself what it’s going to be like. You’ll get a feel for each color’s handling characteristics and how much of each parent color to use to get what you want. Also, keep in mind that the photos I use are all taken with my phone and I’m not the world’s best photographer.

Try these out for yourself and keep them in a sketchbook so you can refer back to them later. Love Cobalt blue? Leave me a comment and let me know how you like to use it.

Chiara Scuro

Chiara is an acrylic painter, content writer and painting consultant. She's out to chuck a Molotov cocktail at the elitist notion that it takes some innate talent to learn how to paint. Check out her blog, The Painted Pants Lady, and learn how to paint smarter.

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