This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Cobalt Blue is a fascinating colour with a much longer history than many pigments in use today. It's also the only goblin hiding in the Periodic Table.
Cobalt, is symbol Co on the periodic table with an atomic weight of 27. While in it's natural, raw state it's a somewhat burnished silver colour, it is famously known for producing vibrant blues in pigments. The pigments are made by heating aluminum silicates with cobalt and heating to 1200°C. Cobalt Blue has the chemical formula CoAl2O4 (It's also possible to create Cobalt Green: 4CoO.3Al2O3 ). Developed for porcelain in China and as a paint pigment in Europe, it's been in use as a colour since at least the 1700's.
Back in my days managing an art supply store, Cobalt Blue could be a hard sell: there are imitation versions of the colour on the market and they are substantially cheaper, and made up of multiple pigments to imitate the colour. Imitation colours are typically known as "hues", as in Cobalt Blue = real thing, Cobalt Blue Hue = imitation. In addition, most companies have health warnings about Cobalt Blue on the packaging. It can be toxic in huge amounts or damaging to the lungs if applied as in airbrushing.
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In the fine arts, there's a popular trend amongst seasoned painters of using as many monopigment paints as possible. Essentially the idea is this: you know when you were a kid and mixed tons of colours together and you got that nondescript grey-brown colour? If you want to mix purple from your primaries and you take imitation paints Cobalt Blue Hue (usually made of cheaper Phthalo Blue and Zinc White) + Cadmium Red Hue (usually a couple of Napthol Reds) then you are actually mixing 4 pigments together. The theory goes, this will lead to a duller muddier purple than if you had just used real Cobalt Blue and real Cadmium Red. It's a fine rule of thumb, but I am not sure how accurate it actually is or whether anyone has tested it scientifically: different pigments have different tinting strengths based in part on their dispersal in the vehicle (oil, gum arabic or acrylic polymer) so it seems likely to me that a modern imitation paint could potentially be more vibrant than a traditional single pigment one.
But now on to the goblins.
Cobalt ore is notoriously tricky to extract from the ground, and could poison mines in the form of cobaltite (CoAsS), which contains arsenic and sulfur. For this reason in German it was referred to as kobold ore, or "goblin ore". People even prayed for protection from cave-ins caused by kobolds. Kobolds went on to become a diminutive, reptilian race (Lawful Evil) in Dungeons & Dragons, yet are seldom depicted as blue.
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All fine art paints are pigment particles bound in a sticky medium: vegetable oil, gum arabic or acrylic polymer.
Find more of these posts by clicking Pinch of Pigment in the tags below. This is the 2nd in the series.
