Cobalt blue

Cobalt blue is a synthetic inorganic pigment powder, cobalt aluminate (CoAl₂O₄), first made by the French chemist Louis Jacques Thénard in 1802. It gives a clean, bright, deep blue and is one of the most stable blues: lightfast, heat-resistant and unaffected by acids, alkalis and lime, for oil, acrylic, tempera, watercolor, gouache and fresco.

Key facts

• Color: clean, bright deep blue
• Pigment type: Synthetic inorganic pigment (cobalt aluminate spinel)
• Colour Index: PB28 (C.I. 77346)
• Composition: cobalt(II) aluminate, CoAl₂O₄
• Origin: synthetic
• Also known as: Thénard's blue
• Opacity: semi-transparent
• Lightfastness: 8 of 8 on the Blue Wool Scale (supplier data)
• Chemical resistance: alkali 5/5, lime 5/5, acid 5/5 (supplier data, 5 = best)
• Suitable for: oil, acrylic, egg tempera, watercolor, gouache, fresco (lime)
• Packaging: 50 g jar · 1 kg bag

How to use

Mull the powder with a muller or palette knife into linseed oil, egg yolk, gum arabic or acrylic medium. Grind gently: very fine grinding lowers its saturation. Its tinting strength is relatively low, so mixtures need a generous amount, and in oil it is semi-transparent and well suited to glazes. It is also listed for fresco and silicate binders. In thick oil layers kept in dim light, yellowing linseed oil can make it look greenish. Store dry in a closed jar.

About cobalt blue

Cobalt ores were used to color glass and ceramics long before the pigment existed, and impure cobalt blue decorated Chinese porcelain for centuries. The pure aluminate pigment was developed by Louis Jacques Thénard in 1802, and commercial production began in France in 1807. It remains one of the most popular blue pigments, valued for its stability to light and heat and its compatibility with other pigments.

Safety

Contains cobalt compounds: may cause allergic skin or respiratory reactions; harmful if inhaled. Handle dry pigment with care: avoid breathing the dust and contact with eyes. Wear a dust mask (FFP2/N95) and gloves when working with the powder, work in a well-ventilated area and wipe up spills with a damp cloth. Do not eat, drink or smoke while working; wash your hands afterwards. Keep away from children and pets. For artistic and technical use only — not for cosmetics, food or tattoos.

FAQ

Is cobalt blue lightfast and suitable for fresco?

Yes. Our supplier rates it 8 of 8 for lightfastness and 5 of 5 for alkali and lime resistance, and lists it for fresco and silicate binders.

How do Cobalt blue, Cobalt cornflower blue and Cobalt indigo differ?

All three are cobalt aluminate blues (PB28). Cobalt blue is the bright, deep standard; Cobalt cornflower blue is a lighter, softer shade modified with zinc and magnesium; Cobalt indigo is the darkest grade, almost navy.