Cobalt indigo

Cobalt indigo is a synthetic inorganic pigment powder, cobalt aluminate (CoAl₂O₄), in a very dark grade: a deep navy blue that looks almost black in the jar. It is lightfast and resistant to lime, alkalis and acids, and suits oil, acrylic, tempera, watercolor, gouache and fresco, especially for deep shadows and dark glazes.

Key facts

• Color: very deep navy blue, almost black in masstone
• Pigment type: Synthetic inorganic pigment (cobalt aluminate spinel)
• Colour Index: PB28 (C.I. 77346)
• Composition: cobalt(II) aluminate, CoAl₂O₄
• Origin: synthetic
• 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: 25 g jar · 1 kg bag

How to use

Mull it into linseed oil, egg yolk, gum arabic or acrylic medium with a palette knife or muller, grinding gently, since over-grinding lowers the saturation of cobalt blues. Use it pure for deep shadows and dark glazes, or mix it with white for cool blues. In thick oil layers kept in dim light, yellowing linseed oil can give it a greenish cast. It is lime-resistant and also listed for fresco and silicate binders. Store dry in a closed jar.

About cobalt indigo

Cobalt aluminate blue was developed by Louis Jacques Thénard in 1802 and produced commercially in France from 1807. It is made by calcining cobalt and aluminum oxides at high temperature into a spinel crystal, which makes it one of the most heat- and chemically stable blues; our supplier gives heat resistance up to 1100 °C. The name "indigo" describes only the deep color: the pigment contains no indigo dye.

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

Does Cobalt indigo contain indigo dye?

No. The name describes the color only. It is cobalt aluminate (PB28), an inorganic pigment.

How does it differ from Cobalt blue?

Both are cobalt aluminate (PB28). Cobalt indigo is our darkest grade, a navy blue that looks almost black in masstone; Cobalt blue is a brighter, mid-to-deep blue.