Jarosite

Jarosite is a natural yellow mineral pigment powder made from jarosite, a potassium iron sulfate that forms where iron sulfides such as pyrite weather. It gives a light, slightly lemony ochre-yellow and is used in oil, egg tempera, watercolor, gouache and acrylic as a historic natural yellow.

Key facts

• Color: light ochre-yellow with a lemon note; natural batches vary slightly
• Pigment type: Natural mineral pigment
• Composition: jarosite, potassium iron sulfate hydroxide, KFe₃(SO₄)₂(OH)₆
• Opacity: semi-opaque
• Lightfastness: 8 of 8 on the Blue Wool Scale (supplier data)
• Chemical resistance: acid 5/5 (supplier data, 5 = best); not alkali-resistant (lime and other alkalis turn it into brownish iron hydroxide)
• Suitable for: oil, acrylic, egg tempera, watercolor, gouache
• Not recommended for: fresco (lime), lime and cement plaster (not alkali-resistant)
• Packaging: 50 g jar · 1 kg bag

How to use

Mull the powder into the binder with a palette knife or muller: linseed oil, egg yolk for tempera, gum arabic for watercolor and gouache, or acrylic medium. Jarosite is a soft mineral (2.5–3.5 on the Mohs scale), so it disperses easily without long grinding. Use it pure for light yellows or with white and other earths for warm light tones. Keep it away from lime, cement and other alkaline materials, and store the powder dry: in persistent damp, jarosite slowly turns brownish as it alters to iron hydroxide.

About jarosite

Jarosite was first described in 1852 by the German mineralogist August Breithaupt and named after the Barranco del Jaroso in the Sierra Almagrera, southern Spain. It forms where iron sulfides oxidize under acidic conditions. It has been identified as a yellow pigment in ancient Egyptian painting, for example in Middle Kingdom paint at Karnak. In 2004 NASA's Opportunity rover found jarosite on Mars, a sign that water was once present there.

Safety

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 jarosite the same as yellow ochre?

No. Yellow ochre is colored by hydrated iron oxide-hydroxide, FeO(OH), mixed with clay and sand; jarosite is a potassium iron sulfate. It gives a lighter, more lemony yellow and, unlike ochre, it does not tolerate lime.

Can I use jarosite in fresco?

No. Jarosite is not alkali-resistant: in wet lime it slowly breaks down into brownish iron hydroxide. For yellows in fresco, choose a natural yellow ochre.

Is jarosite lightfast?

Yes. It is rated 8 of 8 on the Blue Wool Scale (supplier data). Its weak points are alkali and persistent damp, not light.