PO 61 – isoindole orange

Poor lightfastness

Manufacturer/ product code:  Kremer / 23800
Chemical Description:  tetrachloro-isoindolinone
Color: orange
Color Index: PO 61
Date:  —

PO 61 – Isoindole Orange is a synthetic organic pigment belonging to the isoindolinone (often shortened to isoindole) pigment family, a class developed in the second half of the 20th century to overcome the lightfastness limitations of earlier azo oranges. PO 61 occupies a strong, saturated orange region, typically leaning toward a warm, slightly reddish orange with high chroma and excellent color clarity.

Chemically, isoindole pigments are characterized by a rigid heterocyclic structure that enhances molecular stability and resistance to photochemical degradation. In PO 61, this structure results in high tinting strength, good transparency to semi-transparency, and clean mixing behavior. Compared with traditional monoazo oranges such as Hansa Orange, PO 61 maintains its brilliance while offering significantly improved permanence, making it suitable for demanding applications.

PO 61 has been widely used in industrial coatings, automotive finishes, plastics, and high-quality printing inks, where both vivid color and durability are essential. In artists’ paints, it provides a modern alternative to cadmium and early organic oranges, delivering intense orange hues without the toxicity or opacity of heavy-metal pigments. Its transparency makes it especially effective for glazing and for producing luminous mixtures with yellows and reds.

In mixtures, PO 61 blends cleanly and predictably. Combined with yellows, it yields bright, energetic oranges, while mixtures with reds deepen toward vermilion-like tones. When paired with blues or greens, it produces neutralized browns and muted shadows without excessive greying, a property valued in both artistic painting and color matching.

From a conservation science perspective, PO 61 demonstrates very good to excellent lightfastness and chemical resistance. Its aging behavior is generally stable, with minimal discoloration under museum lighting conditions. Analytical identification is typically achieved through techniques such as Raman spectroscopy, FTIR, and reflectance spectroscopy, where its spectral features distinguish it from azo and quinacridone oranges.