HISTORICAL PIGMENT ARCHITECTURE
1901 BASF Synthesis & The Industrial Vat Dye Revolution
The discovery of Indanthrone Blue (Color Index PB60) in 1901 by Swiss-German chemist René Bohn at BASF in Ludwigshafen opened a new era in synthetic dye and pigment chemistry. While attempting to synthesize an anthraquinone-based indigo derivative, Bohn melted 2-aminoanthraquinone with potassium hydroxide, yielding a dark blue compound of unprecedented chemical resistance.
Marketed under the trade name Indanthren Blue RS (from 'indigo of anthracene'), PB60 was the founding member of the legendary Indanthren family of vat dyes—famed for remaining entirely unfaded even after prolonged boiling in alkaline bleach and continuous outdoor solar exposure.
Adapted into fine art oil, acrylic, and watercolor media during the 20th century, PB60 filled a major historical void: a permanent, lightfast replacement for fugitive plant Indigo and unstable Prussian blue mixtures. In the modern atelier, it is revered for its deep, atmospheric midnight-blue chroma.
CHEMICAL SPECIFICATION & OPTICAL DATA
Polycyclic Anthraquinone Physics, π-Stacking & Midnight Optics
Chemically formulated as C₂₈H₁₄N₂O₄ (indanthrone), PB60 consists of a large, planar polycyclic organic structure containing two coupled anthraquinone units linked by two secondary amine groups. In the solid crystal lattice, these flat molecules pack in dense π-π stacking arrangements with strong intermolecular hydrogen bonds.
Spectrophotometric transmission curves show that PB60 absorbs heavily across the entire yellow, orange, and red spectral regions (560–680nm), while transmitting a narrow, deep blue-violet band between 430nm and 470nm. In thin glazes, it exhibits a subtle warm-red undertone, giving it a distinctive midnight-indigo character.
Carrying a permanent ASTM Lightfastness Class I rating, PB60 is completely resistant to ultraviolet solar radiation, atmospheric ozone, and chemical pollutants. Its nanoscale organic particles produce smooth, non-granulating, deeply staining washes that penetrate paper fibers with velvet-like richness.
ART HISTORICAL HERITAGE
Art Historical Heritage: Nocturnes, Marine Vistas & Tonalist Depths
Since its adoption by fine art color makers, Indanthrone Blue has been celebrated as the definitive pigment for painting twilight, moonlit nocturnes, and stormy maritime atmospheres. Tonalist and modern landscape painters embraced PB60 to render deep, shadow-filled evening skies without the harsh cyan bias of phthalocyanine blues.
In 20th-century watercolor painting, PB60 replaced historic plant Indigo. Artists valued its ability to create velvety darks in a single transparent wash, providing dramatic tonal contrast without requiring heavy, muddy multiple layers.
Modern botanical artists and portraitists rely on PB60 to mix subtle, lifelike shadow tones, delicate denim blues, and rich chromatic blacks when paired with warm earth pigments like Burnt Sienna.
STUDIO CRAFT & MATERIAL PRACTICE
Atelier Practice: Deep Midnight Washes & Paraffin Boundaries
In Coloartura's atelier in Ermelo, Indanthrone Blue (PB60) is formulated into concentrated watercolor washes and laid across 300gsm cold-pressed cotton paper. Its non-granulating, staining organic chemistry allows it to form flat, velvety midnight fields that sink deep into the cotton fibers.
When flooded against hydrophobic wax crayon resist lines, PB60 creates clean, high-contrast borders where the deep indigo pigment meets the white wax barriers. This dynamic balance between dark atmospheric pooling and rigid geometric containment defines Coloartura's 1:1 square museum compositions.
