PB 66 – synthetic indigo

First indigo synthesis dates back to 1882 but it was commercially produced from 1901

Manufacturer/ product code:  Kremer / 36009
Chemical Description: synthetic indigo
Color: blue
Color Index: PB 66
Date:  from 1882

PB 66, known as synthetic indigo, is an organic blue pigment chemically identical to the natural dye indigo, indigoid (C₁₆H₁₀N₂O₂), but produced by industrial synthesis rather than extraction from plants of the Indigofera genus. Synthetic indigo was first manufactured on a commercial scale in the late 19th century, following the work of Adolf von Baeyer, and it rapidly replaced natural indigo due to its lower cost, consistent quality, and large-scale availability.

As a pigment, PB 66 is obtained by rendering indigo insoluble and finely divided, allowing its use in paints, printing inks, and colored plastics. It is characterized by a deep, slightly reddish blue that becomes more intense in thicker applications. Compared to many synthetic organic blues, indigo has relatively low tinting strength and moderate opacity, giving it a subdued, velvety appearance rather than a brilliant chroma.

From a conservation standpoint, PB 66 shows moderate lightfastness. While more stable than many early synthetic dyes, indigo is susceptible to photochemical degradation, particularly under strong light and in the presence of oxygen, leading to fading and a shift toward gray or greenish tones. It is generally stable in neutral conditions but can be affected by strong oxidizing or reducing environments. Being an organic pigment, PB 66 cannot be detected by elemental techniques such as XRF and requires molecular methods for identification, including Raman spectroscopy, FTIR, or chromatographic analysis. The presence of synthetic indigo in an artwork is significant for dating, indicating a late 19th- or 20th-century origin or later restoration materials.