Indigo

Natural organic pigment

Pigments sorted by Colour | Type | First date of use | Elements | Pigment timelines

Composition and Properties of Indigo

It is an organic pigment of very dark blue colour with the following chemical formula:

indigo-chemical-formula

The pigment is not destroyed by dilute acids and alkalis and is the most lightfast of all the natural organic dyes. No incompatibilities have been reported for use in oil painting. It was employed in mixtures with yellow pigments such as orpiment or yellow ochre to produce a green color.

Collection of the historical dyes of the Technical University of Dresden, Germany

Pigment

Painted swatch

Names

Alternative names

Woad

Color Index

Natural Indigo: NB 1, CI 75780
Synthetic Indigo: PB 66, CI 73000

Word origin

From Latin indicum “indigo,” from Greek indikon “blue dye from India,” literally “Indian (substance),” neuter of indikos “Indian,” from India.

From Online Etymology Dictionary

Indigo

German

Indigo

French

Indaco

Italian

Anil, indigo

Spanish

Preparation of Indigo

Preparation of the Natural Dye

The starting material for the preparation of this pigment is a multitude of plants. The plant Indigofera anil is found mainly in India and woad (Isatis tinctoria) grows in Europe.

Woad plant, Isatis tinctoria

The relevant substance contained in all of these plants is indican (indoxyl glucoside).

indican

Indican

To separate the glucose rest (right part of the indican molecule above) from the indoxyl (on the left) the plant parts, e.g. leaves of woad are fermented for several weeks. The fermentation process brings about the cleavage of the glycoside bond and leads to a separation of the two parts of the indican molecule and to the isolation of indoxyl.

Indoxyl

Oxidation of indoxyl by atmospheric oxygen leads then to the final product.

Indoxyl_Oxidation

Oxidation of indoxyl

Video: 'Setting up an Organic Indigo Dye Vat' by The Wild Dyery

Preparation of Synthetic Indigo

The first synthesis of indigo by Bayer and Emmerling dates back to 1870. Today indigo can be prepared industrially or in the laboratory from o-nitrobenzaldehyde and acetone. The laboratory preparation is an easy two-step synthesis and is often used in teaching organic chemistry (1,2).

References

(1) Deuber, R. and Lipscher, J. Herstellung von Indigo, Färbervorgang, SwissEduc

(2) Royal Society of Chemistry Website (Learn Chemistry), Microscale synthesis of indigo dye

Video: 'Indigo Dye Synthesis and Fabric Staining Experiment' by Scott Milam

Image courtesy of Historische Farbstoffsammlung TU Dresden

History of Use

The pigment had been in use since antiquity but its use in oil painting ceased at the beginning of the eighteenth century. The following graph gives the frequency of its use in the paintings of the Schack Collection in the Bavarian State Art Collections in Munich (1).

indigo_history_of_use

References

(1) Kühn, H., Die Pigmente in den Gemälden der Schack-Galerie, in: Bayerische Staatsgemäldesammlungen (Ed.) Schack-Galerie (Gemäldekataloge Bd. II), München 1969.

(2) Clark, R. J. H.; Cooksey, C. J.; Daniels, M. A. M.; Withnall, R., Indigo, woad, and Tyrian Purple: important vat dyes from antiquity to the present. Endeavour 1993, 17 (4), 191-199. https://doi.org/10.1016/0160-9327(93)90062-8

(3) Mira S. de Roo, The Trade in Blue During the 17th Century: An Examination of Western European Pigment Trade in Azurite, Indigo, Lapis Lazuli, and Smalt During the 17th Century Through Works in the National Gallery, London, Master of Philosophy, University of Glasgow, 2004.

Examples of use

Antonis van Dyck, Lord John Stuart and his Brother, ca 1638

Lord Bernard’s blue satin dress: undermodelling is done in indigo in the shadows and indigo and lead white in the mid-tones. The surface was then scumbled (applying a very thin opaque layer of paint so that the underlying colour is still partly visible) with azurite and azurite mixed with indigo.

Johannes Vermeer, Christ in the House of Martha and Mary, 1654-5

The blue cloak of Christ is painted in indigo and the blue-green skirt of Mary contains indigo mixed with lead white and yellow ochre.

Video: 'The World’s Earliest Known Use of Indigo Dye Found in Peru' by wwwAAASorg

Timeline for Blue Pigments

Identification

Fiber Optics Reflectance Spectrum (FORS)

FORS-spectrum-Indigo

Spectra by A. Cosentino, Cultural Heritage Science Open Source (CHSOS)

Infrared Spectrum

IR-spectrum at Coblentz Society, Inc., “Evaluated Infrared Reference Spectra” in NIST Chemistry WebBook, NIST Standard Reference Database Number 69, Eds. P.J. Linstrom and W.G. Mallard, National Institute of Standards and Technology, Gaithersburg.