Indigo
Description
A natural dark blue dye obtained from Indigofera tinctoria plants native to India, Java, Peru, and other tropical areas. The use of indigo was first mentioned in Indian manuscripts in the 4th century BCE. It was exported to Europe in Roman times but did not become plentiful until sea routes opened up in the 17th century. The natural material is collected as a precipitate from a fermented solution of the plant. The coloring component, indigotin, is extracted as a colorless glycoside, but turns blue with oxidation. Synthetic indigo was first produced in 1880 by Adolf von Baeyer. Made from anthranilic acid, the synthetic colorant is chemically identical to natural indigo and has almost entirely replaced the natural dyestuff. Indigo is a fine, intense powder which may be used directly as a Pigment in oil, tempera, or watercolor media. The exposed pigment can fade rapidly in strong sunlight. Indigo is still used to dye jeans, where its fading and uneven coloring have become favorable characteristics.
- See also [(Sukumo-Ai)] in Uemera Dye Archive
Synonyms and Related Terms
2,2'-biindolinyliden-3,3'-dion; Indigofera tinctoria; Natural Blue 1; CI 75780 (natural); Vat Blue 1; CI 73000 (synthetic); Pigment Blue 66; indigotin; indicum (Pliny); indigo (Esp. Fr., Dan., Ned., Port., Sven.); Indigo (Deut.); anil (Esp.); Indiko (Gr.); indaco (It.); aneel; anile; ai (Jap.); rams (Tibetan); neel (Assam); blue ynde; blue inde; anneil; India blue; intense blue; rock indigo; stone blue; indigo carmine; intense blue; indico; indicoe; indego; nil
.PNG)
Risks
- Discolored by reducing agents and bleaches.
- Fisher Scientific: SDS
Physical and Chemical Properties
- Soluble in nitrobenzene, phenol, chloroform, glacial acetic acid. Insoluble in water, ethanol, acetone, ethyl acetate, pinene.
- Absorption max = 599 (in xylene).
- ISO R105 Lightfastness Classification = 3-4
- Microscopically, indigo has fine, translucent dark blue, rounded particles that are weakly birefringent and appear red under Chelsea filter.
- Composition = C16H10N2O2 (CAS = 482-89-3, molecular weight= 262.26)
- Melting Point = 390-392 C
- Refractive Index = >1.662
Comparisons
Characteristics of Common Blue Pigments
Additional Images
Indigo piece (Indigofera tinctoria)
Indigo powder (Indigofera tinctoria)
Paper dyed with indigo
Paper dyed with indigo at 50x
Indigo at 100x (visible light left; UV light right)
Indigo blue at 500x
Colorant Extraction
The traditional method for extracting indigo dye from the plant leaves involves a complex process of fermentation, oxidation, and precipitation. The core principle behind natural indigo extraction is a chemical reaction that makes the dye, which is initially water-insoluble, into a water-soluble form. While the process varies across cultures, it requires harvesting mature indigo-bearing plants, such as Indigofera and woad, placing the cut leaves into a vat with hot water, then letting it set about 24 hours. This allows the solution to ferment, resulting in the water-soluble indican to be hydrolyzed into a sweet-smelling dye pre-cursor (indoxyl) that colors the water yellow. The plant remains are filtered out and the pH of the solution is raised to about 11 with lime or soda ash. This solution is then vigorously aerated by whipping in order to oxidize the indoxyl into the indigotin. The water-insoluble pigment forms a blue sludge on the bottom and sides of the container, which is then scraped off, formed into cakes and left to dry for later use.
Resources and Citations
- Irfan Laskar1 and Shahida Ansari1, 'Illustrated Manuscripts at Auniati Satra of Majuli Island, Assam', Heritage: Journal of Multidisciplinary Studies in Archaeology 9: 1116-1139, 2021-22.
- H.Schweppe, "Indigo and Woad", Artists Pigments, Volume 3, E. West FitzHugh (ed.), Oxford University Press: Oxford, 1997.
- Pigments Through the Ages: Indigo
- Analytical strategies for natural dyestuffs in cultural heritage objects - EU-ARTECH European research project - http://www.organic-colorants.org
- Nicholas Eastaugh, Valentine Walsh, Tracey Chaplin, Ruth Siddall, Pigment Compendium, Elsevier Butterworth-Heinemann, Oxford, 2004
- The Dictionary of , Grove's Dictionaries Inc., New York, 1996 Comment: 'Pigment'
- Palmy Weigle, Ancient Dyes for Modern Weavers, Watson-Guptill Publications, New York, 1974
- Matt Roberts, Don Etherington, Bookbinding and the Conservation of Books: a Dictionary of Descriptive Terminology, U.S. Government Printing Office, Washington DC, 1982
- Dictionary of Building Preservation, Ward Bucher, ed., John Wiley & Sons, Inc., New York City, 1996
- The Merck Index, Martha Windholz (ed.), Merck Research Labs, Rahway NJ, 10th edition, 1983 Comment: entry 4977
- A.Scharff, 'Synthetic dyestuffs for textiles and their fastness to washing', ICOM-CC Preprints Lyon, Getty Conservation Institute, Los Angeles, 1999
- Hermann Kuhn, Conservation and Restoration of Works of and Antiquities, Butterworths, London, 1986
- Thomas B. Brill, Light Its Interaction with and Antiquities, Plenum Press, New York City, 1980
- R., M.Curran, C.Bailie, 'Identification of Traditional Organic Colorants Employed in Prints and Determination of their Rates of Fading', Woodblock Prints, Allen Memorial Museum, Oberlin College, Oberlin, 1984
- F. Crace-Calvert, Dyeing and Calico Printing, Palmer & Howe, London, 1876
- Random House, Webster's Encyclopedic Unabridged Dictionary of the English Language, Grammercy Book, New York, 1997
- Encyclopedia Britannica, http://www.britannica.com Comment: "indigo" [Accessed May 6, 2004].
- Website: at www.colour-index.org
- Book and Paper Group, Paper Conservation Catalog, AIC, 1984, 1989
- The American Heritage Dictionary or Encarta, via 98, ., 1998
- AAT: http://www.getty.edu/research/tools/vocabulary/aat/, J. Paul Getty Trust, Los Angeles, 2000