Indigo Dye: Water Won't Dissolve It, So the Vat Changes It

Cloth lifted from a Yunnan indigo vat looks green at first, then turns blue in the air [1]. That happens because indigo will not dissolve in water, so it cannot soak into a fiber as blue [1]. The vat changes it first, into a pale leuco form that dissolves there [2]. In the air, oxygen turns that form back into blue indigo, now inside the fiber [2].

A 1995 US patent calls the dissolved leuco form yellow [2]. In the two traditional fermentation vats that Li and colleagues studied in Yunnan and Hunan, the liquid turned yellow-green once it was ready to dye [1][3].

Indigo won't dissolve in water: what that means

The patent, US 5,378,246, calls indigotin "the true coloring matter of indigo" [2]. It describes a dark blue powder or bronze-blue crystals with the formula C16H10N2O2, and the ChEBI chemical database gives indigo the same formula [2][4]. Li and colleagues put the problem plainly. Indigo is a water-insoluble pigment, so it must be reduced to leuco-indigo before it can get into the fibers [1].

Wider than a phone screen: swipe sideways along the diagram, or open it full size.

The change is a reduction: in the patent's words, indigotin "combines with two atoms of hydrogen" and becomes "a colorless body, known as indigo-white or the leuco form" [2]. That form "is insoluble in water, but dissolves in alkali, with a yellow color" [2]. In commercial vats an alkali and a reducing chemical do this work [2]. This is a historical process, described for history only — these materials are toxic or caustic.

Oxygen undoes the change "with great ease," and water is given off as indigotin forms again [2]. Both Li papers describe leuco-indigo as dissolving in water [3][1]. Li and colleagues place that dissolving in the warm liquid of the vat, and the patent says the leuco form will not dissolve in plain water [1][2].

• indigo-states.csv (CSV, indigo-states.csv) · file notes (JSON): where the values come from, version and license Data: Li, Cunningham, Fan and Wang, Identity blues: the ethnobotany of the indigo dyeing by Landian Yao (Iu Mien) in Yunnan, Southwest China, Journal of Ethnobiology and Ethnomedicine 15: 13, 2019, CC BY 4.0, https://doi.org/10.1186/s13002-019-0289-0; Li, Shi, Huang, Tong, Wu and Wang, Fermentation Blues: Analyzing the Microbiota of Traditional Indigo Vat Dyeing in Hunan, China, Microbiology Spectrum 10(4): e01663-22, 2022, CC BY 4.0, https://doi.org/10.1128/spectrum.01663-22; US Patent 5,378,246, Indigo dye process, Sally Gurley, filed 1993, granted 1995, https://patents.google.com/patent/US5378246A/en; ChEBI (Chemical Entities of Biological Interest), EMBL-EBI, record CHEBI:749117, CC BY 4.0, https://www.ebi.ac.uk/chebi/: each described state of the dye, one description per row

Inside the vat

A fermentation vat leaves the reducing to microbes [3]. Modern dyeing uses reducing chemicals instead, which Li and colleagues say can be harmful [3]. The Dong vat they followed in Gaobu Village, Hunan, held indigo paste, rice wine, a mix of plants and a lye of rice-straw ash [3]. This is a historical process, described for history only — these materials are toxic or caustic.

That vat fermented for about 27 days before its liquid turned yellow-green and could dye cloth [3]; a Landian Yao vat in Guangming Village, Yunnan, took about a week [1]. The color matches the dissolved dye, which the patent calls yellow and Li and colleagues call "yellowish-green in reality" [2][1].

Japan's sukumo vat starts from sukumo, which Li and colleagues list as one form of the dye [3]. The abstract of a 2022 study by Nakagawa and colleagues describes sukumo as composted indigo leaves, and says microbes reduce its insoluble indigo to leuco-indigo [5].

Where the vat's indigo came from

Before any dyeing, the indigo had to be made from leaves. The Encyclopédie of Diderot and d'Alembert shows such a works, an indigoterie, in its first volume of plates, published in Paris in 1762 [6]. Its 1765 article on indigo says the plant was very common in the Antilles and Saint-Domingue [7].

The plate's key names three stepped tanks [6][7]. The cut plant steeped in the trempoire, the liquid was beaten in the batterie, and the settled blue was left to rest in the reposoir [6][7]. From their Yunnan fieldwork, Li and colleagues explain such beating: it lets in the oxygen that turns indoxyl, released from the soaking leaves, into indigo [1].

The key calls the man carrying the plant, and the men beating the tank with buckets on long poles, "negres" [6]. The house behind them belongs to the "maître de l'habitation," the plantation's master [6]. Another 1765 article in the Encyclopédie, by Le Romain, is about Black people "considérés comme esclaves" (considered as slaves) in the American colonies [8]. It says land growing sugar cane, cotton, indigo and other crops needed "un nombre d'esclaves proportionné à leur étendue," slaves in proportion to its size [8].

Out into the air

The patent calls the next step in commercial vat dyeing "skying" [2]. After each dip the yarn goes into the open air, where "the leuco dye is oxidized to the insoluble indigotin form in the fibers" [2]. In the Yunnan village, Li and colleagues report, cloth looked green when first taken out and slowly turned blue [1].

Wider than a phone screen: swipe sideways along the diagram, or open it full size.

1. In the vat: the liquid is yellow-green in the fermentation vats studied, and the fiber takes up the dissolved leuco dye [1][3][2].
2. Lifted out: in the Yunnan vat, the cloth looks green at first [1].
3. In the air: oxygen turns the dye back into insoluble indigo inside the fibers [2].
4. Back into the vat: the patent repeats its process "to increase the dye uptake on the fibers and produce a deeper shade of blue" [2]. Landian Yao dyers used to dip cloth at least ten times [1].

Is woad the same dye as indigo?

The pigment is the same; the plant differs. ColourLex, a reference on pigments, lists woad among indigo's other names [9]. It says indigo can be made from many plants, among them woad (Isatis tinctoria) in Europe, and that all of them hold the same starting compound, indican [9].

The Landian Yao used three plants, chiefly Strobilanthes cusia with two Indigofera species [1]. The Hunan vat's paste came from Strobilanthes cusia and Polygonum tinctorium [3]. Li and colleagues also list the medieval woad vat of Europe and the sweet indigo vat of India among traditional fermentation recipes [3]. In commercial dyeing, plants had given way by the 1990s: "synthetic indigo has essentially replaced natural indigo," the 1995 patent says [2].

What survives: the fade record

A dyed cloth rarely says how it was dyed. The Met catalogues this ceremonial cloth as gold and silver on indigo-dyed cotton [10]. The catalogue data read here do not say whether a dye analysis found the indigo, and in the photograph the ground looks almost black [10].

This site calls the checkable part the fade record: an object's documented color history. It holds only recorded storage, tests and treatment, and marks everything else not verified. For this cloth, the records read here hold no dye test, so the indigo rests on the catalogue's word.

The Glen Affric tartan shows what a test adds. V&A Dundee reported in 2023 that dye analysis confirmed indigo or woad in its green; the other colors stayed inconclusive [11]. The tartan entry reads that record in full, and a shade alone cannot settle arsenic green either.

Cite this page

Sources

1. 1Shan Li, Anthony B. Cunningham, Ruyan Fan and Yuhua Wang. "Identity blues: the ethnobotany of the indigo dyeing by Landian Yao (Iu Mien) in Yunnan, Southwest China." Journal of Ethnobiology and Ethnomedicine 15 (2019): 13. https://doi.org/10.1186/s13002-019-0289-0 (full text) License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/)
2. 2Sally Gurley. "Indigo dye process." US Patent 5,378,246, filed 10 May 1993, granted 3 January 1995. https://patents.google.com/patent/US5378246A/en (full text)
3. 3Shan Li, Yuru Shi, Hui Huang, Yan Tong, Shaohua Wu and Yuhua Wang. "Fermentation Blues: Analyzing the Microbiota of Traditional Indigo Vat Dyeing in Hunan, China." Microbiology Spectrum 10, no. 4 (2022): e01663-22. https://doi.org/10.1128/spectrum.01663-22 (full text) License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/)
4. 4ChEBI (Chemical Entities of Biological Interest), EMBL-EBI. "Indigo" (CHEBI:749117). https://www.ebi.ac.uk/chebi/CHEBI:749117 (full text of the database record) License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/)
5. 5Kasumi Nakagawa, Michiki Takeuchi, Manami Tada, et al. "Isolation and characterization of indigo-reducing bacteria and analysis of microbiota from indigo fermentation suspensions." Bioscience, Biotechnology, and Biochemistry 86, no. 2 (2022): 273–281. https://doi.org/10.1093/bbb/zbab209 (abstract only)
6. 6Denis Diderot and Jean le Rond d'Alembert, eds. Encyclopédie, Recueil de planches, vol. 1. Paris, 1762. "Agriculture et économie rustique: Indigoterie et Manioc," explanation of the plate. ARTFL Encyclopédie, University of Chicago. https://artflsrv04.uchicago.edu/philologic5/encyclopedie0226/navigate/18/22 (plate explanation in full; short quotation only)
7. 7"Indigo, autrement appellé Inde." In Encyclopédie, ou Dictionnaire raisonné des sciences, des arts et des métiers, vol. 8, pp. 679–681. Paris, 1765. ARTFL Encyclopédie, University of Chicago. https://artflsrv04.uchicago.edu/philologic5/encyclopedie0226/navigate/8/3019 (full text; paraphrased)
8. 8Le Romain. "Negres, considérés comme esclaves dans les colonies de l'Amérique." In Encyclopédie, ou Dictionnaire raisonné des sciences, des arts et des métiers, vol. 11, pp. 80–83. Paris, 1765. ARTFL Encyclopédie, University of Chicago. https://artflsrv04.uchicago.edu/philologic5/encyclopedie0226/navigate/11/441 (opening and the section on plantation work read; short quotation only)
9. 9ColourLex. "Indigo." https://colourlex.com/project/indigo/ (full text; paraphrased, not quoted)
10. 10The Metropolitan Museum of Art. "Ceremonial cloth," India, Deccan, Aurangabad (?), late 18th century, painted and printed gold and silver leaf and opaque watercolor on indigo-dyed cotton, 2003.177 (Purchase, Friends of Asian Art Gifts, 2003). Catalogue data row on the Wikimedia Commons file page. https://commons.wikimedia.org/wiki/File:MET_DP156678.jpg (collection record (catalogue data, CC0))
11. 11V&A Dundee. "Scotland's oldest tartan discovered by Scottish Tartans Authority." 26 March 2023. https://www.vam.ac.uk/dundee/info/scotland-s-oldest-tartan-discovered-by-scottish-tartans-authority (full text; paraphrased)