Editing Indigo dye (section)

==Dyeing technology==
[[File:Leucoindigo.svg|thumb|class=skin-invert-image|Indigo white (leuco-indigo)]]
[[File:IndigoDyedYarn.JPG|thumb|Yarn dyed with indigo dye]]

===Indigo white===
Indigo is a challenging dye because it is not [[soluble]] in water. To be dissolved, it must undergo a chemical change ([[redox|reduction]]). Reduction converts indigo into "white indigo" ([[leuco dye|leuco]]-indigo). When a submerged fabric is removed from the dyebath, the white indigo quickly combines with [[oxygen]] in the air and reverts to the insoluble, intensely colored indigo. When it first became widely available in Europe in the 16th century, European dyers and printers struggled with indigo because of this distinctive property. It also required several chemical manipulations, some involving toxic materials, and presented many opportunities to injure workers. In the 19th century, English poet [[William Wordsworth]] referred to the plight of indigo dye workers of his hometown of Cockermouth in his autobiographical poem ''The Prelude''. Speaking of their dire working conditions and the empathy that he felt for them, he wrote:
{{Poem quote|
Doubtless, I should have then made common cause
With some who perished; haply perished too
A poor mistaken and bewildered offering
Unknown to those bare souls of miller blue}}

A pre-industrial process for production of indigo white, used in Europe, was to dissolve the indigo in stale urine, which contains ammonia. A more convenient reductive agent is zinc. Another pre-industrial method, used in Japan, was to dissolve the indigo in a heated vat in which a culture of [[thermophile|thermophilic]], [[anaerobic digestion|anaerobic]] bacteria was maintained. Some species of such bacteria generate [[hydrogen]] as a metabolic product, which convert insoluble indigo into soluble indigo white. Cloth dyed in such a vat was decorated with the techniques of ''[[shibori]]'' ([[tie-dye]]), ''[[kasuri]]'', ''[[katazome]]'', and ''[[tsutsugaki]]''. Examples of clothing and banners dyed with these techniques can be seen in the works of [[Hokusai]] and other artists.

===Direct printing===
Two different methods for the direct application of indigo were developed in England in the 18th century and remained in use well into the 19th century. The first method, known as 'pencil blue' because it was most often applied by pencil or brush, could be used to achieve dark hues. [[Arsenic trisulfide]] and a thickener were added to the indigo vat. The arsenic compound delayed the oxidation of the indigo long enough to paint the dye onto fabrics.{{Citation needed|date=May 2024}}

[[File:IndigoDyePotOnStove.JPG|thumb|Pot of freeze-dried indigo dye]]
The second method was known as 'China blue' due to its resemblance to Chinese blue-and-white porcelain. Instead of using an indigo solution directly, the process involved printing the insoluble form of indigo onto the fabric. The indigo was then reduced in a sequence of baths of [[iron(II) sulfate]], with air oxidation between each immersion. The China blue process could make sharp designs, but it could not produce the dark hues possible with the pencil blue method.

Around 1880, the 'glucose process' was developed. It finally enabled the direct printing of indigo onto fabric and could produce inexpensive dark indigo prints unattainable with the China blue method.

Since 2004, [[freeze drying|freeze-dried]] indigo, or instant indigo, has become available. In this method, the indigo has already been reduced, and then freeze-dried into a crystal. The crystals are added to warm water to create the dye pot. As in a standard indigo dye pot, care has to be taken to avoid mixing in oxygen. Freeze-dried indigo is simple to use, and the crystals can be stored indefinitely as long as they are not exposed to moisture.<ref>{{cite web |title=Directions for Instant Indigo |author=Judith McKenzie McCuin |url=http://www.paradisefibers.com/instantindigo.htm |access-date=2008-05-06 |url-status=dead |archive-url=https://web.archive.org/web/20041116195957/http://paradisefibers.com/instantindigo.htm |archive-date=2004-11-16}}</ref>