Editing Indigo dye (section)

===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>