This section is from the "A Complete Dictionary of Dry Goods" book, by George S. Cole. Also available from Amazon: A complete dictionary of dry goods and history of silk, cotton, linen, wool and other fibrous substances,: Including a full explanation of the modern processes ... together with various useful tables.
Indigo Blue Calico. A fine blue color, which is produced upon cotton by placing in an immense vat 2,000 gallons of water, 20 pounds of ground indigo, 30 pounds of iron filings, 30 pounds of powdered zinc, and 30 pounds of lime. A length of calico is then dipped into the vat for 15 minutes, taken out and exposed to the air for 5 minutes. The piece of calico which is white when it comes out of the vat gradually becomes green and then blue, owing to the oxygen of the air oxidizing the white-indigo and transforming it into blue, which is insoluble in water and "fixed" on the calico. The number of dips the cloth receives is owing to the particular shade of blue the printer requires.
Patterns on indigo blue prints are produced by discharging the blue with a bleaching liquor. The pattern to be discharged is cut out on two metal plates; several thicknesses of cloth are placed between these plates and an immense pressure applied, the liquor being run in on the uppermost plate, which is grooved to receive it. The pressure on the cloth makes clean work by preventing the spreading of the liquor, and the liquor destroys the blue color in all those parts where the patterns in the plates allow it to circulate, thus producing the white figure. If any other color than white is desired, it only remains to print it on over the white pattern that has been discharged from the blue ground. The chemical liquor used to discharge the blue color (in order to produce the white pattern) is sometimes very injurious to the cloth, the discharged portions rotting and wearing out much sooner than the solid blue ground. [See Indigo, Bandanna, Turkey Red, Calico]