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Why Are Jeans Blue but White Inside?

Cover of the Short: Why Are Jeans Blue but White Inside?▶ Watch the Short

The answer

Indigo won't dissolve in water, so dye works change it into a soluble form, dip the warp threads, and the air turns it blue again. It only coats the outside of each thread, so the core stays white (that's why jeans fade), and the crosswise weft threads are left white (that's the inside).

Sources (3)

  1. US 10,711,397 B1, "Yarn material with a white center" (RevoLaze, granted 14 Jul 2020): indigo "is not naturally water soluble"; "the soluble leuco-indigo is transformed to the insoluble oxidized blue indigo after exposure to oxygen"; ring dyeing makes "a layer of dyestuff on the outside perimeter of the yarn cross section", "a lack of full penetration of dye all the way to the core"; partial dye removal exposes "the white core"; "Denim weft yarns are generally white and the warp yarns are indigo dyed." https://patents.google.com/patent/US10711397
  2. US 8,215,138 B2, "Device and continuous dyeing process with indigo" (granted 10 Jul 2012): indigo must be "reduced in an alkaline solution (leuco)", then after squeezing "a passage in air to allow the leuco to oxidize, become blue and then insoluble"; dye sits in "the cortical layer of the yarn (ring dyeing)"; industrial indigo dyeing is of "warp chains for denim fabrics". https://patents.google.com/patent/US8215138B2/en
  3. US 4,283,198 A, "Inert atmosphere indigo dyeing" (granted 11 Aug 1981): "skying", "where the leuco indigo is oxidized to the insoluble form"; fabric or warp yarns pass through the bath. https://patents.google.com/patent/US4283198

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