My significant other’s place of business in Washington, D.C.—Land of The Stuffed Shirts—recently announced that its list of acceptable Casual Friday clothing had expanded to include blue jeans. I’ve never seen so much joy and excitement among government workers over clothing. And it’s all because of the beloved status that the iconic pants have achieved. The average American probably owns at least five pairs of them.
In this week’s issue of C&EN, I wrote a short story about the chemistry that goes into making blue jeans, noting that the cotton yarn used to make denim doesn’t go directly from white to blue during the dyeing process. Interestingly, it passes through a short phase of being yellow. This is because indigo, the dye responsible for blue jeans’ hue, is not soluble in water in its native form. To dye yarn, indigo must be reduced to leucoindigo, white in powder form and yellow when dissolved in a basic solution.
So, when cotton yarn dips into a vat of leucoindigo dye, it comes out yellow, turning blue as oxygen in the air converts the reduced compound into indigo. To see the chemistry in action, click on the video above.
One thing I didn’t have the space to mention in my feature story is that leucoindigo sticks to cotton yarn better when the fibers are pretreated with a strong base such as caustic soda. This is something that indigo dyers often do to yarn before passing it into dye vats. The base swells the cellulose fibers in the cotton and causes them to go from having an alpha structure to a more crystalline, beta structure that has a higher affinity for the dye. This process is called mercerization among textile makers.
Once the cotton yarn used to make denim has been dyed, washed, and neutralized, it is coated with starch to strengthen it for weaving. At this point, the dyed threads are called “warp.” When woven into denim, these blue threads run parallel in the fabric and are put together with white cotton “weft” thread, which runs perpendicular. Look down at your jeans, and you’ll see what I mean.
Georg Schnitzer, an indigo expert at the Singapore-based supplier Bluconnection, explained to me that when denim is woven, it is done in such a way that 75% of the material’s blue warp is on the outside of the pants and 75% of the white weft is on the inside. You can check this out as well, the next time you wash your favorite blues.
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