Indigo is a deep blue dye, originally extracted from plants and now mostly produced synthetically, that has been used to color denim since the fabric's earliest days. In denim production, indigo is applied to the warp yarn, usually in repeated dips that build up color in layers, while the weft yarn is generally left undyed.
What makes indigo behave differently from many other dyes is that it does not fully penetrate the cotton fiber. Instead, it forms a layer mostly on the outside of each yarn, which is why denim can be sanded or brushed to reveal lighter cotton underneath, and why the fabric fades through ordinary wear and washing as that outer layer gradually wears away.
This surface dyeing is the reason raw denim develops fade patterns that trace exactly where the fabric rubs and creases during wear, at the knees, hips and pockets. See our raw denim guide for more on how fades develop, and the entry on rope dyeing for how indigo is applied to yarn at scale.
