A Longer Note About Indigo

by Sarah Lake Upton in


My Short Note About Indigo leaves a lot of questions unanswered: what is “crocking”?  how will it affect the finished piece? is there anything one can do to reduce it? why does it happen in the first place? and etc.  This will hopefully answer some of those questions, but I suspect there will be a follow up piece to this piece at some point because I can be a wicked over-explainer. Also because indigo is honestly magic. 

To understand crocking it helps to understand the basics of dyeing with natural indigo. 

To use most natural dyes one basically makes a broth using plants or plant extracts.  The process of dyeing with natural dyes, after scouring and mordanting the yarn, is not actually that much more complicated than the process of making good soup, though it is more time consuming.  (I dye in stock pots and canning pots on my kitchen stove, so the comparisons to cooking may come more easily than they otherwise should).  There are certain variables that need to be kept track of (pH, heat, time, pre-mordants or after-baths) but they are no more complicated that making sure that the soup is seasoned correctly and has enough rosemary.  The yarn will dry a few shades lighter than whatever color it achieves in the dyepot, but like soup making, one can easily “taste” it along the way, and with experience be able to modify it to achieve a desired result.  

Natural indigo is not at all like that. 

Natural indigo is a vat dye, an un-helpful descriptor (can’t vats be used in most dyeing?) that to a dyer means that a complicated chemical reaction must first take place in order to achieve the desired result.  

The colorant in natural indigo, indigotin,  is not water soluble and will not stick to fiber.  To make natural indigo work as a dye, one must first make it change into a slightly different chemical by making the dye bath very alkali and then removing oxygen from the bath.  Doing this will change the dye bath into a form generally referred to as “indigo white” (even through the bath itself is actually a sickly yellow/green when properly reduced).  Once the bath is the proper pH and weird yellow/green color, the fiber may be added and allowed to sit for a short period of time.  When the fiber is removed from the dye and exposed to air it will be that same weird yellow as the dye bath but then as the dye reacts with oxygen and transforms back to indigotin (now stuck to fibers) it magically becomes blue again.  

 Photo credit Deb Cunningham - this is the same skein of indigo over the course of about ten seconds as the reduced indigo reacts to the presence of oxygen and turns blue again. 

Photo credit Deb Cunningham - this is the same skein of indigo over the course of about ten seconds as the reduced indigo reacts to the presence of oxygen and turns blue again. 

Depth of color is developed through multiple dips in the indigo vat. 

Sometimes the indigo doesn’t fully adhere to the fiber.  Because indigo isn’t water soluble, washing and rinsing will only convince the most unadhered indigo particles to wash out of the fiber, which leave the particles that are hanging on but just a little.  These are the particles that can break free and stain you hands and needles when the yarn is subjected to the mechanical action of knitting.  But, because these are particles of indigo, not indigo white, they still don’t really want to stick to anything.  I tend to think of indigo particles in this form as being more like chalk dust than dye.  It will stick to your hands and needles and if you are doing color work it may temporarily discolor the other yarns you are working with, but because you haven’t gone through the process of transforming indigotin into indigo white, you are basically dealing with very fine colorful dust.  Once the particles have broken free enough to discolor things, they are usually easily washed away.  The only exception to this is the fine pores of bamboo needles, which tend to trap indigo particles.  

Some of you may have come across photos of natural indigo dyers with very blue hands

 Indigo dyes fingernails especially well.

Indigo dyes fingernails especially well.

For instance, this is my hand after a round of indigo dyeing before I found a pair of gloves that I liked.  In this case the indigo in the dyepot is treating the proteins in my skin the same way it treats the proteins in wool, and sticking accordingly.  As noted, this is very different from crocking.  When I knit with crocking yarn I will often get a blue ring around the back of my middle finger where I tension the yarn, and the palms of may hands will become a little blue from where they come in contact with my knitting. 

Some batches of indigo dyed yarn do not crock at all.  Some batches of yarn crock quite a bit.  Sometimes one or two skeins in a dye lot will crock but the others won't.  I do everything I can to reduce crocking, but there is no way for me to eliminate it completely, or predict with batches will and won’t crock.   Generally the mechanical action of knitting knocks the last few clinging particles free, and then with a final soak and rinse during blocking it will be done.