For customers of denim, color is the quality that distinguishes denim from other apparel fabrics, especially the unique appearance of Indigo. Indigo has been in use for perhaps 5000 years, originally as a natural dye and in the last 100 years primarily available in the synthetic form. No other commercially available dye can match the appearance of Indigo, especially when faded to light shades which produces a special appearance known as a “patina” which is similar to the glow of a blue sky. The color of Indigo is associated in lore with notions of intuition, religion, spirituality and loyalty.
Indigo-dyed denim presents unique problems in garment production because of variations in color, especially after washing. Fabric shipments to 
laundry customers can contain literally dozens of visibly different shades after washing which requires special efforts in cutting and sewing to avoid mixing garment panels that produce a garment with parts that do not match in color.
The most basic method of organizing shipments of Indigo-dyed denim is to cut small samples of fabric from each roll of fabric that will go into a shipment and sew them together in a “blanket”. This blanket is then washed so that there is some representation of the eventual color after fading 
during laundering. Then these samples are compared by the customer and “shaded”, sort usually from dark to light. Then, the fabric rolls with the most similar after-wash colors are combined on the cutting table in order to minimize variation in the garments produced. This technique is often favored by denim mills. This approach can be generally satisfactory, however has a number of deficiencies.
First, the fabric samples are taken usually from one end of the roll, when both ends should be sampled. Almost all denim companies fail to control Indigo dyeing so that multiple shades are produced in each dye lot and these changes can occur quickly, so that even within a short fabric roll of 100 meters the washed color can change. By sampling from both ends of a fabric roll it can be determined if the roll needs to be cut again in order to isolate different shades.
Secondly, these washed fabric samples are often graded visually, by human eye, which is unreliable because of its subjective nature. Consistent visual evaluation of color requires careful training and management due to problems with eye fatigue, differences in evaluator skill and carelessness. When a poor job has been done during visual shade sorting, it is impossible to supervise the reliability of the evaluation because it is based on subjective individual opinion. The visual approach is favoured by fabric suppliers and customers because of its simplicity. There is also a conceit factor that interferes with reliable color management because of a common assumption that color is obvious and those that have roles in judging color that they are natural masters of judging color which is rarely true.
Thirdly, the fabric supplier will rely on one wash method, usually a rinse wash to produce these blankets for sorting, while with other procedures such as stonewashing, bleaching or enzyme treatments, the samples will fade differently than in a rinse wash so that samples that are close in color in a rinse only will not match well when subjected to other laundry techniques.
Indigo is the most difficult and complicated of all dyes to apply to cotton. Until the 20th century, it was used primarily for wool and silk which are more suitable for Indigo dyeing. The Indigo dyeing process is inherently unstable as normally practiced. The dye must be reacted with a reducing agent, normally sodium hydrosulfite, which begins to lose strength as soon as it is mixed and the concentration available in the dye mix changes during the dyeing operation unless it is buffered, which results in the Indigo
shade changing during the long periods of dyeing. This in turn, results in from 8 to 15 washed Indigo shades per set, assuming a tolerance of 0.2 Delta E for a visually-noticeable color difference. The lack of dyeing control for Indigo is nearly universal, so that customers have accepted the variation as unavoidable, which is not actually the case since a few companies have mastered the problem and can produce around 95% single shade consistency
While the Indigo dye molecule does not itself change color during dyeing, the yarn does develop different color tones , usually with reddish or greenish effects occurring. These tone differences are almost always a result of variations in reducing agent levels during Indigo dyeing, with greenish 
effects appearing with higher levels of reducing agent and reddish with relatively lower concentrations of reducer. What actually occurs is that the Indigo is more finely dispersed with high levels of reducer which increases the number of points of color (higher chroma) resulting in a greener, brighter, lighter color while low levels of reduction, there are fewer points of color (lower chroma) resulting in a redder, darker and duller Indigo color with the same % of Indigo on weight of yarn. The greener tone is more difficult to wash to lighter shades and is more colorfast to rubbing, while the reddish tone fades quickly and will have greater rub-fastness problems.
The article will be continued in Part II shortly…………
This is a guest post by Harry Mercer. Mr. Mercer has 30 years experience in the denim business including 3 prominent U.S. denim companies.He is an expert colorist for measurement and color matching as well as textile testing. He can be contacted here
