What Makes Blue Jeans Blue

Down the runway they walk: pinched models with jeans in blue, green, black, gray, even vibrant oranges and reds like cardinal feathers. Down the centuries, jeans have gripped the world, favored by workers for their toughness, by the casual person for their stylishness, by the fashion designers for their versatile uses. Although, true to their name, originally dyed blue, today all colors and hues may be found.

And while many modern dyes are synthetic, the classic blue color is attributed to Indigo Dyes, a family of natural dyes, present in the form of Beta-Glucoside found in plants of the genre Indigophera, found across East Asia to South Africa, Guatemala
and Europe. The color was well known even among Egyptians, who wrapped their mummies in cloths dyed indigo. The color itselfindigo bluecarries significant religious and cultural connotations across the globe.

Now, with synthetic dyes being the preferred option, variants on the original chemical makeup have emerged. However, their molecular structure always features the Chromophore group, which is made of alternated double bonds of C, N, O, S, or halogens. These create a molecule which appears as a visible color to the human eye.

Pure blue indigo is made of a crystalline blue powder that has copper reflexes; it is insoluble in alkaline or in acid diluted solutions, but will become soluble in various solvents such as nitrobenzene, aniline, or phenol when they are brought to a boiling point. A weak acid and base character, given the conditions of the solution in question, will also make it soluble.

But it cannot be used in this state. To render the indigo useable, it must be reduced to a color Leuko-Indigo, or White Indigo, which is perfectly soluble in an alkaline solution. It takes quickly to cotton and wool fibers when they are soaked in it. Cloth, once extracted from its time soaking in the leuko-indigo, and once it dries, O2 in the air will oxidize. Slowly, the leuko-indigo will give way to the original blue-indigo, which remains stuck to the fibers of the cloth. This is the commonly-seen color of “blue” jeans. Silk is not a wise choice for this method of dyeing. The leuko-derivatives don’t take well to the specific type of fibers that make up silk.

However, as industry becomes quicker and more efficient as modern technology progresses, indigo may be purchased already in the reduced leuko-indigo form. It can be sold either as a solid or in a solution.

In the factories, among the vats of dye (the fumes of which it’s not advisable to breathe, less because of any danger inhaling them produces and more because dye smells positively foul), an interesting chemistry takes place.

Indigo reduction is performed with NaOH and NaHSO3, or Sodium Hydrogensulfite, ZN, and Glucose. However, less costly and more in tune to the ever-growing swiftness of factory work, such reduction as catalythic reduction, which involed H2 in alkaline at 5-6 ATM, with Ni as a catalyst, is much preferred.

An earlier method, which appeared in 1897, is thus: NAPHTALENE -> PHTALIC ANYDRIDE -> PHTALIMMIDE -> ANTHRANILIC ACID ->


However, a more recent method, patented by a German chemical company (Detsche Gold-und Silber Scheide Anstalt, or DEUGUSSA) begins with Phenylglycine and is inflused with K+ salt and a mixture of NaOH, KOH and NaNH2 (SODIUM AMIDE, a very strong base), at 180-210C. What forms is Indoxyl, which then reacts to the alkalies and forms the corresponding salt and H2O, then fixed by NAnH2:

H2O + NaNH2 -> NH3 + NaOH.

The NH3 is recycled and produces NaNH2 with Na.

Any indigo salt created is dissolved in water, after which indigo precipitates, concentration the solution. Only Na, which is metallic, is added to his process regularly in order to form new NaNH2.

This is the only process still used today.