Indigo

Indigo is derived from the term Indicum (Latin) and Indikon (Greek) meaning “blue dye from India” or “Indian substance”. The term is coiled because India is to be believed the oldest centre of indigo dyeing in the Old World. It was the major commercial place for the supply of indigo across the Greco-Roman civilization. Nowadays, Indigo is frequently referred to as Indigo dye that has a distinctive blue colour. The dye is mostly extracted from Indigofera tinctorial and woad (Isatis tinctoria). Indigofera tinctoria is a common plant in the subtropics of Asian and Africa that produces more dye than woad, which can only be obtained in temperate climates. Indigo dye is one of the oldest natural known dyes mostly used for denim jeans, arts and crafts.

Indigo is the colour between blue and violet with a spectral wavelength of 440 and 420 nanometers. German chemist von Baeyer was the first person to determine the molecular structure for Indigo and was reward the Nobel Prize in Chemistry as a result of it. Indigo is a dark-blue, not soluble in water, crystallized powder that melts at 390 to 392 Celsius. The intensity of the blue is caused by absorption in the yellow section of the spectrum (maximum at 602 nanometers). The molecule (C16H10N2O2) has a weight of 262.27 atomic units of mass being relatively small. It is made of oxygen (red), nitrogen (blue), and hydrogen. Indigo is derived from two indoxyl rings joined at position 7 on each ring.

Indican is a substance that naturally occurs in Indigofera plants. The compound is colourless and soluble in water. It is hydrolyzed to glucose and indoxyl. When it gets exposed the solution to air the indoxyl gets converted to indigo. Indigo is a very difficult dye to use because it cannot be dissolved; it must go through a chemical change. After removing it from the dye-bath, it becomes insoluble because the dye combines very quickly with oxygen. During the preindustrialized era, the Europeans dissolved indigo in urine stale. After the urine treatment, Indigo becomes a soluble substance known as leucoindigo (indigo white) which produces a yellowish green solution. In Japan, indigo was dissolved in a heated vat so that the anaerobic bacteria were maintained. These microorganisms use hydrogen as a metabolic product in which converts insoluble indigo into soluble leucoindigo.

Nowadays, when dyeing indigo to fabrics, the dye does not respond and will not join chemically to fabric; it undergoes a two-stage reduction. It gains electrons on the oxygen atoms and reduction occurs when electrons are transferred from a donor (oxidizing agent) to an acceptor (reducing agent). The indigo dye vat is used to keep the dye in solution so it will infiltrate and stick to fabric fibers. The colour of indigo throughout the dyeing process is strongly dependant on the conditions used. It ranges from red in the vapor phase (О»max = 540) to violet in non-polar organic solvents to blue in organic state in polar solvents. Oxidation occurs when Indigo molecules are exposed to air, heat and strong light. Once oxidized, the molecules are trapped inside the fibers. After much use, fading starts to happen when molecules fall out of the structure that holds them together.

An indigo dye vat helps to keep the dye in solution and transfer the dye pigments successfully to the fabric. Normally, a dye vat operates at about pH 9.5. Its efficiency can be improved by using sodium hydroxide. Ancient Blue Crystal Indigo is a ready-made dye. The dye is immediately ready to use when water is added. This kind of dye is possible to cause skin damage due to its high intensity in pH.

Nowadays, indigo is mostly synthetically produced. There are a number of different ways; the original method is credited to Heumann in the 1800s. The process involves heating phenylglycine-o-carboxylic acid to with sodium hydroxide, which produces indoxyl-2-carboxylic acid. Then, it decarboxylates and oxidizes it in air to form indigo.

In 1901, Pfleger discovered a new way to synthesis indigo. With this process, sodamide is used to treat phenylglycine that produces indoxyl and oxidize in air to form indigo.

A relative of Indigo is Tyrian purple, also known as Royal purple. It is made from a secretion gathered from Murex Trunculus and Murex Brandaris (marine mollusks). The dye requires 12,000 molluscs for only 1.4 grams of pigment thus making it very costly to harvest. Therefore in many cultures, Tyrian purple is only reserved for high-class citizens that could afford it. It is also be used to make dye, but indigo is a lot cheaper and the colour is identical.

The molecule of Tyrian purple