Alphabe Thursday … I is for Indigo

The production of indigo dates back to beyond classical times. The word ‘indigo’ comes from the Greek indikon, latinised indicum, meaning the substance from India; showing that indigo pigment was imported into the Graeco-Roman world.  

The Sanskrit word nila, meaning dark blue, spread from India eastwards into Southwest Asia and westwards to the Near and Middle East, probably both through pre-Islamic trading routes and later during the Islamic era.  

The Arabs conveyed their word nil, or an-nil (with the definite article), further west in the course of their conquests across northern Africa and into southern Spain.  Soon the Spanish and Portuguese transmitted the words anil and anilera to Central and South America in the sixteenth century.  During  the reign of Queen Elizabeth I, a British Act of Parliament referred to indigo and nele, alias blew Inde, while European travellers’ and merchants’ report interchange neel/anyle with indico.  In the seventeenth century indigo became the common name, but indigo’s etymological history survives into modern times with the word ‘aniline’ so called because indigo formed the basis of dyes when chemically synthesised.

Aniline is an oily liquid compound, colourless when pure.  It was isolated in 1826 by distilling natural indigo with lime and discovered in coal tar in 1834.  In 1841 it was discovered that it could be obtained by heating caustic potash with indigo and then it received its name as suggested from the Sanskrit nila and Arabic an-nil.  In 1856 William Henry Perkins revolutionised the dyeing industry by using aniline to produce the first synthetic dye ‘mauveine.’ Obtained from coal tar derivatives it provides the chemical base of many modern synthetic dyes.     

Indigo a strong blue dye produced largely from the leaves of the Indigofera tinctoria; it has been extracted from an estimated forty plant species worldwide.  The term anil was wisely used for many indigo plants.

The dye is produced as a powder or a cake by a lengthy process of steeping and stirring.  To apply the dye to cloth indigo must be rendered soluble in an alkaline liquid containing a reducing agent.  This process of ‘reduction dyeing’ is called vatting and in this state the indigo transforms to a ‘white’ form. Dyeing is carried out in the ‘white’ vat liquid and, after dying , is converted back to blue state by exposure to atmospheric oxygen.  The colourant in indigo is the same as that in woad.

Further reading Dyeing and dyestuffs by Su Grierson

Indigo by Jenny Balfour-Paul