Indigo is the oldest natural dye used in India since ancient time to produce blue colour on natural fibres. Majority of natural dyes need treatment with metal salt either before or after dyeing/printing operation. Indigo is applied in the absence of mordant producing blue shades with good fastness properties.
The natural indigo dye is extracted from the leaves of indigo plant. The species of the plant cultivated in India is known as Indigofera tinctoria. However, in the absence of availability of natural indigo, block printers use synthetic indigo. The chemical structure of the dye extracted from indigo plant leaves and synthetic indigo is same.
Indigo is insoluble in water and belongs to vat dye class. Before dyeing it must be converted into water soluble form by the process known as reduction in presence of alkali. The dyeing is carried out while indigo is water soluble reduced form. After dyeing the fabric is exposed to air during which the water soluble form of indigo gets converted into original water insoluble form producing blue color on the dyed fabric.
In ancient time the reduction process was carried out in wooden vats, therefore, this class of dye is known as vat dyes.
Indigo has poor affinity to cotton, wool or silk. Therefore, deep blue shade on fabric is produced only after repeated dyeing. For this purpose the fabric to be dyed is dipped in reduced indigo solution, squeezed and exposed to air. The procedure of dipping into reduced indigo solution, squeezing and exposure to air is repeated several times (6-10 times) to get deep blue shade. Larger the number of dips, deeper will be the blue colour on fabric.