The natural or vegetable dye sources used in Rajasthan are shown in the Table
Among these Harda, Alizarin, Indigo are most common.
In the event of non-availability of natural madder and indigo their synthetic counter parts i.e. chemical dyes are also used. Many times a combination of direct and resist styles are practiced depending on the design.
Before printing the fabric is desized, scoured and bleached either using traditional methods or modern methods depending on the facilities and expertise available.
In this process the fabric is treated with a solution containing Harda powder. The process of Harda treatment with intermediate drying is repeated 2-3 times till greenish yellow colour is obtained. The fabric is then dried either in sunlight or in shade. The purpose of harda treatment of cotton fabric is to hold the metal on fabric by reaction with tannin present in harda. This metal ion is then subsequently available for reaction with the colouring component/s of the vegetable dyes applied subsequently by dyeing or printing techniques. From the natural dyeing point view only Alum and iron salts are environmentally friendly. The other salts like copper sulphate, sodium dichromate and stannous chloride are not environmentally friendly. Therefore, although they are suitable as mordant for natural dyes they are not recommended on environmental and toxicity grounds.
Black and red are the most popular colours produced by block printing of cotton in Rajasthan. The chemistry involved in the production of these colours is summarized below.
Treatment of cotton or silk or wool with harda followed by treatment with iron salt is commonly used to produce black colour either during dyeing or printing. Harda treated fabric followed by subsequent printing with alum is used to produce variety of shades by subsequent dyeing or printing with other natural dyes.
Black shade – This is available on treatment of fabric with Harda solution. Dry Block print with iron salt solution is often used to produce black outline of the design.
Other shades – Different shades are available on treatment of fabric with harda solution. Dry treatment with alum either by printing or treating the whole fabric with alum solution and dyeing or printing with other natural dye source. During this operation the metal salt chelated with tannin in harda forms another complex with the natural dye source applied by dyeing or printing process.
For example when harda treated fabric is printed with alum, a complex between tannin of harda and aluminium is formed. When this fabric is treated with alizarin by dyeing process, the alizarin forms a complex with aluminium with the formation of red colour.