"Vat dyes" are a special class of dyes that work with a special chemistry. Cotton, wool, and other fibers can be all dyed with vat dyes.
Most vat dyes are less suitable than fiber reactive dyes for the home dyers, as they are difficult to work with; they require a reducing agent to solubilize them. The dye is soluble only in its reduced (oxygen-free) form. The fiber is immersed repeatedly in this oxygen-free dyebath, then exposed to the air, whereupon the water-soluble reduced form changes color as oxygen turns it to the water-insoluble form. Indigo is an example of this dye class; it changes from yellow, in the dyebath, to green and then blue as the air hits it.
Application of vat Dyes
Application of vat dyestuff is not so easy like direct or reactive dyes. The application involves a lot of chemistry - such as oxidation, reduction, pH control and so on. Even the dissolution of the dyestuff involves a great technique of reduction using adequate quantities of caustic soda and sodium hydrosulphite.
There are several methods available for the application of vat dyes to yarn and fabric.