Vat Dyes are based on original Indigo dye that is a kind of natural Dye. Vat dyes are an ancient class of dye. Now a days, Indigo dyes(parent of Vat dyes) are manufactured synthetically. Various kinds of fibers can be washed by using vat dyes and the main categories of fibers are cotton and wool. One thing we should remember that in all Vat dyeing process, Vat dyes are not used. "Tie Dyeing" which is a kind of direct dye application is the opposite of Vat Dyeing. For the home dyers, vat dyes are not so suitable as fiber reactive dyes. This is because it is difficult to work with as these require reducing agent to dissolve in water. These dyes can be dissolved into water only when these are oxidized.
The fibers which have to be dyed are immersed in oxygen-free vat or bucket repeatedly and then these fibers are exposed in the air. When it is done so then the dissolved water in the fibers reduces and the color change is occurred there as oxygen turns the dyes to water insoluble form. For an example, Indigo (a kind of Vat dyes) changes from yellow to green in the vat (dye bath) and when it is exposed in the air then it turns in green.
The commonly used vat dyes are:
Anthraquinone: These kind of dyes are often found in the classes of Vat dyes and Disperse dyes. Its fastness capacity is very high and therefore, the high cost of these dyes is ignores. Mostly these dyes are applied on cotton and cellulose fibers.
Indigoids: These dyes are intensely insoluble in water and therefore they are unable to dye the fibers directly until and unless they are not being oxidized.
Molecular Structure of Some Vat Dyes