Cotton can be dyed with vat dyes. The process of vat dyeing refers to dyeing in a bucket or vat. It can be performed whenever a solid even shade, the same colour over the entire garment, is wanted, using almost any dye, including fibre reactive dye, direct dye, acid dye, etc. One alternative to vat dyeing is direct dye application, such as the process used for tie dyeing. Note that not all vat dyeing is done with vat dyes and the colours shown here in this article are for demonstration purposes only as colours vary with individual monitors and therefore cannot be guaranteed as accurate.
Vat dyes are those dyes which are applied to the substrate in reduced, soluble form and then oxidize to the original insoluble pigment. Vat dye is obtained through oxidation and is usually very bright and will hold up better when bleached than most other dyes. The process is very colour fast in all respects. This is an expensive procedure and is used mainly on high-end products. Vat dyes include the natural dyestuff, indigo, and the artificial dyes are called indanthrene. They are called vat dyes because, being originally insoluble in water, they undergo special preparation in large vats before the cloth is introduced; here they are made soluble, usually by the adding of caustic soda and hyposulphite.
In this mixture or dye liquor the textiles are soaked. Certain chemicals are thereafter added, changing the dyestuff back to the insoluble form in the cloth or fibre. This is called the fixing process. All the vat dyes are fast, especially to washing. The artificial vat dyes have in many instances taken the places of the older mordant dyes, and their future seems promising, for they are cheaper and easier to apply than the mordents, although not so cheap and convenient as the direct dyes.