Vat dyes are those dyes which are applied to the substrate in reduced, soluble form and then oxidize to the original insoluble pigment. Common vat dyes are quinonic dyes and particularly common are anthraquinones and indigoids. These dyes are essentially insoluble in water and incapable of dyeing fibres directly. However, reduction in alkaline liquor produces the water soluble alkali metal salt of the dye. In this leuco form these dyes have an affinity for the textile fibre. Subsequent oxidation reforms the original insoluble dye. Most vat dyes are less suitable than, say, fiber reactive dyes, for the home dyers, as they are difficult to work with; they require a reducing agent to solubilize them. Indigo is an example of this dye class.