Fast textile dyestuff which are insoluble in water but and have to be converted by chemically to be absorbed by the fabric. The dye has to form compounds soluble in Alkalis when reduced, that is when the oxygen is removed. In this state dyestuff is deposited on to the fibres in the dye vat and on contact with the air reverts to its insoluble, stable compound form. Indigo and shellfish or Tyrian purple are the only natural vat dyes. This class of dyes has a good all-round fastness properties.