Indigo and its derivatives are obtained from plants (indigofera species). It is a substance which is insoluble in all kind of solvents. In order to make ist soluble a chemical trick is used that is called the vat process:
Indigo is added to water together with a base and sodiumdithionite as a reducing agent. Then a textile is added so that the reduced dye can be fixed by the fibres. When exposed to air a re-oxidation takes place and the blue colour occurs.

Apparatus: beaker 400 or 600 [ml], tripod, asbestos wire, BB, glass rod, weigh, thermometer

Chemicals: 0,5 [g] Indigo, 3 [g] sodiumdithionite, 5-6 pellets of sodiumhydroxide white cotton textiles

Procedure:

Ca. 250 [ml] of water are heated in a beaker to ca. 70 [°C]. Indigo, sodiumdithionite and sodiumhydroxide are added so that the solution turns yellowish. The textile is added and under casual stirring heated for ca. 5 minutes. The textile is washed with tap water and allowed to dry at the air.