Vat dye
A dye that must be chemically changed to dissolve, then turns back to its colour in the air.
Also: vat dyeing
Some dyes, indigo above all, will not dissolve in water. To use them, the dyer removes oxygen from the dye in a vat, traditionally by fermentation with plant matter, lime or ash. In this state the dye dissolves and turns yellowish green. Cloth dipped in the vat and lifted out changes colour before the eyes as the air turns the dye back to blue, now trapped in the fibre. Repeated dips build a darker shade. Modern vat dyes, made on the same principle, are among the most wash-fast colours for cotton.