The vat dyes are insoluble complex polycyclic molecules
based on the quinone structure (keto-forms). They are reduced with
sodium hydrosulfite in a strongly alkaline medium to give soluble
leuco forms that have a great affinity for cellulose. After the reduced
dye has been absorbed on the fiber, the leuco forms are reoxidized to
the insoluble keto forms. The dyeings produced in this way have high
wash and light-fastness.
A class of insoluble dyes applied
by first reducing them to derivatives that
are soluble in dilute alkali. In this condition
they have a great attraction for certain
fibers, such as cotton. The solution is applied
to the material and the insoluble dye
is regenerated in the fibers by atmospheric
oxidation. Indigo and indanthrene are examples
of vat dyes.
