The exhaustion agents or retarding agents are added to the dye
bath depending upon the dyestuffs taken,
during the entire dyeing period. Excess quantities of both sodium hydroxide (NaoH) and sodium hydro sulphate (Na2s2O4) should be present in the dye bath in order to keep the dye in the soluble form.
At the end of the dyeing the partly or completely exhausted dye bath must be kept in a distinctly reduced condition; otherwise oxidation of the residual vatted dye takes place in the dye bath itself
leading to the appearance of turbidity. This is ensured by adding sufficient sodium hydro sulphate. The dyed goods may then be removed from the dye bath and excess liquor which contains the
unexhausted vat dye, sodium hydroxide, sodium hydro sulphate is removed as for as possible from the goods.
The dyed goods are rinsed with cold water and then subjected to an oxidation treatment by exposure to atmospheric oxygen. This is called “air oxidation” or “airing” but the oxidation may be accelerated by using stronger oxidizing agent such as sodium per borate or hydrogen peroxide or sodium dichromate in the presence of acetic acid. This process is usually referred to as chemical oxidation.
During the oxidation step the sodium salt of leuco vat dye absorbed by the fiber is oxidized and converted into insoluble dye in the fiber. At the same time the vatted dye contained in the residual liquor in the goods being dyed also gets converted into the insoluble form which is loosely deposited on the fibre surface. This loosely deposited dye on the surface of the fiber has to be removed for achieving optimum fastness properties especially rubbing and washing fastness properties. This is achieved by soaping process. The dyed material is treated in hot soap solution or a synthetic detergent solution for 15 – 30 minutes. After the soaping treatment the dyed goods should be rinsed thoroughly and finally the dyed material is dried.