A new vat dyeing process for cotton knits with a modified jet dyeing machine has been developed. Proper conditions with regard to the amount of the reducing agent, oxidizing process, and dyeing temperature were searched for with this machine. The dyeing proceeded stably and reproducibly at 60°C by using the modified machine and process. The conventional vat dyeing method of cotton knits with a jet dyeing machine has been performed under nitrogen gas. In the present study, a jet dyeing machine was modified for dyeing the knit roll in the following points. The air tightness of the machine was made sure at the joint parts and window. The machine was equipped with the inlet and outlet to the liquor circulating system for the additions of liquors containing adequate agents. A sensor for in situ measurements of oxidation and reduction potential and pH was inserted into the liquid circulation system. These measurements made it sure that the dyeing, gentle oxidation by washing water, and final oxidation reach to respective quasi-equilibrium states after certain periods. Using this modified machine, vat dyeing at 60°C was examined to confirm the feasibility of fast and uniform dyeing. Increased amount of hydrosulfite in excess to the required amount for the reduction of the dye was used to consume out oxygen in the air space of the machine. The dyed knit was overflow washed with water, four times of the dyeing liquor and then treated with oxidizing agent to complete the oxidation of the dyed knit. It was found by visual inspection that faultless dyeing of the knit with vat dyes, especially in deep colour, was attained at the temperature leading to spare the dyeing period.
