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Article Highlights• Development of a clean process for indigo dyeing • Substitution of conventional sodium dithionite by green alternatives in reducing indigo• Study of synergy effect of mixture α-hydroxycarbonyls in reducing indigo • Determination of the optimum mixture of α-hydroxycarbonyls for reducing indigo Abstract Textile industries use different chemicals in dyeing processes, consuming large quantities of water and producing large volumes of wastewater. For the particular case of indigo dyeing processes, its reduction is performed chemically by the addition of sodium dithionite. However, this is considered environmentally unfavorable because of the resulting contaminated wastewaters. Therefore, it is important to replace sodium dithionite with other alternatives in order to achieve cleaner processes. α-hydroxycarbonyls have been suggested as possible environmentally friendly alternatives to reduce indigo. However, each one applied alone is unable to attain the dyeing performances offered by the conventional reductant. Thus, the study of the synergy of some selected α-hydroxycarbonyls was proposed. In this paper, a mixture design of experimental (DOE) methods was used to determine the optimum combination of α-hydroxycarbonyls to be applied in the indigo reduction process. Based on the design expert software, quadratic models were established as functions of α-hydroxycarbonyls ratios. The diagnostics of models were investigated by using mixture contour plots. Finally, a model was proposed to predict the optimum conditions leading to dyeing performances exceeding those obtained from the reduction of indigo by the conventional sodium dithionite.Keywords: green reducing agent, α-hydroxycarbonyls, synergetic effect, mixture design, clean process.As a vat dye, indigo is considered the oldest dye known to man. It is an organic compound with a distinctive blue color. Historically, it was naturally extracted from plants, and this was important economically because blue dyes were rare. Nearly all indigo dyes produced today -several thousand tons each year -are synthetic. Indigo and other vat dyes consumption reaches about 33×10 6 kg annually [1]. This