Stoichiometric Optimisation of Vat Dyeing for Sustainable Colour Fastness in Cotton Fabrics

Purpose: Vat dyes are widely recognised for their superior colour fastness on cotton substrates; however, the efficiency of their reduction–oxidation cycle is highly dependent on the stoichiometric balance among the dye, sodium hydroxide (NaOH), and sodium hydrosulphite (Na₂S₂O₄). This study investigates how controlled variations in reagent ratios influence colour fastness, to develop a stoichiometrically grounded optimisation framework applicable to artisanal and semi-industrial dyeing systems in Ghana. Methodology/Design: An experimental factorial design was employed using mercerised cotton fabric as the substrate. All reagent ratios were expressed in a fixed order (dye: NaOH: Na₂S₂O₄), while the mixing sequence was treated as a separate variable. The commonly used 1:2:2 ratio was adopted as a comparative baseline rather than a universal standard. A total of 27 dyeing conditions were tested across three vat dye systems (golden yellow, red, and violet). Colour fastness to rubbing (dry and wet) and washing was evaluated using ISO-based grey scale assessments. Each experiment was replicated three times, and results are reported as mean ± standard deviation. Findings: The results demonstrate that stoichiometric balance between NaOH and Na₂S₂O₄ governs the efficiency of dye reduction, fibre penetration, and subsequent fixation. Ratios approximating equilibrium, particularly 5:1:1 and 2:1:1 (dye: NaOH: Na₂S₂O₄), consistently produced superior fastness grades. In contrast, reagent excess or deficiency disrupted the redox balance, resulting in reduced dye stability and increased colour loss. While no clear sequence-dependent trend was observed, this outcome is interpreted cautiously due to confounding between dye type and mixing sequence. Practical and Social Implications: The study provides an empirically validated, ratio-based guideline that can improve process consistency, reduce chemical excess, and enhance product durability in resource-constrained dyeing environments. The framework supports a transition from intuition-based practice to controlled, evidence-informed dyeing processes. Originality: This research advances the application of stoichiometric theory within textile dyeing by linking quantitative reagent balance to measurable performance outcomes under practical conditions. It contributes a context-sensitive optimisation model that integrates chemical precision with artisanal production systems, extending beyond conventional laboratory-based studies.