Sustainable indigo shade development in enzyme-assisted denim washing using composite handmade stones as a pumice substitute

Purpose This study aims to evaluate composite handmade stones (HMS) as a sustainable substitute for conventional pumice stones in neutral-cellulase-assisted denim washing, with particular emphasis on indigo shade development, fabric performance, stone durability and wastewater characteristics. Design/methodology/approach A full factorial experimental design was used to compare HMS and conventional pumice under controlled stone enzyme washing conditions. Denim fabric performance was evaluated through fabric weight (grams per square meter; GSM), ends per inch (EPI), picks per inch (PPI), tensile strength, tear strength, dimensional stability, color fastness to crocking and spectrophotometric parameters, including color strength (K/S), lightness (L*), red–green coordinate (a*), yellow–blue coordinate (b*) and whiteness index (WI). Stone durability was assessed using absorbency, crushing strength and Los Angeles (LA) abrasion tests. Effluent quality was evaluated through pH, total dissolved solids (TDS), turbidity and chemical oxygen demand (COD). Scanning electron microscope (SEM) was used to compare the surface morphology of treated denim, while environmental impact measurement (EIM) was used to assess the environmental performance of the washing processes. Findings HMS produced colorimetric and fabric performance comparable to conventional pumice, with no statistically significant differences (p > 0.05) in GSM, EPI, PPI, tensile strength, tear strength, K/S and WI. Similar surface fibrillation and controlled abrasion were confirmed by SEM. HMS demonstrated substantially greater durability, with lower crushing and LA abrasion values than pumice, indicating reduced stone degradation and fragmentation. The HMS process also reduced effluent TDS from 1,210 to 1,010 ppm and COD from 194 to 181 mg/L. The EIM assessment showed a reduction in the overall score from 28 to 14, corresponding to a 50% reduction in environmental impact. Originality/value The study demonstrates the technical equivalence and environmental advantages of composite HMS in enzyme-assisted denim washing. Unlike conventional evaluations that focus mainly on aesthetic performance, this work integrates indigo shade development, fabric physical–mechanical properties, stone durability, effluent characteristics and environmental impact. The findings establish HMS as a practical and more durable pumice substitute capable of achieving comparable denim aesthetics while reducing stone fragmentation, wastewater pollutant load and overall environmental impact.