Functional bio-dyeing of natural fabrics using purple gromwell (Lithospermum erythrorhizon) and natural mordants

Amid the accelerating demand for global sustainability, the conventional textile industry is transitioning to using plant dyes through eco-technological innovations. The rhizome-derived naphthoquinone pigments extracted from the roots of gromwell (Lithospermum erythrorhizon Siebold & Zucc.) exhibit dual functionality as plant dyes and bioactive compounds. In this study, we systematically evaluated both the functional attributes and color performance of gromwell-dyed natural fabrics using natural mordants. We used ultraviolet–visible light spectroscopy and GC–MS to explore the pigment composition of gromwell root extracts and employed an orthogonal experimental design to optimize the extraction and dyeing conditions. We assessed the dyeing properties and functional performance of nine natural and nine metallic mordants by measuring the color depth (K/S value), color characteristics, color fastness, ultraviolet protection, antioxidant and antibacterial activities of dyed fabrics. Shikonin was the main pigment in gromwell roots, and optimal extraction conditions were achieved with a material-to-liquor ratio of 1:10 in 90 % (v/v) ethanol following incubation at 70 °C for 140 min. Optimal dyeing conditions were observed following incubation at pH 7 for 80 min at 80 °C for cotton and pH 7 for 70 min at 70 °C for hemp. Although natural mordants enhanced dyeing properties and antioxidant activity, they resulted in slightly weaker ultraviolet protection than metallic mordants. All mordant-treated fabrics showed excellent colorfastness, whereas color fastness was only moderate for untreated fabrics, except in response to washing. Dyed fabrics without or with mordant have antibacterial effects on S. aureus and E. coli, especially using CuSO₄.