Natural Dyeing of Silk with Dalbergia sissoo Bark and Leaves Through a Comparative Box–Behnken Response Surface Methodology

Abstract

The textile industry is one of the largest consumers of synthetic dyes and a principal contributor to industrial water pollution. This study examines the natural dyeing of silk fabric using aqueous extracts from the bark and leaves of Dalbergia sissoo (D. sissoo), widely available across South Asia, as a potential substitute for synthetic colourants. A four-factor Box–Behnken design was applied to optimize the dyeing conditions for each plant part. Dye concentration, mordant type (alum, tannic acid and gallnut), mordant method (pre, meta and post) and mordant concentration were the variables for the bark dataset, while sodium chloride concentration replaced the mordant method for the leaves dataset. The dyed samples were characterized in terms of colour strength (K/S), CIELAB colour coordinates and the principal fastness properties, and the K/S response was visualized through three-dimensional response surface and contour plots. The bark extract produced shades from pale yellow to deep reddish-brown with K/S values between 0.34 and 3.59, the deepest obtained with gallnut pre-mordanting at 30% dye and 30% mordant concentration. The leaves extract produced brighter yellow to yellow–green shades with K/S values between 0.62 and 1.44, the deepest obtained with alum at 15% mordant concentration combined with 20 g/L sodium chloride. Rubbing fastness and multifiber staining of both substrates fell between grades 4 and 5, and the colour-change rating between 1/2 and 2/3. The results establish D. sissoo as a viable dual-source natural colourant delivering two distinct colour palettes from a single tree species.