Mordant Selection in Natural Dyeing of Cotton with Cinnamomum verum Bark Extract: Colour Strength against Rubbing Fastness

Abstract

Natural dyes are applied with mordants, and the metal salts commonly used for the purpose differ substantially in environmental burden: copper salts in particular are toxic to aquatic life and are discharged in dyehouse effluent. This study compares alum and copper sulphate as pre-mordants for Cinnamomum verum bark extract on 100% bleached cotton, across aqueous and methanolic extraction systems at three dyebath liquor

ratios, evaluating both colour strength by the Kubelka- Munk function and rubbing fastness by ISO 105-X12.

Copper sulphate gave the higher colour strength in every comparison, reaching a peak K/S of 7.6 against 3.5 to 4.5 for alum. Rubbing fastness ran in the opposite direction: alum-mordanted fabrics reached grade 4 dry and grade 3 wet, while several copper-mordanted fabrics fell to grade 1/2 wet. The deeper shades obtained with copper sulphate were thus accompanied by poorer resistance to rubbing. Depth and mordant identity vary together in this design, so the poorer rubbing fastness is not attributed to copper as such, but the practical consequence stands: the greater depth available from copper sulphate cannot be had without the accompanying loss of rubbing fastness, and copper is the more environmentally burdensome of the two metals. Where depth of shade is not the governing requirement, alum is preferable on both counts. Substituting a plant colourant for a synthetic one while retaining a metal mordant moves the environmental burden rather than removing it.