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.
