Production of concentrated natural dyes is a pre-requisite for a re-introduction of plant colorant based
dyes into modern textile dyeing operations. Aluminium salts such as Al2(SO4)3.14-15H2O or KAl(-
SO4)2.12H2O can be used to precipitate extracted plant dyes from aqueous extracts at pH 5.0e5.5. Onion
peel, Canadian Goldenrod and pomegranate peel were studied as representative sources for dye
extraction. As an average 5% w/w of the extracted dry plant material could be collected as precipitate.
After dissolving these residues in diluted oxalic acid, the quality of the dye lake was characterised by
photometric analysis of the total phenol content in the dry using the Folin-Ciocalteau method, determination
of the aluminium content and measurement of the absorbance at 400 nm. Representative
values of TPH in the dry solid dyestuff range from 20 to 40% and representative values for the aluminium
content were determined with 3e5% w/w. Colour strength of the dissolved lakes was determined in
dyeing experiments using different substrates and mordants followed by measurement of CIELab coordinates
and K/S value according Kubelka-Munk function. Compared to the direct use of plant extracts the
colour strength of the lakes is lower, however chroma of the dyeings is higher, as the lake formation also
represents a dye purification step.