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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 Al 2 (SO 4 ) 3 .14-15H 2 O or KAl(SO 4 ) 2 .12H 2 O can be used to precipitate extracted plant dyes from aqueous extracts at pH 5.0–5.5. 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 3–5% 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.

The aqueous extract of ash-tree bark (Fraxinus excelsior L.) was chosen as a model to study the shade reproducibility of dyeing on wool. A meta-mordanting process using FeSO4·7H2O mordant was chosen as a system with particular potential for industrial application. The exhaust dyeing process with immediate use of the extracts as a dyebath and direct addition of FeSO4·7H2O stock solution as a meta-mordant process showed good shade reproducibility and satisfying levelness of the dyed material. An increase of Fe(ii)-mordant above a dyebath concentration of 2–3 g l−1 did not result in further colour depth. Extraction of 1–2 g of bark was found sufficient to dye 1 g wool yarn to the darkest colour possible; use of higher amounts of bark did not yield substantially higher colour depth. The quality of bark and the extraction step were found to be of significant importance for the colour depth; thus, in an optimised process, conditions of extract formation have to be well controlled.

The food and beverage industry releases considerable amounts of wastes which contain natural dyes. Such wastes could serve as a sources for the extraction of natural dyes for textile-dyeing operations. The extraction of brilliant yellow and red colours from fruits and vegetables is of particular interest. Wastes, e.g. pressed berries, pressed grapes, distillation residues from strong liquor production, and wastes and peels from vegetable processing, have been extracted with boiling water and test dyeings on wool yarn were performed. Colour strength, shade and fastness properties of the dyeings have been tested. The extracts were applied as direct dyes and in the presence of iron(II) or alum mordants. The results prove the potential of such wastes as a source for natural dyestuff extraction. To obtain textile dyeings with acceptable fastness properties, however, rigorous selection of dyes and development of suited processes is required. A considerable number of red natural dyes need further research to optimise the low level of fastness to light. Copyright © 2005 Society of Chemical Industry

The production of natural dyes from plant sources available in the moderate climate of Austria (Europe) has been studied. Besides direct agricultural production, additional sources e.g., bark from timber industry and wastes from food industry have been investigated. Model calculations of energy consumption indicate that stabilised plant material e.g., dried plants, is favourable compared to dyestuff extracts. The low dyestuff content of the crops demands extraction of huge amounts of plant material, thus extraction should be performed in water only. Dyeing experiments indicate that based on a selection of dyestuffs and mordants an acceptable quality level can be reached.