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.
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.
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