Reuse of ash-tree (Fraxinus excelsior L.) bark as natural dyes for textile dyeing:: process conditions and process stability

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 FeSO4.7H(2)O 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 FeSO4.7H(2)O 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.