The dyeing process involves three factors: fabric, dye bath and mordant. Some natural dyes have a chemical make-up that will not allow them to bind with various fabrics and require the application of a mordant (the metal salt) solution to the fibres at some point in the dyeing process. This forms a bridge between the dye and the fibre. The metal ions adhere to the wool and serve as points of attachment for the dye molecules.
Mordant comes from the French "mordre" to bite, it therefore cause the colour to 'bite' into the fabric. Mordants are metallic salts that fix a dye to a fabric by basically acting on the dye molecules, breaking apart existing links and forming new ones that will bind with the fabric. It is basically the chemical link that fixes the dye to a substrate by combining with the dye pigment to form an insoluble compound. In the woods we used Potassium aluminium sulphate and Iron (ferrous sulphate) as our mordants. With most mordants, the presence of heat allows this reaction to happen.
The most commonly used mordant dyes have hydroxyl and carboxyl groups and are negatively charged, i.e. anionic. Some other mordant dyes may possess amino groups, and are cationic overall. Despite this, they must still have hydroxyl or carboxyl groups, since lake formation requires it. (The dye lake is an insoluble molecule formed when the complex of dye and mordant are combined, which then attaches to the wool.) This happens best in the presence of heat. Mordants increase the fastness of the dye since the larger molecule is now bonded to the fibre.
Wool is a protein, a naturally occurring polymer made up of amino acid repeating units. Many of the amino acid units have acidic or basic side chains that are ionized (charged).The presence of many charged groups in the structure of wool provides excellent binding sites for dye molecules, most of which are also charged. Wool is highly receptive toward mordants. Due to its amphoteric nature wool can absorb acids and bases equally effectively. When wool is treated with a metallic salt it hydrolyses the salt into an acidic and basic component. The basic component is absorbed at –COOH group and the acidic component is removed during washing.
In a nutshell, the mordant is a chemical which fixes the dye to the wool.
Hunt, A, (2009) Chemistry 4th edn. Oxfordshire: Phillip Allen.
Revise A2 Chemistry for Salters (OCR A Level Chemistry B)
Salters (OCR) Revise A2 Chemistry
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dictionary.reference.com
www. wilipedia.org
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Apart from the 3 books listed above, here is a list of websites used to inform this dye blog: