Saminathan Ratnapandian

Pre-mordanting is where the textile material is first treated with the mordant and then dyed. Intermediate drying allows storage of the mordanted material

and helps in absorption of the dye liquor. Printing with different mordants and then dyeing with a polygenetic dye to obtain a multicoloured pattern is an application of this method. The disadvantage of pre-mordanting is that prolonged storage can damage the textile if the metal salt is corrosive. In addition, the mordant can leach into the dye bath resulting in colour variations.

2. 
   Meta mordanting involves a dye bath containing both dye and mordant. The fabric is dyed in one step. The major disadvantage is that the aggregation of dye-mordant complex in the dye bath interferes with the dyeing process. The large complex cannot be easily absorbed by the substrate resulting in uneven dyeing.
   
3. 
   Post mordanting is a popular method for leather colouration. The mordant is applied after the dye has been applied. This method yields deeper penetration of the dye and a level shade. Prolonged storage of dyed fabric prior to mordanting has to be avoided in order to minimise loss of colour by fading or rubbing.
   

Although the function of a mordant is clear, the exact mechanism involved is difficult to identify [28]. The complexity of dye molecules hinders intimate understanding of the binding mechanism between dye and substrate or the precise role of the mordant in fixing the dye. Hence there is ongoing research in this area in terms of determining the optimum amount of mordant required and developing a theory that explains the precise role of the mordant.

Table 1.1 lists some typical colours obtained using natural colours and their plant sources. Plants yield the maximum variety of natural dyes. The dye can be derived from leaves, flowers, seeds, roots and bark. In certain instances, each of these parts of the same plant yield different colours [38, 39]. Other sources of natural dyes are naturally occurring metal salts termed mineral colours. Some examples are chrome-yellow, iron buff and mineral khaki. Tyrian purple is a well known colour obtained from shell-fish (Murex brandaris) while cochineal is obtained from insects (Dactylopius coccus).

Lichen and fungi can also yield dyes but their slow rate of growth severely limits any commercial exploitation. Some of the important chemical groups present in natural dyes are given in Figure 1.2 [40].
Table 1.1 Typical shades of natural dyes