Dyes can be classified as either natural or synthetic. The only natural dye
still used on a large scale is logwood, which imbues silk, wool, cellulose
acetate, and nylon with a deep black colour. With the exception of a few
inorganic materials used in special processes, all dyestuffs used in textile
applications are synthetic organic chemicals.
Synthetic dyes can be classified according to either their
characteristics or their application to textiles. Manufacturers of dyes usually
classify dyes according to their chemical structure, whereas dyers generally
classify dyes based on the method of application.
According to the dyers' method of classification, there are
direct dyes, which produce fast colours in fibres placed directly in the dye
solutions, and indirect dyes, which produce fast colours only if the fibres are
treated before or after dyeing.