Sulphur dyes are complex heterocyclic molecules or mixtures formed by melting or boiling organic compounds containing amino or nitro groups with Na-polysulphide and Sulphur. Sulphur dyes are so called as they all contain Sulfur linkage within their molecules.

Sulphur dyes are highly coloured, water insoluble compounds and have to be converted in to water soluble substantive forms (lucoforms) before application to the textile materials. This conversion is carried out by a treatment with a reducing agent like dilute aqueous Na2S. Since this lucoform of Sulphur dye is substantive to cellulosic materials. They are absorbed on the fibre surface. Then they are reconverted original water insoluble form of dye by oxidation. This oxidation is carried out by “airing” (exposure to air) or by using an oxidizing agent like Na-dichromate (Na2Cr2O7).

The reducing agents converts the “S” in dye in to –SH group and the Sulphur linkages. Then inside the material the thiols containing –SH groups are oxidized & thus reconverted to original form of dye.

This is shown in the bellow:

Dye-S-S-Dye + 2[H] = Dye-SH + HS-Dye

Dye-SH + HS-Dye +[O] = Dye-S-S-Dye + H₂O

Sulphur gives best result (Bright Tone) when they are used to produce black, Black & brown shades but red shades cannot be obtained by Sulphur dyes.
 
The history of Sulfur dyes may be summarized as below:

1. The first Sulphur dyes where made in 1873 heating saw dust, caustic soda and Sulphur. It occurred by chance when a reaction vessel containing Na2S was leaking and the saw dust was used to wipe the solution coming out. Later a cotton fabric come in contact with this contaminated sawdust and become stained.

2. The real pioneer of Sulphur dyes was vidal who produce vidal black (Name of Sulphur dye) by fusing para-phenylene diamine with Na2S & Sulphur in 1893.

3. In 1897 Kalischer produced Immedial Black FF by heating 2, 4-dinitro-4-dihydroxy diphenylamine with Na-poly sulphide.

4. In 1896 Read Holliday introduced a range of grey, brown and black Sulphur dyes by the action of Sulphur, alkali sulphides and many organic compounds.