Sulfur dye

Dyed cellulosics exhibit a tendering effect on storage under humid atmosphere due to presence of excess free sulfur.

This restricts use of metal vessels except those made of stainless steel:[2] The forerunner of sulfur dyes said to be "Cachou de Laval", which was discovered by Groissant and Bretonniere in 1873 and is prepared by treating products with lignin (like sawdust or straw) with sulfide sources (like sodium hydroxide or sulphide mixed with sulphur).

These experiments demonstrated that deeply colored materials could be readily produced by combining aromatic compounds and sulfur sources.

It is accepted that the sulfide reduces the nitro groups to aniline derivatives, which are thought to form indophenol-containing intermediates that are further crosslinked by reaction with sulfur.

In the presence of a reducing agent and at alkaline pH at elevated temperature of around 80 °C, the dye particles disintegrate, which then become water-soluble and hence can be absorbed by the fabric.

Oxidation can also be effected in air or by hydrogen peroxide or sodium bromate in a mildly acidic solution.

Due to the highly polluting nature of the dye-bath effluent, sulfur dyes are being slowly phased out in the West but they are used on a large scale in China.

[3] Recent advances in dyeing technologies have allowed the substitution of toxic sulfide reducing agents.

Future developments in the field of reducing dye levels by means of electrochemical processes are promising.