What is Sulphur Dyes? Chemistry of dyeing with Sulpher Dye?

Sulphur Dyes:

These dyes are so called because they contain sulphur atoms in their molecules. The fibres mostreadily coloured with sulphur dyes are the natural and man-made cellulosicfibres.

Chemistry of dyeing with Sulpher Dye:

The Sulpher dyes contain Sulpher linkage within their molecules. They are insoluble in water but can be made soluble in water by treating them with reducing agents. This also makes them substantive towards cellulosic fibres. Na2S acts as reducing agent that breaks the Sulpher linkage and break down the longer molecules in to simple components which can penetrate the material (fiber/fabric) surface easily.