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The bleaching is usually accomplished by the use of a hydrosulphite, either in the form of sodium or calcium, but sometimes only the sulphurous acid gas is used.

This is dehydrated sodium hydrosulphite with the chemical formula, Na2S2O4.

It is reduced by hydrosulphite of soda yielding a blue vat, in which cotton and other vegetable fibres are dyed in the same way as in the indigo vat.

Cotton is dyed by means of the "lime and copperas vat," the "zinc powder vat," or the "hydrosulphite vat."

The constituents of the hydrosulphite vat are hydrosulphite of soda, lime and indigo.