Encyclopedia of Practical Receipts and Processes

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Blue (Copperas sulphate of iron)

Copperas (sulphate of iron) is used as a mordant for dyeing blue by ferrocyanide of potassium (prussiate of potassium). The copperas best suited for the blue vat should be of a dark rusty green color, and free from copper, zinc, or alumina. Thus, 10 pounds cotton may be dyed a good rich blue by working it for 15 minutes in a solution of 4 pounds copperas; wring out; and then work through a solution of 4 ounces of the ferrocyanide; finally, wash in cold water containing 1 ounce alum in solution.

Copperas is also used as a dye by the oxidation of the iron within the fibre. Thus:

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