Scientific American 4, 25.1.1868
A French patent has been allowed for a new method of recovering indigo from cotton or woolen rags which have previously been dyed with that substance. The inventor places the rags in a boiler provided with a double bottom and saturates them thoroughly with a solution of caustic soda of 1" Baume. After this the rags are kept for five hours under the action of steam at 45 pounds pressure. By this treatment the indigo is reduced, and dissolved, then precipitated from the soda solution, and finally collected in as pure state as the best sorts in commerce.