In the dipping of China blues, care should be taken to swing the frames during the operation; and when the last dip is given, the piece is to be plunged upon its frame into a fourth vat, containing dilute sulphuric acid of spec. grav. 1·027. This immersion is for the purpose of removing the oxide of iron, deposited upon the calico in the alternate passages through the sulphate of iron and lime vats. They are then rinsed an hour in running water, and finally brightened in the above dilute sulphuric acid, slightly tepid. Sometimes they are subjected to a soap bath, at the temperature of 120°. By the addition of nitrate of lead to the indigo vat, the blue becomes more lively. Some use the roller dyeing apparatus for running the pieces through the respective baths instead of the square frames. (See [Wincing].) But the frame-dip gives the most evenly dyes, and preserves the vats in good condition for a much longer time.

The various phenomena which occur in the dipping of China blues, are not difficult of explanation with the lights of modern chemistry. We have, on the one hand, indigo and sulphate of iron alternately applied to the cloth; by dipping it into the lime, the blue is deoxidized, because a film of the sulphate of iron is decomposed, and protoxide of iron comes forth to seize the oxygen of the indigo, to make it yellow-green, and soluble, at the same time, in lime-water. Then, it penetrates into the heart of the fibres, and, on exposure to air, absorbs oxygen, so as to become insoluble and fixed within their pores. On dipping the calico into the second vat of sulphate of iron, a layer of oxide is formed upon its whole surface, which oxide exercises an action only upon those parts that are covered with indigo, and deoxidizes a portion of it; thus rendering a second dose soluble by the intervention of the second dip in the lime-bath. Hence we see that while these alternate transitions go on, the same series of deoxidizement, solution, and re-oxidizement recurs; causing a progressively increasing fixation of indigo within the fibres of the cotton. A deposit of sulphate of lime and oxide of iron necessarily falls upon the cloth, for which reason the frame should be shaken in the lime water vat, to detach the sulphate; but, on the contrary, it should be held motionless in the copperas bath, to favour the deposition of as much protoxide upon it as possible. These circumstances serve to account for the various accidents which sometimes befall the China blue process. Thus the blues sometimes scale off, which may proceed from one of two causes:—1. If the goods are too dry before being dipped, the colour swells, and comes off in the vats, carrying along with it more or less indigo. 2. If the quantity of sulphate of lime formed upon the cloth be considerable, the crust will fall off, and take with it more or less of the blue; whence arise inequalities in the impression. The influence of temperature is important; when it falls too low, the colours take a gray cast. In this case it should be raised with steam.

VI. The decolouring or enlevage style; not by the removal of the mordant, but the destruction of the dye. The acid, which is here mixed with the discharge paste, is intended to combine with the base of the chloride, and set the chlorine free to act upon the colour. Among the topical colours for this style are the following:—

1. Black.—Take one gallon of iron liquor of spec. grav. 1·086.
1. Black.—One pound of starch; boil together, and while the paste is hot, dissolve in it
1. Black.—One pound of tartaric acid in powder; and when cold, add
1. Black.—Two pounds of Prussian blue, prepared with muriatic acid, see [p. 226.]
1. Black.—Two ounces of lamp black, with four ounces of oil.
2. White discharge.—Take one gallon of water; in which dissolve
2. White discharge.—One pound and a half of oxalic acid,
2. White discharge.—Three pounds of tartaric acid; add
2. White discharge.—One gallon of lime juice of spec. grav. 1·22; and thicken with
2. White discharge.—Twelve pounds of pipe clay, and six pounds of gum.
3. Chrome-green discharge.—
3. Chrome-green disTake one gallon of water, thicken with 18 ounces of starch; boil
3. Chrome-green disand dissolve in the hot paste
3. Chrome-green disTwo pounds and a half of powdered nitrate of lead,
3. Chrome-green disOne pound and a half of tartaric acid,
3. Chrome-green disTwo pounds of Prussian blue, as above.
4. Blue discharge.—Take one gallon of water, thicken with
4. Blue discharge.18 ounces of gum; while the boiled paste is hot, dissolve in it
4. Blue discharge.Two pounds of tartaric acid, and mix one pound of Prussian blue.
5. Chrome-yellow discharge.—This is the same as the chrome-green given above, but without the Prussian blue.
6. A white discharge on a blue ground, requires the above white discharge to be strengthened with 8 ounces of strong sulphuric acid, per gallon.
7. White discharge for Turkey red needs to be very strong.
7. Take one gallon of lime juice of sp. grav. 1·086; dissolve in it
7. Five pounds of tartaric acid; thicken with
7. Eight pounds of pipe-clay, four pounds of gum; then dissolve in the mixture
7. Three pounds of muriate of tin in crystals; and add, finally,
7. Twenty-four ounces of sulphuric acid.
8. Yellow discharge for Turkey red.—
8. Yellow Take one gallon of lime juice of spec. grav. 1·086; in which dissolve
8. Yellow Four pounds of tartaric acid,
8. Yellow Four pounds of nitrate of lead; thicken the solution with
8. Yellow Six pounds of pipe-clay, and three pounds of gum.
9. For green discharge, add to the preceding 24 ounces of Prussian blue, as above.

The decolouring or chlorine bath is usually formed of wood lined with lead, and has an area of about 5 feet square, with a depth of 6 feet. A square frame, mounted with a horizontal series of rollers at top and bottom, may be let down by cords, at pleasure, into the cistern. The pieces are introduced and guided in a serpentine path, round the upper and lower rollers alternately, by a cord.

This bath is filled with a solution of chloride of lime, of the spec. grav. 1·045, whose decolouring strength is 65° by Gay Lussac’s indigo chlorometer. It ought to be made turbid by stirring before putting in the goods, which should occupy three minutes in their passage. The piece is drawn through by a pair of squeezer cylinders at the end of the trough, opposite to that at which the piece enters. With black, white, and blue impressions of all shades, the goods are floated in a stream of water for an hour; then rinsed and dried. When there is yellow or green, the pieces must be steeped in water, then merely washed by the wince, and passed through solution of bichromate of potash, containing from 3 to 5 ounces of the salt per piece. Here the pieces are winced during 15 or 20 minutes, rinsed, and next passed through dilute muriatic acid to clear the ground; then rinsed and dried.

Discharge by the intervention of the chromic acid.

After having dipped the pieces to the desired shade, they are padded in a solution of bichromate of potash; dried in the shade without heat; and then printed with the following mordant:—