The stannic indigotate in paste, or as it is generally called precipitate of indigo, is prepared by turning into the clear portion of a strong copperas vat an acid solution of protochlorate of tin, and filtering it upon woollen filters,—as much as possible away from the air. It would be better to prepare a strong tin vat by heating a mixture of indigo, caustic soda, and protochlorate of tin, and to precipitate by chlorohydric acid.[9]
------------------------------------------------------------- [9] Mr. T. P. Shepard gives in his valuable “Receipts for Calico Printing,” published in 1872, the following:—
NO. 52. INDIGO PRECIPITATE FOR FAST BLUE AND GREEN.
10 pounds quicklime, slacked with 6½ gallons water; then 2 pounds ground indigo finely rubbed in water are stirred in; then add 6 pounds copperas dissolved in 5 gallons of water; then add 5 gallons hot water and 15 gallons cold water.
Stir well from time to time, until the liquid has assumed a yellow color and deep blue veins or streaks appear on its surface. When this moment arrives, draw off the clear liquor, and precipitate every ten quarts of it with
½ pound tin crystals, dissolved in ½ pound muriatic acid.
To the remainder of the mixture of lime and indigo, 15 gallons of water may be added, and the whole stirred; and when settled, the indigo may be precipitated from the clear liquor as before. This operation may be repeated a second time before all the indigo is exhausted.
The indigo precipitate is to be collected on a muslin filter, and well squeezed out. -------------------------------------------------------------
The deposit is made into a paste with gum water; a salt of tin is often added to prevent oxidation. It is important to prevent the transformation of the indigo white into indigotine before printing. This indigotine would not fix itself on the fabric. Moreover, after printing, it is necessary to hasten the dissolution of the indigo white to enable it to penetrate the fibre. It is sufficient for this end to pass it through milk of lime. The stannic combination is immediately destroyed; the colorable matter unites itself with the lime, and the color passes into a pale gray with apple green. The indigo white becomes momentarily soluble; but the presence of the excess of lime and the thickening, as well as the attractive affinity of the thickening, prevent any running.