Persoz gives the following general receipt:—
“1 part by weight of indigo, finely rubbed. 1 „ „ „ „ Nordhaussen acid. 1 „ „ „ „ ordinary sulphuric acid. Leave for forty-eight hours, then heat until a drop turned into water will dissolve without producing a precipitate. Leave to cool, and dilute with water till the strength is brought to 18 Beaumé.”
Napier says that he has found the following method of preparing sulphate of indigo, in quantities for use, very satisfactory: “The indigo is reduced to an impalpable powder, and completely dried by placing it on a sand bath or flue for some hours at a temperature of about 150° F. For each pound of indigo six pounds of highly concentrated sulphuric acid are put into a large jar, or earthen pot, furnished with a cover. This is kept in as dry a place as possible, and the indigo is added gradually in small quantities. The vessel is kept closely covered, and care taken that the heat of the solution does not exceed 212°F. When the indigo is all added, the vessel is placed in such a situation that the heat may be kept up at about 150°F., and allowed to stand, stirring occasionally, for forty-eight hours. These precautions being attended to, we have uniformly found that any failure occurring was clearly traceable to the impurity of the indigo or weakness of the acid used.”
The processes for producing and separating the two acids derived from the combination of sulphur and indigo are minutely given by Berzelius, in vol. i. of his “Traité de Chemie,” who states this curious fact illustrative of the peculiar affinities of wool with certain dyeing substances. Wool or flannel thoroughly scoured, when immersed in the blue solution of indigo with sulphuric acid, acts as a base: it combines gradually with the acid blue, and becomes itself colored of a deep blue. When saturated with color, it is withdrawn. Fresh wool is introduced until the bath yields no more color. If sublimed or perfectly pure indigo is used, there remains in the bath nothing but free sulphuric acid. The wool thus plays the part of a base with which the blue acids combine. The dyed wool is afterwards washed and treated in feeble alkaline bath (ammonia), which redissolves the blue. This method of purifying the Saxon blue is still practised by French manufacturers.