Figure 23 also shows an excellent machine for indigo dyeing on cloth. In this the vat has a frame carrying guide rollers, round which the cloth passes, so that it travels several times through the vat liquor in its passage from one end of the vat to the other, the amount of liquor in the vat being so arranged that the cloth is entirely immersed the whole time. After going through the liquor the cloth passes between a pair of squeezing rollers, in order to have any surplus liquor taken out, then it traverses the space between sets of guide rollers arranged over the vat, during which time the indigo becomes oxidised and the blue develops, while finally it is plaited down on a table. The illustration clearly shows the working of the machine.
Dyeing Wool with Indigo Extract.--Sulphonated indigo, prepared by dissolving indigo in sulphuric acid, is sold under the name of "indigo extract," or "indigo carmine," in two forms--paste (containing, perhaps, 25 to 30 per cent. actual colour) and powder. Both forms are freely soluble in water, although some makes are more so than others. This quality of solubility is dependent upon the proportion of sulphuric acid which may have been used in the preparation of the extract. When this is small, what is termed indigo monosulphonic acid only is formed, which is but slightly soluble in water, and gives red shades. If a larger proportion of acid be used, then the indigo disulphonic acid is formed, which is fairly easily soluble in water, and gives bluer shades than the former.
As all forms of indigo extract are regular articles of commerce, details for their preparation will not be given here. It will suffice to say that indigo is heated with strong sulphuric acid until test samples show that the indigo has been completely dissolved, and it is then diluted with water and filtered. Sometimes it is sold in this condition under the term "chemic," but if this be used in dyeing wool it gives rather unsatisfactory results. When "sour extract" is required, the liquor filtered out is next treated with salt until all the colour has been precipitated out, when it is filtered off, drained, pressed and sold. Should "neutral" or "sweet" extract be required, then the acid liquor is neutralised with soda, and the product is salted out as before, drained and pressed to a suitable consistence. It is then sold as "indigo extract," or dried, at 150° F., to a powder, which is known as "indigo carmine".
All forms of indigo extract are dyed on wool from baths of Glauber's salt and sulphuric acid, and therefore they can be classed with the acid-dyeing coal-tar colours. Indigo extract is notable for its level dyeing and penetrative properties, but it is not fast to light or milling.
Messrs. Read Holliday & Sons have a powder form of indigo extract which will be found very useful and to give better shades than the usual run of paste extract, while it only takes about one-fifth the quantity to give a similar shade. Working at the boil should be avoided with indigo extract, as tending to make the shades greenish in tone; from 170° to 180° F. will usually be found hot enough to dye good shades.
Indigo extract is not much used by itself in dyeing blues on wool, but it is extensively employed along with other dye-stuffs to produce an immense variety of shades--drabs, greens, fawns, greys, lilacs, etc., of which some examples will be given later on.
Indigo Blue.--Prepare a bath with 10 lb. indigo extract, 5 lb. sulphuric acid, and 10 lb. Glauber's salt. Work just under the boil to shade.
Sky Blue.--The dye-bath contains 1 lb. indigo extract, 2 lb. sulphuric acid, and 10 lb. Glauber's salt. Work at about 160° F. to shade.
Dyeing Wool Blue with Logwood.--This method of dyeing blue on wool has lost much of its importance since the introduction of the artificial dyes, but it is still employed when a blue fast to milling is wanted. Logwood gives dark navy blue shades. The process is as follows: The wool is first mordanted by boiling for one and a half hours in a bath of 3 lb. bichromate of potash and 2-1/2 lb. of tartar. The operation must be so carried out that the non-oxidising green chrome mordant is developed on the fibre, and therefore the boiling must be thorough. In place of tartar, argols and oxalic acid are frequently used, while lactic acid or lignorosine might be employed. The dyeing is done in a bath of 20 to 25 lb. logwood, or 5 to 8 lb. logwood extract; the bath is started cold, heated slowly to the boil, and kept at that heat for one to one and a half hours. Between the mordanting and dyeing the wool should be well rinsed.
Dyeing Blue with Coal-tar Dyes.