First Method.--This method is used in applying the now large and increasing group of azo dye-stuffs, which are characterised by being able to dye unmordanted cotton from a simple boiling bath. The dye-stuffs that are applied by the method now to be described include such as Benzopurpurine, Chrysamine, Chrysophenine, Titan red, Titan yellow, Benzo brown, Diamine red, Diamine brown, Diamine blue, Congo blue, Congo red, etc. The dyeing is done in a bath at the boil. If the bath contained only the dye-stuffs there would be a liability for the dyeing to be uneven, to prevent which a saline compound, such as salt, is added. Taking it all round, salt is the best body to add as it suits all colours very well indeed. Then come Glauber's salts; borax and phosphate of soda can also be used, but, owing to their slight alkaline properties, they are not so good as the neutral salts, like the two first named. When these colouring matters are dyed on cotton some of them dye best in a bath containing potash or soda, but these bodies, for reasons previously pointed out, are not available in wool dyeing, and should never be used. Wool dyes best in a slightly acid bath, and this may be taken advantage of in dyeing the yellows and blues of this group by adding a small quantity of acetic acid. The reds, as a rule, are affected by acids, and, therefore, it is not possible to use an acid bath with Benzopurpurine, Congo red, with the possible exception of the Titan reds and scarlets, Diamine scarlet, Benzo fast scarlet, Purpuramine, which are faster to acetic acid than the other reds of this class of dye-stuffs.
Probably the best plan of dyeing these colours is to first heat the bath to about 160° F., then enter the goods, and turn over two or three times to ensure that they are thoroughly impregnated with dye-liquor. The bath is now raised to the boil, and, steam being turned off, the goods are handled without further steam until the desired shade is obtained. Another plan is to enter the goods when the bath is at about 150° F., and, after raising to the boil, to work for half to one hour at that heat; but the plan first described gives rather better results, and is far preferable. The dye-baths, as a rule, are not completely exhausted, except when very pale shades are being dyed; in no case is it necessary to throw the dye-bath away, but simply to add the required amount of dye-stuff for a new batch; with those colouring matters which are not entirely exhausted from the bath a smaller amount, generally about three-fourths only, is required to be added, with about one-third the quantity of salt which was added to the first bath. Of course it is not advisable to keep the same bath or liquor in work always, but after about twenty or thirty batches of goods are dyed to throw it away and start a fresh liquor.
As a rule it will be found that these dye-stuffs are more thoroughly taken up from the bath than is the case in dyeing cotton; thus often with the same amount of dye-stuff in proportion to the material used the wool will dye rather a deeper shade than will cotton. In some cases, especially with the blues and violets, the shade is greatly different on wool from what it is on cotton, being generally redder and much stronger. (See the chapter on Union Dyeing.) While the shades are somewhat faster to light on wool than they are on cotton, they are no faster to soaping and in some cases not so fast. What may be the function of the salt, or other such added substance, is not very clear, probably it plays the same part as to similar bodies in dyeing the basic dye-stuffs. The dye-stuffs which are referred to above are all derived from coal-tar, and in the recipes which follow many examples of their use will be found.
There are but few natural dye-stuffs that have any direct affinity for wool. Turmeric, saffron, anotta, are about the only representatives, and these are not of much importance in wool dyeing by themselves, although they are sometimes used in conjunction with other natural dye-stuffs, when they are applied by a process which is adapted more especially for the other dye-stuff which is used.
Second Method.--The method of wool dyeing now being dealt with does not differ essentially from that described above, but as it is applied to quite a different class of dye-stuffs it is thought better to consider it as a second method. The dye-stuffs made use of in this method are what are called the basic coal-tar colours, and it may be remarked in passing that there are no natural colouring matters having the same properties. These dye-stuffs are derived from a number of so-called colour bases, such as Rosaniline, Pararosaniline, Methylrosaniline, Phenyl-rosaniline, and Auramine base. Many of these are colourless bodies containing the Amidogen group NH2, which imparts to them basic properties enabling them to combine with solids to form salts, and these salts have a strong colouring power. They form the commercial dye-stuffs Magenta, Saffranine, Thioflavine T, Auramine, Benzoflavine, Brilliant green, Methyl violet, etc., and these are salts (usually the hydrochloride) of colour bases. All these basic dye-stuffs have strong affinity for the wool fibre, and will immediately combine with it, dyeing it in colours which resist washing, etc., to a considerable extent, although there are great differences between the various members of the group in this respect. It has been shown that what takes place in dying wool with these colouring matters is that the colour base combines with the fibre the acid of the dye-stuff remaining in the dye-liquor.
Although it is possible to dye wool with the basic dyes from a plain bath containing water only, yet the results are not satisfactory, especially when working on a large scale; and for dyeing pale shades especially, the affinity of the dye-stuff for the fibre is so great that the first portions of the goods which are entered into the dye-bath have a great tendency to absorb all the dye-stuff, or the larger proportion of it, so that uneven dyeing is the result, one end of the piece of cloth being darker than the other end. This defect is particularly accentuated when pale tints are being dyed, the colouring matter being completely absorbed before all the goods are entered into the bath, but it may be remedied by adding the dye-stuff to the bath in small quantities at intervals during the process of dyeing. The best and most satisfactory method, however, is to add to the bath 10 per cent. of the weight of the wool of Glauber's salt, or some other neutral alkaline salt, which addition almost entirely prevents any defect of uneven dyeing. How these assistant mordants act is somewhat uncertain, the explanation generally given is that they exert a slightly solvent action on the dye-stuff, and so prevent it from going upon the fibre too readily. This is scarcely an adequate explanation, but in want of a better it will have to stand.
The affinity of the basic dyes for wool increases with increase of temperature. This is a property that has an important bearing on the method of dyeing, and to any person who pays some attention to theory in its practical applications it indicates the most rational method of working, which is to enter the goods into the bath cold, or, at the most, at a hand heat, then, after working a short time to get the goods thoroughly impregnated with the dye-stuff, to gradually raise the temperature to the boil and work for from half an hour to an hour longer, even if before this time the dye-bath be exhausted. The reason for giving a fair length of time in the bath is to get the colour properly fixed on the fibre. The combination of the dye-stuff and the fibre is a chemical one, and, as stated above, the dye-stuff has to be decomposed so that the base may combine with the essential constituent of the wool fibre, while it is obvious that this decomposition and then the union of the colour base with the wool must take time, and as it is effected more easily and completely at the boiling point, it is advisable to work the goods in the bath so as to fully insure that they are given the necessary time for the chemical change to take place.
The dye-bath is generally completely exhausted of colour, but if fairly clean it need not be thrown away, but used for another batch of wool by simply adding more Glauber's salt and dye-stuff. After a time the bath gets too dirty to used, when it may be thrown away, and a new dye-liquor made up.
In dyeing for pale shades it is best to add the dye-stuff in small quantities at intervals during the process of dyeing, and to run the goods quickly through the bath, so as not to give the dye-stuff too much opportunity to become absorbed by a portion of the goods only.
Working according to the hints given above, the dyeing of wool with the basic coal-tar colours may be carried out in a very satisfactory manner.