The Dyeing of Woollen Fabrics

by Franklin Beech

The group of adjective colouring matters may be subdivided into two divisions, not depending upon any differences in the mode of application, but upon certain differences in the results they give. Perhaps the best example of an adjective dye-stuff is Alizarine. This body has a faint red colour, but of itself possesses absolutely no colouring power. When, however, it is brought into combination with such metallic oxide as alumina, iron and chrome, then it forms coloured bodies, the colour of which varies with the metal with which it is in union, thus with alumina, it is a bright red; with iron, a dark violet, almost black; with chrome, a deep red; with tin, a scarlet; and so on. This is a representative of the true adjective dyes, which comprise most of the so-called Alizarine dye-stuffs, and logwood, fustic, and most of the natural dye-stuffs. Another division of the group includes a few colouring matters of recent introduction, like Azo green, Alizarine yellow, Galloflavine, Anthracene yellow, Flavazol, etc., which, while forming insoluble colour lakes with metallic oxides, do not give different colours with different metals.

This class of dye-stuffs, owing to their forming these insoluble colours, gives really fast colours, capable of resisting lengthened exposure to light and air, and resisting washing, acids and alkalies. Of course there are differences between the various members of the group in this respect, and even the resisting power of an individual member depends a good deal on the metal with which it is combined, and the care with which the process of dyeing has been carried out.

In the dyeing of these adjective dye-stuffs, upon the various fibres, and on wool in any particular, the object is to bring about in any convenient way the formation on the fibre of the metallic combination of the colouring principle and the mordant, and it is obvious that if a satisfactory result is to be obtained, then this must be done in a very thorough manner. There are three ways in which this combination of colouring principle and mordant may be brought about in dyeing wool with these bodies, we may either mordant the wool first, and then apply the dye-stuff, or we may impregnate the wool with the dye-stuff first, and then fix or develop the colour afterwards, or, lastly, we may carry on both operations in one process. Each of these methods will now be discussed, and their relative advantages pointed out.

The mordanting method is one of the most generally useful. It consists in first causing a combination of the metal with the wool fibre. This is carried out by boiling the wool in a solution of the metal, such as bichromate of potash, chrome alum or chrome fluoride when chrome is to be used as a mordant, with alum or sulphate of alumina when alumina is required to be deposited on the fibre, and with copperas when iron is to be the mordant. It is best to add a little oxalic acid, cream of tartar, or tartaric acid to the mordanting bath, which addition helps in the decomposition of the metallic salt by the wool fibre, and the deposition of the metallic oxide on the wool. With bichromate of potash, sulphuric acid is often used, much depending upon the character of the mordant required. Some dye-stuffs, such as logwood for blacks, work best when the wool is mordanted with chromic acid, which is effected when sulphuric acid is the assistant mordant. Other dye-stuffs, such as fustic, Persian berries and Alizarine yellow, are best dyed on a basic chrome mordant, which is effected when tartar or oxalic acid is the assistant mordant used, or when some other form of chrome compound than bichrome is employed.

The actual mordanting is done by boiling the wool in a bath of the mordant, the quantity of which should be varied according to the particular mordant that is being employed and to the quantity of dye-stuffs which is to be used. It is obvious that for a fixing deep shade of, say, Alizarine on the wool, a larger quantity of mordant will be required than to fix a pale shade; sometimes this point is overlooked and the same amount of mordant employed for pale or deep shades. The best plan of carrying out the mordanting is to enter the wool in the cold bath or at a hand heat, and then raise to the boil and continue the boiling for one hour; of course the goods should be kept turned over during the process to facilitate the even mordanting of the wool. A great deal of the success of dyeing with the dye-stuffs now under consideration depends upon the efficiency with which the mordanting has been carried out. If this is at all unevenly done then no amount of care in the succeeding dyeing process will lead to the development of an even dyeing. After the mordanting is finished the goods should be rinsed with water, but it is not necessary to dry them.

The next stage in the process is the actual dyeing operations, which is done by immersing the mordanted wool in a bath of the dye-stuff or mixture of dye-stuffs.

The fundamental principle is to bring about the combination between the colouring principle of the dye-stuff and the metallic oxide which has been deposited on the wool in the previous mordanting process. As neither of these bodies, however, is very energetic it follows that the action must be a slow one, and, therefore, time is a highly important factor in the dyeing of wool by the mordanting process. The combination between the dye-stuff and the mordant is influenced also by temperature, and is most active at the boiling point of water. It is, therefore, needful to conduct this operation at that temperature, but it would be a wrong way to introduce the mordanted material into a boiling bath of the dye-stuff; nothing would conduce to uneven dyeing so much as that course. The best method of working, which, moreover, is most particularly applicable to the series of Alizarine dye-stuffs, is to enter the goods in a cold bath of the dye-stuff, and to work them for a short time to get them thoroughly impregnated, a condition which is essential if even dyeing is the goal aimed at, then to raise the temperature of the bath gradually to the boil, the goods being in the meantime well worked. The dyeing is continued for from one to one and a half hours at the boil.

It is important in dyeing by this process, especially when using Alizarine, to keep the temperature of the bath as uniform as possible, and the goods well worked. Alizarine, and some other members of this class, are rather sensitive to heat, and if a dye-vat be hot at the bottom and cold at the top uneven dyeing is sure to be the result; this is due to the greater affinity of the Alizarine for the mordant at the high than at the low temperature, and thus more is fixed on to the wool. The remedy for this is to so construct the heating arrangements of the vat that the temperature shall be as uniform as possible, while the goods should be kept continually turned over, and every portion of them brought into intimate contact with the dye-liquor. The continuance of the dyeing operations for one and a half to two hours after the vat has reached the boil is necessary to properly develop and fix the colour on the fibre; a short boil leaves the goods of a poor shade, without any solidity about it, and the colour is loose, while a longer boil brings up a solid shade and a fast colour.

Although it is not absolutely necessary to add any acid to the dye-bath during the dyeing operations, yet as the Alizarines and most of this class of dye-stuffs dye better in a slightly acid bath it is advisable to add a small quantity of acetic acid, say about one pint to every 100 lb. of goods; this serves to correct any alkalinity of the water, which may be due to its containing any lime. Dye-stuffs of the acid class, such as indigo extract, Cloth red, Acid magenta, etc., may be used along with the Alizarine dye-stuffs, in which case the addition of acid to the dye-bath becomes necessary, but too great an excess of acid should be avoided, as it interferes somewhat with the dyeing of the mordant dyes.

This is by far the best and most generally used method of applying these mordant dyes. It is not a costly process, being indeed economical, as it only requires just the right amounts of drugs and dye-stuffs, and there is the minimum loss of material in the mordanting and dye-baths. Shades can be brought up with the greatest ease, although it is well in the dyeing to add rather less dye-stuff than is actually required, and to add more when it is seen how the shade is coming up. The labour is the most important item in the mordanting and dyeing method.