The Dyeing of Woollen Fabrics

by Franklin Beech

Various methods may be adopted to cause the indigo to become dissolved. These may be divided into two groups: (1) Fermentation vats, in which the action of reducing agents is brought about through the influences of the fermentation of organic bodies, such as woad, bran, treacle, etc; (2) Chemical vats in which the reducing effect is brought about by the reaction of various agents on one another.

Of such vats the copperas and lime and the hydrosulphite vats are examples. The fermentation vats, when in good order, work well and give good results, but they are most difficult to prepare or set. The chemical vats are the easiest to work, and (especially the hydrosulphite vats) are coming to the fore, and are gradually driving out the fermentation vats.

The actual method of dyeing with the indigo vat is the same with all methods of preparation. The material to be dyed is well wetted or wrung out in water. It is then dipped into the vat, handled a few minutes to ensure its thorough impregnation, then lifted out, the surplus liquor wrung out, and the material exposed to the air, when the indigo white on it soon absorbs oxygen and turns into blue indigo.

With these few preliminary remarks the methods of setting the various indigo vats will now be described in detail.

Woad Indigo Vats.--This is one of the most difficult of the various methods of setting vats. There are so many opportunities for it to go wrong, and to be able to set a woad vat successfully will go far to make a man a successful indigo dyer. No two woad vat dyers use exactly the same recipe in setting a woad vat, and each considers he has a secret art by means of which he ensures the successful working of this vat, and this he jealously guards. All these differences in the manner of setting the vat are brought about not by any radical differences in the materials used, but by some unnoticed differences in other surroundings; differences in the mean temperature of the water used, in the general conditions of the atmosphere of the indigo shed and in other similar circumstances, all of which have a material influence on the development of the vat, but which are, in the majority of cases, overlooked by the indigo dyer, the result being that a method of working which is successful in one place would not be so in another. The fermentation processes depend upon the reducing action brought about by certain organisms of the nature of the yeast plant which grow and develop in such vats.

To ensure the proper growth and development of these organisms every condition must be perfect, correct temperature, proper proportions of food for them to live on, and a certain degree of alkalinity or acidity of the vat, and these points are most difficult to regulate as they will vary very much from time to time.

A successful vat maker is one who closely observes his vats, and the way in which they are working, and who, as the result of such observations, is able to tell in what way his vats are deficient, so that he may know how to supply that deficiency.

The following method of setting a woad vat may be adopted. It is calculated for 100 gallons of liquor. The vat is filled with hot water, and 80 lb. of woad are allowed to steep overnight in it, having first been well stirred into the water, so as to ensure that every part is wetted out. The next morning there is added 8 lb. madder, 12 lb. bran, 5 lb. quick-lime (previously slaked with water), and 2-1/2 lb. soda. These are thoroughly stirred together, then from 5 to 7-1/2 lb. indigo is stirred in. The indigo should have been previously ground into a fine paste with water. The temperature of the vat should now be maintained at from 115° to 125° F. for two to three days, at the end of which time it ought to be in a state of quiet working. Should it be found that the fermentation is going on too rapidly, a little lime may be thrown in, which will retard it. On the other hand, if it should not be going on with sufficient energy, this may be remedied by adding a little bran, or better, a little treacle.

When in perfect condition the vat should have a slight smell of ammonia. If this is not noticed it indicates that the vat is deficient in alkalinity, and a little more lime should be added. Soda may be used in the place of lime, but it is so much more energetic in character that any additions of it have to be made with great care, or the vat will become too alkaline in character, and the fermentation will go on too rapidly, the ammoniacal odour is lost, and a peculiar putrid smell takes its place. As soon as this is noticed, lime ought to be added to retard the fermentation and to develop the ammoniacal smell. The colour of a good well-set vat is olive brown.

When all the indigo is dissolved and the colour of the vat is a clear olive yellow to brown the vat is then ready for dyeing, and may be used for a long time, until, in fact, the deposit gets too large and the wool becomes dirtied. But it must not be continually worked, or it will give bad shades and loose colours. When in a bad condition it will usually turn of a dark brown colour, and give dull greenish shades. To remedy this there should be added some bran, treacle, and a little madder, as well as indigo, and the vat should be left for a day, at a temperature of 130° F., to get up to full strength again. Every night when in work indigo ought to be added to the vat in proportion to that consumed during the day, with bran and lime, the latter in not too great amount, just sufficient to keep it of the necessary alkalinity.