Cotton is boiled off at actual boiling heat for two hours, with 8 per cent. of its weight of carbonate of soda and a little soft soap, which treatment is sufficient for dark colours.
For light colours it is necessary that the cotton be bleached. Wool is scoured with soda and soap in the proportion of 10 lb. soda and 2 lb. Marseilles soap for 100 lb. wool. Silk is scoured by boiling for one and a half hours in a boiling bath with 30 per cent. of its weight of soap. For light colours a second boiling should be given, with 15 per cent.
The careful cleaning of wool previous to dyeing is of exceptional importance. Raw wool is cleaned with carbonate of soda and ammonia. For 50 lb. wool to be cleaned 6 lb. carbonate of soda and 1-1/2 lb. ammonia are added to a bath of 150 gallons water. The wool is laid down in it for twenty minutes at 35° C., taken up, squeezed, treated for fifteen minutes in another bath, with 5 lb. carbonate of soda and then rinsed. The first bath must be renewed as often as possible, because it contains all the impurities. In the case of woollen yarn 30 lb. require two tubs of 40 gallons capacity. The first tub is to contain 35 gallons water and 2 lb. ammonia at 10° Be. After working the skeins for three minutes in it they are left to stand for fifteen minutes, then wrung out, and the operation is repeated in the second tub. Finally, the yarn is rinsed several times in soft water.
Woollen piece goods are treated in a large wooden tub at 40° C. with 4 lb. carbonate of soda and 2 lb. carbonate of ammonia for 80 lb. material. The pieces are moved about for twenty minutes, laid down in the bath overnight, again turned for ten minutes and hydro-extracted. They may also be handled for forty minutes in a bath of 2 oz. ammonia for 100 lb. wool at 60° C., and then for twenty minutes in clear water at 60° C.
After wetting or preparatory treatment, it will be best to proceed immediately to dyeing; if the fibres be left in a heap for too long a time, there is danger that they may become heated, or at least that the moisture may be irregularly distributed by the occurrence of partial drying, causing an uneven fixation of the colour in the first stages of dyeing. The first two conditions of successful dyeing are, therefore, a suitable wetting out and scouring. The dyer, however, must not be less careful to see that the dye-bath is what it ought to be.
Whenever possible the dye-stuff must be dissolved separately, or at least the bath not entered before the dye-stuff is well dissolved. Artificial dye-stuffs require particular attention to this point, because the presence of undissolved particles is the cause of irregularities, such as streaks, or, at least, specks. The solution is mostly made hot as follows: After pouring water at 180° F. upon the dye-stuff, stir gently, strain through flannel or through a very fine sieve, and pour more water upon the residue until nothing more is dissolved. As is well known, the artificial dye-stuffs often contain insoluble matter, resins, etc. It is therefore advisable to use only soft water for this operation.
The solutions of artificial dye-stuffs are ordinarily made at the rate of 1 to 5 lb. per 10 gallons of water, 2 lb. being the proportion mostly employed. This depends more or less on the solubility of the dye-stuff. Old solutions sometimes contain crystals of the dye-stuff which have separated out. These should be redissolved by heating before the solution is used. But it is best to make only such a quantity of solution as will suffice for immediate requirements.
With paste colours care should be taken to keep them in closed vessels in such a manner that they will not become hard by evaporation, and they should not be kept in any place where they are likely to freeze in winter time. In such an event it is not an uncommon circumstance for the casks or other vessels containing them to burst, with a consequent loss of dye-stuff. Before any of the paste is withdrawn from the cask, it is advisable to stir well up with a wooden stirrer.
In adding dye-stuff during the actual dyeing operation, it is advisable to add the dye-stuff to the bath in two or three portions, always taking out the goods before adding each lot of dye-stuff, and stirring up the contents of the bath before re-entering the goods. Another important condition of obtaining a level dyeing is to proceed slowly, beginning with a weak bath at a moderate temperature, and rising gradually to a boil. If necessary to retard the dyeing from the commencement, then an assistant mordant is added to the dye-bath, in the shape of soda crystals or phosphate of soda for the benzidine colours on cotton; bisulphate of soda or Glauber's salt in dyeing with azo colours or acid colours on wool; or tartar may be used in most cases with good effect, causing the wool to have a softer feel. Finally, the evenness of the dyeing is much increased by the frequent turning over of the material in the dye-bath, so managing this in the case of wool as to avoid felting.
When dyeing with a mordant, the dyer should see that the mordanting operation is thoroughly well done, for as much care is required for the mordanting as for the actual dyeing; in fact, if anything, the mordanting should be done with rather more care, as if it be at all defective no amount of care in the following dyeing operations will ensure a level dyeing. Chrome mordanted wool should be dyed without delay, as it is rather sensitive to light, especially the yellow sort, which gradually changes into the green sort of chromed wool.