All worsted and woollen yarns used for the surface of carpets, with the exception of those whereon the colours are printed, as in Tapestry carpets, are dyed; and fast and accurate dyeing is of cardinal importance in carpet manufacture.
Practically all worsted yarns, and some woollen yarns, however, are supplied to carpet manufacturers by the spinners in oil or in grease; that is to say, still impregnated with the oil which has been put into them in the process of spinning. The freeing of the yarn from this oil by washing is a necessary preliminary to dyeing, inasmuch as the presence of oil would prevent the dye liquor permeating the wool fibres.
Scouring is effected by soap and hot water, generally by machines of one type or another. The yarn in skeins is sometimes thrown upon swifts, which revolve over a vat of suds. The skeins are carried through the liquor and well rinsed by the rotation of the swifts. Another method employed is to pass the skeins, tied together in a long chain, through vats in which they are steeped in the suds, and through rollers which squeeze them more or less dry. Or the skeins may travel along a series of aprons, underneath a shower, between rollers, or through bowls of suds. The precise method adopted is not of great importance provided that the scouring solution has full opportunity to operate upon the wool fibres, so that the yarn is delivered ultimately free of oil and of soap. For this reason, a final bath of pure water is desirable.
The solvents principally used are ammonia, alkali, and an alkaline soap of some kind. Cotton-seed oil soap is generally regarded as one of the most suitable.
The scoured yarn is now ready for dyeing, and passes, without being fully dried, to the sticks or frames of the hand-vat or dyeing machine.
The three main considerations to be kept in view in the dyeing of worsted and woollen yarns for carpets are levelness of colour, accuracy of matching, and fastness of colour to light. The second and third are matters which concern the skill of the head dyer and the quality of the dyestuffs which he employs; the first depends upon the handling of the yarn by the operative in the hand vat or dyeing machine; and the essential factor, apart from the proper temperature of the dye-liquor, is motion.
Dyeing by hand, although superseded almost entirely by machine-dyeing, is still employed in some cases, particularly where only small quantities are required. The dye-vat is of wood, and should be a well-made piece of joinery to withstand the wear and tear incidental to its use. Round the bottom on the inside is fitted a tube of copper or some acid-resisting metal, perforated with small holes, through which steam is passed to heat up the water. The head dyer weighs out the dyestuffs estimated to produce a shade a fraction below the required shade, so as to admit of final adjustment after testing. This, of course, applies equally to hand or machine-dyeing. For hand vats, the skeins of scoured yarn are hung on a set of poles, which, when filled, are placed on brackets above the vat. When the dyestuff has been put in, and the water brought to the required temperature, a pair of workmen, each holding one end of the poles, standing on each side, sets them on the edge of the vat and lets the free ends of the skeins sink down into the liquor. When all the poles are in place, the operators start on the pole at one end, and, raising each skein in turn, draw it over a few inches, so that the part which has hitherto remained outside the liquor is now immersed. When the skeins on one stick are finished, that stick is pushed a little way along and the next is treated similarly. When the whole set is done, the operation is repeated. The essence of the treatment is to keep the yarn moving. If the skeins were allowed to remain in the same place, even after the first moving, there would be “stick marks” at the point where the skein rested on the pole, because the liquor would not have had as much access to this place as to the rest of the skeins.
After an hour or so, or when sufficient time has been allowed for the dyestuffs to permeate the yarn fibres, the poles are lifted on to the brackets, and a skein is taken out and submitted to the head dyer for matching. The head dyer will compare it with his standard, and, in the case of dyeing to match a cut-pile or tufted fabric such as Wilton or Axminster, he will probably make a tuft from a thread of the sample skein, so as to compare the shade of the cut ends as well as of the outside of the thread. He will then make the necessary corrections or additions to the dye-bath if the exact shade has not been reached, and the operation will be continued until he is satisfied. The poles are then finally lifted, removed from above the vat to a horse; the yarn is then stripped from the poles and passed to the hydro-extractor prior to going on to the stove or drying machine.
The object of dyeing machines, of course, is to effect the same treatment of the yarn as is done by hand as just described, but in a more expeditious way. A machine can, in fact, do nearly twice as much as a man, and in a more regular manner. That is to say, one man can mind a machine of the same capacity as a hand vat which requires two men; but it is desirable, if not, indeed, necessary, that he should have assistance in loading and unloading.
Given the necessity for keeping the yarn in motion while immersed in the dye liquor, the problem of the designer of dye machines has been to find the most simple and effective way of doing this, keeping in view the need for convenience in preparing the yarn for the bath, dropping it in, and removing it.