With this exception, silk fabrics must be degummed or scoured before they can be dyed and finished. This process removes the harshness and leaves a white, soft, flimsy fabric that has a pleasant touch but would be undesirable for general use in clothing. The degumming process is done as part of the dyeing and finishing operation and the process is also called "boiling-off." The result ant liquid is a valuable adjunct in the dye bath as it contains a large amount of the natural sericin—it is known as boil-off liquor.
Silk is usually boiled-off in a solution of 30% of its own weight of neutral soap dissolved in sufficient pure water to give a 1% solution. If degummed in skeins, the hanks of raw silk hung on a wooden rod are immersed in this solution which is maintained at a temperature just below the boiling point, the hanks being turned round to expose all parts equally to the solvent action of the hot solution. If woven in the gum, the fabrics are immersed in the same type of solution. The temperature is about
C. After one hour the silk is removed, centrifuged, then treated similarly in a second bath with half the quantity of soap. It is finally rinsed and dried in a hydro-extrac tor. According to the amount of gum to be boiled off the soap solutions are made strong or weak; but care has to be exercised not to overdo the scouring, whereby loss of strength, substance and lustre would result. The perfect scouring of silks removes from 20 to 27 per cent of their weight, according to the character of the silk and the amount of soap or oil used in the working. Scouring renders all common silks, whether white or yellow in the raw, a brilliant pearly white, with a delicate soft flossy texture, from the fact that the fibres which were agglutinated in reeling, being now degummed, are separated from each other and show their individual tenuity in the yarn. Silks to be finished white are at this point bleached by exposure in a closed chamber to the fumes of sulphurous acid, and at the close of the process the hanks are washed in pure cold water to remove all traces of the acid. Instead of "sulphuring" a dilute solution of hydrogen peroxide containing sodium silicate may be used for bleaching.
Silk hosiery, from the time of Queen Elizabeth of England, has typified a desirable luxury to both men and women, but after the World War of 1914-18, silk hosiery for women be came an everyday necessity. Knitted silk fabrics have also been in demand, especially for underclothing.
Silk knitting is generally done by the weft or warp method. The first forms loops running crosswise of the fabric and links each loop into the one on the preceding row. The second is really woven as well as knitted because parallel yarns have to be ar ranged in two tiers on the loom first. Loops are formed in both directions, lengthwise and crosswise. Weft-knitted silks are the jersey type. Warp-knitted silks are glove silks, used for the bet ter grades of silk knitted underwear. There are two types, tricot and Milanese. Tricot is less expensive than Milanese but is not run-proof. Milanese, because of its construction, is free of runs but a break in one thread when the fabric wears thin will develop into a hole. Hosiery of silk is made by the weft method. It is knitted as a flat fabric and shaped by drawing two or more stitches together. These contractions appear as dots along the seam at the back of the leg and are termed "fashion marks." The hosiery is called "fashioned" or "full-fashioned." The flat fabric is sewn with a seam under the foot and at the back of the leg.
Silk has a special affinity for dyestuffs and finishing materials, and the handling of the fibre either as yarn or fabric has been the pride of dyers through out the ages. In the mediaeval period of European history, Flor ence was noted for its dyed silks. In France, the patronage of Louis XIV developed many new ways of dyeing silks and in 1630 reference was made in a French publication to the metallic weighting of silk. The finishing of silk fabrics to give them smoothness, lustre, stiffness and rustle was the subject of constant experimentation and skill by the workmen of Italy, France and England. When the silk industry became established in the United States, many of the artisans from Europe brought with them their guild and family secrets for the treatment of the precious fibre. Many formulae have been published for the dyeing and finishing preparations but every dyer treasures his own method of handling the silk in the dye bath, how long it should remain in the weight ing solution or sizing bath. And that pride of individual artisan ship continues to the present day. In many respects, the dyeing and finishing of silk remains a trade secret and much of the suc cess or failure of the finished fabric is dependent upon the knowl edge and experience of the men supervising the work.