The quality of raw silk as it reaches the manufacturer has been determined largely by care and skill used in reeling the silk filaments from the cocoon. The process is one which requires great patience and skill, as the requirements of a perfectly reeled thread are that it shall be even and contain no knots. Since all the cocoons do not furnish threads of exactly the same length, and the fiber varies in fineness in different parts of the cocoon, the reeler must know just when to add a new thread and just how to keep the threads even.
In the filiatures where silk is reeled hand work is supplemented by a machine-turned reel, the basins in which the gum is softened are heated by steam, and many improvements have been made over the old processes still in use in parts of China, where the results are much inferior in quality. When the pupa of the silkworm has been killed the cocoons are sorted, and those of like fineness, color, luster, and degree of perfection are placed together. These cocoons are then plunged into a basin of warm water to soften the gum which holds the fibers together, and the outside loose fibers which cannot be reeled are removed. The reeler then takes a brush and finds the ends of the outside fibers, which are carefully unwound until one is found which is continuous. The fibers from four or five cocoons are united and passed through an agate ring. To make the fibers stick together the thread is then twisted about one coming from another ring, separated from that thread again, and passed through a second ring onto the reel. By the use of improved machines the reeler is enabled to watch several sets of cocoons at once. He must see that a broken fiber is immediately replaced by another, and when the fiber draws near the end and becomes too weak he must replace it by a better.
The skein of silk taken from the reel is twisted, and constitutes the raw silk of commerce. The individual fibers of the thread are stuck together by the silk gum; the silk is harsh, due also to the gum, but the skein is, nevertheless, beautiful. These skeins are packed into bundles called books, are baled, covered with matting, and sent to the manufacturer.
Because of the length of the silk fiber, and the fact that it is already a thread in form, the processes which it must undergo at the factory are much more simple than those required for cotton and wool. Although there have already been four or five fibers united into one thread in reeling, this is not yet of sufficient strength for weaving, and several of these threads must be united and twisted. The processes of doubling or twisting as carried on at the factory are known as throwing.
Throwing may be divided into five processes:
Rewinding the raw silk from skeins onto bobbins or cops.
Cleaning by passing the thread through two fixed plates, so closely adjusted as to stop the machinery if there is a knot or irregularity. In some hand-reeled Chinese silks there are many of these irregularities, which must be removed, as the luster and beauty of finished silk depend on perfect evenness of thread.
Doubling, uniting threads from several bobbins to form one thread.
Spinning, giving a twist to the thread, the amount of twist varying according to the use for which the thread is designed.
Tram is the thread used for filling in silk cloth and consists of two or more single threads having no twist, combined and twisted just enough to hold for weaving. Organzine is the thread used for warp; and since it must be stronger than the filling in order to stand the strain of the loom, it is made up of two threads, one twisted in one direction, the other in the other direction, and then the two twisted together. As organzine must have strength it is made from the best quality of silk, but because of the amount of twist it lacks luster. The filling thread, since it does not require such great strength, has less twist and a higher luster. Singles is the name given to a thread which has no twist, and it may be used as filling for cloth which is to be dyed in the piece.
The completion of the throwing process leaves the silk in a form suitable for weaving, but there is still present the silk gum, which hides the luster and makes the silk harsh. True silk fiber, or fibroin, constitutes about 60 to 70 per cent of raw silk; the other 20 to 30 per cent is sericin, or silk gum. This sericin is soluble in warm, soapy water, and must now be removed from the silk thread. The coloring matter of silk is partly in the sericin, partly in the fiber, so that the process which removes the gum also removes part of the color.
Not all silk requires the complete removal of the silk gum, but silks are classified according to the degree of cleansing.
Boiled-off silk is silk which has all of the gum removed.
Souple silk has one-sixth of the gum removed. Ecru silk has one-twelfth of the gum removed.
Boiled-off silk goes through three operations: