1. Thrown Silk
Necessity for Throwing
The ordinary raw silk, as it comes from the filature and is sold in the market, is composed of from two to eight filaments adhering together by virtue of the natural gum secreted by the worm. This thread is too thin and delicate for many uses and so must be submitted to further twisting and doubling—a process called “throwing.” It must be borne in mind, however, that there are several types of fabrics in which raw silk is used without going through the throwing operation. This is usually the case, for example, with the warp threads for crepes, charmeuse, and messaline.
“Throwsters”
Although some of the larger silk mills have their own departments for this process, as a general rule it is done by independent concerns known as “throwsters”—who specialize in this particular operation. Machinery of a very complicated and accurate type is necessary, capable of operating at very high speed. It has been found that the investment called for to equip such a mill is too large in most cases to allow the manufacturers to incorporate this operation in their factories, particularly if it happens that the spindles are not fully employed during the whole of the year.
Throwing Machines
Organzine and Tram
There are two main classes of thrown silk—organzine and tram—the former has a heavy twist and is used for the warp; while the latter is given only a light twist and is used for filling. Within these two general types there are many special grades, according to the type of material for which they are to be used, such as crepe, chiffon, sewing thread, floss silk, embroidery silk, etc.
Soaking the Raw Silk
The raw silk as it comes to the throwing mill is done up in books of skeins and packed in bales, as described in a previous chapter. The skeins, after being weighed and examined, are first soaked for about twelve hours in warm soapy water, which softens the natural gum, after which they are thoroughly dried. This, of course, reduces the moisture content below the customary 11 per cent, which is later restored by natural absorption.
Winding
The first step in the actual throwing is the winding of the thread on bobbins. The skeins are put on swifts—a light type of reel—and wound at a very high rate of speed on the proper size of bobbin. It is here that the winding strength of the silk meets its severest test, for constant breaks during the winding involve considerable expense when speed is such an important factor.
Twisting
The bobbins of silk are next taken to the twister, which combines two or more strands and gives the thread the necessary amount of twist for the special type that is desired. In the case of tram a very light twist is given, about two or three turns to the inch. For organzine the single threads are first given sixteen turns to an inch in one direction, then doubled and twisted fourteen turns in the other direction.
Crepe
Twisting is the basis of the manufacture of the various kinds of crepe. The filling or tram used is given a very hard twist, about sixty-five turns to the inch—either in right or left-hand direction. These threads are later woven alternately and so produce the crinkled effect.
Thrown silk is classified in a similar way to raw silk, except that usually market quotations on thrown silks mention not only the classification but also the purpose for which it is to be used (tram or organzine), the size in deniers, the number of threads and twists to the inch.