We have covered in the preceding chapters the culture of raw silk, its reeling, and finally its marketing in the countries where it is to be manufactured into the finished textile. We now come to the mechanical phases of the industry, by which raw silk is made first into yarn and then into fabric. Whereas, in the case of cotton and wool, we have seen that yarn is manufactured by various modifications of the spinning process, this is not true in the case of silk. The spinning process is here supplanted by an operation known as “throwing,” and is resorted to only in the manufacture of yarn from the various kinds of silk waste.

1. Thrown Silk

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.

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.

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.

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.

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.

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.

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.

2. Spun Silk

The above processes refer only to throwing of raw silk. In the case of waste, schappe, frisons, etc., an entirely different method must be used, comprising a series of operations very similar to the carding, combing and spinning of wool.[8] The gum, of which silk waste contains a very large proportion, must first be boiled off in a hot soapy bath or allowed to ferment in vats, the latter method being used more generally in Europe. After degumming, the waste silk is next washed and dried and allowed to absorb the 11 per cent moisture content, as in the case of the thrown silk. This is followed by picking, combing, and dressing to remove any dirt or other foreign matter and to separate the fibres and arrange them in parallel order.

The silk is next separated into short laps, then drawn out into thin slivers by means of drawing frames, and thoroughly combed to make the fibres smooth and uniform. These slivers are finally made suitable for winding on bobbins by being passed through roving frames. Then follows the actual spinning process, which twists the roving into yarn. Different types of yarn are produced by varying the amount of twist and the number of strands used. Single yarn is made by twisting a single strand on itself, while for 2-ply, two yarns are twisted together, and so forth, as in the case of cotton or wool.

After being cleaned and examined, the yarn is wound into skeins of about 5 ounces.

The principal use of spun silk is in mixed fabrics, in conjunction with wool, cotton, or raw silk, the spun silk thread generally being used as filling rather than warp. The better grades are made into velvet and plush and various types of knit goods. Although high quality spun silk often has the strength and wearing quality of thrown silk, it never can compete with the latter in regard to lustre.

Spun silk is described as to size in one of two general ways. In the English system the number of the yarn is the number of hanks of 840 yards weighing a pound, with a second number indicating the ply. Thus, size “20-2” would designate a 2-ply yarn, a pound of which contained 16,800 yards. The French system is more or less on the same principle, the main difference being that the number of the yarn indicates the thousands of metres weighing a kilogram.

3. Marketing Thrown and Spun Silk

Thrown and spun silk are bought and sold in very much the same way as is raw silk, although the market is not as extensive nor is the volume of trading as large. The following tables of quotations are from a recent silk journal. A comparison with the quotations given in Chapter III will show the appreciation in value of the various gradings through the throwing and spinning processes.

THROWN SILK

SPUN SILK

Although far from approaching raw silk, the importations of silk waste to this country reach quite substantial figures, as the following table shows:

(Courtesy of Silk Association of America)