In this process the silk yarn is softened in a soft-water solution of a good cocoa-nut or olive oil soap. The hanks are hung in vats and the water is gradually heated to 200 ° F. During this treatment the silk swells, becomes sticky, and the gum washes off, leaving the fiber more glossy. One-half to one hour is the time required, after which the silk is squeezed, rinsed in a slightly soapy bath, and put into bags of hemp or flax.
The yarn is boiled for one-half hour in a 10 per cent soap solution, and is then washed well in clear water. This removes the last traces of gum.
If the silk is to be white or light in color the natural coloring matter present in the fibroin must be removed. Treatment in a dilute bath of hydrochloric or nitric acid for about fifteen minutes, then a thorough washing and rinsing, are followed by exposure to sulphur fumes. This destroys the coloring matter, but leaves the silk harsh and rough. It must be softened by immersion in a cream of tartar solution for an hour and a half.
Silk is sometimes bleached in a solution of hydrogen peroxide or sodium peroxide. This method is more expensive than the former.
Souple silk is scoured in a bath at a lower temperature and containing less soap than that used for boiled-off silk. Only part of the gum is removed, but the fiber swells and becomes more absorbent.
Ecru silk has the coloring matter removed, but little of the gum. The process for this is much bleaching but little washing. The method used by the manufacturer to make up for the loss of weight resulting from the removal of the silk gum will be discussed later.
Fig. 28. Silk Cocoons and Skeins.
A. Cocoons B. Silk reeled as it comes to market C. Silk from imperfect cocoons D. Same silk as C after carding E. Skeins of silk spun from same.
Silk weaving does not differ essentially from any other weaving. Because of the extreme fineness of the thread the loom is more difficult to set up, and must be run rather more slowly. A great variety of effects may be produced by variations of weaves. Damask, satin, velvet, different crepes and brocades depend on the weave for their character. The Jacquard loom again offers almost endless possibilities, and the designer is ever at work to produce some new effect which will appeal to the public eye.
The finishing processes for silk differ from those used with other materials. Little washing or scrubbing is necessary, and most silks are finished almost entirely in a dry state. Washing is necessary with some silks, but those which have been heavily weighted in the yarn are not washed. Singeing in a gas flame, or shearing with a machine arranged on the same principle as the lawn mower, removes the fuzzy ends which would diminish the luster. Sizing may be sprayed upon the surface of the cloth, after which it is passed through heated cylinders to dry and to press out wrinkles. For certain surfaces the silk is pressed between layers of finishing paper; then, to take away the papery feeling, is passed between wooden rollers in which buttons are driven. Singeing and brushing, and a final tentering or stretching on rows of hooks to give the desired width, prepare the cloth for folding, measuring, and making into bolts for the market.
The finishing processes differ greatly for different varieties of goods. Silk may be dyed or printed before or after weaving, a moire effect may be produced by embossing the cloth with heavy engraved rollers, velvets must have the pile steamed, panne velvets have a special method of pressing, and many other processes give a great variety of results.
Besides the best quality of cultivated silk and the wild silks, there is an inferior grade of yarn known as spun silk. In the process of silk culture it is necessary that a number of moths emerge from the cocoon to provide eggs for the next lot of worms. As has been stated, the cocoon is injured by the exit of the moth, and cannot be used for reeled silk. In the process of reeling there is also much waste, such as the outsides of cocoons, which must be removed before the thread for reeling has been reached, defective cocoons, and the last inner layer, which cannot be successfully reeled.
Although the fibers from these forms of waste silk cannot be reeled in one continuous thread, they are, nevertheless, valuable and are not wasted. They are given quite a different treatment, however, from the perfect fibers. First, the gum is removed by thorough washing; then the tangled mass of short fibers is carded, as cotton or wool would be carded, then drawn out and spun into a thread. The resulting yarn, to distinguish it from reeled silk, is called spun silk.
Spun silk is usually not so strong as reeled silk, and is not so lustrous because of the many ends of silk projecting from the surface of the yarn and because of its harder twist. Frequently the two kinds of silk are combined in a material, the spun silk being used for the warp and the reeled silk for the filling.
Poorer grades of this silk, too short or too weak to be spun, are made into hat braids or a poor quality of dress trimmings.
Because of the high cost and beauty of silk there have been many attempts to find substitutes for it. Efforts have been made to spin the spider's web and to use the filaments spun by other moths, but so far these have been unsuccessful. A single exception is the byssus of the shellfish, Pinna. This byssus is a tassel-like appendage by which the mussel attaches itself to the rocks, and it may be combed out and spun into a thread which is used sometimes for gloves, purses, and other small articles. The natural color of this silk is olive green or brown.
Chemists have experimented for years to find a substitute for silk, and have produced several artificial silks from different substances. The most successful of these is Chardonnet silk, so called from the man who succeeded ill producing it. This silk is made from cellulose steeped in concentrated sulphuric and nitric acid and dissolved in a mixture of equal parts of alcohol and ether. This solution, collodion, is then forced through very fine capillary tubes, from which it comes in threads, which coagulate in the air. As this cellulose is highly explosive it must be denitrated before being spun into yarn.
Fig. 29. Artificial Silk.
Chardonnet silk has a high luster, considerable tensile strength, and, although yellowish in color, may be bleached with chloride of lime and dyed readily. The greatest objection to the use of it is that it does not withstand the action of water well. It is used, however, for braids, neckties, and for fancy articles which need not come in contact with water. In making beautiful tapestries in a studio in New York, artificial silk has been found satisfactory, as resistance to water is not essential in this sort of material.
Other varieties of artificial silk have been made from solutions of cellulose in ammoniacal copper oxide, or chloride of zinc, and from filaments of gelatin treated with formaldehyde. The Chardonnet and other silks made by practically the same method are, however, most satisfactory, and have partially met an ever-increasing demand.
Thus it is that in the past two or three generations many processes have been discovered whereby the cost of silk has been decreased and the supply increased, or, rather, the supply has been made to go much farther. The unfortunate part of the change is that the wearing qualities of silk have been greatly decreased, but since the public demands quantity and variety at small cost it obtains that which it demands.
Barker, A. F. Textiles. Hurst, G. H. Dyeing and Printing of Silk. Matthews, J. M. Textile Fibres.
Posselt, E. A. Wool, Cotton, Silk, from Fibre to Finished Fabric. Silk. Pamphlet. Nonotuck Silk Company.