CHAPTER VWEAVING AND FINISHING
The manufacture of thrown and spun silk into the finished material, whether by weaving or knitting, varies with the different types of fabric desired. But the several processes are based on the same general principle and are very similar to those used for cotton and wool. To avoid repetition we would refer to Pages twenty-six to twenty-nine and Pages thirty-one to thirty-five, where full descriptions will be found.
Boiling Off
The first step in the finishing of fabrics is dyeing—unless it has already been done before weaving. A considerable proportion of silk is dyed in skein or yarn form before weaving, but in either case the methods of dyeing are practically the same. Preparatory to dyeing it is necessary to boil off the natural gum by means of hot soap baths. If the silk is to be dyed in dark colors a considerable amount of the gum is allowed to remain. Such silk is known as souple silk and is used principally for filling. If all the gum has been removed, it is called bright silk.
Bleaching
All silk, after boiling off, should be nearly pure white. Raw silk, as mentioned in a previous chapter, is often bright yellow, but as this color is entirely in the gum, it is lost by boiling off. In the case of uneven coloration in the raw silk, it is necessary to resort to bleaching before dyeing in order to get uniformity in the finished product. This applies particularly to wild silks, such as Tussah, which are bleached by being placed in an air-tight room filled with a chemical preparation, such as sulphur gas or sodium or hydrogen peroxide. A further washing and drying is then necessary before the skeins or goods can be dyed.
Jacquard Loom
Jacquard Loom
Jacquard Loom
Dyeing
Mordant Dyes
There is no standard dye used for silk, some manufacturers preferring one type and others, another. It may be said, however, that as a general rule silk is dyed by the direct method, without the use of an intermediate or mordant agent as in the case of cotton. This is madepossible through the natural absorptive powers of silk fibre. Mordant dyes are sometimes used, it is true, salts of tin or iron being employed as the agent; but this method is useful principally when it is desired to weight the silk and restore what was lost in boiling off. As this loss generally amounts to about 20 or 25 per cent and as raw silk is sold by weight, it is easy to see that this, unless recovered in some way, would reduce the manufacturer’s profit considerably.
Winding Thrown Silk into Skeins for Dyeing
Winding Thrown Silk into Skeins for Dyeing
Winding Thrown Silk into Skeins for Dyeing
Weighting Silk
The process of weighting silk has been greatly abused, as it is comparatively simple to adulterate it, without discovery, to such an extent that the properties of the fibre are seriously impaired. Weighted silk never has the strength or wearing quality of the unadulterated product. However, a certain amount of weighting—to restore a part of the degumming loss—is generally recognized as legitimate, although the percentage permissible is often a matter for discussion.
Both skein and piece silk are dyed by being immersed in vats containing the hot dye solution. The silk is run over rollers which insure even coloration throughout. After dyeing it is dried, stretched on a tenterframe and then submitted to dry heat which sets the dye and adds luster to the silk.
Printing
Printing is also used for coloring and pattern effects in very much the same way as for cotton goods. After printing the goods are submitted to a dry steam heat which sets the colors.
Finishing
Beyond dyeing or printing, silk goods require very little finishing other than calendaring by passing through rollers. Some types of fabric require softening and others, hardening—accomplished in a variety of ways too numerous to describe. Many of these are secret processes perfected and patented by individual companies. A multitude of different effects can be obtained by these various treatments of the surface of the goods.