WEAVING AND FINISHING
1. WEAVING
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.
2. FINISHING
[Sidenote: _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.
[Sidenote: _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.
[Illustration: _Jacquard Loom_]
[Sidenote: _Dyeing_]
[Sidenote: _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
made possible 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.
[Illustration: _Winding Thrown Silk into Skeins for Dyeing_]
[Sidenote: _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 tenter frame and then submitted to dry heat which
sets the dye and adds luster to the silk.
[Sidenote: _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.
[Sidenote: _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.