Silk is frequently known as the fiber of luxury. It is the most
expensive to cultivate, the most beautiful and the strongest fiber.
Since it is the most expensive fiber to buy, and the demand for it
is so great, the temptations to adulterate are also naturally very
great. The long, strong, lustrous silk fiber which bleaches and dyes
beautifully and is fine as a spider’s web, is not to be duplicated. The
best grade or “reeled silk” is taken from the cocoon in one continuous
thread which may be several hundred yards long. In manufacturing
reeled silk many defective cocoons are found in which the fibers are
not perfect, or are broken. The silk from these cocoons may be treated
like a short fiber and spun into threads varying in strength according
to the length of the fibers. This so-called “spun” silk has not the
high luster nor strength of “reeled” silk, but is often used as warp
with reeled silk filling, or in imitation pongee, and back of satins,
velvets and in many other ways.
In olden times the price of silk was much greater than now, but the
material was much more durable. Silks which have been laid away for a
hundred years are still in fairly good condition. Now our silks are
much cheaper, but the result is that when they are put away, even for a
few months they may fall into bits, and their wearing quality can not
be compared with the good old silks of long ago. The reason for this
change is not hard to find. The cost of raw silk is about thirty times
that of raw cotton and the waste at least five times that of cotton.
The manufacturer must make up in some way if he is to sell silk at the
prices demanded by the public.
Silk has a very great ability to absorb dyes and metallic salts without
apparently changing the quality of the material, and since dyes and
metallic salts are much cheaper than pure silk, the manufacturer
makes great use of these materials. Loading is the common name for
this process of treating silk and it is common practice to add thirty
percent of foreign material, just the percent lost by the silk when the
gum is removed, while it is possible to add two hundred fifty or even
three hundred percent.
When we buy novelties and do not care how short their life is to be,
these heavily weighted silks answer the purpose very well, but when we
wish for durability and the silk begins to crack and split or to become
shiny after a few wearings we realize the disadvantage of our modern
methods. Practically no silk can be found on the market entirely free
from loading, but there is a great difference in the amount present.
Burning is the simplest test for good silk; a thread of pure silk will
burn slowly leaving as it burns a very small amount of crisp ash in a
ball at the end of the thread. Heavily weighted silk burns and leaves
the ash in the form of the original thread; this ash of course drops to
pieces readily. Fig. 5A shows a piece of taffeta sold for one dollar
a yard. Fig. 5B shows the result of burning the silk. This ash, left
in the shape of the original sample, is made up of metallic salts,
dyestuffs, etc. A very small percent of ash would be left from the silk
itself.
Another method of adulterating silk is with cotton. The fibers are
not spun together here as the cotton and wool, but the threads of the
two materials are woven together. In satins, velvets and brocades the
cotton is entirely covered by the silk threads on the surface, and
appears as the back of the cloth. In cheap silks a fine cotton thread
sometimes forms either warp or filling.
Pongee is a material made from the cocoon of the uncultivated silk
worm; rajah, tussah and other uneven, coarse materials are from the
same source. These silks are very strong, but do not have a high
luster. Mercerized cotton looks quite silky and is sometimes mixed with
these silks, or a material of mercerized cotton and spun silk may be
sold for pongee, or even a material entirely of mercerized cotton.