DYEING
224. Dyes. One of the most important and lucrative industrial
processes of the world to-day is that of staining and dyeing. Whether
we consider the innumerable shades of leather used in shoes and
harnesses and upholstery; the multitude of colors in the paper which
covers our walls and reflects light ranging from the somber to the
gay, and from the delicate to the gorgeous; the artificial scenery
which adorns the stage and by its imitation of trees and flowers and
sky translates us to the Forest of Arden; or whether we consider the
uncounted varieties of color in dress materials, in carpets, and in
hangings, we are dealing with substances which owe their beauty to
dyes and dyestuffs.
The coloring of textile fabrics, such as cotton, wool, and silk, far
outranks in amount and importance that of leather, paper, etc., and
hence the former only will be considered here; but the theories and
facts relative to textile dyeing are applicable in a general way to
all other forms as well.
225. Plants as a Source of Dyes. Among the most beautiful examples
of man's handiwork are the baskets and blankets of the North American
Indians, woven with a skill which cannot be equaled by manufacturers,
and dyed in mellow colors with a few simple dyes extracted from local
plants. The magnificent rugs and tapestries of Persia and Turkey, and
the silks of India and Japan, give evidence that a knowledge of dyes
is widespread and ancient. Until recently, the vegetable world was
the source of practically all coloring matter, the pulverized root of
the madder plant yielding the reds, the leaves and stems of the indigo
plant the blues, the heartwood of the tropical logwood tree the blacks
and grays, and the fruit of certain palm and locust trees yielding the
soft browns. So great was the commercial demand for dyestuffs that
large areas of land were given over to the exclusive cultivation of
the more important dye plants. Vegetable dyes are now, however, rarely
used because about the year 1856 it was discovered that dyes could be
obtained from coal tar, the thick sticky liquid formed as a by-product
in the manufacture of coal gas. These artificial coal-tar, or aniline,
dyes have practically undisputed sway to-day, and the vast areas of
land formerly used for the cultivation of vegetable dyes are now free
for other purposes.
226. Wool and Cotton Dyeing. If a piece of wool is soaked in a
solution of a coal-tar dye, such as magenta, the fiber of the cloth
draws some of the dye out of the solution and absorbs it, becoming in
consequence beautifully colored. The coloring matter becomes "part and
parcel," as it were, of the wool fiber, because repeated washing of
the fabric fails to remove the newly acquired color; the magenta
coloring matter unites chemically with the fiber of the wool, and
forms with it a compound insoluble in water, and hence fast to
washing.
But if cotton is used instead of wool, the acquired color is very
faint, and washes off readily. This is because cotton fibers possess
no chemical substance capable of uniting with the coloring matter to
form a compound insoluble in water.
If magenta is replaced by other artificial dyes,--for example,
scarlets,--the result is similar; in general, wool material absorbs
dye readily, and uniting with it is permanently dyed. Cotton material,
on the other hand, does not combine chemically with coloring matter
and therefore is only faintly tinged with color, and loses this when
washed. When silk and linen are tested, it is found that the former
behaves in a general way as did wool, while the linen has more
similarity to the cotton. That vegetable fibers, such as cotton and
linen, should act differently toward coloring matter from animal
fibers, such as silk and wool, is not surprising when we consider that
the chemical nature of the two groups is very different; vegetable
fibers contain only oxygen, carbon, and hydrogen, while animal fibers
always contain nitrogen in addition, and in many cases sulphur as
well.
227. The Selection of Dyes. When silk and wool, cotton and linen,
are tested in various dye solutions, it is found that the former have,
in general, a great affinity for coloring matter and acquire a
permanent color, but that cotton and linen, on the other hand, have
little affinity for dyestuffs. The color acquired by vegetable fibers
is, therefore, usually faint.
There are, of course, many exceptions to the general statement that
animal fibers dye readily and vegetable fibers poorly, because certain
dyes fail utterly with woolen and silk material and yet are fairly
satisfactory when applied to cotton and linen fabrics. Then, too, a
dye which will color silk may not have any effect on wool in spite of
the fact that wool, like silk, is an animal fiber; and certain
dyestuffs to which cotton responds most beautifully are absolutely
without effect on linen.
The nature of the material to be dyed determines the coloring matter
to be used; in dyeing establishments a careful examination is made of
all textiles received for dyeing, and the particular dyestuffs are
then applied which long experience has shown to be best suited to the
material in question. Where "mixed goods," such as silk and wool, or
cotton and wool, are concerned, the problem is a difficult one, and
the countless varieties of gorgeously colored mixed materials give
evidence of high perfection in the art of dyeing and weaving.
Housewives who wish to do successful home dyeing should therefore not
purchase dyes indiscriminately, but should select the kind best suited
to the material, because the coloring principle which will remake a
silk waist may utterly ruin a woolen skirt or a linen suit. Powders
designed for special purposes may be purchased from druggists.
228. Indirect Dyeing. We have seen that it is practically impossible
to color cotton and linen in a simple manner with any degree of
permanency, because of the lack of chemical action between vegetable
fibers and coloring matter. But the varied uses to which dyed articles
are put make fastness of color absolutely necessary. A shirt, for
example, must not be discolored by perspiration, nor a waist faded by
washing, nor a carpet dulled by sweeping with a dampened broom. In
order to insure permanency of dyes, an indirect method was originated
which consisted of adding to the fibers a chemical capable of acting
upon the dye and forming with it a colored compound insoluble in
water, and hence "safe." For example, cotton material dyed directly in
logwood solution has almost no value, but if it is soaked in a
solution of oxalic acid and alum until it becomes saturated with the
chemicals, and is then transferred to a logwood bath, the color
acquired is fast and beautiful.
This method of indirect dyeing is known as the mordanting process; it
consists of saturating the fabric to be dyed with chemicals which will
unite with the coloring matter to form compounds unaffected by water.
The chemicals are called mordants.
229. How Variety of Color is Secured. The color which is fixed on
the fabric as a result of chemical action between mordant and dye is
frequently very different from that of the dye itself. Logwood dye
when used alone produces a reddish brown color of no value either for
beauty or permanence; but if the fabric to be dyed is first mordanted
with a solution of alum and oxalic acid and is then immersed in a
logwood bath, it acquires a beautiful blue color.
Moreover, since the color acquired depends upon the mordant as well as
upon the dye, it is often possible to obtain a wide range of colors by
varying the mordant used, the dye remaining the same. For example,
with alum and oxalic acid as a mordant and logwood as a dye, blue is
obtained; but with a mordant of ferric sulphate and a dye of logwood,
blacks and grays result. Fabrics immersed directly in alizarin acquire
a reddish yellow tint; when, however, they are mordanted with certain
aluminium compounds they acquire a brilliant Turkey red, when
mordanted with chromium compounds, a maroon, and when mordanted with
iron compounds, the various shades of purple, lilac, and violet
result.
230. Color Designs in Cloth. It is thought that the earliest
attempts at making "fancy materials" consisted in painting designs on
a fabric by means of a brush. In more recent times the design was cut
in relief on hard wood, the relief being then daubed with coloring
matter and applied by hand to successive portions of the cloth. The
most modern method of design-making is that of machine or roller
printing. In this, the relief blocks are replaced by engraved copper
rolls which rotate continuously and in the course of their rotation
automatically receive coloring matter on the engraved portion. The
cloth is to be printed is then drawn uniformly over the rotating roll,
receiving color from the engraved design; in this way, the color
pattern is automatically printed on the cloth with perfect regularity.
In cases where the fabrics do not unite directly with the coloring
matter, the design is supplied with a mordant and the impression made
on the fabric is that of the mordant; when the fabric is later
transferred to a dye bath, the mordanted portions, represented by the
design, unite with the coloring matter and thus form the desired color
patterns.
Unless the printing is well done, the coloring matter does not
thoroughly penetrate the material, and only a faint blurred design
appears on the back of the cloth; the gaudy designs of cheap calicoes
and ginghams often do not show at all on the under side. Such
carelessly made prints are not fast to washing or light, and soon
fade. But in the better grades of material the printing is well done,
and the color designs are fairly fast, and a little care in the
laundry suffices to eliminate any danger of fading.
Color designs of the greatest durability are produced by the weaving
together of colored yarns. When yarn is dyed, the coloring matter
penetrates to every part of the fiber, and hence the patterns formed
by the weaving together of well-dyed yarns are very fast to light and
water.
If the color designs to be woven in the cloth are intricate, complex
machinery is necessary and skillful handwork; hence, patterns formed
by the weaving of colored yarns are expensive and less common than
printed fabrics.