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Simple Science

221. Wool and Silk Bleaching
Animal fibers like silk, wool, and feathers, and some vegetable fibers like straw, cannot be bleached by means of chlorine, because it attacks not only the coloring matter but the fiber itself, and leaves it shrunken and inferior. Cotton and linen fibers, apart from the small amount of coloring matter present in them, contain nothing but carbon, oxygen, and hydrogen, while animal fibers contain in addition to these elements some compounds of nitrogen. The presence of these nitrogen compounds influences the action of the chlorine and produces unsatisfactory results. For animal fibers it is therefore necessary to discard chlorine as a bleaching agent, and to substitute a substance which will have a less disastrous action upon the fibers. Such a substance is to be had in sulphurous acid. When sulphur burns, as in a match, it gives off disagreeable fumes, and if these are made to bubble into a vessel containing water, they dissolve and form with the water a substance known as sulphurous acid. That this solution has bleaching properties is shown by the fact that a colored cloth dipped into it loses its color, and unbleached fabrics immersed in it are whitened. The harmless nature of sulphurous acid makes it very desirable as a bleaching agent, especially in the home.

Silk, lace, and wool when bleached with chlorine become hard and brittle, but when whitened with sulphurous acid, they retain their natural characteristics.

This mild form of a bleaching substance has been put to uses which are now prohibited by the pure food laws. In some canneries common corn is whitened with sulphurous acid, and is then sold under false representations. Cherries are sometimes bleached and then colored with the bright shades which under natural conditions indicate freshness.

Bleaching with chlorine is permanent, the dyestuff being destroyed by the chlorine; but bleaching with sulphurous acid is temporary, because the milder bleach does not actually destroy the dyestuff, but merely modifies it, and in time the natural yellow color of straw, cotton, and linen reappears. The yellowing of straw hats during the summer is familiar to everyone; the straw is merely resuming its natural color which had been modified by the sulphurous acid solution applied to the straw when woven.
222. Why the Color Returns
Some of the compounds formed by the sulphurous acid bleaching process are gradually decomposed by sunlight, and in consequence the original color is in time partially restored. The portion of a hat protected by the band retains its fresh appearance because the light has not had access to it. Silks and other fine fabrics bleached in this way fade with age, and assume an unnatural color. One reason for this is that the dye used to color the fabric requires a clear white background, and loses its characteristic hues when its foundation is yellow instead of white. Then, too, dyestuffs are themselves more or less affected by light, and fade slowly under a strong illumination.

Materials which are not exposed directly to an intense and prolonged illumination retain their whiteness for a long time, and hence dress materials and hats which have been bleached with sulphurous acid should be protected from the sun's glare when not in use.
223. The Removal of Stains
Bleaching powder is very useful in the removal of stains from white fabrics. Ink spots rubbed with lemon juice and dipped in bleaching solution fade away and leave on the cloth no trace of discoloration. Sometimes these stains can be removed by soaking in milk, and where this is possible, it is the better method.

Bleaching solution, however, while valuable in the removal of some stains, is unable to remove paint stains, because paints owe their color to mineral matter, and on this chlorine is powerless to act. Paint stains are best removed by the application of gasoline followed by soap and water.
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.

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