Dyes and dyeing

by Charles Ernest Pellew

The whiteness of the pattern depends, of course, upon the skill with which the paste has been applied, and the care taken to prevent it from washing off before or during the dyeing process. It is difficult, though not absolutely impossible, to get as sharp and clear-cut results as those of the Japanese, for instance. But, on the other hand, with a dark background it is often, indeed generally, more pleasing to have the white patterns softened and not standing out too vividly.

In our laboratory we have had considerable success with this process. And some of our friends and students have used it with very good results upon articles of clothing, which, made of linen, calico, etc., must be fast to severe washing as well as to light.

Of course, it is perfectly easy to alter the color of the background, as in other classes of resist work, such as Tied and Dyed work, for instance, or Batik, by either starting off with colored cloth which is protected all through by the resist paste, or else by covering the stencilled and dyed goods, afterwards, with some shade which will soften and harmonize both pattern and background. For this covering shade, which need not be very fast to washing, but must be distributed uniformly over the whole cloth, the student will find the Salt colors very useful.

Discharge Stencilling.—Though it is not certain whether this process is known to, and used by, the Japanese, it is not a difficult matter, with modern dyes and modern chemicals, to get interesting results with it. There are two distinct and separate ways open to the dyer for discharging, i.e., destroying his dyestuffs, whether they are dyed on cloth, or whether, as is not infrequently the case with amateurs, they are present as a stain on his hands and fingers. In each case, however, care must be taken, as may easily be imagined, to use such chemicals as will spare the materials, whether cotton and linen, or nails and skin, while attacking the coloring matter.

(a)Discharge by Oxidation. Chlorine Compounds, Bleaching Powder, etc.—In the first place, chemists have long known that certain chemicals, more particularly the powerful gaseous element known as chlorine and certain of its compounds, have the power of permanently destroying coloring matters by oxidizing or burning them.

At first this was done by using chlorine itself, or a water solution of chlorine. Later, however, it was found that on passing chlorine into some caustic alkali, like quicklime, or caustic soda, or caustic potash, these would absorb immense quantities of chlorine which would be again given out, as desired, on the addition of acid, or even, though very slowly and gradually, by the action of the carbonic acid gas in the air.

The lime compound, which contains more chlorine than the others, and has the great advantage of being dry, has long been known as chloride of lime or as bleaching powder, and has been, and is, commonly used from one end of the world to the other as a quick, ready, cheap source of chlorine either for bleaching or for disinfection. The potash and soda compounds, known respectively as Labarraque’s solution and Javelle water, are less active and powerful than bleaching powder, but have the same general properties.

Over a hundred years ago, very soon after the discovery of the bleaching properties of these compounds, chemists began to use them, not only for decolorizing and whitening raw cotton and linen cloth, but also for discharging the color in patterns from dyed goods. The process was not a difficult one, and is used to this day to some extent in the calico printing mills. The cloth is first dyed to shade, fixed, and dried. The pattern is then printed on with a paste containing some solid organic acid, like citric acid or tartaric acid, dissolved in it. After drying, the printed cloth is passed through a bath of bleaching powder in water, possibly with a little weak alkali added, to be sure that no free chlorine is present; and wherever the bleaching powder meets the acid the cloth is decolorized, but the rest of the cloth comes out of the bath without being much, if at all, altered in color. Of course, on coming out of this bath the cloth must be thoroughly washed to get rid of any traces of chloride of lime, which otherwise, on exposure to the air, would play havoc with the rest of the colors.

This process worked very well with the old vegetable dyes, and, every now and then, some craftsman, of an experimental turn of mind, revives it for stencil work. The dyed cloth is stencilled with a paste made of wheat flour boiled with a saturated solution of citric acid, it is dried, and then passed through a bath of bleaching powder in water, say two or three tablespoonfuls to the gallon. It is generally best to stir in a few drops of a soda solution to the bath, till all smell of chlorine has gone, or else the background may be affected. The stencilled cloth is dipped in this bath, and kept there for a few minutes, until the bleaching process is well under way, and then taken out, and washed in hot soap and water, and rinsed well.

Advantages and Disadvantages of Bleaching Powder Discharge.—The chief advantage of this process is that it is very cheap and the materials can be bought at almost any grocery. The disadvantages are, however, important. As long as it is confined to easily discharged, comparatively fugitive, colors, it will destroy the color all right in the stencilled parts, although the bleaching powder bath is apt to attack the color in the body of the cloth, and the outlines of the pattern are apt to be soft and irregular because of the escaping chlorine, where the citric acid is acting.