Dyes are usually classified into four groups, according to their general properties.1
"a. Acid dyes.
1 Matthews. Laboratory Manual of Dyeing and Textile Chemistry, p. 58.
"b. Basic dyes.
"c. Substantive dyes.
"d. Mordant dyes.
"This classification in a general way is based on the chemical nature of the dyestuff and its reaction towards the fiber. The following is a brief summary of these properties:
"c. Substantive dyes. Of neutral chemical nature: dye both animal and vegetable fibers directly; mostly applied to cotton and somewhat to both wool and silk.
"d. Mordant dyes. Of neutral chemical nature; dye neither animal nor vegetable fibers directly, but require a metallic mordant; mostly applied to wool."
Some other dyes not included in these groups are the mineral pigment dyes, as Prussian blue, vat dyes, such as indigo, and sulphur dyes.
The acid dyes are cheap, and are most commonly used for wool. The dye is dissolved in an acid bath, with considerable Glauber salt. The cloth is put in at a low temperature and the water gradually heated. With an alum mordant on cotton the acid dyes give colors fast to light but not to washing.
Basic Dyes are much used for silk, since this fiber has a great affinity for them, and the colors produced have much depth and brilliancy. An after-treatment with tannic acid and tartar emetic increases the fastness to washing.
Basic colors are not used as much for wool as formerly, since acid dyes give better results. With cotton the cloth is first mordanted with tannic acid, then with salts of antimony or iron, and finally dyed.
Hard water may not be used with the basic dyes, as the color is precipitated and dye spots result.
Direct Cotton Colors, Or Substantive Colors, may be used for either animal or vegetable fibers, and for this reason are largely used for union goods. Their worst fault is a lack of fastness to light and washing, a difficulty which may be lessened by an after-treatment of the material. This after-treatment usually consists in developing on the fiber by some chemical treatment an insoluble dyestuff from the more or less soluble dye already present.
Mordant Dyes are dyes which cannot be used on any fiber without a mordant. Madder, logwood, alizarin (Turkey red), are common examples of these dyes. Many of the old natural dyes belonged to this class. They are usually rather fast colors and are more difficult to dye.
Indigo is an interesting example of vat dyes. These dyes are in an insoluble form, and must be reduced to a soluble form before they can be applied to the material. Indigo as it comes to market is a blue powder or paste prepared from the root of the plant Indigofera, or prepared synthetically from coal-tar products. The two substances are chemically identical, but the synthetic product is purer than the natural one.
When indigo is reduced by action of copperas and quicklime or other agents, it yields a grayish-white product, indigo white, whose calcium salt is soluble in water. The cotton is impregnated with indigo white, then exposed to the air, when oxidation takes place producing the blue indigo again in insoluble form on the fiber. Sulphur Dyes are those requiring sodium sulphide to produce a solution. They are especially fast to light and are used largely for linens and silks. They require after-treatment with potassium dichromate or by exposure to air.
These different classes of dyes include hundreds of different dyestuffs with many names. Firms such as Cassella, Metz, Badische, and others have their distinctive names for different dyestuffs. These and many other large firms are German, but have American agents.
The use of the different classes of dyestuffs requires considerable knowledge of chemistry and long practice in handling. The difficulties in securing even colors, fast colors, true shades, and always the same shades are many.
With certain classes of dyes hard water must not be used, as the calcium and magnesium in the water precipitate the dyestuff, while impurities like iron or organic matter may cause trouble. Careful solution of the dyestuff that it may be evenly distributed, regulation of the temperature of the solution, and the proper use of assistants are all important precautions.
Sometimes a material may be dyed with a dyestuff of one class, then topped, or dyed again in another kind of dyestuff. Basic dyes may be used on cotton, on a tannin mordant, the cotton being then dipped in a direct cotton dye. The color acid of the direct cotton or substantive dye unites with the color base of the basic dye and forms an insoluble compound.
In dyeing cotton with basic dyes two mordants may be used. The cotton may be saturated with tannic acid solution, then with an antimony salt, a tin salt, or an iron salt, and finally dyed. An acid bath is used for cotton and a neutral bath for wool.
With so large a choice of dyestuffs and so many methods of using them, it will readily be seen that the handling of dyes, if the best results are to be produced, is no task for the amateur. Because of the greater cost involved in dyeing fast colors, the manufacturer is tempted to produce fugitive ones. Chemicals used to assist the dye process may weaken the fabric if they are allowed to act too long, or if they are not entirely removed from the cloth when the dyeing is complete.
Very crude, as well as the most refined, colors may be obtained from these dyestuffs. It remains with the dye expert to decide which. That aniline dyes may be used for the most artistic products has been proved in the Herter Studios in New York, where silks, wools, linens, and cottons for the most wonderful tapestries, rugs, and hangings are dyed in the chemical dye pot.
A recent development in amateur dyeing is the use of oil paints and gasoline. The process is very simple. The desired color may be obtained with little difficulty by any one who understands the mixing of paints. Only a limited amount of material may be successfully dyed in this way, but it is especially suitable for nets, laces, and other delicate materials.
The process consists in mixing the paints until the desired hue is obtained, then dissolving in gasoline. The fabric is dipped several times in the solution and dried.