the action of strong caustic soda ('mercerizing' process) while in a state of great tension, in order to give it a permanent silky lustre. Dyeing is not only an art, it is also a branch of applied chemistry. One fundamental principle is, that the colouring matter and other necessary substances must be applied in a state of solution, and while in direct contact with the fibre they must be rendered insoluble, so that they are precipitated within or upon the fibre and fixed permanently. The method of effecting this varies greatly, according to the fibre and the colouring matter employed. As a rule, the vegetable and the animal fibres are dyed by very different methods. The affinity of the animal fibres for certain colouring matters is often so great that they are readily dyed by simple immersion in hot colour solutions; but this simple process is not generally sufficient. According to the method of their application in dyeing, the following groups of dye-stuffs may be distinguished: Acid Dyes, Basic Dyes, Direct Dyes, Developed Dyes, Mordant Dyes, Vat Dyes. A dye is substantive to a particular fibre when it dyes that fibre directly, and adjective when the presence of a third substance known as a mordant is necessary.
The acid dyes are so called because they are of an acid character and are applied in an acid dye-bath. As a rule, they are only suitable for dyeing the animal fibres, e.g. wool and silk, also leather, horn, feathers, &c., and they are substantive to these materials. The purple vegetable dyestuff orchil belongs to this class. The acid dyes derived from coal-tar are very numerous, and yield a great variety of hues—red, orange, yellow, green, blue, violet, brown, and black. The basic dyes are so called because their essential constituents, to which they owe their dyeing power, are organic bases. The bases themselves are colourless and too insoluble in water to be of use, hence they are employed in the form of their soluble coloured salts, usually the hydrochlorides of the colour-bases. Their solutions are precipitated by tannic acid, because it combines with the colour-bases to form insoluble tannates. Wool, silk, and animal substances generally have a direct attraction for colour-bases, and hence these fibres are readily dyed by simple immersion in hot aqueous solutions of the basic dyes. Cotton and linen, on the other hand, are not dyed so readily; they need first to be mordanted or impregnated with the mordant tannic acid. Most of the colours of this class are fugitive to light, and all but one, viz. barberry root, are derived from coal-tar products.
The direct dyes are so called because they dye cotton without the aid of any mordanting process. The first of this class derived from coal-tar was congo red, discovered in 1884; at present this group includes a very great variety of fast colours, and forms, indeed, one of the most important and valuable series of dye-stuffs employed. Cotton, linen, and the vegetable fibres generally are dyed in the simplest possible manner by merely boiling them in a solution of the dye-stuff, with or without the addition of a little soap, carbonate or sulphate of soda, &c. Wool and silk are frequently dyed in the same manner as cotton. Very few vegetable dye-stuffs belong to the direct colours, e.g. Safflower, Turmeric, Saffron, Annatto (see the articles). They are all fugitive, and are now of little or no importance to the dyer. The coal-tar colours of this class, on the other hand, are extremely numerous. The developed dyes are formed in situ upon the fibre by the successive application of two or more substances. They include aniline black, a permanent black produced by the oxidation of aniline, and the ice colours, which are azo dyes derived from certain coal-tar products containing nitrogen.
The mordant dyes form one of the most important classes of colouring matters, for they include not only most of the vegetable dye-stuffs, e.g. madder, logwood, fustic, &c., but also many valuable fast coal-tar colours, commonly known as the alizarin dyes, after their typical representative, alizarin. These mordant dyes have by themselves very little colouring power, as a rule, and if employed alone in dyeing give little or no result. If applied, however, in conjunction with metallic salts, notably those of chromium, aluminium, iron, tin, and copper, they each yield a variety of colours, according to the metallic salt employed. In employing them two distinct operations are usually involved: first, that of applying the metallic salt or mordant, called the mordanting process; and second, that of dyeing proper, in which the mordanted material is boiled in a solution or decoction of the dye-stuff. During the dyeing operation the colouring principle of the dye-stuff combines with the metallic salt already upon the material, and the colour is thus produced and fixed upon the fibre. The method of mordanting varies with the fibre and the metallic salt employed. The vegetable dye-stuffs of this class include Madder, Sapanwood, Camwood, Barwood, Old Fustic, Young Fustic, Quercitron Bark, Persian Berries, Weld, Logwood (see these separate articles). Madder was formerly the most important and highly valued of the dye-stuffs of this class, being especially employed to produce the fine 'Turkey-red' dye; but it is now entirely superseded by the coal-tar colour alizarin. Similarly, the employment of cochineal (an insect dye) has also greatly diminished through the introduction of the cheaper colours. Camwood and barwood are almost entirely used in wool-dyeing, either in
conjunction with the indigo-vat or for the purpose of dyeing various shades of brown. Old fustic is the most important of the yellow mordant dye-stuffs, and the colours are fast although not very brilliant. Quercitron bark is an excellent dye-stuff employed by wool-dyers for the production of bright orange and yellow colours. Persian berries and weld, a species of wild mignonette, are both excellent dye-stuffs, but their employment is now limited. Logwood is largely employed by wool, silk, and cotton dyers for dyeing black and dark-blues, which, although fast to washing, are only moderately so towards light. The important vegetable dye catechu (q.v.) is used in dyeing cotton and wool brown. On wool, catechu yields khaki browns in single bath by using copper sulphate as the mordant. On silk, it is largely employed for weighting purposes in the process of dyeing black. Although dyewoods are still much employed, they are being steadily replaced by coal-tar colours. The vat dyes are insoluble in water, but yield reduction products which are soluble in aqueous alkali, and can be readily reoxidized to the dye-stuff. In this class may be included the sulphur dyes, substances of uncertain composition obtained by fusing certain compounds containing nitrogen with sulphur and sodium sulphide, which are now extensively and increasingly employed, especially as direct dyes for cotton. Indigo, a typical vat dye, is prepared both artificially and from natural sources. It is a dark-blue powder quite insoluble in water, but when reduced it yields indigo-white which dissolves in aqueous alkali, the solution thus obtained being called an indigo-vat. Cotton, wool, or silk steeped for some time in the clear yellow solution of such a vat, and then exposed to the oxidizing influence of the air, is dyed a permanent blue. The indigo-white absorbed by the fibre loses its acquired hydrogen, and thus insoluble indigo-blue is regenerated within and upon the fibre.
In the classification adopted above, the following mineral colours employed in cotton dyeing belong to the group of developed dyes, since they are formed on the fibre: chrome yellow, obtained by immersing cotton successively in solutions of acetate of lead and bichromate of potash; iron buff (oxide of iron), produced by the successive application of sulphate of iron and carbonate of soda; Prussian blue, developed by passing the buff-dyed cotton through an acidified solution of potassium ferrocyanide; manganese brown (oxide of manganese), deposited similarly to iron buff. The mineral colours are very useful for certain purposes, and are very fast to light.—Bibliography: Crookes, A Practical Handbook of Dyeing and Calico-printing; W. P. Dreaper, Chemistry and Physics of Dyeing; Knecht, Rawson, and Loewenthal, Manual of Dyeing; Rawson, Gardner, and Laycock, A Dictionary of Dyes, Mordants, &c.
Dyer, John, English poet, born in Carmarthenshire about 1700, died in 1758. Educated at Westminster School, he became a painter, but, not succeeding in that vocation, took orders and was appointed to a small living. In 1727 he published his poem of Grongar Hill, in 1740 The Ruins of Rome, and in 1757 The Fleece, a didactic poem in five books.
Dyer's-broom, a European and now also N. American shrub (Genista tinctoria), formerly used with woad for dyeing green.
Dyer's-weed, Resēda Luteŏla, a British plant of the same genus as mignonette, otherwise called Yellow-weed, Weld, or Woad, nat. ord. Resedaceæ. This plant grows in waste ground; it affords a beautiful yellow dye, and is cultivated for that purpose.—Dyer's Greenweed is Genista tinctoria.
Dying Declaration, a deposition made by one who is in prospect of death. Such declarations are as a general rule admissible as evidence only in criminal and not in civil cases, and must be made, according to English, though not Scottish law, in the full consciousness of the danger of death.