DYEING is the art of staining or coloring yarn or cloth. It has been practiced among eastern nations from time immemorial; and in the sacred writings, we read of the vest ments of the high-priest being dyed purple, of linen cloths being dyed blue, purple, and scarlet, and of rams' skins being dyed red. The famous Tyrian purple is believed to have been discovered by an inhabitant of Tyre fifteen hundred years B.C. ; and immediately afterwards the Tyrian purple became the badge of royalty, and cloth dyed with it com manded a princely price. The Egyptians, Greeks, and Romans practiced the art of dyeing; and gradually it became more and more wide-spread as civilization advanced, the discovery of America and other lands materially increasing the number of dye stuffs. In earlier times, dyeing was much more extensively followed as a domestic art than it is at the present time. In the Highlands of Scotland, however, females are still in the habit of dyeing cloth brown by immersing it in a solution of copperas (sulphate of iron), and then treating it with a decoction of sumach, logwood, and crottel (parmelin onapha lodes), a lichen which covers many rocks and trees in moist situations; black, by immers ing the cloth or yarn in an infusion of the bark of the alder-tree (alnus glutinosa), along with copperas and a little sumach; yellow, by the common heather (calluna vulgaris) and alum; red, by the roots of bed-straw (gallant verum) and alum, etc.
The dye-stuffs (q.v.) employed in the various processes of dyeing are numerous, and when two or more are associated together, many different shades and colors are pro duced besides the original color yielded by each. The dyeing materials are procured from the mineral, vegetable, and animal kingdoms, and are often very costly. The arrangements connected with dyeing operations are at times simple, whilst at other times they are compleX, and require the greatest care and skill on the part of the dyer. In communicating the deep indigo blue to woolen cloth and yarn, a vat is taken, about 6 or 7 ft. in diameter, and 8 to 9 ft. in depth, and nearly filled with water, along with 18 to 22 lbs. of indigo, 10 to 20 lbs. of madder, 7 to 9 lbs. of bran, and generally 7 to 9 lbs, of woad. After the requisite boiling, and the addition of 7 or 8 lbs. of lime, to form an alkaline liquid in which the indigo can be held in solution, the whole is well closed over with tightly fitting wooden covers; and in a day, the putrid fermentation of the woad and bran proceeds, the result of which is to abstract the oxygen from the blue indigo, the color of which is gradually reduced till it assumes a yellowish color, and the solu tion then contains indigo white. If woolen yarn or cloth is now dipped in this liquid,
it comes out of a yellow tint, from the attachment of the white indigo solution; but when exposed to the air, the oxygen immediately begins to act upon the white indigo, combining with it, so as to form oxidized or blue indigo, and as the process of oxidation proceeds, the yarn or cloth becomes first of a greenish and then of a blue color. If the cloth be again soaked in the yellowish solution, and subsequently exposed to the air, the depth of the blue color may be increased step by step, till it arrives at that deep shade of blue so well known, especially in the coarser qualities of woolen cloth. In the dyeing of cotton with indigo, the vat is prepared differently. The indigo is first ground into a thin paste with water, and afterwards placed in a vat with protosnlphate of iron and milk of lime. The lime (Ca0) takes the sulphuric acid (SOO from the sulphate of iron (FeOSO3), forming sulphate of lime (CaOSO9), and liberating protoxide of iron (FeO), which immediately abstracts the oxygen from the blue indigo, reducing it to white indigo, and the latter dissolves in the excess of lime present in the vat, yielding a color less solution. When cotton cloth or yarn is dipped in this, it comes out of the vat almost colorless; but on exposure to the air, the indigo becomes reoxidized, and the cloth passes to a green, and ultimately to a deep-blue shade. The cloth or yarn is then washed in water, and afterwards soaked in very dilute sulphuric acid, to which remove any oxide of iron remaining attached, and rewashed in water, when the blue color becomes more bright and clear.
In the fixation of color upon cloth, recourse is often had to a mordant (see CALICO PRINTING), which acts as a middle agent, and attaches the color to the cloth. The prin cipal mordants are alum, cream of tartar, and salts of tin. Previous to the application of any color, the cloth or yarn must be well cleansed from grease, oil, etc., by scouring in soda or in soap, and except where the material is to be dyed of a dark color, the goods are also subjected to the process of bleaching. In the case of fabrics which require a smooth surface, the preliminary operation of singeing off the loose hairs is resorted to (see CALICO-PRINTING).