DYES AND DYEING 



DYES AND DYEING 



271 



The goods are first treated with a solution of one 

 of the chemicals, and then on working in another 

 solution the pigment is produced. In calico-print- 

 ing, any pigment can be fastened mechanically as 

 in ordinary printing, except that gum arable, dex- 



Fig. 376. Safirou {Orocus sativus). 



Source of a yellow dye. 



trin, starch, albumen, and the like, are employed 

 instead of varnishes. 



Definitions. 



Lakes are insoluble compounds of alumina and 

 coloring matters. If these are formed by them- 

 selves, a color-lake or pigment is produced ; but if 

 a fabric is first impregnated with alum or other 

 metallic salts for which the fiber has an affinity, on 

 subsequent treatment in the coloring solution the 

 color-lake is produced in and on the fiber, which is 

 then said to be dyed. Several other metallic oxids 

 also possess similar properties, often giving differ- 

 ent colored precipitates with the same dyestuffs. 

 These metallic compounds are called "mordants" 

 (from the French mordre, to bite). Tannic acid 

 forms insoluble compounds with an entire series 

 of coloring matters and is similarly used. 



Although dyeing has been practiced from time 

 immemorial, and by all nations of the globe, no 

 satisfactory theory has been advanced to explain 

 the process. Mechanical attraction, chemical affin- 

 ity and " solid solution " are given as explanations, 

 all having experimental evidence in support. In 

 wool dyeing, the chemical affinity theory best 

 elucidates the process. 



Classifi/Mtimi of dyestuffs. 



The dyestuffs may be classified either according 

 to their chemical composition, in accordance with 

 the fibers for which they are most suitable, or with 

 the methods used in their application. The first 

 classification is of importance to the chemist, while 

 the last is best for practical purposes, and is shown 

 in the following grouping : 



(a) Direct cotton colors. These dye cotton in full 

 shades without the aid of mordants ; in conjunction 

 with them, certain salts, such as glauber salt or 

 common salt, are used to aid in the ^ 

 absorption of the dye, as these salts ^ 

 tend to force it out of the solution. M^, 

 Alkaline salts, such as soda, soap or .^m 

 phosphate of soda, have an opposite j^ 

 effect and tend to retard the dyeing 1^ 

 process and to prevent uneven dyeing. '%IS' 

 The direct cotton colors also act as 

 mordants, combining with the colors 

 of the following class. These dyes 

 may be converted into others by treat- 

 ment with certain chemicals, thus 

 making a new dye on the goods. 



(b) Basic colors. Colors of a basic 

 nature, which form compounds with 

 tannic acid, insoluble in water, and 

 which dye the vegetable fibers with 

 the aid, and animal fibers without the 

 aid, of mordants. 



(c) Acid colors. Colors of an acid 

 nature, which dye the animal fibers 

 without the aid of mordants. 



(rf) Mordant colors. Colors which 

 are dyed with the aid of metallic mor- 

 dants. Most of the natural coloring 

 matters come under this head. 



(e) Sulfur colors. Colors of recent 

 discovery. Most of them are insoluble 

 in water, but soluble in water contain- 

 ing sodium sulfid. They are used 

 for vegetable fibers as direct col- 

 ors, and are similarly applied. 



(/) Miscellaneous colors. These 

 include those having little in com- 

 mon, and require individual 

 treatment. Some of the most . 

 important come under this 

 head. 



(1) Indigo. See same in list 

 of natural coloring matters. 



(2) Eosines and rhodamines. 

 Especially valuable for pro- 

 ducing brilliant pigments in 

 conjunction with metallic pre- 

 cipitants, for making artifi- 

 cial vermilion, etc. 



(3) Aniline black is pro- 

 duced by impregnating the 

 cotton yarn or cloth with ani- 

 line and the proper amounts of 

 the required chemicals ; on 

 after-treatment, oxidation ^e. 377. 

 takes place and the color is "^fLf/Tl'V^^^^ 

 tormed. Other colors of much source of a yellow dye. 



