S24 



DYEING. 



.,'. ..,,.,. III. Of Mineral Sulstanliit Colours. 



M 

 .. j- -_.. 92. The greater numlwr of mineral substances used in 



dyeing, are employed chiefly with a view of attaching 

 colouring matters to stuffs, or of heightening their lustre 

 after they have been so applied. Some of them, however, 

 particularly iron and copper, ;re occasionally employed 

 for gii MIL; substantive colours. 



'* 93. Iron, in every form of its solution, has a strong 



affinity for linen and cotton, and readily combines, in the 

 tate of an oxide, with their fibres. The colours which 

 it afford vary with the degree of oxidation ; but as the 

 oxide, in all its combinations, is disposed to attract more 

 oxygen from the atmosphere, these colours all terminate 

 in the rusty colour commonly called iron-mould. This 

 absorption of oxygen, being a kind of slow combustion, 

 renders the oxide corrosive, and gradually injures the 

 texture of the cloth. The rigidity which it occasions in 

 the fibres, also renders them more brittle, and less du- 

 rable. Hence iron-moulds at last produce holes. This 

 injurious property of the oxide of iron, is in some de- 

 gree counteracted by combining the oxide with other 

 substances ; but there is reason to fear, that, in every 

 condition, the oxide of iron is hurtful to stuffs. It is to 

 this property that the rottenness so generally complained 

 of, with respect to the black dye, is to be ascribed. 



Acetate of gj.. The oxide of iron is usually applied to linen or 

 cotton in combination with some acid. Vinegar was 

 usually employed for this purpose; and the resulting 

 salt, the acetate of iron, has been long known to dyers 

 under the name of iron liquor. Another acid has of Iste 

 been frequently substituted for the vinegar, called the 

 pyroligneous, which is distilled from wood, and is in 

 reality the acetic acid in combination with a portion of 

 empyreumatic oil. This compound acid dissolves iron 

 better than the acetic, aiid forms a salt which is more 

 useful in dyeing than the acetate. The oxide combines 

 with the acid at different stages of oxidation ; but the 

 resulting solution forms the most intimate and perma- 

 nent union with the fibres of linen and cotton, when the 

 oxidation is greatest. M. Chaptal has ascertained that 

 the different buff colours, and the imitations of nankeen, 

 may be greatly improved by combining the oxide of 

 iron with illumine. He accordingly first impregnates the 

 linen or cotton with a solution of the oxide of iron in the 

 pyroligneous, or someother vegetable acid, marking three 

 degrees on the areometer of Beaume, and, after wringing 

 it properly, plunges it immediately into a solution of pot- 

 ash marking two degrees, with which a saturated solution 

 of alum has been mixed, but so as not to precipitate the 

 alumine. By this mixture, the colour of the oxide is 

 heightened, and the cloth acquires an agreeable, smooth, 

 and soft appearance.. See. Ann. de Chim. xxvi. p. 270. 



95. The application of potash conjointly with an oxide 

 of iron, is stated by Dr Bancroft to have been practised 

 at Manchester for almost half a century. The iron was 

 usually, but improperly, dissolved in aquafortis, as this 

 acid acts with great energy on the fibres of linen and 

 cotton, and soon destroys their texture. 



96. The colours produced by iron, though very dura- 

 ble, are liable to be affected by the infusion of tea, and 

 other Mibstances which contain the gallic acid. The spots 

 occasioned by these may, however, be removed by an 

 oxymuriate of potash or lime. 



Capper. 97. Dr Bancroft has proposed to employ the oxides of 



copper as substantivecolours, though the advantage to be 

 derived from their ue in this rcsj)ect is extremely doubt- 

 1 



ful. He mentions that he had produced accidentally, Sntwtantht 

 and fixed permanently upon calico, a brownish red oxide ' 

 of copper, nearly resembling the ruby copper ore in co- 

 lour, which withstood repeated washing with soap, and six 

 weeks exposure to the atmosphere, without alteration. 

 He thinks that this colour may prove useful by simple 

 topical application, in calico printing ; but he has not 

 examined it sufficiently to give a decided opinion on the 

 subject. 



<)8. The green colour exhibited by most of the prepa- 

 rations of co|)|wr aiiscs from the absorption of oxygen, 

 for which the oxides of this metal have a strong affinity. 

 It does not appear that any of the solutions of copper or 

 its oxides, can be applied simply to linen or cotton, so as 

 to resist the action of soap, though they suffer but little 

 change from exposure to the sun and air. If liquid am- 

 monia be saturated with copper, and thickened with gum, 

 it may, by simple topical application, be fixed upon 

 linen or cotton, when, by an evaporation of a part of the 

 volatile alkali, and the absorption probably of both oxy- 

 gen and carbonic acid, its blue colour will be changed in- 

 to a green resembling that of verdigrcase, or rather that 

 of the malachite, which will sufficient! v resist the impres- 

 sion of sun and air, and bear a considerable number of 

 washings with soap, without being affected thereby. It 

 may therefore, says Dr Bancroft, be usefully employed 

 in this way, especially upon fine muslins, by reason of 

 the great delicacy of its colour, and the facility of its ap- 

 plication. Perm. Col. i. p. 320. 



99- The oxides of various other metals have been pro- 

 posed to be used in dyeing ; but it does not appear that 

 any of them can be usefully or extensively employed as 

 colouring matters upon cloth. 



CHAP. III. 



Of Adjective Colouring Mailers. 



I. Of Animal Adjective Colours. 



100. The most important and valuable colouring mat- 

 ter belonging to this class, is theeocei/.v rncli of Lii 



or cochineal. We have already described the methods 

 which are employed for rearing this insect, and shall 

 therefore confine ourselves at present to an account of 

 its qualities as a dye. See COCHINEAL. 



101. Two kinds of cochineal are employed in dyeing: 

 they are distinguished in Mexico by the Spanish terms 

 gratia Jfna, and gratia si/lrntra. The first variety, being 

 of a larger sixe, and yielding a greater quantity of co- 

 louring matter, is reckoned the most valuable; the other 

 is smaller, and covered with a downy substance, resem- 

 bling cotton, which increases its weight, without being of 

 any use in dyeing. 



102. Cochineal which has been properly prepared, 'and 

 carefully kept, is of a greyish colour, inclining to purple. 

 The greyish colour is occasioned by a powder, which co- 

 vers the insect in its natural state, and part of which 

 still adheres to it : the purple shade is owing to tin 1 co- 

 louring matter extracted by the hot water in which the 

 insect has been killed. Cochineal retains its properties 

 for a very long time if it be properly kept. Hellot says, 

 that he made some experiments with a quantity of it 

 which was 130 years old, and that its properties as n 

 dye were unchanged. 



103. The colouring matter of cochineal may be ex- 

 tracted by water. The decoction i of a crimson colour, 

 inclining to violet. A little sulphuric acid being added 



Adjective 

 colouring 

 matters. 



Cochineal* 



External 



Effects of 

 different 

 agents ilpo 

 the lolutim 

 ufcochiow 



