CHEMISTRY APPLIED TO THE ARTS. 



leaves an insoluble basic salt of copper in the 

 fibres of the cloth. 



2. The formation of an insoluble compound of 

 the colouring-matter with the substance of the 

 fibre. This method applies only to the animal 

 fibres (silk and wool) which are capable of forming 

 stable, insoluble compounds with some colouring- 

 matters, such as picric acid and the aniline 

 colours. 



3. The action of two solutions successively in- 

 troduced to the fibre. This is by far the most 

 important method of dyeing, and may be conveni- 

 ently considered under two heads : (a.) Where a 

 chemical decomposition or a double decomposition 

 takes place ; and (t>.) where the substance first 

 introduced (in this case called the mordant) be- 

 comes insoluble in the fibre, or is decomposed 

 there, producing an insoluble substance ; and 

 then the dye-stuff, into a solution of which the 

 fabric is dipped, enters into combination with this 

 insoluble substance derived from the mordant. 

 "We shall give a few examples of each of these. 



(a.) Carthamine, the colouring-matter of saf- 

 flower, is soluble in alkalies ; cloth dipped in such 

 -a solution is stained or coloured red by it, and the 

 colour can be easily washed out ; but if, after 

 being dipped in the alkaline solution of carthamine, 

 the cloth is treated with an acid, the alkali is 

 neutralised, and the carthamine precipitated in the 

 fibres. Cotton is dyed yellow by dipping it, first, 

 an a solution of acetate or nitrate of lead, and then 

 passing it through a solution of chromate of potash. 

 Here double decomposition takes place in the 

 >fibres acetate or nitrate of potash and insoluble 

 yellow chromate of lead (chrome yellow) being 

 produced. Prussian blue is dyed in a similar way ; 

 the fabric being treated successively with solutions 

 of ferric chloride or acetate, and ferrocyanide of 

 potassium, acetate of potash or chloride of potas- 

 sium, and insoluble ferric ferrocyanide (Prussian 

 tlue), are formed. One of the methods employed 

 in dyeing silk black may be taken as another 

 instance. The silk is dipped in a solution of tannin, 

 which combines loosely with the substance of the 

 silk, and then in a solution of a ferric salt. The 

 black compound of ferric oxide and tannin, which 

 is the basis of common black ink, is thus formed. 



(6.) When a fabric is dyed by successively dip- 

 ping it into two liquids, the first is usually called 

 the mordant. We shall, however, here restrict 

 this term to substances which are introduced in a 

 state of solution into the fibre ; there become 

 insoluble, or give rise, by decomposition, to an 

 insoluble body, which unites with the colouring- 

 matter when the fabric is dipped in the dye-vat. 



The chief substances used as mordants are 

 salts of alumina, ferric and ferrous salts, stannous 

 chloride, stannic chloride, stannate of soda, 

 albumen from eggs or from blood, and gluten of 

 wheat. We shall shortly consider the mode of 

 action of these substances. The soluble salts of 

 alumina, especially the acetate, have a tendency 

 to form basic salts. Thus, if a solution of acetate 

 of alumina is boiled, an insoluble basic acetate is 

 precipitated. Under some circumstances, the 

 solution may even lose the whole of its acetic 

 acid, leaving hydrate of alumina. Similarly, when 

 a mixed solution of sulphate of alumina and 

 acetate of alumina is boiled, acetic acid separates, 

 and insoluble basic sulphate of alumina is pre- 

 cipitated. Now, when cloth is steeped in such 



solutions, and then heated, or even exposed for 

 some time to the air, the change just described 

 takes place ; acetic acid is given off, and an in- 

 soluble basic salt^ of alumina or the hydrate of 

 alumina itself is left deposited in the fibres. 

 Ferric salts act in exactly the same way, insoluble 

 basic salts, or even ferric hydrate, being produced. 

 ' Tin salt ' (stannous chloride with water of crys- 

 tallisation, SnCl 2 ,2HCl) is a crystalline, colourless 

 solid, formed by the action of aqueous hydro- 

 chloric acid on metallic tin. It dissolves in 

 water, but a dilute solution undergoes a change, 

 oxychloride (SnCl 2 ,SnO,2H 2 O) being precipitated 

 as an amorphous white powder, and hydrochloric 

 acid remaining in solution. In the cloth, this 

 change also takes place, and the insoluble oxy- 

 chloride, partially oxidised to hydrated stannic 

 oxide (SnO 2 ,H 2 O), remains in the fibre. Stannic 

 chloride (called by the dyers ' physic ') has the 

 composition SnCl 4 when pure ; as used for dyeing, 

 it generally contains some stannous chloride. It 

 dissolves in water; but when the solution is 

 diluted, hydrated stannic oxide is precipitated, 

 and hydrochloric acid left in solution. ' Pink 

 salt' is a compound of stannic chloride and 

 chloride of ammonium (SnCl 4 ,2NH 4 Cl) ; its dilute 

 solution undergoes, when heated, the same change 

 as has been just described in the case of stannic 

 chloride. As stannic oxide is a very feeble acid, 

 its compounds with bases, such as stannate of 

 soda (Na^jSnOa), are easily decomposed. If a 

 piece of cloth, dipped in a solution of stannate of 

 soda (' preparing salt '), be exposed to the air, the 

 atmospheric carbonic acid is sufficient to precipi- 

 tate stannic hydrate (H. 2 O,SnOj). 



We see, then, that all these metallic salts, when 

 used as mordants, deposit in and on the fibre 

 insoluble substances, which are either basic salts 

 or hydrated oxides. It has already been mentioned 

 that many natural colouring-matters form insoluble 

 compounds, called ' lakes/ with metallic oxides. 

 When, then, the mordanted cloth is dipped in a 

 solution of a dye-stuff of this kind, an insoluble 

 lake is produced wherever the basic salt or 

 hydrated oxide has been deposited. The colour 

 of the lake depends both upon the mordant and 

 upon the dye-stuft Alumina and tin mordants 

 give colours resembling those of the colouring- 

 matter itself, but iron mordants, as a rule, change 

 the colour. Thus, madder gives a red with an 

 alumina mordant ; but with iron, a black, if the 

 mordant is used in strong solution ; and a violet, 

 if dilute. Wool mordanted with alumina is dyed 

 blue by indigo carmine (p. 343), sulphindigotate 

 of alumina being formed. 



Albumen and gluten are used as mordants 

 when cotton has to be dyed with aniline colours. 

 It has already been stated that silk and wool 

 absorb these colours without the intervention of 

 a mordant ; and by surrounding the vegetable 

 fibre with a thin layer of a substance resembling 

 in some respects the substance of silk or wool, 

 cotton can be made to do so likewise. This treat- 

 ment of cotton or linen with albumen or gluten is 

 sometimes called ' animalisation,' because the 

 vegetable fibres are thus made to resemble animal 

 fibres in their power of taking up colours. As 

 tannin forms insoluble coloured compounds with 

 the aniline colours, cotton is sometimes dyed by 

 dipping it, first, in a solution of tannin, and then 

 in a solution of the aniline colour. 



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