210 



DYEING. 



(N M.T- 



I for 

 tab- 



Ami colour- 

 ing; mat- 

 ters. 



Attraction 

 cf metallic 

 oxides for 

 colouring 



Mordants 

 <hould ge- 

 nerally be 



cannot decompose each other solely l>y their own at- 

 traction!), become capable of acting on each other when 

 their attractions an- a .i-tfil liy that of wool ; th.it the 

 tartar appears principally useful for BodantiBC the 

 too powerful action of the alum upon the wool, where- 

 by it is injure*!, for tartar is not usetl in the uluming 

 of silk and thread, which have less action on the alum 

 than wool has. 



206. Berthollet has shewn the attraction of alum for 

 animal substances by an experiment not U-s-, decisive: 

 having mixed a solution of glue with a solution of alum, 

 he precipitated the earthy base by an, alkali, when the 

 alumine it'll down, and carried along with it a portion 

 of combined glue. The decomposition of alum is not 

 o readily produced by vegetable substances though it 

 is partially effected by the assistance of the astringent 

 principle. When a stuff is impregnated with the latter. 

 and placed, after it has been allowed to dry, in a solu- 

 tion of alum, a combination is established between the 

 alumine and the tannin. 



207. The attraction of alumine for the greater num- 

 ber of colouring substances may also be shewn by di- 

 rect experiment. If a solution of colouring matter be 

 mixed with a solution of alum, a precipitation general- 

 ly takes place, consisting of the colouring particles com- 

 bined with alumine. The precipitate is Known in the 

 arts under the name of lac or lake, and is more readily 

 formed when an alkali is added to detach the acid, and 

 leave the alumine at greater liberty to unite with the 

 colouring matter. An excess of alkali ought not to be 

 added, however, because alkalies dissolve the greater 

 number of the lakes. M. Thenard has shewn that alu- 

 mine may even be intimately mixed with several me- 

 tallic colouring matters, as prussiate of iron, oxide of 

 cobalt, &c. 



208. The metallic oxides have also a strong attrac- 

 tion for colouring matters, and when they are presented 

 to the latter in combination with an acid, they frequent- 

 ly quit the solvent, and are precipitated like the alu- 

 mine in combination with the colouring matter. This 

 class of bodies has also the property of combining with 

 animal substances, and of thus serving as a bond of 

 union between these and colouring particles. But not 

 only have the oxides a strong attraction for colouring 

 particles, and animal matters, but their solutions in 

 acids possess properties which render them no less fit 

 to be employed as mordants ; and indeed it is generally 

 in this state that they are used in dyeing. 



'209. In order that a substance may be fit for being 

 employed as a mordant, it is not always sufficient that 

 it should possess an affinity to the colouring principle 

 and the stuff; it must also be perfectly white, other- 

 wise its colour mixing with tliat of the colouring prin- 

 ciple, would produce an intermediate or mixed colour. 

 In some cases, indeed, this may be an advantage, and 



O{ 



tii ""* - 



a substance may be employed both as a colouring mat- 

 ter, and a mordant. Thus the oxide of iron, which, if 

 it were employed alone, would produce a nankeen or 

 buff colour, communicates to cotton a violet with mad- 

 der red ; and in like manner, the oxide of till not only 

 fixe-, but greatly exalts several colouring matters. 

 Mordants ought also to be little liable to change, by 

 the action of air and water, the two agents to which 

 dyed stuffs are most frequently exposed. The metallic 

 oxides, from their disposition to attract additional por- 

 tions of oxygen from the atmosphere, and on that ac- 

 count to sutler changes themselves, as well as to affect 

 the shades of colouring matters, are less valuable as mor- 

 dant- than they would otherwise be. 



210. Neutral salts, particularly nitre and muriate of 

 soda, (common salt,) act as mordants, and modify co- 

 lours ; but the nature of their action has been little at- 

 tended to, and is imperfectly imderstood. Berthollet 

 found that the muriate of soda was contained in the 

 precipitate* produced from some kinds of colouring 

 matter, and that these precipitates retained a consider- 

 able degree of solubility ; he conceives that a small 

 part of the salt attaches itself to the colouring principle 

 and the stuff. Salts with calcareous bases also modify 

 colours ; but as the changes to which they give rise 

 may be produced by the addition of a small quantity of 

 lime, it is probable that they suffer a partial decomposi- 

 tion, and that when they are used in dyeing, a little of 

 the calcareous base enters into combination with the co- 

 louring particles and the stuff. 



211. The two mordants most extensively employed .,_ 



,. . %,, *. J Aluminous. 



at present, are salts of alunnnc and tin. 1 he former m0 rdant. 



is prepared, according to Berthollet, by dissolving in 

 eight pounds of hot water, three pounds of alum, and 

 one pound of acetate of lead, (sugar of Itucl,) to which 

 two ounces of potash, and afterwards two ounces of 

 powdered chalk, are added. The alum is decomposed 

 by the acetate of lead, because the oxide of lead com- 

 bines with the sulphuric acid of the alum, and forms an 

 insoluble compound, which is therefore precipitated ; 

 while the alumine is attracted by the disengaged ace- 

 tous acid, and forms the acetate of alumine. The 

 chalk and the potash are added to neutralize the e\ 

 of the sulphuric acid, and thus to assist the operation of 

 the acetic acid. The substitution of the acetic acid for 

 the sulphuric, in the preparation of the aluminous mor- 

 dant, is attended with several advantages. The acetate of 

 alumine being much more soluble than alum, the same 

 quantity of solvent will hold suspended a greater quan- 

 tity of the former than the latter, and thus present the 

 mordant in a more concentrated slate to the stuff; 

 while, on account of its great solubility, it will not be 

 apt to crystallize when mixed with starch or v-m, to 

 prepare it for being applied to the block, in topical 

 dyeing. The alumine is also retained less powerfully 



These experiments of Berthollet, which seemed to establish the fact, that a powerful affinity subsist* been alumine anil wool, 



haic been questioned, or rather their accuracy has been distinctly denied by Thcnard and Hoard. By a series of experiments, of which tiny 

 have given a minute statement, in a memoir read at the Physical and Mathematical class of the French National Institute, thi-y a*ccminl, 



er when dissolved in water and boiled with wool ; that, in this boiling, the wool ccm'iincb 



that alum and tartar do not decompose each other 



with the alum without decomposing it in any degree, and also with the tartar ; and that equal parts of alum and tartar would dissolve in two 

 n'fths lew of water than would be required to dissolve them separately. They found that wool, as it is commonly cleansed for being nhimed, 

 was not deprived of the carbonate of lime naturally combined with it ; and tliat this wool being boiled the usual time, with one fourth of 

 its weight of alum, and one sixteenth of its weight of cream of tartar, rendered the bath or water troubled or muddy, anil produced a copi- 

 ous white sediment, which, being collected, washed, and examined, w;is found to consul chiefly of a sulphate of lime, and a saturated sul- 

 phate of alumine ; that when wool had been properly cleansed, and freed from the carbonate of lime, no sulphate was obtained ; and that, 

 when in this state, it was boiled in pure water, with the proportions just mentioned of alum and tartar, the residuum of the hath after eva- 

 poration was found to consist of alum, cream of tartar, and a compound difficultly rrystalliztble, composed of tartrile of potash and animal 

 matter. The wool itself afforUeJ, by repeated washings, alum and a small quantity of cream of tartar, >r combination of 



larlaric acid, alum, and animal matter. The mulls obtained by these chemists have led them to conclude, that, in tl.c pincers of aluming 

 animal substances, the alum combines with them entire without suffering decomposition. They have also drawn a similar conclusion wftk 

 respect to stuffs of vegetable origin, viz. cotton and linen. 





