ij3 Compound Colours prtduceJ on Cotton hy the Oxyae of Iron, 



ing to the nature of the violet which may be defired. The cotton muft be carefully waflicd 

 and maddered twice in fucceflion, after which it is brightened in a bath of foap. 



When a true violet of a foft and firm colo.ur is required, the folution of iron is not to be 

 ufed till after the galling. The irw i? Aen precipii^^ated In a bluifli oxyue, which combining 

 with the red of the madder affords a fuperb violet, more or lefs deep according to the ftre;;»gtli 

 of the galling and of the martial folution. 



It is very difficult to obtaiii an uniform colour by this proccfs ; and a very even viplet is 

 C(^4i'i?'^cd in the m^nufidtories as a maftsr-piece of the art. It is generally imaginedvth'^t 

 this important problem of the art of dying can only be folved by well-condu6ted manipulations. 

 But I am w;cll convinced that the great caufe of the unevennefs of this dye is, that the iron 

 liepofitcd upon the cotton receives an oxydation by mere expofure to the air, .which varies in 

 tb/c different parts of the cottpo- TH? threads which are at the outfide of the mafs becoiae 

 ftrongly oxyded, while the interior part, being defended from the aclion of the air, undergoes no 

 change. Whence it fiajlows, that the i^ner part acquires a faint Qiade, while the outer furfkce 

 prefents a violet almoft black. The only means of remedying this ineonvcmeuce;, is to wafh 

 the cotton on ta^ng it .ppjt of the folution of iroij, and to 0ia44e/ it while wet. The cotton is 

 thus rendered more even and velvety. 



The fdvents for iron are nearly the fame for this colour, as for the yellow of which we have 

 riceady treated. 



I fupprefs all obfervation on the manipulations, in order to confine myfelf to mere chemical 

 report ; and on this confideration, I fliall add an obfervation to direft the artift in the brighten- 

 ing of violet upon cotton. 



The red of madder, and the oxyde af iron, being depofited an the fluff, determine the violet 

 colour. This colour inclines to red or blue, accordingly as the one or the other of the two 

 principles predominates. The dyer knows by experience, how difEcult it is to obtain a com- 

 bination which fliall produce any defired tone or colour ; particularly, when it is required to 

 be uniform, bright, and folid. It may, neverthelefs, be effefted, not only by varying the pro- 

 portions of the two principles, but alfo by varying the procefs of brightening. It is only ne- 

 ceflary to know the following fails : namely, that foda deftro) s the iron, while foap by ftrong 

 boiling attacks the red of the madder in preference. Hence the dye may be made to incline 

 to red or blue, accordingly as the brightening is effedled with one or the other of thefe 

 mordants. Thus cotton taken out of the madder, waflied, and boiled with 30,00 parts of 

 foap, will afford a fuperb violet j though, if foda had been ufed, it would have turned out merely 

 a prune colour. 



The oxyde of iron precipitated upon piece-goods, unites ftill more advantageoufly with 

 the yellow afforded by aflringents; and an infinite number ot (hades may be obtained by vary- 

 ing the force of the mordants. In this cafe, it is lefs a combination, or folution, of principles, 

 than a fimple mixture, orjuxtapofition of the colouring matters upon the ftuff. The oxyde 

 of iron may be more intimately combined with the aftringent principle by a boiling heat, 

 which brings it to the ftate of black oxyde, as our colleague Berthollet has obferved. 

 It is alfo pofEble to render theie fame colours brown, and give them a variety of tinges, 



from 



