MORDANT 3G3 



have for the stuffs. Of all the acids which can bo employed to dissolve alumina, for 

 example, vinegar is the one which will retain it with least energy, for which reason 

 the acetate of alumina is now generally substituted for alum, because the acetic acid 

 gives up the alumina with such readiness that mere elevation of temperature is suffi- 

 cient to effect the separation of these two substances. Before this substitution of the 

 acetate, alum alone was employed ; but without knowing the true reason, all the 

 French dyers preferred the alum of Home, simply regarding it to be purest ; and it is 

 not many years since they have understood the real grounds of this preference. This 

 alum has not, in fact, the same composition as the alums of France, England, and 

 Germany, but it consists chiefly of cubic alum having a larger proportion of base. 

 Now this extra portion of base is held by the sulphuric acid more feebly than the 

 rest, and hence it is more readily detached in the form of a mordant. Nay, when a 

 solution of cubic alum is heated, this redundant alumina falls down in the state of 

 a subsulphate, long before it reaches the boiling point. This difference had not, how- 

 ever, been recognised, because Eoman alum, being usually soiled with ochre on the 

 surface, gives a turbid solution, whereby the precipitate of subsulphate of alumina 

 escaped observation. When the liquid was filtered, and crystallised afresh, common 

 octahedral alum alone was obtained ; whence it was most erroneously concluded, that 

 the preference given to Roman alum was unjustifiable, and that its only superiority 

 was in being freer from iron. See ALUM. 



Here a remarkable anecdote illustrates the necessity of extreme caution, before we 

 venture to condemn from theory a practice found to be useful in the arts, or set about 

 changing it. When the French were masters in Rome, one of their ablest chemists 

 was sent thither to inspect the different manufactures, and to place them upon a level 

 with the state of chemical knowledge. One of the fabrics, which seemed to him 

 furthest behindhand, was precisely that of alum, and he was particularly hostile to the 

 construction of the furnaces, in which vast bojlers received heat merely at their 

 bottoms, and could not be made to boil. He strenuously advised them to be 

 modelled upon a plan of his own ; but, notwithstanding' his advice, which was no 

 doubt very scientific, the old routine kept its ground, supported by utility and reputa- 

 tion, and very fortunately, too, for the manufacture; for had the higher heat been 

 given to the boilers, no more genuine cubical alum would have been made, since it is 

 decomposed at a temperature of about 120 F., and common octahedral alum would 

 alone have been produced. The addition of a little alkali to common alum brings it 

 into the same basic state as the alum of Rome. 



The two principal conditions, namely, extreme tenuity of particles, and liberty of 

 action, being found in a mordant, its operation is certain. But as the combination to 

 be effected is merely the result of a play of affinity between the solvent and the stuff 

 to bo dyed, a sort of partition must take place, proportioned to the mass of the solvent, 

 as well as to its attractive force. Hence the stuff will retain more of the mordant 

 when its solution is more concentrated, that is, when the base diffused through it is 

 not so much protected by a large mass of menstruum ; a fact applied to very valuable 

 uses by the practical man. On impregnating in calico-printing, for example, different 

 spots of the same web with the same mordant in different degrees of concentration, 

 there is obtained in the dye-bath a depth of colour upon these spots intense in propor- 

 tion to the strength of their various mordants. Thus, with the solution of acetito of 

 alumina in different grades of density, and with madder, every shade can be produced, 

 from the fullest red to the lighest pink ; and, with acetate of iron and madder, every 

 shade from black to pale violet. 



We hereby perceive that recourse must indispensably be had to mordants at different 

 stages of concentration : a circumstance readily realised by varying the proportions of 

 the watery vehicle. (See CALICO-PHINTIXG and MADDER.) When these mordants are 

 to be topically applied, to produce partial dyes upon cloth, they must be thickened with 

 starch or gum, to prevent their spreading, and to permit a sufficient body of them to 

 become attached to the stuff. Starch answers best for the more neutral mordants, and 

 gum for the acidulous ; but so much of them should never be used as to impede the 

 attraction of the mordant for the cloth. Nor should the thickened mordants be of too 

 desiccative a nature, lest they become hard, and imprison the chemical agent before it 

 has had an opportunity of combining with the cloth, during the slow evaporation of 

 its water and acid. Hence the mordanted goods, in such a case, should be hung up to 

 dry in a gradual manner, and when oxygen is necessary to the fixation of the base, 

 they should be largely exposed to the atmosphere. The foreman of the factory ought, 

 therefore, to be thoroughly conversant with all the minutiae of chemical reaction. In 

 cold and damp weather he must raise the temperature of his drying-house, in order to 

 command a more decided evaporation ; and when the atmosphere is unusually dry and 

 warm, he should add deliquescent correctives to his thickening. But, supposing the 

 application of the mordant and its desiccation to have been properly managed, the 



