OF MORDANTS.
201. The colouring particles seldom have such a de cided affinity for the substances usually submitted to the operations of dyeing, as to form with them a per manent union; it frequently happens, indeed, that the same solvent which deposits the colouring matter on the stuff, will, with equal facility, re-dissolve and erase it. We have already stated, however, that the colour ing matters possess very strong attractions for earths and metallic oxides, particularly for the earth of alumine and the oxides of iron ; and as these bodies also unite readily with the fibres of which stuffs are usually fabri cated, a triple compound may thus be formed, consist ing of the stuff, the colouring matter, and the chemi cal agent having an attraction for both. The substan ces which serve as a bond of union between the colour ing matter and the stuff, are -called morIants, a term applied to them from the mechanical action which they were supposed to exert upon the latter, to prepare it for the reception of the dye. As the theory concern ing the action of mordants, which first suggested the term, is now completely exploded, Mr Henry has pro posed to substitute in place of mordant, the word basis. We shall sometimes employ the one term and some times the other.
202. The word mordant is also applied in a more ex tensive sense, not only to denote those substances which promote the union of the colouring matter with the cloth, and render that union permanent, but all those agents which affect, in any respect, the colouring mat ter in its new state of combination. Dr Bancroft has proposed to apply the term alterants to this class of bo dies, their object being not so much to fix, as to vary and modify the shades of adjective colouring matters.
203. The nature of mordants deserves to be investi gated with the utmost attention, for though almost all the substances included under this appellation have been discovered by accident, an accurate analysis of their ac tion must tend greatly to improve the principles of dye ing, and give something like a scientific form to its pro cesses.
204. A mordant is not always a simple agent ; new combinations are sometimes formed by the_ingredients of which it is composed, so that the substances employed are not the immediate agents, but the compounds to which they give rise. " The substances which com pose a mordant, are sometimes incapable," says Bet- thollct, 44 of decomposing each other solely by their own attractions; but the attraction of the stuff for one of their constituent parts brings about a decomposition and new combinations, and sometimes this effect is not produced or completed without the aid of the attraction of the colouring particles. This appears to be the case in the mixture of alum and tartar, one of the most com mon mordants employed in the dyeing of wool." 205. " I dissolve" continues he, " equal weights of alum and tartar ; the latter salt, by this mixture, acqui red a greater degree of solubility than it naturally pos sesses, but by evaporation and a second crystallization, the alum and the tartar were separated, so that they had not decomposed each other. I boiled for an hour half
an ounce of alum with an ounce of wool, a precipitate was formed, which I washed carefully ; it consisted chiefly of small filaments of wool incrusted with earth; to this I added sulphuric acid, and evaporated to dry ness, dissolved it, and obtained crystals of alum. Some carbonaceous particles separated from it. I evapora ted the liquor in which the wool had been boiled, but obtained from it only a few grains of alum ; the remain der would not crystallize. I re-dissolved it, and pre cipitated the alumine by an alkali : the precipitate was of a slate colour ; it grew black upon a red hot coal, and emitted alkaline vapours.
"By this experiment," says he, " we sec that the wool had decomposed the alum; that a part of the alumine had combined with its most detached filaments, which were least retained by the force of aggregation ; that a part of its animal substance had been dissolved and pre cipitated by the alkali, from the triple combination which it had formed. I made the same experiment with half an ounce of alum, and two drams of tartar ; no precipitation took place. I obtained by evaporation a small portion of the tartar, and some very irregular crystals of alum, the rest would not crystallize ; this 1 1 et nth water pie) ,pit red by potash, and t.1.1),, by c )aporation a salt %% ',it It burned like tartar." I It infers front the whole, " that the wool ha b writ a decompositnal of the alum, that it had muted »ith a part of the alumine, and that even the part 1)1 alum) which retained its alumine had dissolved sown: it the arum nl mattei ; that the tartar and alum, which Cannot drt.0111posc other solely by their own at tractions, heroine capable of acting on each other when their attractions are assisted by that of wool; that the tartar appears principally useful for moderating the too pow c t fur action of the alum upon the wool, where IA it is injured, for tartar is not used in the aluming of silk and t tread, which have less action on the alum than wool has.• 2 )6. Ilerthollet has shewn the attraction of alum for animal substances by an experiment not less decisive: having mixed a solution of glue with a solution of alum, he precipitated the earthy base by an alkali, when the alumine Gil down, and carried along with it a portion of combined glue. The decomposition of alum is not so readily produced by srgrw,wbfr 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 ahnoine and th • tannin.