DYEING.
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many experiments in different processes of dyeing haw been pub-lished by Beckmann, Poerner, Vogler, and Francheville. InEngland two very valuable essavs on dyeing have been writtenby l)elaval and Iienry ; to which may be added, the excellenttreatise on the philosophy of permanent colours, by Dr. Ban-croft.
Sir Torbern Bergman seems to have been the first, who referredthe pluvnomena of dyeing entirely to chemical principles. Hav-ing dyed some wool and some silk in a solution of indigo, in very-dilute sulphuric acid, he explains the effects he observed in theoperation, by attributing them to the precipitation, occasioned bythe blrle particles having a stronger attraction for the particles ofthe wool and silk, than for those of the acidulated water: he re-marks, that this attraction of the wool is so strong, as to deprivethe liquor entirely of the colouring particles; but that the weakerattraction of the silk can only diminish the proportion of these par-ticles in the bath: and he shews, that both the durability of thecolour, and the degree of intensity it is capable of acquiring, de-pend on these different attractions. This is, in fact, the true lightin which the phamomena of dyeing should be viewed; they arereal chemical phenomena, which ought to he analysed in the sameway as all those dependant on the actions which bodies exert, inconsequence of their peculiar nature. It is evident, that the co-louring particles of bodies possess chemical properties, that distin-guish them from ail other substances; and that they lume attrac-tions peculiar to (hem elves, by means of which they unite withacids, alkalies, metallic oxides, or calces, and some earths, princi-pally alumine or pure clay. They frequently precipitate oxidesand alumine, from the acids which held them in solution; at'othertimes they unite with the salts and form supra compounds, orcompounds of more than two ingredienti, which combine with thewool, silk, cotton, or linen. And with these, their union is ren-dered much more close by means of alumine, or metallic oxide,than it would be without their intervention. The difference inthe attractions of the colouring partic les lor wool, silk, and cotton,is sometimes so great, that they will not unite with one of thesesubstances, while they combine very readily with another: thus,cotton receives no colour in a batli which 'lyes wool scarlet. Du-fay got a piece of stuff'made', the warp of which was wool and thewool cotton, which went through the process of hilling, that hem glit be certain, that the wool and the cotton received exactlythe same preparation; but the wool took ti e scarlet dve, and thecotton remained white. It is this difference of attraction, whichrenders it necessary to vary the preparation and the process, ac-cording to the n. tore of tiie substance which is intruded to hedyed of a particular colour. And these considerations ought todetermine the means to be pursued for the improvement of the artof dyeing. It is highly proper to endeavour to ascertain, what arethe constituent principles of the colouring particles. And in thisinquiry, the most essential circumstances are, to determine theaffinities of a colouring substance, first, with the substances whichinay be employed as menstrua; Jdly, with those which may bytheir combinations modify the colour, increase its brilliancy,'andhelp to strengthen its union with the stuff to he dyed; 3dlv, withthe different agents which may change the colour, and principallywith the external agents—air and light. Tiie qualities of the un-combined colouring particles are modified when they unite witha substance; and it this compound unites with a stuff', it undergoesnew modifications. 'Unis the properties of the colouring particlesof cochineal are modified, by being combined with tiie oxide oftin, and those of tiie substance resulting from this combination, arcagain modified by their union with tiie wool or silk; so that theknowledge we may acquire, by the examination of colouring sub-stances in their separate states, can only inform us respecting thepreparations that maybe made of them: that which we acquirerespecting their combinations with substances which serve to fixthem, or to increase their beauty, may inform us what processesin dyeing ought to lie preferred or tried; but it is only by directexperiment made with the different substances employed in dye-ing, that we can confirm our conjectures, and properly establishthe process. These facts shew, that the changes produced byacids and alkalies on many vegetable colours, such as the chemistsemploy, in order to discover the nature of different substances,are owing to the combinations, which take plant between these co
louring particles, and the acids and alkalies. The compounds re-sulting trom these may be compared to neutral salts, which possessqualities different from those of their component parts, but inwhich one of these parts may he in excess, and its qualities conse-quently predominant. 1 his slate of combination is observablebetween the colouring particles of cochineal and acidulous tarlriteof potash, or cream of tartar; by evaporating slowly a solution oftliis salt in a decoction of cochineal, crystals are lormed, whichretain a fine ruby colour, much more bright and intense than thatof the liquor which formed them. There are also some acids,particularly tiie nitric, which, after having combined with the co-louring particle;, change the colour which they at first produced,make it yellow, and destroy it. They then act by means of oneof their principles, tiie oxygen, or vital air. Blue colours are it ttiie only ones which become red by the addition of acids, andgreen by that of alkalies; most red colours, as that of the rose,for instance, are heightened by acids, and made green by alkalies;and some green colours, such as that of the green decoction ofburdock, according to the experiments of Mr. Nose, and thegreen juice of buckthorn, as is evident from tiie trials of Mr.Becker, are reddened bv acids. This property, which is commonto most of the ordinary colours of vegetables, seems to prove, thatthere is a great analogy between their colouring parffcles; and itis not witliou. foundation, that Linnaus supposed, that the red invegetables was owing to an acid, and indicated its presence; butthere are also many vegetables which contain acid in a disengagedstate, without their possessing a red colour. It is therefore evi-dent, that the colouring particles have attractions for acids, alka-lies, earths, and metallic oxides, which constitute a part of theirchemical properties; and in consequence of which, their coloursare more or less varied: therefore these particles form, with thestuff on which they are fixed, a compound which retains onlysome ot their original properties: they are also modified by theirunion witli alumine, or pure clav, metallic oxides, and someother substances; as are also those new compounds, when theyare further combined with the stuff': all these modifications areanalogous to what is observed in other chemical combinations.
Of Mordants.
Mordant is a term that appears to have been first introduced bythe French dyers; and is applied to those substances that serve asintermedia between the colouring particles, and the stuff to liedyed, either lor the purpose of facilitating, or of modifying theircombination. Basis would seem to lie a more appropriate term.Mordants deserve the greatest attention; as by their means co-lours are varied, brightened, made to strike, and rendered moredurable. We shall therefore examine the nature of tiie action ofthe principal bases or mordants, and endeavour to determine howtheir attractions serve to unite the colouring particles with thestuff, and how they affect the qualities of the colours. A basis ormordant is not always a simple agent, for new combinations aresometimes formed by tiie ingredients that compose it; so that thesubstances employed are not the immediate agents, but the com-pounds which they have formed. Sometimes the mordant is fix-ed with the colouring particles, and sometimes the stuff'is impreg-nated with it; on other occasions, both these modes are united;and we may dye successively with liquors containing differentsubstances, the last of which only can act on the particles withwhich the stuff' is impregnated. ’ The principal substances em-ployed as mordants are, aluminous salts, lime, metallic, oxides,some astringent substances, and animal mailers. The art of print-ing linen affords many processes, in which it is easy to observethe effects of mordants: therefore, to elucidate this subject, weshall mention a few examples. 'Flic basis employed for linens in-tended to receive different shades of red, is prepared by dissolvingin 8lb. of hot water, 31b. of alum, and lib. of aretite of lead, orsugar of lead, to which ffoz. of potash, and afterwards 2oa. ofpowdered chalk are added. The alum is decomposed by tiie ace-tite of lead, because tiie oxide or calx of lead combines with thesulphuric or vitriolic acid, and forms an insoluble salt which isprecipitated; tiie base of tiie alum, alumine, at the same timecombines with the acetous acid, or vinegar, and produces an ace-tile of aluntinc; and tiie chalk and potash answer tiie purpose ofsaturating the excess of acid. One of the advantages, which re-sult from the formation of tiie acetite of alumine, is, that the alu-mine is retained in it by a much weaker attraction than in tiie<> alunt;