214

DYEING.

cnce in tillin' coarseness or fineness, when subjected to ebullition, issufficient to produce differences in the results; and because the co-lourin'; particles have a greater or less disposition to combine withthe stuff, according to the proportion of sulphate of iron that liasbeen made use of. Solutions of iron in different acids may pro-duce differences in the results, according to the state of oxygena-tion of the iron in them, according as the proportion of that metalis greater or less, and according to the strong or weak action,which ihe different acids, when disengaged, are capable of exert-ing on the new ly formed compound. And, in the dyeing of astuff, considerable alterations may arise from its greater or less at-traction for the colouring particles. We shall now consider theastringent principle in regard to its property of combining withvegetable and animal substances, particularly the latter, bilk ac-quires by galling, which is an operation that consists in macerat-ing a stuff in a decoction of some astringent substance, a weightwhich cannot betaken from it, or diminished beyond a certain de-gree by repeated washing ; after which operation, the stuff, whenput into a solution of iron, is dyed black, because the astringentprinciple, decomposing the sulphate of iron, forms a triple com-pound with the oxide of iron and the stuff which is dyed. A gall-stuffed is likew ise capable of combining with other colouring parti-cles, the colours of w liich acquire fixity, if they do not naturally pos-sess it; so that the astringent communicates its durability to thetriple compound, or perhaps the more complex one which isformed; hut by this union the colour generally becomes of adeeper shade. The astringent principle, by < ombining with ani-mal substances, renders them incapable of corruption, and tendsto render their texture more compact; and in this the art of tan-ning consists. To account for the colours, which proceed fromthe union of the colouring particles with the basis which a mor-dant gives them, we must attend to the proportion in which thecolouring particles unite to that basis: thus the solution of tin,which produces a very copious precipitate with a solution of co-louring particles, and which thereby proves that the oxide of tinenters in a large proportion into the precipitate, has a much greaterinfluence on the colour of the precipitate, by the whiteness of itsbasis, than the solution of zinc, or that of allttm, w hich generallyproduce much less copious precipitates. The precipitates pro-duced by these two last substances retain very nearly the naturaltint which the colouring particles afforded. It is, therefore, ne-cessary to distinguish, in the action of mordants, the combina-tions that may take place by their means, between the colouringparticle-, the" stuff', and the intermedium; the proportions of thecolouring substances and intermedium; the modification'of co-lour, which may arise from the mixture of the colour of the co-louring particle's, and of that of the basis to which they are united;and the changes which the colouring particles may sutler, from thecombustion that may be produced by the substance that is em-ployed as an intermedium. It is evident also, that astringents donot derive their characteristic property from an acid, or from anyother individual and uniform principle, blit from the propertywhich they possess, of uniting with the oxide of iron, of reducingit to the state of a black oxide, and of acquiring themselves a darkcolour, l>y the combustion they thereby experience. Galls rea-dily undergo a slight combustion, w hich gives them a deep browncolour; but tliis combustion, which requires but a small quantityof oxygen, soon ceases without their properties being in any degreeinjured. But galis owe their stability to the large proportion ofcharcoal they contain; and as they have the property oi combin-ing with some vegetable substances, with several colouring mat-ters, and particularly with animal substances, they serve as inter-media fur them, and impart to them that stability which they pos-sess in so very high a degree. In the opinion oi Dr. Bancroft, anable writer on dyeing, M. Bertiiollet, in ascribing the decays ofvegetable and animal colouring matters in general, to effects'orchanges similar to those of combustion, has gone much furtherthanks warrantable by facts. It cannot, he thinks, be his inten-tion, that we should apply the term of combustion to alterationswhich result from a simple addition of oxygen, to colouring mat-ters, without a destruction or separation of any of their componentparts; though many of the decays and extinctions of these coloursevidently arise only from such simple additions of oxygen. Thenitric, sulphuric, and other acids, containing oxygen, have flicpower not only of weakening, but of extinguishing, for a time, thecolours of many t'uigent matters; not by any effect which can

properly be denominated a combustion, but rather by a change intheir several attractions for particular rays of light; but none oftheir parts being destroyed, or carried away, the addition of analkali, or of calcareous carbonate, will generally undo such altera-tion, and restore the original colour, by decomposing ami neutra-lizing the acid or oxygen, which had caused the alteration. Dr.Bancroft also observes, that until further discoveries shall havebeen made, we are only authorised to conclude, that the perma-nent colours of natural bodies do not depend upon their thick-nesses, sizes, or tiie densities of their several parts or particles, butupon certain affinities or attractions (chemical or physical) bywhich they are disposed to absorb and conceal some of the rays oflight, and to rellect or transmit other rays, giving the sensations orperceptions of their respective colours; that the contact of lightgreatly contributes towards producing these affinities or attrac-tions; but it only docs this (as far as ivc vet know) either liv pn -nit-ting a combination or a separation of the basis of vital air’in lhedifferent coloured or colouring substances. But though most oftiie changes of colour, in permanently coloured bodies, evidentlydepend on changes in their respective portions of oxygtn, he isfar from thinking that this cause operates exclusively m all cases,or that cliemical knowledge is yet far enough advanced, to justifyeven an attempt towards a complete hypothesis respecting thesemost abstruse and most interesting phtrnomenu. It may not beimproper here to add that tiie most pennant dyes we have, arecochineal and gum-lac, for tine red and scarlet colours; indigoand woad, for blue; and, when mixed with different proportionsof cochineal, or gum-lac, for purple and violet colours. Dyers*weed, and some other vegetables, for yellow ; and madder forcoarse reds, purples, and blacks. Tin: fading colours are far morenumerous, and include Brazil-wood, log-wood, red-wood, fustic,turmeric root, anolto, archil, &c. For further information on thetheory of dyeing we refer to tiie works oi the authors above-men-tioned, and particularly to Bertliollet’s treatise, as being the mostcomplete one as yet offered to the public on this subject.

Of the Practice of Dyf.ixo.

The whole of the operative part of dyeing depends on the ap-plication of certain colours, which tiie woikmen call primitive,and which are live in number, namely, blue, red, yellow, fawn, orroot-colour, and black. Each of these furnishes a variety of in-termediate sltades, both according to the nature of tiie ingredients,and the acid or alkaline substances with which they are mixed.Two only of these live colours, should be prepared with ingre-dients producing no colour of themselves : hut which, by theirpeculiar acidity, and the lineness of tiie earth they contain, disposetiie pores of the substance to receive (lie dye. The colourswhich more particularly require such auxiliary process, are redand yellow, together with those derived from them. Black is ob-tained by a particular preparation ; but blue and faw n colour re-quire none, at least for wool ; it being only necessary to scourand soak tliis substance well; then to immerse it in the dyeing vat,stirring it well about, and permitting it to remain for a'longer orshorter time, in proportion as the colour is intended to be moreor less deep. Tiie ingredients used in dyeing blue consist of pas-tel, woad, and indigo.

1. 1’astei., Tinctariu,] is prepared by gathering it when

ripe, suffering it to rot, and then wot king it up into hulls for dry-ing ; which weigh in general from 150 to 200 pound,, and resem-ble a collection of small dry lumps of earth, intermixed with thefibres of plants. In order to extract the colour, it is necessary toprovide large wooden vats, from 12 to 16 feet in diameter, and 6or 7 feet high, or of a magnitude proportioned to the quantity in-tended to be used. The preparation of the blue-vat is the mostdifficult process in tiie art of dyeing; and tiie practical directionsgiven by those w ho understand it, are either detective, or mis-stat-ed. The copper-cauldron should lie placed as near to the vat aspossible, anil tilled with pond-water: to which, if it be not suf-ficiently putrid, may be added two or three pounds of hay, toge-ther with eight pounds of brown madder, or of the bark of tiieroot. The lire should he lighted about three o’clock in tiie morn-ing, and the mixture boil for an hour and a half, or two hours,when the liquor is, by means of a spout conveyed into the vat, inwhich a peck of wheaten bran is previously infused. The pastel-balls are next to be put in, separately, while the liquor is runninginto the vat, in order that they may be the more easily broken

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