DYE

DYE

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mentioned, and is made by digefting this colour-ing matter for above twenty-four hours, with agentle heat, in about four times its weight offtrong fulphuric acid. This acid (which thusconcentrated, and with the aflifiance of heat,would charr and deflroy moil vegetable matters)produces but little apparent alteration on indigo,but dilfolves it into a fluid of an inky blacknefswhen undiluted, but when largely mixed withwater, it produces a very beautiful tranfparentblue liquor, of a brighter colour than the alka-line folutions of indigo, and capable of givingvery line dyes to cloth, filk, or cotton. Thegreat inconvenience however of the Saxon blueis the extreme difficulty of rendering it a tole-rably fall colour, for when applied in the com-mon way it is foon deftroyed by walhing, efpe-cially with foap, fo that the indigo in the Hatein which it exills in this folution appears tohave but a very feeble affinity with animal orvegetable fibre, a remarkable contrail to thehabitude of the fame colouring matter whendeoxygenated and diflblved in alkalies.

The colour of the Saxon blue undergoes nonotable change like that of the common indigovats, the Huffs dyed in it coming out blue andnot green. The colour takes upon woollencloth with great rapidity, fo that it is very diffi-cult to dye it uniform through a whole piece.Some fcientific dyers have thought that this dyewas more permanent when the folution wasfaturated with an alkali juft fhort of the pointat which the indigo begins to feparate, for anentire faturation of the acid will precipitate theindigo nearly unchanged. Other artifts havethought alum of ufe in this cafe in fixing thecolour. The a£lion of acids upon indigo will befurther mentioned under the article Indigo.

PruJJian Blue is fo beautiful a colouring mat-ter for painting, that many attempts have beenmade to fix it permanently on cloth of differentkinds. Pruffian blue, or pruffiat of iron, is natu-rally a fall colour, refilling the adlion of lightand air for any length of time, but when dif-folved in any of the alkalies, or in lime-water,the blue is immediately deftroyed, and a paleftraw colour fubftituted. The folution is atriple compound of pruffic acid, iron, and thealkali or lime ufed. If any acid be added to theabove pruffiat to the full faturation of the alkalior lime, the blue colour is reftored without im-mediately precipitating the pruffian blue.

Two methods have been adopted of preparingthis colour for dyeing, the one, by mixing itwith muriatic acid, in which cafe it is only fuf-pended and not diffolved; the other, by diffol-

ving it in alkalies or lime, and faturating thefolution to that point at which the blue colouris reftored. The pruffian blue in either cafe hasfuflicient affinity with the fibre of cloth to unitewith it rapidly and firmly.

Some have ufed an acetite of iron as a kindof mordant to the cloth before the application ofthe pruffiated alkaline folution. It is neceflaryin all cafes where this folution is ufed, that it beneutralized with an acid to diminifh the ftrongaffinity between the alkali and colouring matter,which without this affiftance the cloth alonecould not overcome.

Two capital inconveniences have hithertobeen found to attend the ufe of this fubftanceas a blue dye, the one the extreme difficulty ofmaking it take at all uniformly on a moderateextent of cloth; for an immerfion of a very fewminutes is found fuflicient to give the full effedlof this colour, and hence the dye is depofitedfo rapidly as to be conftantly wavy and un-even. The other defefl is that it is readily dis-charged by foap, the alkali in the foap havingthe fame effect on the colour fixed in the fibresof cloth as it has on pruffian blue in fubftance,fo that after a very few wafhings the colour be-comes of a dirty yellowilh brown. Stuffs dyedwith this colour therefore muft be walhed withbran, or oatmeal, or in any other way in whichno alkaline fubftance is employed.

A balls of other colours is often ufed indyeing with pruffian blue.

Very beautiful greens are produced with thiscolour, and the permanent yellows.

Dr. Bancroft, in his curious experiments onthis colouring matter, obferves, that when cot-ton is printed with a mixture of adjeflivecolours, and an iron mordant without alum, andafterwards dyed with pruffian-blue folution, therefult is not a mixture of the blue with theadjedlive colour already applied (for example,green, where the latter is a yellow) but isfimply that colour which the pruffian folutionwould produce with the iron mordants withoutthe adjective dye: or in other words the pruf-fian blue appears merely to difplace the firftapplied colour, and unite with the iron mordant.It has however the capital advantage of dyeingmuch more uniformly than in the common way,fo that in this inftance the adjective colour ap-pears to divide and diftribute the mordant moreaccurately than could be done without it. Onthe other hand, where the adjective colour isfixed with an alum mordant, the pruffian bluecannot difplace it, and therefore unites with it,forming an intermediate fliade of colour.