It is not unfrequently desirable to know, with regard to dyed stuff, in what manner it has been dyed, and what dyeing material has been employed. This cannot always be de-elded by the appearance ; for example, in the case of a dark blue, the question rises whether the ground is pure indigo or pure logwood, or a mixture of both, or whether Prussian potash-blue is not present, &c. For this purpose recourse must he had to chemical Te-agents. In order to ascertain what mordants have been used, the most accurate method is to incinerate a sufficiently large piece of the stuff. and examine the ash. BLUE COLORS.—These may consist of indigo, logwood, prussian blue or ultramarine. Indigo blue is fixed on cloth in various ways,—1st, in the blue vat; 2nd, as so-called China or English blue, blue patterns upon a white ground, fixed, according to the principle of the blue vat, with lime and sulphate of iron ; 3rd, as pencil-blue, the indigo being deoxidized by means of oxide of tin and potash; and 4th, as soluble indigo. The first three blues are not acted upon by dilute acids or alkalies. By chlorine and nitric acid, on the contrary, they are destroyed. When the stuffs decolorized by chlorine are wesiie-A cid -lipped in a solution of logwood, the first two remain colorless because they contain no mordanb, while the stuffdyed with pencil-blue becomes red on account of the tin which it contains. j The blua of soluble indigo, and that obtained with cyanide of potassium, agree in being destroyed by alkalies: at the s?.me time, however, the blue of indigo leaves a white ground, while that of thrf cyanide leaves a rusty yellow ground on account of the iron mordant employed. In order to remove all doubt, a few drops of acidulated solution of cyanide of potassium should be added, which, if iron is present, reproduces the blue color. This confirmatory test should always be used in the case of green colors. Prussian blue may be recognized by its being decolorized by alkalies, but not by chlo-riJe of limo, while the latter re-agent destroys indigo-blue. The appearance alone if sufficient to indicate whether the blue is ordinary prussian blue or ' bleu de France," prepared with stannate of potash. Logwood blue may easily be recognized from the fact that it is destroyed by weak acids, and becomes red ; in most cases this is a sufficient ground tor interring the presence of logwood, &c. When the color to be examined is a mixed one, for example, logwood-blue, with prussian blue or indigo, the color of the logwood is first destroyed by dilute acid, the stuff washed and treated with chlorine to ascertain whether the ground color is indigo or prussian blue. Ultramarine may usually be recognized by its peculiar tint; alter incinerating the stuff it remains unaltered in the ash. Hydrochloric acid decomposes it, disengaging at the same time an unpleasant odor of sulphuretted hydrogen. When the ultramarine is imprinted with varnish, the stuff must be rroistened with ether before the hydrochloric acid will act. RED COLOKS.—With the exception of sat-ilower, the red coloring matters require a preparation of alumina or tin. Safflower may be easily recognized by its color being discharged by caustic potash or soda. Madder colors when treated with hy- drochloric acid, acquire a yellow or orange tint without any shade of puce; upon then being treated with milk of lime, the color becomes violet at those places where the hydrochloric acid has acted. The violet is permanent, and by boiling with soap, passes into rose color. The madder-red colors are less susceptible of alteration by acids the more they have been brightened by soap and the higher the temperature at which this took place. The great durability of the Turkish red is owing to this fact. The red and rose colors from madder are separable into several kinds—Turkish-red and rose, ordinary madder-ied and rose, the true topical red, and the colors from garancine and garanceaux. Turkish red may be known by its brightness and indestructibility by acids. Ordinary madder red, when brightened, scarcely differs in any particular, from a true topical color. The only difference is in the mode of preparation. As the topical color is prepared before printing with tin, and alter printing the stuff is steamed, the white is somewhat yellowish, and becomes colored in a decoction of logwood. The red and rose from garancine and garanceaux differ from the above colors in not bearing brightening with soap, acids, and alkalies. When treated with hydrochloric acid, they pass into orange, and do not then give a violet, but a dull blue color, with milk of lime. The tone of color is sufficient to distinguish between colors produced from garancine or garanceaux, the latter possessing an orange shade. When the red is accompanied by violet, the distinction is still more easy, because garancine yields a violet, which is nearly as beautiful as that from madder, while the violet of garanceaux is more reddish-gray. The red colors trom Brazil-wood and cochineal, when treated with hydrochloric acid and tin salt, become gooseberry red ; and then milk of lime produces a violet of little permanence, which disappears entirely on subsequent boiling with soap, while the madder colors acquire their greatest brilliancy by this treatment. The red from cochineal differs from that of Brazil-wood in tone and in its behavior with concentrated sulphuric acid; the former becomes bright-cherry-red, the latter orange. YELLOU- COLORS.—The yellow of quercitron is discharged by chlorine and sulphurous acid, and it is not sensibly changed to orange by either caustic potash or tin salt. The yellow of back thorn-berries is likewise destroyed by chlorine; caustic potash renders it bright yellow. Heated with tin salt, it passes into orange ; with sulphuric acid it acquires a stone color. The orange and nankeen colors from fustic and fustet are changed to red by sulphurio acid; treated with potash they acquire a color resembling that of Catechu; they are discharged by nitric acid. The yellow from sumach acquires greater brightness with tin salt; with nitric acid, it becomes red; sulphuric acid does not produce much alteration; sulphate of iron changes it to gray. The yellow from arnotto is but little affected by chlorine; concentrated sulphuric acid changes it to bluish-green ; with nitric acid it assumes a darker color, and then disappears entirely. Chrome-yellow is unaltered by heating with weak hydrochloric acid, but destroyed by the concentrated acid. It is destroyed by caustic alkalies; boiling potash converts it into orange. Chrome-orange becomes greenish yellow when treated with weak acids. BLACK COLORS.—Log-wood black contains iron as a mordant, sometimes iron and alumina. In the latter case it has a shade of blue. Such a color is discharged by chlorine, a yellow resulting from the iron ground remaining. Treated with hydrochloric acid and tin salt, it becomes red, with the former more cherry-red, with the latter violet-red. The blacks from astringent substances are easily recognizable by the shade of olive which they present. When treated with hydrochloric acid, they acquire a dull orange color; tin salt dissolves the iron, and changes the color to a dirty olive. Chrome black may be known by its behavior with chloride of lime, which destroys the other kinds of black while it changes chrome black to a chestnut-brown. The examination of mixed colors is somewhat more complicated ; but as they are for the most part constituted of the substances already mentioned, it will not be difficult, by means of the above re-actions, to ascertain in what manner and with what materials such colors have been produced.
This article was originally published with the title "Means of Detecting the kind of Dyes used in the Coloring of Textile Fabrics" in Scientific American 8, 44, 350 (July 1853)