This section is from "Scientific American Supplement". Also available from Amazon: Scientific American Reference Book.
By JULES JOFFRE.
The reagents employed are a solution of caustic potassa in ten parts of water; hydrochloric acid diluted with an equal bulk of water, or occasionally concentrated; nitric acid, ammonia, ferric sulphate, and a concentrated solution of tin crystals. The most convenient method of operating is to steep small portions of the cloth under examination in a little of the reagent placed at the bottom of a porcelain capsule. The bits are then laid on the edge of the capsule, when the changes of color which they have undergone may be conveniently observed. It is useful to submit to the same reagents simultaneously portions of cloth dyed in a known manner with the wares which are suspected of having been used in dyeing the goods under examination.
By the action of caustic potassa, the reds are divided into four groups: 1, those which turn to a violet or blue; 2, those which turn brown; 3, those which are changed to a light yellow or gray; 4, those which undergo little or no change.
The first group comprises madder, cochineal, orchil, alkanet, and murexide. Madder reds are turned to an orange by hydrochloric acid, while the three next are not notably affected. Cochineal is turned by the potassa to a violet-red, orchil to a violet-blue, and alkanet to a decided blue. Lac-dye presents the same reactions as cochineal, but has less brightness. Ammoniacal cochineal and carmine may likewise be distinguished by the tone of the reds obtained.
A characteristic of madder reds is that, after having been turned yellow by hydrochloric acid, they are rendered violet on treatment with milk of lime. A boiling soap-lye restores the original red, though somewhat paler. Artificial alizarine gives the same reaction. Turkey-reds, however, are quite unaffected by acid. Garancine and garanceux reds, if treated first with hydrochloric acid and then with milk of lime, turn to a dull blue.
Madder dyes are sometimes slow in being turned to a violet by potassa, and this shade when produced is often brownish. They might thus be confounded with the dyes of the fourth group, i.e., rosolic acid, coralline, eosine, and coccine. None of these colors gives the characteristic reaction with milk of lime and boiling soap-lye. If plunged in milk of lime, they resume their rose or orange shades, while the madder colors become violet. Murexide is turned, by potassa, gray in its light shades and violet in its dark ones. It might, then, be confounded with orchil, but it is decolorized by hydrochloric acid, which leaves orchil a red. Moreover, it is turned greenish by stannous chloride.
A special character of this dye (murexide) is the presence of mercury, the salts of which serve as mordants for fixing it, and may be detected by the ordinary reagents.
The second group comprises merely sandal wood or sanders red, which turns to a brown. On boiling it with copperas it becomes violet, while on boiling with potassium dichromate it changes to a yellowish brown.
The third group includes safflower, magenta, and murexide (light shades). If the action of the potassa is prolonged the (soft) red woods enter into this group. Safflower turns yellow by the action of potassa, and the original rose shade is not restored by washing with water. Hydrochloric acid turns it immediately yellow. Citric acid has no action. Magenta is completely decolorized by potassa, but a prolonged washing in water reproduces the original shade. This reaction is common to many aniline colors. These decolorations and recolorations are easily produced in dark shades, while in very light shades they are less easily observed, because there is always a certain loss of color. Stannous chloride turns magenta reds to a violet. Hydrochloric acid renders them yellowish brown (afterward greenish?). Water restores the purple red shade.
The fourth group comprises saffranine, azo-dinaphthyldiamine, rosolic acid, coralline, pure eosine and cosine modified by a salt of lead, coccina, artificial ponceau, and red-wood.
Saffranine is detected by the action of hydrochloric acid, which turns it to a beautiful blue; the red color is restored by washing in water. Azo-dinaphthyl diamine is recognized by its peculiar orange cast, and is turned by hydrochloric acid to a dull, dirty violet. Rosolic acid and coralline, as well as eosine, are turned by hydrochloric acid to an orange-yellow: the two former are distinguished from eosine by their shade, which inclines to a yellow. Potassa turns rosolic acid and coralline from an orange-red to a bright red, while it produces no change in eosine. If the action of potassa is prolonged, modified eosine is blackened in consequence of the decomposition of the wool, the sulphur of which forms lead sulphide. Coccine becomes of a light lemon-yellow on treatment with hydrochloric acid. Washing with water restores the original shade. It affords the same reactions as eosine, but its tone is more inclined to an orange.
Artificial ponceau does not undergo any change on treatment with hydrochloric acid, and resists potash. Red wood shades are turned toward a gooseberry-red by hydrochloric acid, especially if strong. This last reaction not being very distinct, red-wood shades might be mistaken for those of artificial ponceau but for the superior brightness of the latter. If the action of potassa is prolonged, the red-wood shades are decolorized, and a washing with water then bleaches the tissue. Rocelline affords the same reactions as artificial ponceau, but if steeped in a concentrated solution of stannous chloride it is in time completely discharged, which is not the case with artificial ponceau.
Violets are divided into two groups: those affected by potassa, and those upon which it has no action. The first group embraces logwood, orchil, alkanet, and aniline violets, including under the latter term Perkin's violet, (probably the original "mauve"), dahlia, Parme or magenta violet, methyl, and Hofmann's violets. The action of potassa gives indications for each of these violets. Logwood violet is browned; that of orchil, if slightly reddish, is turned to a blue-violet; that of alkanet is modified to a fine blue. Lastly, Perkin's mauve, dahlia, and methyl violet become of a grayish brown, which may be re-converted into a fine violet by washing in abundance of water. When the shades are very heavy, this grayish brown is almost of a violet-brown, so that the violets might seem to be unaltered.
The action of hydrochloric acid distinguishes these colors better still if the aid of ammonia is called in for two cases.
The acid turns logwood violet to a fine red, and equally reddens orchil violet. But the two colors cannot be confounded, first, because the two violet shades are very distinct, that of orchil being much the brighter; and secondly, because ammonia has no action on logwood violet, while it turns orchil violet, if at all reddish, to a blue shade. Hydrochloric acid, whether dilute or concentrated, is without action on alkanet violet. If the acid is dilute, it is equally without action on Perkin's violet and dahlia. If it is strong, it turns them blue, and even green if in excess. Hofmann's violet turns green even with dilute acid, but prolonged washing restores the original violet shade. Dahlia gives a more blue shade than Perkin's mauve. The action of acid is equally characteristic for methyl violet. It becomes green, then yellow. Washing in water re-converts it first to a green, and then to a violet.
The second group includes madder violet, cochineal violet, and the compound violet of cochineal and extract of indigo. These three dyes are thus distinguished: Hydrochloric acid turns the madder violet-orange, slightly brownish, or a light brown, and it affords the characteristic reaction of the madder colors described above under reds. Cochineal violets are reddened. Sometimes they are decolorized, and become finally yellow, but do not pass through a brown stage.
The compound violet of cochineal and extract of indigo presents this characteristic reaction, that if boiled with very weak solution of sodium carbonate the liquid becomes blue, rather greenish, while the cloth becomes a vinous-red--Moniteur Scientifique.--Chem. News.