MINERAL DYES
Tribes and nations in different parts of the world seem, at a comparatively early date, to have found out the art of coloring and staining textiles with mineral compounds. Iron springs, containing iron salts in solution, are found in many countries; and such springs are always noteworthy from the taste of the waters, and the color of the sediments left when the water stands exposed to the air.
Therefore discovery of the fact that those waters would impart a permanent and quite pleasing orange or reddish-brown color to textiles was perfectly natural.
Iron Buff.—Accordingly, in different parts of the world, people learned to dip cloths in these springs and then expose them to the air, thus dyeing them this iron rust color, commonly called by dyers “iron buff.” When iron became a common metal, it was found that any soluble salt of iron would act as a dyeing solution, just as well as a natural iron spring; and hence we find use made, in widely separated countries, of iron salts for dyeing.
This iron buff is used to this day, though of course it has lost the importance it had in the past. The red sails of the fishermen in the Mediterranean show this color; and it is a useful and interesting dye for weavers of hand-made rugs, curtains, and the like, because of its pleasing tone and great permanence. On the other hand, it is very likely to rub; and it fills the fibre of the cloth with mineral matter, thereby making the material stiff and hard to sew or cut.
Preparation.—Our colonial ancestors made this color cheaply enough. They carefully saved all the scraps of iron and steel that they could find—old horseshoes, broken knife blades, etc., etc.—and placed them in a barrel half filled with vinegar and water. Little by little the iron dissolved in the acid and, when it was strong enough, the housewife would soak her homespun cloth, or other material, in the solution, warming and stirring it, and making it absorb as much of the liquor as possible. Then she would take it out, wring it thoroughly, rinse it slightly, and dip it for a minute or two in another barrel half filled with a water extract of wood ashes.
PLATE II. JAPANESE TOWELLING, SHOWING IMPRESSION OF FRESH DAMP LEAVES
After removing from the solution and wringing again, the goods were shaken out and exposed to the air for some minutes, during which time the color would develop—in other words, would make its final change to yellow or orange, or even to brownish-red, according to the amount of iron absorbed by the fibre.
The process, nowadays, is much the same, excepting that, for the first or iron bath, it is cheaper and easier to use a solution of the green crystalline iron salt, known as copperas, or asferrous (iron)sulphate. This can be obtained at, or through, any drug store at a very low price, as it is not necessary to buy a chemically pure product. The ordinary commercial salt is as pure as the work requires; this dissolves quite readily in warm water.
The amount of copperas to be used, to dye a particular lot of material a particular shade, can only be determined by experience and experiment. It is always easy to build up a color, i.e., to deepen its shade if it is too light, by dipping the fabric over again in the same dye-bath. Indeed there is a general rule to be observed in dyeing all colors like this iron buff or the manganese brown—as well as the sulphur and indigo colors, which will be described later—that are developed, or fixed, by exposure to the air. Whenever dark shades of these colors are desired, they should be produced by successive dippings in weak baths, rather than by one or two dippings in strong baths. This avoids rubbing, as far as possible, and lessens the injury to the cloth fibre. In general, it is best to start with a dye-bath containing some three or four tablespoonfuls of copperas to one gallon of hot water.
For the second, or fixing, bath—that is, the alkali bath—it is now customary to use a solution of soda instead of the extract made from wood ashes. Either cooking soda (bicarbonate of soda) or the stronger washing soda or soda crystals, known to the chemist as carbonate of soda, will be satisfactory, and instead of soda the corresponding potash salts may be used, though these are usually more expensive. It is possible, too, to use a bath of the so-called caustic soda, or caustic potash, known to the chemist as hydroxide of soda and hydroxide of potash. But these, as the name implies, must be handled with care because, when strong, they are likely to burn the hands and clothes. Careful analyses of dyed mummy cloths show that the ancient Egyptians were accustomed to use for their second or fixing bath, a solution of slaked lime, or lime water.
Khaki.—By mixing in the first bath of copperas or other iron salt an equal quantity of chrome alum, and then fixing and developing as above, a certain amount of greenish chromium oxide is deposited in the fibre along with the oxide of iron. This gives rise to the shade known as “khaki.” Sometimes shaded a little with manganese brown, this was the regular dye for the army uniforms, until the recent introduction of the extremely fast and very satisfactory vat dyes.
Uses.—Iron buff is chiefly used for cotton, linen, and other vegetable fabrics; on them it gives pleasant, warm shades of orange and reddish-brown. But on wool, and especially on silk, it is not so satisfactory, owing to its tendency to roughen and injure the fibre. Indeed, in the case of silk, it is likely to greatly diminish, or even to destroy, the lustre. On cotton and linen, however, it has great fastness to light and to washing. Indeed, every one who has tried to get rust stains out of a garment or a piece of table linen knows how hard a matter it is to get rid of the color.
Another important reason for using this dye is that the coloring agents are very cheap, and are easily obtained in any quantities. It has, however, some serious disadvantages, one of which is that the color, especially in dark shades, is very liable to rub. This can best be obviated by building up the shades with successive dippings; and by thoroughly washing the finished goods in a hot soap bath. The dyed goods are pretty certain to be a little stiff, and therefore hard to sew or cut, owing to the fact that the final color is composed of iron rust. When vegetable fibres are filled with a mineral matter they are naturally stiffer and harder than they were originally.
Then there is the final objection on the part of professional dyers to this color, as well as to all the other developed colors, i.e., those colors fixed by exposure to the air. It is not easy to get a smooth, even color with them, and it is very difficult to dye to shade. For handicraft work, where these two points are of minor importance as compared with the beauty of the color, this objection is not so serious, but where it is necessary to dye large amounts of yarn or cloth to a definite shade with this, or similar, dyes, it is, as a rule, far easier to use a dyestuff which does not materially change its shade after the goods leave the dye-bath.
Iron Grey.—Soon after the discovery, in different localities, of the iron buff color, it was discovered that by the action of various vegetable extracts upon the iron salts, dark grey stains could be produced which, under certain conditions, would be fairly fast to light and washing.
This color was, later, found to be due to the combination with iron of the peculiar vegetable acid called tannic acid or tannin. This is found in small quantities in the juices of twigs and leaves of many varieties of plants, and, until the introduction of the modern dyestuffs, this process offered the chief method of obtaining grey or black shades upon cotton. At present it is rarely, if ever, used for that purpose, but the compound is still the basis of most of the writing inks on the market.
To make this color, the cloth is soaked for some time in a solution of an iron salt—nitrate of iron, formed by boiling a solution of copperas for a minute or two with a few drops of nitric acid, is preferable to the untreated copperas—and then, after being wrung and slightly rinsed, it is plunged into a bath containing tannic acid. This can be made by dissolving a few tablespoonfuls of the dry tannic acid in some water, or by making a hot infusion of the leaves, twigs, or bark of any plant or tree containing it. Tea leaves contain much tannin, and so do unripe English walnuts and butternuts. Acorns, oak leaves with nut galls on them, the green twigs of alders, and hazelnut bushes, have all been used to form this color.
PLATE III. SAME TOWELLING AS IN PLATE II, AFTER IMMERSION IN IRON SPRING. THE ROUND WHITE PATTERNS ARE MADE BY TIEING
The grey color quickly develops and, after rinsing, the material can be dried and pressed, or dipped again to obtain a deeper shade, first into the iron and then into the tannin bath. The color is a pleasant, soft shade of grey or, if dyed deeply, a black. It is fast to washing, and fairly so to light, though it may become rusty on standing; like the iron buff, it is not fast to acids.
Some interesting examples of the dyeing of cotton cloth with iron buff and iron grey are shown in Plate [I]. They came from the mineral springs at Arima, near Kobe in Japan, where the waters are so saturated with iron salts, that comparatively short immersion, and exposure to air, will bring out a deep orange shade. The Japanese, not content with dyeing their goods plain colors, have for many generations utilized these springs in the production of figures and designs on the cloth. Plate [V] is an example of stencil work, where the white patterns are made by covering parts of the cloth with a “resist paste” which protects whatever it is in contact with from the action of the coloring agent.
Plate [II] shows a piece of soft calico on which impressions of leaves have been made by placing fresh juicy leaves between two pieces of cloth, and beating them with wooden mallets.
Plate [III] shows the same piece of cloth as in Plate [II], after immersion in the iron spring, and exposure to air. The tannin from the leaf juice converts some of the iron oxide into iron grey; while the white figures are made by tying the cloth with string or tape (Tied and Dyed work) before dyeing it.
So far as we can tell, these two were the only mineral colors known to the ancients. Several other mineral colors, however, were in common use by the cotton dyers in the days preceding the introduction of modern dyestuffs, but it is hardly worth while to dwell here on many of them. Yellow and orange shades were obtained by impregnating cloth with lead salts, and then developing with a bath of chromate or bichromate of potash, with more or less caustic alkali added for the darker shades.
Prussian blue, too, was used as a substitute for the more expensive indigo. This was formed by using the nitrate of iron for the first bath, and then developing the color with a bath of yellow prussiate (ferrocyanide) of potash. These colors, however, are so far inferior in their application, and in fastness to light and to rubbing, to the colors now at our command, that they have disappeared entirely for textile work, though they are still widely used for pigments.
Manganese Bronze (Manganese Brown, Bistre).—There is one good mineral color, however, which came into use early in the last century and which, while hardly ever used by professional dyers, is of interest to craftsmen. This color, in its chemical composition, greatly resembles the iron buff. It is quite cheaply produced by first impregnating the cloth to be dyed with a solution of a manganese salt (manganese chloride is the cheapest), and then, by means of a second bath of alkali, forming a deposit on the fibre of pink manganese hydroxide—corresponding to the greenish ferrous hydroxide—which, on exposure to the air, absorbs oxygen and forms the final brown color.
Unfortunately the alkali used in this case must be caustic alkali—potassium hydroxide orsodium hydroxide—and not one of the mild alkalies like the carbonates or bicarbonates, which will do for the iron color. And, therefore, although it is rather more expensive, and is somewhat liable to weaken the fabric, it is generally more convenient to obtain this color by a one-bath process. A purple solution of the salt known aspermanganate of potash, is prepared and the cloth dipped. After being immersed it is wrung carefully and shaken out, and the red or purplish color gradually changes into the final brown. As soon as this change has taken place the goods should be plunged into a hot soap bath and thoroughly scoured, both to remove any loosely adhering particles of color which cause rubbing, and to prevent tendering of the cloth.
The latter danger, however, is always present with this process and, therefore, full shades should not be dyed excepting on heavy, strong goods like rugs or very coarse yarns or cloth. Even then it should be done carefully and by successive dippings, with a careful washing, after the color has been developed in the air, between each bath.
This injury to the cloth which, hitherto, has been the great drawback to the permanganate process, can be avoided by dipping the goods, as soon as possible after leaving the dye-baths, into a solution containing glucose, as, for instance, two or three spoonfuls of Karo (corn syrup) or molasses in each gallon of hot water. Directly the purple-stained cloth touches this solution the color changes to brown, without affecting the strength of the materials.
This color, like the other mineral colors, is rarely, if ever, to be used on silk, being altogether too likely to injure the texture and the lustre of the material.
In at least one instance, however, it has been used on animal fibres with considerable success. During the critical part of the Boer war, it was at one time necessary for England to put as many of her troops as possible—especially her mounted troops—into the field. Among others the Scots Greys, distinguished at Waterloo and made famous in many other bloody campaigns as a fine old fighting regiment, were ordered to the front. There is a tradition, dating back over two hundred years, that the horses of this regiment must all be either white or grey in color. Some heaven-sent genius at the Horse Guards—the English War Department—hinted quite forcibly to the authorities that to send out a cavalry regiment on white horses to face the Boer sharpshooters, was rather a dangerous experiment. The authorities, therefore, consulted a well-known dyeing chemist. He advised them to send down, on the troopship, some kegs of permanganate; and to instruct the officers and men to sponge each horse with a weak solution of the salt, every day at “Stables.” This was done, and, in consequence, long before reaching Cape Town, the skin and hair of every horse was thoroughly colored a soft, quiet shade of brown.
The color produced by permanganate varies, according to the strength of the solution, or rather with the number of dips in comparatively weak solutions, from a light brownish tan to a full, rich, soft, seal brown. Pleasant shades, too, can be obtained by dyeing first with the iron rust dye and then covering with the permanganate. This color is discharged, not only from textiles but from the hands, by soaking in a solution ofsodium hydrosulphite (commonly used in dyeing indigo) and then washing.