Dyeing

Dyeing is the art of imparting colours to textile and other materials, such as cotton, silk, wool, and leather. It has been practised among eastern nations from time immemorial; and in the Old Testament, we read of the purple-dyed vestments of the high-priests, of linen cloths dyed blue, purple, and scarlet, and of rams' skins dyed red. The famous Tyrian purple, obtained from one or two species of shell-fish, is believed to have been discovered by an inhabitant of Tyre fifteen hundred years B.C.; afterwards this purple became the badge of royalty, and cloth dyed with it commanded a princely price. Purple of various shades was dyed not only at Tyre, but at Tarsus, Alexandria, and other places on the shores of the Mediterranean, though other colours were of course employed. The Egyptians, Greeks, and Romans practised the art of dyeing. There is an ancient Roman dyer's workshop with its apparatus to be seen among the ruins of Pompeii. Gradually the art became more and more widespread as civilisation advanced. In earlier times, dyeing was much more extensively followed as a domestic art than it is at present, but in some outlying parts of Europe and even in the Highlands of Scotland, the colours imparted to home-made fabrics are still to some extent obtained from native vegetable dyes. Many of the ordinary dyestuffs and dyeing agents have been used in England for more than four centuries, and to these America added cochineal, as well as some important dyewoods and barks. Dyeing with colours obtained from natural products had reached a high state of perfection when Perkin, in 1856, introduced the first of the coal-tar colours. Since that date the progress of artificial colour making has been so rapid, and the application of the new dyes made so simple, that it now seems doubtful whether many of the older dyestuffs and processes can continue much longer in use. The experience acquired in dyeing with any colouring material must not be undervalued on account of its partial disuse; and some illustrations, though of less practical importance than they were lately, may here be given, in describing the general principles of dyeing.

Assuming that the textile material has been subjected to the cleansing and whitening operations described under BLEACHING; that the water is soft and clear, and the vessels free from rust and perfectly clean, the next point to consider is the nature of the fibre. Very often if this is of animal origin, such as silk or wool, a simple immersion in a bath containing the colour will dye the fabric; but colour so applied to a vegetable substance as cotton, linen, or jute, would be easily washed away. The fibre in the latter case requires some special preparation to make the dye adhere, and a mordant is employed for this purpose (see CALICO-PRINTING). Mordants are usually mineral salts applied to the yarn or cloth, so as to leave their bases in intimate contact with the fibre. A class of mineral colours may be first described, as their production depends on simple reactions similar to those occurring in the use of mordants. One of these colours, chrome yellow, has been already noticed under CALICO-PRINTING. They are the result of an interchange of the bases and acids of two soluble salts in the material of the fibre, one of the new salts being soluble and readily washed out, the other insoluble and the substance having the colour.

Prussian blue (q.v.) may be taken as an instance of this mode of dyeing. A bath to dye 7 lb. of silk is made up as follows: 10 gallons water; 24 oz. nitrate of iron solution, specific gravity 1.6; 4 oz. stannous chloride. And another bath with 10 gallons water; 6 oz. yellow prussiate of potash; 3 oz. sulphuric acid. The first bath has a temperature of 130° F. The silk is turned through it till it is thoroughly penetrated with the liquor, then washed, and transferred to the second bath, which is also warm. The silk, without washing, is returned to the first bath, after which it is again washed and placed in the second bath. The first bath is now strengthened with 6 oz. of nitrate of iron and 2 oz. of stannous chloride, and the second bath with 3 oz. of sulphuric acid and 2 oz. of yellow prussiate of potash. Once more the silk is returned to the first bath, washed, and transferred to the second bath. After this steep, it is wrung out and left for six hours, when it is washed, raised, and dried in the air.

There are several things to be learned from this process. (1) The solutions must be dilute; (2) several operations are required to get an equal shade; (3) an acid solution is necessary to prevent iron oxide from depositing on the cloth, and this is made more certain by strengthening it in the last dipping; and (4) time is given for any action of the air before the final washing is given to the dyed silk. These or similar matters require attention in dyeing generally.

Buff is produced on cotton by a bath of nitrate of iron, followed by one of dilute and clear lime-water, washing and drying. In this case peroxide of iron is left in the fibre, and forms the colouring material; the nitrate of lime being readily soluble in water, washes out. This is an instance, however, in which the cloth has been mordanted as well as dyed, and if we wish to give it, say, a black or dark purple colour, it only requires to be immersed in a bath of logwood.

Supposing that in place of nitrate of iron we had used a solution of alum or other soluble salt of alumina, as the sulphate (alum cake) or acetate (red liquor), we should have had the hydrate of alumina deposited in the fibre. As, however, this is simply white, the appearance of the cloth would not have altered; but if placed in a logwood bath, a pink or red colour would be the result, the tint and depth varying with the strength of the solutions. In such a case the cloth is dyed with an aluminous mordant.

The mordants most largely used are the salts of iron, alumina, and tin. Acetates and sulphates of both oxides of iron, as well as of alumina, and the two chlorides of tin and stannate of soda, form the greater portion of the materials employed. The method of using the acetates of iron and alumina as mordants has been given in CALICO-PRINTING. Suppose that cloth is impregnated with acetate of iron, which is a combination of the peroxide of iron with acetic acid. This oxide is a feeble base, and the acid is volatile. Consequently, when the cloth is placed in a hot chamber filled with moist air, the acetic acid is expelled, and the oxide of iron is left in the fibre, which is what the dyer requires. The application of the acetate of alumina as a mordant is explained in the same way.

In the case of salts with non-volatile acids (nitrate of iron or sulphate of alumina, for example), lime-water, caustic soda, carbonate of soda, or similar bodies are used to precipitate the mordants in the material to be dyed, as in the instances given above. But in some kinds of dyeing the assistance of an alkali is not necessary to decompose the salt, as merely boiling it with the fibre is sufficient to separate the base and liberate the acid, the cloth retaining the former, and the latter adding to the acidity of the bath. If fresh alumina were added to the bath to combine with the liberated acid, the process could go on indefinitely, but when a certain amount of free acid has accumulated, the fibre ceases to effect the decomposition of the salt. In practice this is not the method followed, but a potash salt of an organic acid is added to the bath, the potash of which neutralises the strong mineral acid; the weaker organic acid being set free, which has not the same power of preventing the absorption of alumina by the fibre. Tartar or argol (impure acid tartrate of potash) is much used along with alum for wool-mordanting, and also in silk-dyeing with tin mordants.

Several salts of tin are much used as mordants—the most important being stannous chloride or muriate of tin—also called ‘tin salts’ and ‘tin crystals.’ Stannic chloride or perchloride of tin is likewise used, and a solution of the metal in hydrochloric and nitric acids, called ‘tin spirits’ and ‘oxymuriate of tin,’ is in general use. The latter is a mixture of stannous and stannic salts, and requires very great care in its preparation. The salts of tin are decomposed readily by the fibre, and the tin spirits require to be used when freshly made, as deposits soon appear in the solutions. Stannate of soda is also much employed in dyeing. When the fibre is charged with it, the insoluble stannic acid is liberated with dilute sulphuric acid.

The mordants mentioned above are employed chiefly in cotton-dyeing with the vegetable dyes or the similar artificial alizarin colours. Wool and silk are not usually mordanted in the manner described, and the following observations apply to cotton and other vegetable fibre. The dye-baths or ‘dye-becks’ have been noticed in CALICO-PRINTING. Often, in dyeing, copper boilers are used with an ordinary fireplace for heating them. The dyestuffs are used either in powder or raspings among the water, or their extracts are employed. The dye solutions are generally warm or boiling, and the goods immersed in them require to be kept in constant motion, or nearly so, to insure uniformity of absorption.

Dyeing of Cotton.—The following is a brief outline of the processes in use for a few important colours:

Black is produced by steeping the goods in a decoction of sumac; then passing them through a solution of acetate of iron. After washing, they are next passed through a decoction of logwood.

Brown is usually obtained by passing the cloth through a decoction of cutch or Catechu (q.v.), and afterwards through a solution of bichromate of potash. Logwood, fustic, or any of the red colouring stuffs, can be afterwards added according to the shade of brown wanted.

Purples and lilacs are got from logwood and alizarin with mixtures of iron and alumina. With madder colours, reds and pinks are got by the use of alumina and tin.

Reds are also got from various dyewoods, as sappan-wood, peachwood, barwood, &c., with tin or alumina, the cloth being first soaked in an astringent, as sumac or gall-nuts. The coal-tar colours, safranine, &c., have almost entirely superseded these woods in the dyeing of reds.

Yellow is got from fustic, quercitrin bark, Persian berries, &c., with tin or alumina mordants. Better shades of yellow are now, however, produced by auramine and other coal-tar dyes. Blue colours are not obtained from the natural dyeing materials with the usual mordants, and green is produced by dyeing a yellow such as fustic over cloth already rendered blue.

The production of Prussian blue on silk has been fully given, and the same method is applicable to cotton. For the dyeing of indigo blue, see CALICO-PRINTING.

Turkey-red is a very bright and permanent colour on cotton, obtained till recently from madder, but now almost entirely from alizarin, by a special process in the treatment. An oil mordant, as it is termed, is employed in combination with the fibre. Formerly a coarse olive-oil was made into an emulsion with a weak solution of crude pearl-ashes, through which the cloth was passed, then wrung out and hung up in a stove. The oil absorbs oxygen, and thickens into a varnish containing free fatty acids. The operation was repeated six or eight times. A Turkey-red oil is now sold which is chiefly the oleic acid of castor-oil in combination with ammonia or soda, and of this oil the cloth requires only one or two applications. The cloth is also mordanted with alumina as usual, and then passed into the dye-bath, which is gradually raised to boiling. The bath may be charged with ground madder, natural alizarin, or the artificial product, and generally sumac or some substance containing tannin is added. The goods are brightened by boiling in soap solution, and finally in a bath of tin spirits.

In dyeing cotton with coal-tar colours (with the exception of the azo group), the goods are first mordanted by passing them through a solution of tannic acid, and then through one of tartar emetic; or they are first passed through a decoction of sumac, and afterwards through a solution of stannate of soda. The goods are usually put into the dye-beck in a cold state, and gradually raised to a heat of about 120° F. as the dyeing proceeds. With the azo group of colours (see below) no previous mordanting is required, but a little sulphate of soda or common salt is added to the dye-beck to make the colour go on more evenly. These azo colours are dyed at a boiling heat. The reds of this group are not very satisfactory on cotton.

Aniline black is produced on cotton yarn directly by the oxidation of aniline with bichromate of potash and hydrochloric acid.

Dyeing of Wool.—This fibre absorbs both colours and mordants so much more readily than cotton, that for the most part the methods of dyeing it differ from those that have been described. Sulphuric acid has little action on wool as compared with cotton, consequently many operations in wool-dyeing are conducted in acid solutions at high temperatures, where cotton would be destroyed. An instance of producing a Prussian blue on worsteds may be given: 'Worsted, 100 lb. Make cold solutions of 9 lb. red prussiate of potash, 2½ lb. tartaric acid, 2½ lb. oxalic acid, and 2 lb. tin composition. Pour these together and add the mixture so produced to about 300 gallons of water at 100° F., and further add 12 lb. good oil of vitriol. Enter the goods, turn well, heat up slowly to the boiling-point, and boil for half an hour.'

Here it will be seen no salt with an iron base is present to form a Prussian blue, and consequently this constituent must be got from the decomposition of the red prussiate by the oil of vitriol, the wool absorbing the colour as it is formed. A blue produced in the way already given for silk would be comparatively cheap, but none of the mineral colours obtained by the mutual interchange of acids and bases in the fibre are successful with wool. Wool is dyed to a much larger extent than either cotton or silk with dark colours, and for these logwood and the astringent dyes catechu, nut-galls, sumac, &c., are chiefly used along with indigo, alizarin, and fustic for special shades. The wool is first, as a rule, boiled with bichromate of potash, sulphate of copper, and oil of vitriol, then with the dyes—for instance: 'Black. For 55 lb. wool. Boil with 17 oz. each chromate, bluestone, and oil of vitriol for 1½ hour. Dye in 22 lb. logwood and 4 lb. fustic, boiling 1 hour.' Sometimes the chromate is mixed with tartar for the first treatment of the wool, with or without sulphuric acid. Woaded blacks are the best for woollen cloths. By this method the wool or cloth is first dyed in the indigo vat a light or medium shade. It is then dyed a chrome black by a process the same or similar to that just given in which bichromate of potash and logwood are the chief ingredients used.

Generally when dyes on wool are required to stand milling (see WOOLLEN MANUFACTURE), the goods are first mordanted by boiling them for an hour in a solution of bichromate of potash and tartar. Brown, olive, drab, and similar colours are dyed with madder or alizarin, camwood, fustic, and logwood, in proportions varying with the shade required.

Until quite recently, the most important of the materials for dyeing wool, next to indigo, was cochineal. It gives with tin and alumina mordants very brilliant pinks, crimsons, and scarlets. To produce scarlet the cloth is boiled in tin spirits—generally with addition of cream of tartar—until mordanted with stannic oxide, then washed and boiled in the ground cochineal till the solution gets colourless, that is, till all the dye is absorbed. A second boiling is given with cochineal mixed with a little more mordant and tartar. Benzidine reds are now replacing cochineal.

With coal-tar dyes almost every conceivable colour can be obtained on wool. In most cases it is only necessary to add a little sulphate of soda and sulphuric acid to the dye-beck, no previous mordanting being necessary. Tin spirits and tartar brighten a number of the colours.

In dyeing wool, alizarin blue solution is said to give beautiful deep blue shades, as fast as indigo to light and milling, and is regarded as a great success as a substitute for the natural dyeware; artificial indigo is as yet too expensive a competitor.

Silk, when dyed dark colours, may be used without the same bleaching operations for the removal of the gum, required for brighter tints. The dyes and mordants for these are much the same as for wool, but the baths are usually hot soap solutions containing the dyes. Cochineal gives a poppy red with a tin or alumina mordant, and annotto in alkaline solution an orange yellow with the latter. Archil and safflower give violets and pinks without a mordant, but they are fugitive.

Silk is dyed with the coal-tar colours by a simple immersion in a solution in water or, if necessary, in spirit. Usually the soap solution of the silk gum is taken, and the colour brightened by rinsing in acetic acid. For the oxyazo dyes sulphuric acid is added to the bath.

DYESTUFFS.—The principles of the art of dyeing being already stated under the head of DYEING, it is only necessary to notice here, in the first place, the more important of the older dyestuffs of commerce, and then to give some account of the remarkable group of artificial dyes known as the 'coal-tar colours.' The first of these was introduced in 1856, and the number of them now in use is considerable. Still many of the older colouring materials to a large extent hold their ground, and it is by no means certain that they will ever be entirely displaced by purely chemically prepared dyes. Natural dyestuffs are chiefly products of the vegetable and animal kingdoms.

Vegetable dyes are obtained from all parts of plants, such as the roots, the wood, the bark, the leaves, the flowers, and the seeds or fruits. That is to say, of certain plants one or other of these is the dye-yielding part, but sometimes the whole plant is employed. The number of plants which yield colouring materials such as could be applied in the tinctorial arts is very great, and if we include those employed by savage races, the number of these dyestuffs actually used is still large. Those, however, which are well known are not very numerous. A fuller account of a few of the more important of those noticed below is given under their separate heads.

Madder (q.v.), from the root of Rubia tinctorum, has been used for dyeing red and for producing, along with other dyes and with certain mordants, compound colours since ancient times. Until the discovery of a process of making artificial Alizarin (q.v.)—the chief colouring principle of madder—it was largely cultivated in the Levant. Madder, and not artificial alizarin, is still used for dyeing calico in Persia and some parts of India, but a comparatively small quantity is now brought to western Europe. There are other two species of Rubia yielding dyes somewhat resembling madder which are employed in India. These are R. cordifolia and R. sikkimensis, the dye from the former being called 'manjit' or Indian madder.

Garancine is a red dyestuff prepared by treating spent madder with sulphuric acid. It is of less importance now than formerly.

Safflower, from the flower-heads of Carthamus tinctorius, yields both a red and a yellow dye, but it is only the former that is useful. The red, or rather pink, is a beautiful, though not a permanent, colour applied to the dyeing of silk, and more sparingly to cotton. It is a costly material, and before the introduction of aniline colours, the cultivation of safflower was an important industry in India.

Brazil-wood (q.v.), obtained from one or more species of Cesalpinia, according to some authorities from C. brazilicensis, according to others from C. echinata. Peachwood and Lima-wood are probably mere varieties, and the colouring matter from all three is supposed to be identical. These red-woods, as they are called, are chiefly used for cheap calico prints, and to some extent for dyeing silk.

Archil (q.v.) and Cudbear (q.v.) are dyes prepared from lichens. The colouring principle in each appears to be the same, and in fact there is no essential difference between the two stuffs. Archil is of a purple colour, and is most useful, along with other colouring matters, in the dyeing of wool various shades of brown and chocolate.

Panama Crimson.—This dye is used by the natives of the Isthmus of Panama for dyeing their straw-hats a fine crimson tint, which is said to withstand in a remarkable manner the action of sun and rain. It is obtained from a vine, but is scarcely known in commerce.

Chrysammic acid is a dye obtained by treating Aloes (q.v.) with nitric acid. From it a purple can be obtained on silk, black on wool, and pink on linen. It can be used with advantage along with aniline dyes.

Barwood and Camwood (q.v.), which are produced by the same tree, are employed in the ground state along with proper mordants for dyeing wool quiet reds and reddish browns; also for producing an imitation of Turkey-red on cotton. On wool the colours are permanent, but the dye on cotton is less so.

Quercitron.—From the bark of Quercus tinctoria a useful yellow dye is obtained. An extract of this bark called 'flavine' is used by woollen-dyers. Quercitron bark is most largely used as the yellow part in compound colours.

Fustic (q.v.).—There are two dyestuffs called by this name; the one is 'young fustic,' and the other 'old fustic.' The latter is the produce of Morus tinctoria, a large tree growing in Central America, and is the more important. The wood of this tree is ground, or an extract of it is made, and used like quercitron bark in the dyeing of compound colours on wool and cotton, for which it furnishes the yellow part.

Persian Berries, the fruit of Rhamnus infectorius, and perhaps other species. They are known also as Yellow Berries, French Berries, and Turkish Berries. The fruit is not much larger than a pea. The dye is employed for wool, but most largely in the printing of calico as the yellow part in such colours as green or orange. A decoction of the berries is made. The colour obtained is bright, but not very permanent.

Turneric, the root of Curcuma longa, a plant largely cultivated in South Asia. It is rich in yellow colouring matter, which is, however, very fugitive. It is one of the few dyes which will fix itself on vegetable fibre without the help of mordants. Turneric is largely used for colouring test-paper for chemical purposes, but its use as dye for textiles is falling off.

Annatto or Arnott (q.v.), a preparation from the seeds of Bixa orellana. It produces a buff colour upon cotton, and a flesh colour upon silk, no mordants being required. The colours obtained from this dye are fugitive. The chief use of it is to improve the appearance of other dyed colours.

Indigo (q.v.) is obtained from two or three species of Indigofera, chiefly I. tinctoria. The indigo-plant is found wild over most parts of India, but generally near places where it has been cultivated. The cultivated plant is cut just as the flowers begin to appear; from two to four crops are taken from the same cuttings. The process of extracting the dye consists in steeping the plant for twelve hours, or rather more, in a vat with water, after which it is transferred to another vat where men agitate the liquid with sticks, thereby effecting the oxidation of the green colouring matter into blue particles of indigotin, which settle down as a sediment. This is next boiled for five hours and repeatedly passed through a strainer, by which the dye-particles are separated. After drying, the dye is pressed into slabs $3\backslash frac\{1\}\{2\}$ inches thick, from which the cubes of commerce are cut. For other plants from which indigo can be obtained, see INDIGO. This dye is one of the oldest known, and is still largely used for the dyeing of wool and cotton. See CALICO-PRINTING.

Woad.—This is believed to be the blue dye with which the ancient Britons stained or coloured their skins. It is obtained from the leaves of the cruciferous plant Isatis tinctoria, which has been long cultivated in Great Britain. In England woad is still used along with indigo in the dyeing of wool, but it is no longer employed in France or Germany.

Logwood (q.v.).—This well-known dyestuff consists of the heart-wood of Hæmatoxylon campechianum, a tree indigenous to Central America. Applied in small quantity to textile materials, the colour which logwood gives to them is a fugitive blue. At some places on the Continent it is used along with a mordant containing a large proportion of alum to dye wool a blue colour. It is employed to some extent as the blue part of compound colours in the dyeing of cotton, but the chief use of logwood is in the production of different shades of black on cotton and wool, for which suitable mordants are necessary. This dye stands best when applied in large quantities for dark colours.

The following vegetable dyes are of some importance in India, a few of them being also used in Europe: Sappan-wood (Cesalpinia Sappan) yields a red colour; Sanders-wood (Pterocarpus santalinus), a pink; Catechu (q.v.) (an extract of Acacia Catechu), browns, drabs, and grays; the roots of Morinda citrifolia, a useful red; the twigs of Strobilanthes flaccidifolius, Assam indigo; the fruit-rind of the pomegranate (Punica granatum), a yellow; and the powder called 'kamala,' from the fruit of Mallotus philippinensis (otherwise called Rottlera tinctoria).

Several plants, either indigenous or naturalised, in Great Britain yield dyes. Among others, weld, the stalks of Reseda luteola, was used for dyeing yellow up to recent times. Dyer's Broom (Genista tinctoria) also yields a yellow colouring matter. From Rhamnus catharticus and R. frangula green and yellow dyes are obtained. The cultivation of the common yellow bedstraw (Galium verum) for red and yellow dyestuffs was at one time attempted.

The following are the chief dyes derived from animal substances:

Cochineal (q.v.), obtained from the insect Coccus cacti, is the most important red colouring matter for animal fibres. See above.

Kermes.—A colouring matter which, though not obtained from the same insect, is identical with cochineal. Several species of Coccus, of which the most common is C. ilicis, yield kermes. This dyestuff is chiefly employed for dyeing woollens and leather in the countries of which the insect is a native—viz. Spain, Turkey, Morocco, and the south of France.

Lac-dye.—In the washing of stick-lac (see LAC), the colouring matter secreted by the lac insect (Coccus lacca) is dissolved in the water, and recovered by boiling down the washings. The red dye is then made up in the form of small cakes. Lac-dye usually produces duller red colours than cochineal, but from a pure extract of it the same, or very similar, colours are obtained. It is still used as a dye in India, but very little now in Europe.

Murcide.—This beautiful and delicate purple dye, prepared by the action of dilute nitric acid upon uric acid and treatment of the product with ammonia, was largely used in 1855 and 1856 for dyeing wool and silk, and for printing upon calico. The uric acid required for its production was got from guano. Murexide had only been tried for a year or two when its manufacture was rendered unprofitable by the introduction of aniline colours.

The only mineral dyes of much importance are Prussian Blue and Chrome Yellow (q.v.). The method of producing the former is given above at p. 136, and the latter under CALICO-PRINTING, where the purely pigment colours applied to cotton cloth, such as artificial ultramarine, are likewise noticed.

Coal-tar Colours.—The dyes now manufactured from products obtained in the distillation of coal-tar are extremely numerous, and new ones are daily added to the list. With the exception of anthracene, from which artificial alizarin is prepared, the raw materials chiefly used are the naphtha or benzene and carbolic acid; naphthalene, a crystalline solid body, has lately been coming much into use as well, more especially for dyes competing with cochineal. These substances, although not constituting a large percentage of the tar, are more than sufficient in amount for any supply that could be required, the quantity of tar from the gas-works in Great Britain alone being nearly half a million tons per annum. The total annual value of these colours produced in England, Germany, and France is about £4,000,000.

The rapid development of this industry is the result of the progress of organic chemistry, and these dyes can only be understood and classified from a chemical point of view. Leaving out artificial alizarin and indigo at present, these colours may be arranged in three divisions: (1) Aniline Dyes.—These are compound amines, bodies of the nature of bases. (2) Phenol Dyes.—These are derivatives of carbolic acid or phenol and similar chemical bodies, and have more or less acid properties. (3) Azo Dyes.—These are bodies containing azote or nitrogen connecting two groups, and may be neutral, but the groups may also be either of a basic or acid nature.

1. Aniline Dyes.—The preparation and properties of aniline have been described under that head. A base of the same series, toluidine, and another from naphthalene, are used along with it for certain colours. The method of preparation for all these bodies is nearly alike—viz. by the action of nascent hydrogen on the requisite nitro-compound and distillation of the product with soda. Iron filings and hydrochloric acid are generally taken to obtain the hydrogen.

When a mixture of aniline and toluidine is heated with arsenic acid for several hours to a temperature of about 380° F., a mass is left from which rosaniline is separated, the compounds of which chiefly form the aniline dyes. Stannic chloride can be used in place of arsenic acid, but the latter gives better results; careful attention is required in the separation of arsenic from the dyes. The residue after cooling is powdered and treated with boiling water, which dissolves arseniate and arsenite of rosaniline, and leaves a residue containing other colouring matters. When the solution has common salt mixed with it, double decomposition takes place, and the arsenical salts remain in solution, while rosaniline as hydrochloride is precipitated. This substance is not insoluble in water, but in water containing salt it is precipitated or 'salted out.'

The mixture of bases treated, called 'aniline oil,' combine with oxygen from the arsenic acid, which removes hydrogen in the form of water, the residues coalescing into the more complex molecule of the colouring matter. Another process—the one now chiefly used—for the preparation of rosaniline is heating aniline with nitrobenzene, protochloride of iron, and iron filings; in this case the oxygen is derived from the nitrobenzene. The residue is treated as in the previous process. The product is purified by crystallisation from water, and the hydrochloride of rosaniline so obtained is the colour magenta.

Rosaniline itself is colourless, and crystallises in needles or plates. It separates from magenta on adding soda to its solution, not being very soluble in water; it dissolves more readily in alcohol. It forms salts with acids, and these are the aniline red dyes. Magenta, or fuchsin, is chiefly hydrochloride, roseine the acetate, azaleine the nitrate, but pure simple salts are not usually sold. The salts have usually a green metallic lustre, and red in thin plates by transmitted light. The solutions have an intense crimson colour, and are not fluorescent.

Rosaniline is accompanied, as usually manufactured, by an almost similar substance—pararosaniline. The difference is that of homologues of the same series, and as pararosaniline is the lower, we shall use it in the following comparative formulae, showing the replacements producing the various colours.

Methane (marsh-gas), CH₄, is the simplest of a series of hydrocarbons, and contains 1 atom of carbon and 4 of hydrogen. The hydrogen atoms may be replaced one after another by various elements or compound groups. In chloroform 3 are replaced by chlorine, giving the formula CHCl₃; and if this body, under certain conditions, acts on benzene, C₆H₆, we get hydrochloric acid and a hydrocarbon named tri-phenyl-methane.

The substitution products, or their compounds, derived from this hydrocarbon are the various aniline dyes.

By treating tri-phenyl-methane in the same manner as benzene is treated to get aniline, we get a tri-amido base termed paralenicaniline. By the addition of an atom of oxygen to this body we get pararosaniline, which by solution in acids, as already stated, forms the aniline reds. The following are the respective formulae:

Aniline Blue.—When the hydrogen atoms in the amido groups (NH₂) become replaced in pararosaniline by phenyl, C₆H₅, methyl, CH₃, ethyl, C₂H₅, or similar groups, the aniline blue colours are produced. The ordinary spirit soluble blue has an atom in each of the amido groups replaced by phenyl, and is the hydrochloride of triphenylated pararosaniline, and has the following formula:

This blue is obtained by heating rosaniline to a high temperature with a large excess of aniline along with some benzoic acid—the action of which is not understood. Ammonia is formed during the operation, and escapes along with the excess of aniline, which is distilled off. When the action ceases, the product is cooled, and excess of hydrochloric acid added, which forms an insoluble compound of the base, giving, when washed and dried, the spirit blue.

It will be seen by the formula that there are still two free atoms of hydrogen in the amido groups.

These can be substituted by methyl or ethyl, and blues of a purer shade obtained. All the varieties are only soluble in spirit, and to a small extent; they give very pure blue colours.

Soluble Blue.—The spirit blue, so called from being insoluble in water, is to a large extent converted into a compound soluble in water, termed 'soluble blue,' thereby giving it a much wider range in its application. The compound is a sulphonic acid salt, and is prepared in the usual manner by mixing with sulphuric acid, gently warming, and after a time pouring the mixture into cold water, in which the free acid is insoluble. After washing, it is cautiously dissolved in soda solution, and salted out. It is then dried gently, forming a brownish cake. It dissolves readily in water. This is monosulphonate of the dye, and called alkali blue.

Water Blue or Cotton Blue is a trisulphonate, and is prepared by longer heating at a temperature a little over $212^\{\backslash circ\}$ F. The excess of sulphuric acid used is separated from the solution by milk of lime, this sulphonic acid being soluble in water. It is converted into a soda or ammonia salt.

Methyl Violet.—If five of the amido-hydrogen atoms in rosaniline are substituted by methyl, this colour is produced. The methyl groups are substituted in the aniline, and the process of oxidation is then nearly the same as in the preparation of magenta. Pure di-methyl-aniline is treated with chloride of copper, and some common salt is used to moderate the action. After the mass is cold it is carefully treated with water, to form a strong solution of the salt, in which the colour is insoluble. After the salt is carefully drained off, the colour is dissolved in water and any copper removed by sulphuretted hydrogen. The salt or form in which this colour is sold is often a double chloride of the colour base with chloride of zinc, which is crystalline. The double zinc salts are frequently used for a similar purpose, as they crystallise readily, giving the product a definite form and appearance, and the oxide of zinc is readily soluble in acids and in caustic alkalies, and having no colour does not interfere with the dyes.

Benzyl-rosaniline Violet.—The methyl groups in the violet described can be replaced by heating the colour with the chloride of benzyl, a body prepared with toluidine, the benzyl group, $C\_7H\_7$, producing a bluer colour.

Malachite Green.—This is a colour belonging to a class having only two amido groups in tri-phenyl-methane. The formula will best illustrate its structure. That of brilliant green is also given.

In malachite green 4 hydrogen atoms are replaced in the amido groups by methyl, and in brilliant green by ethyl. These colours are chiefly sold as zinc double salts or oxalates. They generally have a rich metallic lustre, and are readily soluble in water.

Helvetia green and others are sulphonic acid derivatives of these colouring matters.

2. Phenol Dyes.—When the hydrogen atoms of benzene are replaced by hydroxyl, OH, bodies of an acid or semi-acid kind are formed, called phenols. These are mono-acid, di-acid, &c., according to the number of hydrogen atoms substituted. This is seen in the following formulæ:

Naphthalene yields the most important bodies of this class, called naphthols. A general method for their preparation is to melt the sulphonic acid of the hydrocarbons with caustic soda, and add to the solution of the residue an excess of hydrochloric acid, which separates the phenol.

Some of the important yellow dyes are nitro compounds of these bodies. They are got by cautiously mixing the phenols with strong nitric acid, and generally finishing with the aid of heat. Practically better results are got by first forming the sulphonic acid.

Pieric Acid is tri-nitro-phenol. Its formula is $C\_6H\_2(NO\_2)\_3OH$. It is sparingly soluble in water, to which it gives an intensely bitter taste, recognisable in fibres which have been dyed with it. It crystallises in thin yellow laminae. The salts form fine crystals, and are more or less explosive.

Naphthol Yellow, Martins' Yellow.—This is di-nitro-naphthol, and was the first colour of value made from naphthalene. It is insoluble in water, but gives fine yellow or orange coloured salts, crystalline and soluble in water. They closely resemble the pierates. A sulphonic acid of this yellow is also used as a dye.

Rosolic Acid, Aurin.—If tri-phenyl-methane contained phenol instead of amine groups, as in rosaniline, it would represent these dyes. In fact, the rosolic acids and rosanilines are convertible into each other. These dyes are now of little importance in practice.

Phthaleins.—These form a very important class of dye-yielding materials, and are formed by the union of phenols with the anhydride of phthalic acid. Phthalic anhydride is obtained from naphthalene by first forming a chlorine addition product, and then oxidising with nitric acid. Crude phthalic acid so obtained is converted into water and the anhydride of phthalic acid (which is volatile) by heating.

Gallein was the first discovered of these colours, and was produced by heating pyrogallic acid with the anhydride. It is chiefly made into cerulein.

Cerulein is obtained from gallein by heating with sulphuric acid to $400^\{\backslash circ\}$ F. till the colour changes to brownish-green, then, on cooling, mixing with a large quantity of cold water. The treatment has removed an atom of water. The cerulein forms a blackish powder. It is insoluble in water, but dissolves in alkalies with a beautiful green colour.

Fluorescein, Eosin.—This is the resorcin phthalein, and is prepared by heating the materials to $400^\{\backslash circ\}$ F. till water ceases to be given off. The mass remaining is fluorescein. From the remarkable fluorescence of its compounds it derives its name, and an alkaline solution of it is taken as a striking example of this phenomenon. It is slightly soluble in water, with a yellow colour, and in the dry state is a reddish crystalline powder.

Fluorescein itself is scarcely used as a dye. But when part of its hydrogen is substituted by bromine, chlorine, or iodine, the beautiful dyes called cosins are formed. The tetrabrominated eosin, or rather its potash salt, has been most largely used. It forms red crystals with yellowish-green reflections. The solutions are rose-coloured with intense green fluorescence. One of the most beautiful colours of this group is the replacement by 2 chlorine and 4 iodine atoms, called phloxine. Methyl and ethyl ethers of this body are also dyestuffs.

3. Azo Dyes.—The first section of the coal-tar colours consisted of aniline and similar bodies coalesced into more complex basic forms. The second of phenols, with substitutions yielding acid compounds. The third section, now to be considered, in its simplest form is neutral, but by reactions with amines (bases) or phenols (acids) can produce colouring bodies with the chemical properties of either of the other sections. This class is obtained by a reducing action on nitro compounds, leaving two residues in combination by the mutual affinity of the nitrogen atoms—hence the name, from azote (nitrogen). The construction will be understood from the following equation :

We have here, it will be seen, simple phenyl groups combined with two nitrogen atoms, and from the absence of amido or hydroxyl groups, the substance is neither basic nor acid. If in place of nitrobenzene a nitro derivative of a different hydrocarbon had been taken, a corresponding azo compound would have been produced. The principal development of the coal-tar colours of late years has been connected with this reaction. It can be seen that by manipulating the hydrocarbon groups with amido and hydroxyl groups as with the bodies in the other sections of the colours, any number of azo dyes may be obtained.

Most of these dyes from benzene and the lower members of its series are yellow or brown, but when hydrocarbons with more carbon atoms are used, such as cymol and naphthalene, reds and blues are produced; some of the scarlets having almost displaced the colours from cochineal.

Bismark Brown, Phenylene Brown, &c.—This is an example of a colour with amido derivatives, and is selected as being the first of the azo colours having a manufacturing success. It is prepared by the action of nitrous acid on the di-amine of phenylene. The reaction is shown in the equation :

These colours are very easily prepared; as a rule the colour precipitates when its components are brought together. Besides the dyeing of cotton and wool, this colour is much used for dyeing leather. It is used as the hydrochloride, a dark powder.

Fast Yellow is the potassium salt of a sulphonic acid. Its construction is seen by the formula :

Oxyazo dyes are prepared with phenols, and have become the most important of the coal-tar colours. They are nearly all sulphonic acid compounds, and used in the form of soda salts of these acids. The yellow and orange colours are sold as Tropæolins; fast red, Roccellin; claret red, Bordeaux; scarlets, Biebrich, Crocëin, &c. An example may be given of the composition of one of these colours, first known as Meister's scarlet, now sold as scarlet G :

This dye is the sulphonic acid itself, but usually an alkaline salt is employed in the dyeing process, as the acids are more or less insoluble. A compound of the acids with alkaline bisulphites has lately been used with some success.

The benzidine and allied colours recently introduced belong to the tetrazo group of the oxyazo dyes. These have the valuable property of dyeing cotton without a mordant. Almost any shade of blue, green, yellow, and red can be obtained from them. They stand scouring and milling, and are considered fast to light on wool; but some, at least, are not permanent on cotton. There exists a strong belief that the benzidine colours will be those most largely made from coal-tar products in the future since they are so simply applied. See PHENYL COMPOUNDS.

Anthracene Dyes.—These are only a small group, and are regarded rather as artificial productions of the natural colours of madder than coal-tar colours. Artificial alizarin is now, however, manufactured on a large scale, and has almost completely taken the place of the natural product. In England it was first made in 1870. In the manufacture of these dyes anthracene is first converted into anthraquinone by oxidation with solution containing chromic acid, and then into sulphonic acid, or rather sulphonic acids, for there are several formed. This requires fuming sulphuric acid, and a temperature of 320° F. The acids are converted into soda salts, and may be separated by crystallisation. Mono-acid produces blue, di-acids more of the red and orange colours.

The soda salts of the sulphonic acids are now mixed with a small proportion of chlorate of potash, and heated under pressure with caustic soda to 400° F. for twenty-four hours. Water is added in sufficient quantity to keep the soda liquid, and the mixture requires to be constantly stirred, or the materials would not come together. The mass obtained is ground and dissolved in water neutralised with hydrochloric acid, when the alizarin separates. It is filtered and pressed.

Alizarin as sold contains three colouring matters—alizarin, giving blue; anthrapurpurin, red; and flavopurpurin, orange shades. They may all be produced separately from the different sulphonic acids. Their properties as dyes are similar.

There are several complicated processes for preparing artificial indigo from coal-tar derivatives.

See Benedikt on Coal-tar Colours (1886); Crookes on Dyeing and Tissue-printing (1882); O'Neil on Calico-printing, Dyeing, &c. (1878); the technical journals; Nietzki, The Chemistry of the Organic Dye-stuffs (trans. 1892); and A Manual of Dyeing, by Knecht, Rawson, and Löwenthal (3 vols. 1893).