564
DYE I N G
copper sulphate (5 per cent.). After squeezing, the material is boiled for about fifteen minutes in a solution of bichromate of potash (l oz. per gal.), then washed and dried. By repeating the operations two or three times, deeper shades are obtained. During the boiling with catechu the cotton attracts the active principles catechin and catechu-tannic acid, but it thus acquires only a pale brown colour ; in the bichromate of potash, however, these are oxidized to form insoluble japonic acid, which permeates the fibre, and a deep brown colour is thus developed. Catechu browns are fast to a variety of influences, e.g., washing, alkalis, acids, &c., but less so to light. Catechu has been recently much employed, in conjunction with copper sulphate, for dyeing the so-called khakibrown on woollen material for military clothing. On silk, catechu is much used for weighting purposes in dyeing black. Mineral Colours.—These include Chrome Yellow, Iron Buff, Prussian Blue, and Manganese Brown. Chrome Yellow is only useful in cotton-dyeing as a self-colour, or for conversion into Chrome Orange, or, in conjunction with Indigo, for the production of fast green colours. The cotton is first impregnated with a solution of lead acetate or nitrate, squeezed, and then passed through a solution of sodium sulphate or lime water to fix the lead on the fibre, as sulphate or oxide of lead. The material is then passed through a solution of bichromate of potash. The colour is changed to a rich orange by a short, rapid passage through boiling milk of lime, and at once washing with water, a basic chromate of lead being thus produced. The colour is fast to light, but has the defect of being blackened by sulphuretted hydrogen. Iron Buff is produced by impregnating the cotton with a solution of ferrous sulphate, squeezing, passing into sodium hydrate or carbonate solution, and finally exposing to air, or passing through a dilute solution of bleaching powder. The colour obtained, which is virtually oxide of iron, or iron-rust, is fast to light, but readily removed by acids. Prussian Blue is applicable to wool, cotton, and silk, but since the introduction of coal-tar blues its employment has been very restricted. The colour is readily obtained on cotton by first dyeing an iron-buff, according to the method just described, and then passing the dyed cotton into an acidified solution of potassium ferrocyanide, when the blue is at once developed. A similar method is employed for silk. Wool is dyed by heating it in a solution containing potassium ferricyanide and sulphuric acid. The colour is developed gradually as the temperature rises ; it may be rendered brighter by the addition of stannous chloride. On wool and silk, Prussian blue is very fast to light, but alkalis turn it brown (ferric oxide). Manganese Brown is applied in wool, silk, and cotton dyeing. The animal fibres are readily dyed by boiling with a solution of potassium permanganate, which, being at first absorbed by the fibre, is readily reduced to insoluble brown manganic hydrate. Imitation furs are dyed in this manner on wool-plush, the tips or other parts of the fibres being bleached by the application of sulphurous acid. Cotton is dyed by first impregnating it with a solution of manganous chloride, then dyeing and passing into a hot solution of caustic soda. There is thus precipitated on the fibre manganous hydrate, which by a short passage into a cold dilute solution of bleaching powder is oxidized and converted into the brown manganic hydrate. This manganese bronze or brown colour is very susceptible to, and readily bleached by, reducing agents; hence by the application of these, in conjunction with various dyestuffs, the calico-printer is able to produce a pleasing variety of so-called coloured discharge patterns. With respect to the question of colour, we meet with two kinds of substances in nature, those which possess colour and those which do not. Why this dye°ng ^ difference 1 The physicist says the former are bodies which reflect all the coloured rays of the spectrum composing white light—if opaque, they appear white •, if transparent, they are colourless. The latter are bodies which absorb some of the spectrum rays only, reflecting the remainder, and these together produce the impression of colour. A black substance is one which absorbs all the spectrum rays. The fundamental reason, however, of this difference of action on the part of substances towards light remains still unknown. All substances which possess colour are not necessarily dyestuffs, and the question may be again asked, Why1? It is a remarkable circumstance that most of the dyestuffs at present employed occur among the so-called aromatic or benzene compounds derived from coal-tar, and a careful study of these has furnished a general explanation of the point in question, which briefly
is, that the dyeing property of a substance depends upon its chemical constitution. Speaking generally, those colouring matters which have the simplest constitution are yellow, and as the molecular weight increases, their colour passes into orange, red, violet, and blue. In recent years chemists have begun to regard the constitution of nearly all dyestuffs as similar to that of Quinone, and some even believe that all coloured organic compounds have a quinonoid structure. According to O. N. Witt, a colourless hydrocarbon, e.g., benzene, becomes coloured by the introduction of one or more special groups of atoms, which he terms the colour-bearing or chromophorous groups—e.g., N02, - N : hi -, &c. Benzene, for example, is colourless, whereas nitro-benzene and azo-benzene are yellow. Such compounds containing chromophorous groups are termed chromogens, because, although not dyestuffs themselves, they are capable of generating such by the further introduction of salt-forming atomic groups—e.g., OH, NH2. These Witt terms auxochromous groups. In this way the chromogen tri-nitro-benzene C6H3(N02)3 becomes the dyestuff, tri-nitro-phenol (picric acid) C6H2(N02)3(0H), and the chromogen azo-benzene C6H5 • N : N • C6H5 is changed into the dyestuff amido-azo-benzene (Fast Yellow) C6H5 • N": N • C6H4(NH9). These two dyestuffs are typical of a large number which possess either an acid or a basic character according as they contain an hydroxyl (OH) or an amido (NH2) group, and correspond to the Acid Colours and Basic Colours to which reference has already been made. Other important atomic groups which frequently occur, in addition to the above, are the carboxyl (COOH) and the sulphonic acid (HSOs) groups; these either increase the solubility of the colouring matter or assist in causing it to be attracted by the fibre, &c. In many cases the free colour-acid or free colour-base has little colour, this being only developed in the salt. The free base rosaniline, for example, is colourless, whereas the salt magenta (rosaniline hydrochloride) has a deep crimson colour in solution. The free-acid Alizarine is orange, while its alumina-salt is bright red. It may be here stated that the scientific classification of colouring matters into Nitro-colours, Azo-colours, &c., already alluded to, is based on their chemical constitution, or the chromophorous groups they contain, whereas the classification according to their mode of application is dependent upon the character and arrangement of the auxochromous groups. The question of the mordant-dyeing property of certain colouring matters containing (OH) and (COOH) groups has already been explained under the head of Artificial Mordant Colours. The peculiar property characteristic of dyestuffs, as distinguished from mere colouring matters, namely, that of being readily attracted by the textile fibres, notably the animal fibres, appears then to be due to their more or less marked acid or basic character. Intimately connected with this is the fact that these fibres also exhibit partly basic and partly acid characters, due to the presence of carboxyl and amido groups. The behaviour of magenta is typical of the Basic Colours. As already indicated, rosaniline, the base of magenta, is colourless, and only becomes coloured by its union with an acid, and yet wool and silk can be as readily dyed with the colourless rosaniline (base) as with the magenta (salt). The explanation is that the base rosaniline has united with the fibre, which here plays the part of an acid, to form a coloured salt. It has also been proved that in dyeing the animal fibres with magenta (rosaniline hydrochloride), the fibre unites with the rosaniline only, and liberates the hydrochloric acid. Further, in dyeing cotton with magenta it is necessary to prepare the cotton previously with the mordant tannic acid, with which the base rosaniline unites to form an insoluble salt.
