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Bleaching of printed papers without destroying the texture of the leaves. —Steep the leaves in a caustic solution of soda, and afterwards in one of soap. Arrange the sheets alternately between cloths in the same manner as paper-makers dispose their sheets of paper when delivered from the form. Put the leaves in a press, and they will become whiter, unless they were originally loaded with printers' ink or size. If this should not completely effect the whitening of the leaves, repeat the process a second, or even a third time.
Bleaching coloured rags to make white paper. Soak or macerate the rags sufficiently; put them into a solution of caustic alcali, and then into the oxygenated muriatic acid; and, lastly, steep them into diluted sulphuric acid.
Dyeing is the art of extracting the colouring principle from different substances, and transferring them to wool, silk, cotton, or linen. When other matters are coloured, the process is called staining
In dyeing, the colouring matter is not merely deposited on the stuff, but is firmly attached to it by chemical combination depending on an affinity subsisting between them.
If the colouring matters were merely spread over the surface of the fibres of the cloth, the colours produced might be very bright, but they would not be permanent, since they would be rubbed off, and would disappear when the cloth was washed, or even by exposure to the weather. Dyeing is, therefore, a chemical process, consisting in combining a certain colouring matter with fibres of
cloth. The colouring matters are, for the most part, extracted from animal and vegetable substances, and have usually the colour which they give to the cloth.
The particles of these colouring matters appear to be transparent, because the original colour of the cloth will appear through them. The colour of dyed cloth, therefore, does not depend upon the dye alone, but also upon the previous colour of the cloth. Thus, if the cloth be black it will not receive a dye of any colour; and hence it is necessary, that the cloth should be white, if we wish to dye it of a very bright colour.
The colouring matter, or dye-stuff, must be dissolved in some liquid, that the particles may be precipitated upon the cloth ; and it is essential that its affinity for this solvent should not be so strong as for the cloth to be dyed.
Thus the facility with which cloth imbibes a dye depends upon two things ; namely, the affinity between the cloth and the dye stuff, and that between the dye stuff and its solvent. Much of the accuracy of dyeing depends upon preserving a due proportion between these two affinities. If the affinity between the dye-stuff and cloth, compared with that between the dye-stuff and the solvent be too great, the cloth will receive the dye too quickly, and the colour will be apt to be unequal : and if the affinity between the dye-stuff and the solvent be greater than between the dye-stuff and the cloth, the latter will scarcely receive the dye, or, at least, very faintly.
Wool has the strongest affinity for colouringmatters ; silk the next strongest ; cotton has much less affinity; and linen has the least of all. Hence the dye-stuff for cotton or linen should be dissolved
in substances which they have less affinity for, than when silk or wool are to be dyed. Thus iron dissolved in the sulphuric acid may dye wool; but when it is intended to dye cotton and linen by iron, the latter should be dissolved in the acetous acid.
There are few colouring substances that have, of themselves, so strong an affinity for cloth as to answer the purpose of dyeing so as to remain permanent; and, on this account, an intermediate substance is employed, that has a decided attraction for both the colouring matter and the cloth, thus serving as a bond of union between them. This substance is previously combined with the cloth, which is then dipped into the solution containing the dye-stuff. The dye-stuff combines with the intermediate substance, which being firmly combined with the cloth, secures the permanence of the dye. Substances employed for this purpose are denominated mordants. Instead of this some prefer the term basis.
The most important part of dyeing consists in the proper choice, and the proper application of mordants; as upon them, the permanency of every dye depends. What has been said respecting the application of colouring matters applies equally to the application of mordants. They must be previously dissolved in some liquid, which has a weaker affinity to them than the cloth has, to which they are to be applied ; and the cloth must be dipped, or even steeped in this solution, in order to saturate itself with the mordant. The mordants are earths, metallic oxides, tan, and oil.
Of the earths, alumine is the most useful. It is applied in the state of sulphate of alumine, or common alum; and in that of acetate of alumine.
When alum is used as a mordant, it is dissolved in water, and sometimes a quantity of tartar is added. The cloth is put into this -solution, and kept till it has absorbed as much alumine as is necessary. It is then taken out, and is washed and dried. A quantity of alumine has by this process combined with the fibres of the cloth, which is perceived by the latter weighing more than before. The addition of the tartar, or tartrate of potash, is made on two accounts; the potash which it contains combines with the sulphuric acid of the alum, and thus prevents that very corrosive substance from injuring the texture of the cloth : the tartareous acid, on the other hand, combines with part of the alumine, and forms a tartrate of alumine, which is more easily decomposed by the cloth than alum.
Acetate of alumine is used as a mordant for cotton and linen, which have a much less affinity than wool for alumine. The alumine is retained less powerfully in a state of combination by the acetic than by the sulphuric acid; and, therefore, cotton and linen are better able to separate it and attach it : also the acetic acid being volatile, gradually leaves the earthy basis, and allows the alumine to unite to the stuff.
This mordant is now prepared by pouring acetate of lead into a solution of alum; on which a double decomposition takes place; the sulphuric acid combines with the lead, and the sulphate of lead precipitates in the form of an insoluble powder, while the alumine combines with acetous acid, and remains in the liquor.
This mordant gives a richer colour than alum.
Lime is also sometiines employed as a mordant : but it does not answer so well in general, not giving so good a colour. It is used either in the state
of lime water, or as sulphate of lime dissolved in water.
Although all the metallic oxides have an affinity for cloth, only two, the oxides of tin and of iron, are much used as mordants.
The oxide of tin is one of the most valuable mordants, and is the only one by which scarlet, the brightest of all colours, can be produced. It was first brought to London by Karsten, a German, in 1543, which period forms an epoch in the history of dyeing
Proust has shown that tin has two oxides. The first, or grey oxide, consists of seventy parts of tin, and thirty oxygen: the second, or white oxide, of sixty parts of tin, and forty oxygen. The first oxide absorbs oxygen rapidly from the air, and becomes converted into the white oxide. It is, therefore, the white oxide alone that is the real mordant; since if the first were applied to cloth, as it probably often is, it must soon be converted into the white oxide by absorbing oxygen.
Tin is used as a mordant in three states; dissolved in nitro-muriatic acid, in acetous acid, and in a mixture of sulphuric and muriatic acids. That commonly used by the dyers, and called by them spirit of tin, is the nitro-muriate. It is prepared by dissolving granulated tin in very dilute nitric acid, or what is called single aquafortis : and a quantity of muriate of soda, or muriate of ammonia, is added. These salts are decomposed by the nitric acid, and the muriatic acid is set free. sometimes to economize the nitric acid, a quantity of sulphuric acid is added, just sufficient to saturate the base of the muriate of soda.
When nitro-muriate of tin is used as a mordant, it is dissolved in a large quantity of water, and