Salt-Cake in Dyeing.

Scientific American 5, 30.1.1869

Written for the Scientific American by Dr. M. Reimann.

In England, as well as in Germany, salt-cake, or sulphate of soda, has been employed for some considerable time already as an expedient in dyeing wool.

The practical dyer, when asked concerning the adcantages of this substance, which seems to possess so little importance, for the dyeing process, can state no reasonable ground for its employment, only in rare cases you will perharps hear that the bath dyes more equally when the sulphate of soda is added to it. Even the chemist, on regarding the matter somewhat superficially, does not observe what purpose the sulphate of soda serves in the dyer's bath. He considers it one of the number of utterly useless substances employed by the dyers in accordance with the prescriptions of some hand-book.

Nevertheless, if we regard the matter carefully in the following discussion, we shall see that sulphate of soda can be employment is based on the most interesting chemical and physical principles. At the same time we shall be obliged to advance into the comparatively unknown region of the dyeing theory, the practical use of which we shall soon recognize.

The sulphate of soda, which scarcely ever treated of in books of dyeing, because of its chemical indifference for coloring matters, elevates, as every soluble substance does, the specific weight, and thus also the boiling point of the solution. This property already, when taken into consideration, renders it important for many dyeing processes. It is possible, for instance, to change the shade of aniline violet into blue or red, according as the temperature of the solution is more or less elevated.

When the dyeing is performed in an acid bath (the dyers very frequently add sulphuric acid to their baths), the sulphate of soda combines with the free sulphuric acid in the bath, and forms with it bisulphate of soda, a crystallizable solid salt. In this manner the bath retains its acid reaction without the presence of free sulphuric acid in the bath. Hence, when half-woolen cloths are dyed, the cotton in them, ex-tremely sensiticve to the action of the mineral acids, will be very well preserved.

Dissolved in water in great quantities, the sulphate of soda diminished the capacity of the bath to dissolve the added coloring matters in as great a degree as though there were no such salt present; this, too, is highly important for many dyeing processes.

Several practical examples will demonstrate the advantages of sulphate of soda more conclusively than a whole series of theoretical observations. The red coloring matters as the cudbear, and more especially the magenta, and the red dyeing woods, possess, as is well known, the property of combining only with the greatest difficultu with the fiber when dyed in an acid bath. Therefore, whenever the substances are employed in the acid bath - and often this is necessary - the greatest part of the coloring matter is wasted and lost if the common process is employed. The same applies also to the yellow wood.

If, however, the said coloring matters be dyed in an acid bath according to this new method, a twofold result will be attained. By adding sulphuric acid, the dyeing power of the quantity of sulphate of soda which is employed, it is possible to control the combination of the pigment with the texible fiber. Therefore, by means of the sulphate of soda various shades can be produced.

This fact is of great importance in many sorts of dyeing. There are some kinds of yarn, especially the long slubbing wool, which have the property of felting when exposed too frequently to a change of temperature; they can then no longer be worked into weft yarn. Nevertheless, the wool must be exposed to such a change of temperature, for, in the preparing the shades, it is taken out of the bath at times, so that new coloring matter may be added to the part already in the bath.
In all these cases it would be unnecessary to take out the yarn if we were to add a little more coloring matter and acid, and shades could be produced by gradually adding sulphate of soda to the bath. By this process a great deal of manual labor may be spared, and the dyer enabled to work with far more security and comfort. Should at any time too much coloring matter have gone upon the fiber, the fault can readily be corrected by the addition of a little acid.

The truth of the above assertions is most easily preceptible in dyeing Magenta. As antoher example, let us regard the dying of shades, for which the wool must first be boiled in a solution of chrome salt, in the most cases in bichromate of potash. This is often done for red, brown, and grey, which are produced by means of logwood, red and yellow wood. When the wood i boiled in a bath of bichromate of potash, and especially when to this, as is commonly the practice, sulphuric acid is added, the colors of the logwood and red wood attack the fiber very quickly, and therefore often spread unequally. Hence, dyers must begin to dye at low temperatures, and must increase the heat very slowly. If to such a dyeing bath but a small quantity of acid is added, the effect of the coloring matters in it is almost nothing, it is, therefore, possible to dye with the boiling bath without fear of an unequal spreading of the coloring matters. It is only necessary to add, whilethe coloring matter is fixing on the fiber, sulphate of soda in small quantities, the coloring matter will combine with the fiber, while the sulphate of soda absorbs the free acid. It is therefore possible to produce shades without removing the goods from the bath, if we take care that the quantity of coloring matter which is at first added to the bath is not too small.

A simialt effect can be produced by adding the sulphate of soda at the beginning of the dyeing process. For sulphate of soda we may, in this case, employ even common salt. In this case the salts employed will, when dissolved in the fluid, precipitate the dissolved coloring matter, which is then contained in the bath, in a very fine state of division, or the salts will prevent the coloring matters from dissolving, according as these latter or the salts were first introduced into the bath.

For the process this is quite indifferent. The pigments fix on the fiber in the same measure as they are dissolved. Fresh coloring matter will only then be dissolved, when the portion already in dissolution is already consumed. The dyeing is more wqual, if the coloring matters are not dissolved in the bath, but are contained in it in a state of minute division, as every dyer knows who has ever employed aniline blue, soluble in water. This pigment, because of its ready solubility in acids, often fixes too quickly if the dyeing is carried on in an acid bath, and therefore dyes at times unequally. It is therefore best to dye from a neutral or weak alkaline fluid, and then to produce the blue by adding an acid. The same pigment soluble only in alcohol is precipitated as soon as its solution is added to the bath, and therefore dyed more equally, though more slowly still. In many cases also it is advantageous to employ sulphate of soda where small quantities of indigo carmine are used to give somewhat more of blue to a shade. The affinity of this coloring matter for wool being very great, small quantities of it may often dye the woolen goods very unequally; to prevent this, and give uniformity to the color, it is necessary to continue the boiling operation for some time. The indigo carmine will dye more slowly and equally in the case of the free acid is carried off by sulphate of soda.
The question now remains whether only the sulphate of soda, the importance of which I have endeavored to prove in the precoding remarks, is able to produce these results, or whether any other agent, can relace it in these processes.

In the preceding I already mentioned common salt as a substitute; and it can be advantageously employed, if either a higher specific weight can be produced, or the dissolved coloring matter be precipitated.

When common salt is employed in an acid bath, the development of hydrochloric acid is highly disagreeable. Cotton is violently attacked by it. Common salt can in turn be replaced for these processes by sulphate of magnesia and other salts which exercise no effect on the chemical constituents of the coloring matters, as, for instance, the compounds of alumina, iron, and tin.

Similar to the effect produced by the sulphate of soda, is that of the corresponding combination with potash, viz.: the sulphate of potash. This salt, however, is more expensive than the soda-salt. The bisulphate of potash is now already frequently employed in dyeing. The bisulphate of soda, which is a residium in some chemical manufacturing processes, for instance, in the production of nitric acid can often be advantageously employed for sulphate of soda and free acid.

To compare the expense of the employment of these substances, we must therefore observe that the sulphate of potash crystallizes without water, while the sulphate of soda contains 55·9 per cent, and the sulphate of magnesia 51·22 per cent of water, which is of course deoid of any value.

Finally, we must state that 100 parts of crystallized sulphate of soda are able to fix 30½ parts of sulphuric acid (of 668 B. s. w), and thus to form bisulphate of soda; or in other words, for every pound of sulphuric acid added to the bath, three pounds of crystals of sulphate of soda must be employed.

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