IRON, is by much the most useful of all the metals, as has been sufficiently proved under the article IRON, Encycl. and under CHEMISTRY in this Supplement. The word is again introduced here, because it affords us an opportunity of laying before our readers some valuable observations by Chapal on the use of the oxys of iron in dyeing cotton.
"The oxyd of iron has such an affinity for cotton thread, that if the latter be plunged in a saturated solution of iron in any acid whatever, it immediately assumes a chamois yellow colour, more or less dark, according to the strength of the liquors. It is both a curious and easy experiment, that when cotton is made to pass through a solution of the sulphat of iron, rendered turbid by the oxyd which remains suspended in the liquor, it will be sufficient to dip the cotton in the bath to catch the last particle of the oxyd, and to restore to the liquor the transparency it has lost. The solution, then, which before had a yellowish appearance, becomes more or less green, according as it is more or less charged.
"The colour given to cotton by the oxyd of iron becomes darker, merely by exposure to the air; and this colour, soft and agreeable when taken from the bath, becomes harsh and ochry by the progressive oxydation of the metal. The colour of the oxyd of iron is very fast: it resists not only the air and water, but also alkaline leys, and soap gives it splendour without sensibly diminishing its intensity. It is on account of these properties that the oxyd of iron has been introduced into the art of dyeing, and been made a colouring principle of the utmost value.
"In order that the oxyd of iron may be conveniently applied to the cotton thread, it is necessary to begin by effecting its solution; and, in this case, acids are employed as the most useful solvents. Dyers almost everywhere make a mystery of the acid which they employ; but it is always the acetous, the sulphuric, the nitric, or the muriatic. Some of them ascribe great differences to the solution of iron by the one or the other acid; but, in general, they give the preference to the acetous. This predilection appears to be founded much less on the difference of the colours that may be communicated by the one or the other salt, than on the different degrees of corrosive power which each exercises on the stuff. That of the sulphat and muriat is so great, that
that if the stuff be not washed when it comes from the bath, it will certainly be burnt; whereas solutions by the acetous, or any other vegetable acid, are not attended with the like inconvenience.
"Iron appears to be at the same degree of oxidation in the different acids, since it produces the same shade of colour when precipitated; and any acid solvent may be employed indiscriminately, provided the nature of the salt, and the degree of the saturation of the acid, be sufficiently known; for the subsequent operations may be then directed according to this knowledge, and the inconveniences which attend the use of some of these salts may be prevented. This, without doubt, is a great advantage which the man of science enjoys over the mere workman, who is incapable of varying his process according to the nature and state of the salts which he employs.
"1. If the sulphat of iron, or any other martial salt, be dissolved in water, and cotton be dipped in the liquid, the cotton will assume a chamoy colour, more or less dark according as the solution is more or less charged. The affinity of the cotton to the iron is so great, that it attracts the metal, and takes it in a great measure from the acid by which it was dissolved.
"2. If the iron of a pretty strong solution be precipitated by an alkaline liquor that shows five or six degrees (by the areometer of Baumé), the result will be a greenish blue magma. The cotton macerated in this precipitate assumes at first an unequal tint of dirty green; but mere exposure to the air makes it in a little time turn yellow, and the shade is very dark.
"It is by such, or almost similar processes, that dyers communicate what is called among workmen an ochre or rust colour. But these colours are attended with several inconveniences to the artist: 1. Strong shades burn or injure the cloth: 2. This colour is harsh, disagreeable to the eye, and cannot be easily united with the mild colours furnished by vegetables."
To avoid these inconveniences, our author made several attempts, which led him to the following practice: He treats the cotton cold in a solution of the sulphat of iron, marking three degrees; he wrings it carefully, and immediately plunges it in a ley of potash at two degrees, upon which he has previously poured to saturation a solution of the sulphat of alumine: the colour is then brightened, and becomes infinitely more delicate, soft, and agreeable. The sulphat no longer attacks the tissue of the stuff; and after the cotton has been left in the bath for four or five hours, it is taken out to be wrung, washed, and dried. In this manner we may obtain every shade that can be wished, by graduating the strength of the solutions. This simple process, the theory of which presents itself to the mind of every chemist, has the advantage of furnishing a colour very agreeable, exceedingly fixed, and, above all, extremely economical. He employs it with great advantage in dyeing nankeens, as it has the property of resisting leys. It becomes brown, however, by the action of astringents.
M. Chaptal made several attempts to combine this yellow with the blue of indigo, in order to obtain a durable green; but as they were all unsuccessful, he infers that there is not a sufficient affinity between the blue of indigo and the oxyds of iron. He found that these oxyds, on the other hand, combine very easily with the
red of madder, and produce a bright violet or plum colour, the use of which is as extensive as beneficial in the cotton manufactory. But if we should confine ourselves to apply these two colours to cotton, without having employed a mordant capable of fixing the latter, the colour would not only remain dull and disagreeable by the impossibility of brightening it, but it would still be attended with the great inconvenience of not resisting leys. We must begin, then, by preparing the cotton as if to dispose it for receiving the Adrianople red; and when it has been brought to the operation of galling, it is to be passed through a solution of iron, more or less charged, according to the nature of the violet required: it is then to be carefully washed, twice maddered, and brightened in a bath of soap.
When a real velvety rich violet is required, it is not to be passed through the solution of iron till it has been previously galled; the iron is then precipitated in a bluish oxyd, which, combined with the red of madder, gives a most brilliant purple, more or less dark according to the strength of the galling and of the ferruginous solution. It is very difficult to obtain an equal colour by this process; and in manufactories, an equal violet is considered as a master-piece of art. It is generally believed, that it is only by well-directed manipulations that it is possible to resolve this problem, of so much importance in dyeing. But I am convinced (says our author), that the great cause of the inequality in this dye is, that the iron deposited on the cotton receives an oxydation merely by exposure to the air, which varies in different parts of it. The threads which are on the outside of the hank are strongly oxydated, while those in the inside, removed from the action of the air, experience no change. It thence follows, that the inside of the hank presents a weak shade, while the exterior part exhibits a violet almost black. The means to remedy this inconvenience is, to wash the cotton when it is taken from the solution of iron, and to expose it to the madder moist. The colour will become more equal and velvety. The solvents of iron are almost the same for this colour as for the yellow colour already mentioned.
The following observation may serve to guide the artist in brightening the violet on his cotton. The red of madder and the oxyd of iron deposited on the stuff determine the violet colour. This colour becomes red or blue, according as either of the principles predominates. The dyer knows by experience how difficult it is to obtain a combination which produces the tone of colour desired, especially when it is required to be very full, lively, and durable. This object, however, may be obtained, not only by varying the proportions of the two colouring principles, but also by varying the process of brightening. The only point is to be acquainted with the two following facts; that the soda destroys the iron, while the soap, by strong ebullition, seizes in preference the red of the madder. Hence it is, that the colour may be inclined to red or blue, according as you brighten with one or the other of these mordants. Thus, cotton taken from the madder dye, when washed and boiled in the brightening liquor with ths of soap, will give a superb violet; whereas you will obtain only a plum colour in treating it with soda.
The oxyd of iron precipitated on any stuff unites also very advantageously with the fawn colour furnished by
by astringents; and by varying the strength of mordants, an infinity of shades may be produced. In this case, it is less a combination or solution of principles than the simple mixture or juxtaposition of the colouring bodies on the stuff. By means of a boiling heat, we may combine, in a more intimate manner, the oxyd of iron with the astringent principle; and then it is brought to the state of black oxyd, as has been observed by Berthollet. It is possible also to embrown these colours, and to give them a variety of tints, from the bright grey to the deep black, by merely passing the cottons impregnated with the astringent principle thro' a solution of iron. The oxyd is then precipitated itself by the principle which is fixed on the stuff.
An observation, which may become of the utmost value for the art of dyeing, is, that the most usual astringent vegetables all furnish a yellow colour, which has not much brilliancy, but which has sufficient fixity to be employed with advantage. This yellow colour is brightened in the series of vegetables, in proportion as the astringent principle is diminished, and the vivacity of the colour is augmented in the same proportion. It is difficult, then, to obtain yellow colours which are at the same time durable and brilliant. These two valuable qualities are to each other in an inverse ratio; but it is possible to unite the colouring principles in such a manner as to combine splendour with fixity. Green oak bark unites perfectly with yellow weed, and sumach with green citron. It is by this mixture that we may be able to combine with the oxyd of iron vegetable colours, the splendour of which is equal to their durability.
Our author concludes his observations with cautioning the dyer against substituting sumach and the bark of the alder tree or oak for gall when dyeing cotton red. "I can safely assert (says he), that it is impossible to employ these as substitutes, in whatever doses they may be used. The colour is always much paler, poorer, and less fixed. I know that the case is not the same in regard to dyeing wool and silk, in which it may be employed with success; and in giving an account of this difference, I think the cause of it may be found in the nature of the gall-nuts. 1. The acid which they exclusively contain, as Berthollet has proved, facilitates the decomposition of the soap with which the cottons have been impregnated, and the oil then remains fixed in their tissue, and in a greater quantity, as well as in a more intimate combination. 2. The gall-nuts, which owe their development to animal bodies, retain a character of animalisation, which they transmit to the vegetable stuff, and by these means augment its affinities with the colouring principle of the madder; for it is well known of what utility animal substances are to facilitate this combination. This animalisation becomes useless in operating upon woollen or silk."