Chapter XI: Dyeing of Wool and Fur; and Optical Properties (3)

_Boric Acid._--At ordinary temperatures and under ordinary conditions boric acid is a very weak acid, but like silicic and some other acids, its relative powers of affinity and combination become very much changed at high temperatures; thus, fused and strongly heated boric acid can decompose carbonates and even sulphates, and yet a current of so weak an acid as hydrogen sulphide, passed through a strong solution of borax, will decompose it and set free boric acid. Boric acid is obtained chiefly from Italy. In a tract of country called the Maremma of Tuscany, embracing an area of about forty square miles, are numerous chasms and crevices, from which hot vapour and heated gases and springs of water spurt. The steam issuing from these hot springs contains small quantities of boric acid, that acid being one of those solid substances distilling to some extent in a current of steam. The steam vapours thus bursting forth, owing to some kind of constant volcanic disturbance, are also more or less laden with sulphuretted hydrogen gas, communicating a very ill odour to the neighbourhood. These phenomena were at first looked upon by the people as the work of the devil, and priestly exorcisms were in considerable request in the hope of quelling them, very much as a great deal of the mere speech-making at the present time in England on foreign competition and its evils, and the dulness of trade, the artificial combinations to keep up prices, to reduce wages, general lamentation, etc., are essayed in the attempt to charm away bad trade. At length a kind of prophet arose of a very practical character in the form of the late Count Lardarel, who, mindful of the fact that the chemist Höffer, in the time of the Grand Duke Leopold I., had discovered boric acid in the volcanic steam jets, looked hopefully beyond the exorcisms of the priests and the superstitions of the people to a possible blessing contained in what appeared to be an unholy confusion of Nature. He constructed tanks of from 100 to 1000 ft. in diameter and 7 to 20 ft. in depth, of such a kind that the steam jets were surrounded by or contained in them, and thus the liquors formed by condensation became more and more concentrated. These tanks were arranged at different levels, so that the liquors could be run off from one to the other, and finally to settling cisterns. Subsequently the strong liquors were run to lead-lined, wooden vats, in which the boric acid was crystallised out. Had the industry depended on the use of fuel it could never have developed, but Count Lardarel ingeniously utilised the heat of the steam for all the purposes, and neither coal nor wood was required. Where would that Tuscan boric acid industry have been now had merely the lamentations of landowners, fears of the people, and exorcisms of the priests been continued? Instead of being the work of the arch-enemy of mankind, was not it rather an incitement to a somewhat high and difficult step in an upward direction towards the attainment, on a higher platform of knowledge and skill, of a blessing for the whole province of Tuscany? What was true in the history of that industry and its development is every whit as true of the much-lamented slackening of trade through foreign competition or other causes now in this country, and coming home to yourselves in the hat-manufacturing industry. The higher platform to which it was somewhat difficult to step up, but upon which the battle must be fought and the victory won, was one of a higher scientific and technological education and training. The chemist Höffer made the discovery of boric acid in the vapours, they would no doubt take note; but Höffer went no further; and it needed the man of both educated and practical mind like Count Lardarel to turn the discovery to account and extract the blessing. In like manner it was clear that in our educational schemes for the benefit of the people, there must not only be the scientific investigator of abstract truth, but also the scientific technologist to point the way to the practical realisation of tangible profit. Moreover, and a still more important truth, it is the scientific education of the proprietors and heads we want--educated capital rather than educated workmen.

_Borax._--A good deal of the Tuscan boric acid is used in France for the manufacture of borax, which is a sodium salt of boric acid. Borax is also manufactured from boronitrocalcite, a calcium salt of boric acid, which is found in Chili and other parts of South America. The crude boronitrocalcite or "tiza" is boiled with sodium carbonate solution, and, after settling, the borax is obtained by crystallisation. Borax itself is found in California and Nevada, U.S.A., and also in Peru, Ceylon, China, Persia, and Thibet. The commercial product is obtained from the native borax (known as "tincal") by dissolving in water and allowing the solution to crystallise. The Peruvian borax sometimes contains nitre. For testing the purity of refined borax the following simple tests will usually suffice. A solution of the borax is made containing 1 part of borax to 50 parts of water, and small portions of the solution are tested as follows: _Heavy metals_ (_lead_, _copper_, etc.).--On passing sulphuretted hydrogen into the solution, no coloration or precipitate should be produced. _Calcium Salts._--The solution should not give a precipitate with ammonium oxalate solution. _Carbonates._--The solution should not effervesce on addition of nitric or hydrochloric acid. _Chlorides._--No appreciable precipitate should be produced on addition of silver nitrate solution and nitric acid. _Sulphates._--No appreciable precipitate should be produced on adding hydrochloric acid and barium chloride. _Iron._--50 c.c. of the solution should not immediately be coloured blue by 0·5 c.c. of potassium ferrocyanide solution.

_Soap._--Soap is a salt in the chemical sense, and this leads to a wider definition of the term "salt" or "saline" compound. Fats and oils, from which soaps are manufactured, are a kind of _quasi_ salts, composed of a fatty acid and a chemical constant, if I may use the term, in the shape of base, namely, glycerin. When these fats and oils, often called glycerides, are heated with alkali, soda, a true salt of the fatty acid and soda is formed, and this is the soap, whilst the glycerin remains behind in the "spent soap lye." Now glycerin is soluble in water containing dissolved salt (brine), whilst soap is insoluble, though soluble in pure water. The mixture of soap and glycerin produced from the fat and soda is therefore treated with brine, a process called "cutting the soap." The soap separates out in the solid form as a curdy mass, which can be easily separated. Certain soaps are able to absorb a large quantity of water, and yet appear quite solid, and in purchasing large quantities of soap it is necessary, therefore, to determine the amount of water present. This can be easily done by weighing out ten or twenty grams of the soap, cut in small pieces, into a porcelain dish and heating over a gas flame, whilst keeping the soap continually stirred, until a glass held over the dish no longer becomes blurred by escaping steam. After cooling, the dry soap is weighed, and the loss of weight represents the amount of moisture. I have known cases where soap containing about 83 per cent. of water has been sold at the full market price. Some soaps also contain more alkali than is actually combined with the fatty acids of the soap, and that excess alkali is injurious in washing silks and scouring wool, and is also not good for the skin. The presence of this free or excess alkali can be at once detected by rubbing a little phenolphthalein solution on to the freshly-cut surface of a piece of soap; if free alkali be present, a red colour will be produced.

LECTURE VII

SHELLAC, WOOD SPIRIT, AND THE STIFFENING AND PROOFING PROCESS

_Shellac._--The resin tribe, of which shellac is a member, comprises vegetable products of a certain degree of similarity. They are mostly solid, glassy-looking substances insoluble in water, but soluble in alcohol and wood spirit. In many cases the alcoholic solutions show an acid reaction. The resins are partly soluble in alkalis, with formation of a kind of alkali salts which we may call resin-soaps.

Shellac is obtained from the resinous incrustation produced on the bark of the twigs and branches of various tropical trees by the puncture of the female "lac insect" (_Taccardia lacca_). The lac is removed from the twigs by "beating" in water; the woody matter floats to the surface, and the resin sinks to the bottom, and when removed forms what is known as "seed-lac." Formerly, the solution, which contains the colouring matter dissolved from the crude "stick-lac," was evaporated for recovery of the so-called "lac-dye," but the latter is no longer used technically. The seed-lac is bleached by boiling with sodium or potassium carbonate, alum, or borax, and then, if it is not pale enough, is further bleached by exposure to sunlight. It is now dried, melted, and mixed with a certain proportion of rosin or of orpiment (a sulphide of arsenic) according to the purpose for which it is desired. After further operations of melting and straining, the lac is melted and spread into thin sheets to form ordinary shellac, or is melted and dropped on to a smooth surface to form "button-lac." Ordinary shellac almost invariably contains some rosin, but good button-lac is free from this substance. The presence of 5 per cent. of rosin in shellac can be detected by dissolving in a little alcohol, pouring the solution into water, and drying the fine impalpable powder which separates. This powder is extracted with petroleum spirit, and the solution shaken with water containing a trace of copper acetate. If rosin be present, the petroleum spirit will be coloured emerald-green.

Borax, soda crystals, and ammonia are all used to dissolve shellac, and it may be asked: Which of these is least injurious to wool? and why? How is their action modified by the presence of dilute sulphuric acid in the wool? I would say that soda crystals and ammonia are alkalis, and if used strong, are sure to do a certain amount of injury to the fibre of wool, and more if used hot than cold. Of the two, the ammonia will have the least effect, especially if dilute, but borax is better than either. The influence of a little sulphuric acid in the wool would be in the direction of neutralising some of the ammonia or soda, and shellac, if dissolved in the alkalis, would be to some extent precipitated on the fibre, unless the alkali, soda or ammonia, were present in sufficient excess to neutralise that sulphuric acid and to leave a sufficient balance to keep the shellac in solution. Borax, which is a borate of soda, would be so acted on by the sulphuric acid that some boric acid would be set free, the sulphuric acid robbing some of that borax of its soda. This boric acid would not be nearly so injurious to wool as carbonate of soda or ammonia would.

The best solvent for shellac, however, in the preparation of the stiffening and proofing mixture for hats, is probably wood spirit or methylated spirit. A solution of shellac in wood spirit is indeed used for the spirit-proofing of silk hats, and to some extent of felt hats, and on the whole the best work, I believe, is done with it. Moreover, borax is not a cheap agent, and being non-volatile it is all left behind in the proofed material, whereas wood spirit or methylated spirit is a volatile liquid, _i.e._ a liquid easily driven off in vapour, and after application to the felt it may be almost all recovered again for re-use. In this way I conceive the use of wood spirit would be both more effective and also cheaper than that of borax, besides being most suitable in the case of any kind of dyes and colours to be subsequently applied to the hats.

_Wood Spirit._--Wood spirit, the pure form of which is methyl alcohol, is one of the products of the destructive distillation of wood. The wood is distilled in large iron retorts connected to apparatus for condensing the distillation products. The heating is conducted slowly at first, so that the maximum yield of the valuable products--wood acid (acetic acid) and wood spirit--which distil at a low temperature, is obtained. When the condensed products are allowed to settle, they separate into two distinct layers, the lower one consisting of a thick, very dark tar, whilst the upper one, much larger in quantity, is the crude wood acid (containing also the wood spirit), and is reddish-yellow or reddish-brown in colour. This crude wood acid is distilled, and the wood spirit which distils off first is collected separately from the acetic acid which afterwards comes over. The acid is used for the preparation of alumina and iron mordants (see next lecture), or is neutralised with lime, forming grey acetate of lime, from which, subsequently, pure acetic acid or acetone is prepared. The crude wood spirit is mixed with milk of lime, and after standing for several hours is distilled in a rectifying still. The distillate is diluted with water, run off from any oily impurities which are separated, and re-distilled once or twice after treatment with quicklime.

_Stiffening and Proofing Process._--Before proceeding to discuss the stiffening and proofing of hat forms or "bodies," it will be well to point out that it was in thoroughly grasping the importance of a rational and scientific method of carrying out this process that Continental hat manufacturers had been able to steal a march upon their English rivals in competition as to a special kind of hat which sold well on the Continent. There are, or ought to be, three aims in the process of proofing and stiffening, all the three being of equal importance. These are: first, to waterproof the hat-forms; second, to stiffen them at the same time and by the same process; and the third, the one the importance of which I think English hat manufacturers have frequently overlooked, at least in the past, is to so proof and stiffen the hat-forms as to leave them in a suitable condition for the subsequent dyeing process. In proofing the felt, the fibres become varnished over with a kind of glaze which is insoluble in water, and this varnish or proof is but imperfectly removed from the ends of the fibres on the upper surface of the felt. The consequence is a too slight penetration of the dyestuff into the inner pores of the fibres; indeed, in the logwood black dyeing of such proofed felt a great deal of the colour becomes precipitated on the outside of the fibres--a kind of process of "smudging-on" of a black pigment taking place. The subsequent "greening" of the black hats after a short period of wear is simply due to the ease with which such badly fixed dye rubs off, washes off, or wears off, the brownish or yellowish substratum which gradually comes to light, causing a greenish shade to at length appear. If we examine under the microscope a pure unproofed fur fibre, its characteristic structure is quite visible. Examination of an unproofed fibre dyed with logwood black shows again the same characteristic structure with the dye inside the fibre, colouring it a beautiful bluish-grey tint, the inner cellular markings being black. A proofed fur fibre, on the other hand, when examined under the microscope, is seen to be covered with a kind of translucent glaze, which completely envelops it, and prevents the beautiful markings showing the scaly structure of the fibre from being seen. Finally, if we examine microscopically a proofed fibre which has been dyed, or which we have attempted to dye, with logwood black, we find that the fibre presents an appearance similar to that of rope which has been drawn through some black pigment or black mud, and then dried. It is quite plain that no lustrous appearance or good "finish" can be expected from such material. Now how did the Continental hat manufacturers achieve their success, both as regards dyeing either with logwood black or with coal-tar colours, and also getting a high degree of "finish"? They attained their object by rubbing the proofing varnish on the inside of the hat bodies, in some cases first protecting the outside with a gum-varnish soluble in water but resisting the lac-varnish rubbed inside. Thus the proofing could never reach the outside. On throwing the hat bodies, thus proofed by a logical and scientific process, into the dye-bath, the gums on the outer surface are dissolved and removed, and the dye strikes into a pure, clean fibre, capable of a high degree of finish. This process, however, whilst very good for the softer hats used on the Continent, is not so satisfactory for the harder, stiffer headgear demanded in Great Britain. What was needed was a process which would allow of a through-and-through proofing and stiffening, and also of satisfactory dyeing of the stiffened and proofed felt. This was accomplished by a process patented in 1887 by Mr. F.W. Cheetham, and called the "veneering" process. The hat bodies, proofed as hard as usual, are thrown into a "bumping machine" containing hot water rendered faintly acid with sulphuric acid, and mixed with short-staple fur or wool, usually of a finer quality than that of which the hat bodies are composed. The hot acid water promotes in a high degree the felting powers of the short-staple wool or fur, and, to a lesser extent, the thinly proofed ends of the fibres projecting from the surfaces of the proofed hat-forms. Thus the short-staple wool or fur felts itself on to the fibres already forming part of the hat bodies, and a new layer of pure, unproofed wool or fur is gradually wrought on to the proofed surface. The hat-forms are then taken out and washed, and can be dyed with the greatest ease and with excellent results, as will be seen from the accompanying illustration (see Fig. 15). This successful invention emphasises the value of the microscope in the study of processes connected with textile fibres. I would strongly advise everyone interested in hat manufacturing or similar industries to make a collection of wool and fur fibres, and mount them on microscope slides so as to form a kind of index collection for reference.

1. Natural wool fibre unproofed.

2. Wool fibre showing proof on surface, filling up the cells
and rendering the same dye-proof.

3. Fur fibre from surface of veneered felt, showing dye
deposited in cells and on the surface, bright and lustrous.

4. Wool fibre as in No. 2, with dye deposited on surface of
proof.

5. Section of proofed and veneered body, showing unproofed
surface.

6. Section of proofed body without "veneer."]

LECTURE VIII

MORDANTS: THEIR NATURE AND USE

The name or word "mordant" indicates the empiricism, or our old friend "the rule of thumb," of the age in which it was first created and used. It serves as a landmark of that age, which, by the way, needed landmarks, for it was an age of something between scientific twilight and absolute darkness. _Morder_ in French, derived from the Latin _mordere_, means "to bite," and formerly the users of mordants in dyeing and printing believed their action to be merely a mechanical action, that is, that they exerted a biting or corroding influence, serving to open the pores of the fabrics, and thus to give more ready ingress to the colour or dye.

Most mordants are salts, or bodies resembling salts, and hence we must commence our study of mordants by a consideration of the nature of salts. I have already told you that acids are characterised by what we term an acid reaction upon certain vegetable and artificial colours, whilst bases or basic substances in solution, especially alkalis, restore those colours, or turn them to quite another shade; the acids do the one thing, and the alkalis and soluble bases do the opposite. The strongest and most soluble bases are the alkalis--soda, potash, and ammonia. You all know, probably, that a drop of vitriol allowed to fall on a black felt hat will stain that hat red if the hat has been dyed with logwood black; and if you want to restore the black, you can do this by touching the stain with a drop of strong ammonia. But the use of a black felt hat as a means of detecting acidity or alkalinity would not commend itself to an economic mind, and we find a very excellent reagent for the purpose in extract of litmus or litmus tincture, as well as in blotting paper stained therewith. The litmus is turned bright red by acids and blue by alkalis. If the acid is exactly neutralised by, that is combined with, the alkaline base to form fully neutralised salts, the litmus paper takes a purple tint. Coloured reagents such as litmus are termed indicators. A substance called phenolphthalein, a coal-tar product, is a very delicate indicator; it is more sensitive to acids than litmus is. Now there are some salts which contain a preponderance of acid in their composition, _i.e._ in which the acid has not been fully neutralised by the base; such salts are termed acid salts. Bicarbonate of soda is one of these acid salts, but so feeble is carbonic acid in its acid properties and practical evidences, that we shall see both monocarbonate or "neutral" carbonate of soda and bicarbonate or "acid" carbonate of soda show evidences of, or, as chemists say, react with alkalinity towards litmus. However, phenolphthalein, though reacting alkaline with monocarbonate of soda, indicates the acidity of the bicarbonate of soda, a thing which, as I have just said, litmus will not do. We will take two jars containing solution of monocarbonate of soda, and in the first we will put some phenolphthalein solution, and in the second, some litmus tincture. The solution in the first jar turns rose coloured, and in the second, blue, indicating in each case that the solution is alkaline. If now, however, carbonic acid be blown into the two solutions, that in the first jar, containing the phenolphthalein, becomes colourless as soon as the monocarbonate of soda is converted into bicarbonate, and this disappearance of the rose colour indicates acidity; the blue solution in the jar containing litmus, on the other hand, is not altered by blowing in carbonic acid. Furthermore, if to the colourless solution containing phenolphthalein, and which is acid towards that reagent, a little reddened litmus is added, this is still turned blue, and so still indicates the presence of alkali. We have, therefore, in bicarbonate of soda a salt which behaves as an acid to phenolphthalein and as an alkali to litmus. Another extremely sensitive indicator is the coal-tar dyestuff known as "Congo red"; the colour changes produced by it are exactly the inverse of those produced in the case of litmus, that is, it gives a blue colour with acids and a red colour with alkalis.

We have now learned that acids are as the antipodes to alkalis or bases, and that the two may combine to form products which may be neutral or may have a preponderance either of acidity or of basicity--in short, they may yield neutral, acid, or basic salts. I must try to give you a yet clearer idea of these three classes of salts. Now acids in general have, as we have seen, what we may call a "chemical appetite," and each acid in particular has a "specific chemical appetite" for bases, that is, each acid is capable of combining with a definite quantity of an individual base. The terms "chemical appetite" and "specific chemical appetite" are names I have coined for your present benefit, but for which chemists would use the words "affinity" and "valency" respectively. Now some acids have a moderate specific appetite, whilst others possess a large one, and the same may be said of bases, and thus as an example we may have mono-, di-, and tri-acid salts, or mono-, di-, and tri-basic salts. In a tri-acid salt a certain voracity of the base is indicated, and in a tri-basic salt, of the acid. Again, with a base capable of absorbing and combining with its compound atom or molecule several compound atoms or molecules of an acid, we have the possibility of partial saturation, and, perhaps, of several degrees of it, and also of full saturation, which means combination to the full extent of the powers of the base in question. Also, with an acid capable of, or possessing a similar large absorptive faculty for bases, we have possibilities of the formation of salts of various degrees of basicity, according to the smaller or larger degree of satisfaction given to the molecule of such acid by the addition of a base. We will now take as a simple case that of hydrochloric acid (spirits of salt), which is a monobasic acid, that is, its molecule is capable of combining with only one molecule of a monoacid base. Hydrochloric acid may be written, as its name would indicate, HCl, and an addition even of excess of such a base as caustic soda (written NaOH) would only yield what is known as common salt or chloride of sodium (NaCl), in which the metal sodium (Na) has replaced the hydrogen (H) of the hydrochloric acid. Now chloride of sodium when dissolved in water will turn litmus neither blue nor red; it is therefore neutral. Such simple, neutral, monobasic salts are mostly very stable. By "stable" we mean they possess considerable resistance to agencies, that, in the case of other salts, effect decompositions of those salts. Such other salts which are decomposed more or less readily are termed "unstable," but the terms are of course only comparative.

Now let us consider a di- or bi-basic acid. Such an one is vitriol or sulphuric acid (H_{2}SO_{4}). The hydrogen atoms are in this case an index of the basicity of the acid, and accordingly the fully saturated sodium salt is Na_{2}SO_{4} or neutral, or better normal, sulphate of soda. In like manner the fully saturated salt of the dibasic acid, carbonic acid (H_{2}CO_{3}), is Na_{2}CO_{3}, ordinary or normal carbonate of soda. But we must observe that with these dibasic acids it is possible, by adding insufficient alkali to completely saturate them, to obtain salts in which only one hydrogen atom of the acid is replaced by the metal of the base. Thus sulphuric and carbonic acids yield NaHSO_{4}, acid sulphate or bisulphate of soda, and NaHCO_{3}, bicarbonate of soda, respectively. An example of a tribasic acid is phosphoric acid, H_{3}PO_{4}, and here we may have three different classes of salts of three various degrees of basicity or base-saturation. We may have the first step of basicity due to combination with soda, NaH_{2}PO_{4}, or monosodium phosphate, the second step, Na_{3}HPO_{4}, or disodium phosphate, and the third, and final step, Na_{3}PO_{4}, or trisodium phosphate. Now let us turn to the varying degrees of acidity, or rather the proportions of acid radicals in salts, due to the varying appetites or combining powers of bases. Sodium only forms simple monoacid salts, as sodium chloride (NaCl), sodium sulphate (Na_{2}SO_{4}); calcium forms diacid salts, _e.g._ calcium chloride (CaCl_{2}); and aluminium and iron, triacid salts, for example, aluminium sulphate [Al_{2}(SO_{4})_{3}] and iron (ferric) sulphate [Fe_{2}(SO_{4})_{3}]. Now in these triacid salts we can remove some of the acid groups and substitute the elements of water, OH, or hydroxyl, as it is called, for them. Such salts, then, only partly saturated with acid, are termed basic salts. Thus we have Al_{2}(OH)_{2}(SO_{4})_{2}, Al_{2}(OH)_{4}SO_{4}, as well as Al_{2}(SO_{4})_{3}, and we can get these basic salts by treating the normal sulphate [Al_{2}(SO_{4})_{3}] with sufficient caustic soda to remove the necessary quantities of sulphuric acid. Now it is a curious thing that of these aluminium sulphates the fully saturated one, Al_{2}(SO_{4})_{3}, is the most stable, for even on long boiling of its solution in water it suffers no change, but the more basic is the sulphate the less stable it becomes, and so the more easily it decomposes on heating or boiling its solution, giving a deposit or precipitate of a still more basic sulphate, or of hydrated alumina itself, Al_{2}(OH)_{6}, until we arrive at the salt Al_{2}(SO_{4})_{2}(OH)_{2}, which is quite unstable on boiling; Al_{2}(SO_{4})(OH)_{4} would be more unstable still. This behaviour may be easily shown experimentally. We will dissolve some "cake alum" or normal sulphate of alumina, Al_{2}(SO_{4})_{3}, in water, and boil some of the solution. No deposit or precipitate is produced; the salt is stable. To another portion of the solution we will add some caustic soda, NaOH, in order to rob the normal sulphate of alumina of some of its sulphuric acid. This makes the sulphate of alumina basic, and the more basic, the more caustic soda is added, the sodium (Na) of the caustic soda combining with the SO_{4} of the sulphate of alumina to form sulphate of soda (Na_{2}SO_{4}), whilst the hydroxyl (OH) of the caustic soda takes the position previously occupied by the SO_{4}. But this increase of basicity also means decrease of stability, for on boiling the solution, which now contains a basic sulphate of alumina, a precipitate is formed, a result which also follows if more caustic soda is added, production of still more basic salts or of hydrated alumina, Al_{2}(OH)_{6}, taking place in either case.

_Mordanting or Fixing Acid (Phenolic) Colours._--But what has all this to do with mordanting? is possibly now the inquiry. So much as this, that only such unstable salts as I have just described, which decompose and yield precipitates by the action on them of alkalis, heat, the textile fibres themselves, or other agencies, are suitable to act as true mordants. Hence, generally, the sources or root substances of the best and most efficient mordants are the metals of high specific appetite or valency. I think we have now got a clue to the principle of mordants and also to the importance of a sound chemical knowledge in dealing most effectively with them, and I may tell you that the man who did most to elucidate the theory of mordanting is not a practical man in the general sense of the term, but a man of the highest scientific attainments and standing, namely, Professor Liechti, who, with his colleague Professor Suida, did probably more than any other man to clear up much that heretofore was cloudy in this region. We have seen that with aluminium sulphate, basic salts are precipitated, _i.e._ salts with such a predominance of appetite for acids, or such _quasi_-acids as phenolic substances, that if such bodies were present they would combine with the basic parts of those precipitated salts as soon as the latter were formed, and all would be precipitated together as one complex compound. Just such peculiar _quasi_-acid, or phenolic substances are Alizarin, and most of the natural adjective dyestuffs, the colouring principles of logwood, cochineal, Persian berries, etc. Hence these substances will be combined and carried down with such precipitated basic salts. The complex compounds thus produced are coloured substances known as lakes. For example, if I take a solution containing basic sulphate of alumina, prepared as I have already described, and add to some Alizarin, and then heat the mixture, I shall get a red lake of Alizarin and alumina precipitated. If I had taken sulphate of iron instead of sulphate of alumina, and proceeded in a similar manner, and added Alizarin, I should have obtained a dark purple lake. Now if you imagine these reactions going on in a single fibre of a textile material, you have grasped the theory and purpose of mordanting. The textile fabric is drawn through the alumina solution to fill the pores and tubes of the fabric; it is then passed through a weak alkaline bath to basify or render basic the aluminium salt in the pores; and then it is finally carried into the dye-bath and heated there, in order to precipitate the colour lake in the fibre. The method usually employed to mordant woollen fabrics consists in boiling them with weak solutions of the metallic salts used as mordants, often with the addition of acid salts, cream of tartar, and the like. A partial decomposition of the metallic salts ensues, and it is induced by several conditions: (1) The dilution of the liquid; (2) the heating of the solution; (3) the presence of the fibre, which itself tends to cause the breaking up of the metallic salts into less soluble basic ones. Thus it is not really necessary to use basic aluminium sulphate for mordanting wool, since the latter itself decomposes the normal or neutral sulphate of alumina on heating, an insoluble basic sulphate being precipitated in the fibres of the wool. (4) The presence of other added substances, as cream of tartar, etc. The best alumina mordant is probably the acetate of alumina ("red liquor"), and the best iron mordant, probably also the acetate ("iron liquor") (see preceding lecture), because the acetic acid is so harmless to the fibre, and is easily driven off on steaming, etc. A further reason is that from the solution of acetate of iron or alumina, basic acetates are very easily precipitated on heating, and are thus readily deposited in the fibre.

_Mordanting and Fixing Basic Colours._--Now let us ask ourselves a very important question. Suppose we have a colour or dyestuff, such as Magenta, which is of a basic character, and not of an acid or phenolic character like the colours Alizarin, Hæmatein (logwood), or carminic acid (cochineal), and we wish to fix this basic dyestuff on the tissue. Can we then use "red liquor" (acetate of alumina), acetate of iron, copperas, etc.? The answer is, No; for such a process would be like trying to combine base with base, instead of base with acid, in order to form a salt. Combination, and so precipitation, would not take place; no lake would be formed. We must seek for an acid or acid body to use as mordant for our basic colour, and an acid or acid body that will form an insoluble precipitate or colour-lake with the dyestuff. An acid much used, and very valuable for this purpose, is tannic acid. The tannate of rosaniline (colour principle of Magenta) is a tolerably insoluble lake, which can be precipitated by Magenta from a solution of tannate of soda, the Magenta being capable of displacing the soda. But tannic acid, alone, does not form very fast lakes with Magenta and the other basic dyestuffs, and so a means of rendering these lakes more insoluble is needed. It is found that tannic acid and tartar emetic (a tartrate of antimony and potash) yield a very insoluble compound, a tannate of antimony. Perchloride of tin, in a similar manner, yields insoluble tannate of tin with tannic acid. These insoluble compounds, however, have sufficient acid-affinity left in the combined tannic acid to unite also with the basic aniline colours, forming very fast or insoluble colour lakes. This principle is extensively used in practice to fix basic aniline colours, especially on cotton. We should first soak the piece of cotton in a solution of tannic acid, and then pass it into a solution, say, of tartar emetic, when the tannic acid will be firmly fixed, as tannate of antimony, on the cotton. We then dip the mordanted piece of cotton into the colour bath, containing, for instance, Magenta, and it is dyed a fine red, composed of a tannate of antimony and Magenta. You now see, no doubt, the necessity of sharply discriminating between two classes of colouring matters, which we may term _colour acids_ and _colour bases_ respectively. There are but few acids that act like tannic acid in fixing basic aniline dyestuffs, but oleic acid and other fatty acids are of the number. A curious question might now be asked, namely: "Could the acid colour Alizarin, if fixed on cotton cloth, combine with a basic aniline colour, _e.g._ Aniline Violet, and act as a mordant for it, thus fixing it?" The answer is, "Certainly"; and thus an Alizarin Purple would be produced, whilst if Magenta were used in place of Aniline Violet, an Alizarin Red of a crimson tone would result.

_Chrome Mordanting of Wool and Fur._--In studying this subject I would recommend a careful perusal of the chapter on "Mordants" in J.J. Hummel's book, entitled _The Dyeing of Textile Fabrics_, and pages 337 to 340 of Bowman's work on _The Wool-Fibre_.

In the treatment of wool or fur with bichrome (potassium bichromate) we start with an acid salt, a bichromate (K_{2}Cr_{2}O_{7}) and a strong oxidising agent, and we finish with a basic substance, namely, oxide of chromium, in the fibres of the wool or fur. If we desire to utilise the whole of the chromic acid in our mordanting liquor, we must add to it some sulphuric acid to set free the chromic acid from the potassium with which it is combined. Bichromate of potash with sulphuric acid gives sulphate of potash and chromic acid. The question of the proper exhaustion of bichromate baths is an important economic one. Now we must remember that this chromic acid (CrO_{3}) oxidises our wool or fur, and must oxidise it before it can of itself act as a mordant by being reduced in the process to hydrated chromic oxide, Cr_{2}O_{3} + 3 H_{2}O. [2 CrO_{3} (chromic acid) = Cr_{2}O_{3} (chromic oxide) + O_{3} (oxygen).] It is this hydrated chromic oxide in the fibre that yields with the Hæmatein of the logwood your logwood black dye. Mr. Jarmain finds that it is not safe to use more than 3 per cent. (of the weight of the wool) of bichromate; if 4 per cent. be used, the colour becomes impaired, whilst if 12 per cent. be employed, the wool cannot be dyed at all with logwood, the phenomenon known as "over-chroming" being the result of such excessive treatment. I think there is no doubt, as Professor Hummel says, that the colouring matter is oxidised and destroyed in such over-chroming, but I also think that there can be no doubt that the wool itself is also greatly injured and incapacitated for taking up colour. Now the use of certain coal-tar black dyes in place of logwood obviates this use of bichrome, and thus the heavy stress on the fibre in mordanting with it. It also effects economy in avoiding the use of bichrome, as well as of copper salts; but even thus, of course, other problems have to be solved before it can be finally decided which is best.

LECTURE IX

DYESTUFFS AND COLOURS

_Classification._--In classifying the different dyestuffs and colouring matters it is, of course, necessary to consider first the properties of those colouring matters generally, and secondly the particular reason for making such classification. The scientific chemist, for example, would classify them according to theoretical considerations, as members of certain typical groups; the representative of medical science or hygiene would naturally classify them as poisonous and non-poisonous bodies; whilst the dyer will as naturally seek to arrange them according to their behaviour when applied to textile fabrics. But this behaviour on applying to textile fibres, if varied in character according to the chemical nature of the colouring matter, as well as the chemical and physical nature of the fabric--and it is so varied--will make such classification, if it is to be thorough-going, not a very simple matter. I may tell you that it is not a simple matter, and, moreover, the best classification and arrangement is that one which depends both on the action of the dyes on the fibres, and also on the intrinsic chemical character of the dyestuffs themselves. Since the higher branches of organic chemistry are involved in the consideration of the structure and dispositions, and consequently more or less of the properties of these dyes, you will readily comprehend that the thorough appreciation and use of that highest and best method of classification, particularly in the case of the coal-tar dyes, will be, more or less, a sealed book except to the student of organic chemistry. But it may be asked, "How does that highest and best method of classifying the dyestuffs affect the users, the dyers, in their processes?" In reply, I would say, "I believe that the dyer who so understands the chemical principles involved in the processes he carries out, and in the best methods of classifying the dyes as chemical substances, so as to be able to act independently of the prescriptions and recipes given him by the dye manufacturers, and so be master of his own position, will, _ceteris paribus_, be the most economical and successful dyer." Many manufacturers of dyestuffs have said the very same thing to me, but, independently of this, I know it, and can prove it with the greatest ease. Let me now, by means of an experiment or two, prove to you that at least some classification is necessary to begin with. So different and varied are the substances used as colouring matters by the dyer, both as regards their chemical and physical properties, that they even act differently towards the same fibre. I will take four pieces of cotton fabric; three of them are simple white cotton, whilst the fourth cotton piece has had certain metallic salts mixed with thickening substances like gum, printed on it in the form of a pattern, which at present cannot readily be discerned. We will now observe and note the different action on these pieces of cotton--(i.) of a Turmeric bath, (ii.) a Magenta bath, and (iii.) a madder or Alizarin bath. The Turmeric dyes the cotton a fast yellow, the Magenta only stains the cotton crimson, and on washing with water alone, almost every trace of colour is removed again; the madder, however, stains the cotton with no presentable shade of colour at all, produces a brownish-yellow stain, removed at once by a wash in water. But let us take the printed piece of cotton and dye that in the Alizarin bath, and then we shall discover the conditions for producing colours with such a dyestuff as madder or Alizarin. Different coloured stripes are produced, and the colours are conditioned by the kind of metallic salts used. Evidently the way in which, the turmeric dyes the cotton is different from that in which the madder dyes it. The first is a yellow dyestuff, but it would be hard to assign any one shade or tint to Alizarin as a dyestuff. In fact Alizarin (the principle of madder) is of itself not a dye, but it forms with each of several metals a differently coloured compound; and thus the metallic salt in the fabric is actually converted into a coloured compound, and the fabric is dyed or printed. The case is just the same with logwood black dyeing: without the presence of iron ("copperas," etc.), sulphate of copper ("bluestone"), or bichrome, you would get no black at all. We will now try similar experiments with woollen fabrics, taking three simple pieces of flannel, and also two pieces, the one having been first treated with a hot solution of alum and cream of tartar, and the other with copperas or sulphate of iron solution, and then washed. Turmeric dyes the first yellow, like it did the cotton. Magenta, however, permanently dyes the woollen as it did not the cotton. Alizarin only stains the untreated woollen, whilst the piece treated with alumina is dyed red, and that with iron, purple. If, however, the pieces treated with iron and alumina had been dyed in the Magenta solution, only one colour would have been the result, and that a Magenta-red in each case. Here we have, as proved by our experiments, two distinct classes of colouring matters. The one class comprises those which are of themselves the actual colour. The colour is fully developed in them, and to dye a fabric they only require fixing in their unchanged state upon that fabric. Such dyes are termed _monogenetic_, because they can only generate or yield different shades of but one colour. Indigo is such a dye, and so are Magenta, Aniline Black, Aniline Violet, picric acid, Ultramarine Blue, and so on. Ultramarine is not, it is true, confined to blue; you can get Ultramarine Green, and even rose-coloured Ultramarine; but still, in the hands of the dyer, each shade remains as it came from the colour-maker, and so Ultramarine is a monogenetic colour. Monogenetic means capable of generating one. Turning to the other class, which comprises, as we have shown, Alizarin, and, besides, the colouring principle of logwood (Hæmatein), Gallein, and Cochineal, etc., we have bodies usually possessed of some colour, it is true, but such colour is of no consequence, and, indeed, is of no use to dyers. These bodies require a special treatment to bring out or develop the colours, for there may be several that each is capable of yielding. We may consider them as colour-giving principles, and so we term them _polygenetic_ colours. Polygenetic means capable of generating several or many. In the various colours and dyes we have all phases, and the monogenetic shades almost imperceptibly into the polygenetic. The mode of application of the two classes of colours is, of course, in each case quite essentially different, for in the case of the monogenetic class the idea is mainly either to dye at once and directly upon, the unprepared fibre, or having subjected the fabric to a previous preparation with a metallic or other solution, to fix directly the one colour on that fabric, on which, without such preparation, it would be loose. In the case of the polygenetic class, the idea is necessarily twofold. The dyeing materials are not colours, only colour generators. Hence in all cases the fabric must be prepared with the twofold purpose--first, of using a metallic or other agent, capable of yielding, with the dye material, the desired colour; and secondly, of yielding it on the fibre in an insoluble and permanent form. Now, though I have gone so far into this mode of classification, because it does afford some information and light, yet I can go no farther without getting into a territory that presupposes a knowledge and acquaintance with the chemical structure of the colouring matters as organic substances, which would be, at present, beyond us. I shall now turn to another mode of classification, which, if not so far-reaching as the other, is at least an exceedingly useful one. The two methods may be combined to a considerable extent. By the latter plan the colours may be conveniently divided into three groups: I., substantive colours; II., adjective colours; III., mineral and pigment colours.

_Substantive Dyestuffs._--The substantive colours fix themselves readily and directly on animal fibres and substances, but only a few amongst them will dye vegetable fibres like cotton and linen directly. Almost all substantive colours may, however, be fixed on cotton and linen by first preparing or mordanting those vegetable fibres. Silk, wool, fur, etc., act like fibre and mordant together, for they absorb and fix the substantive colours firmly. In our experiments we saw that turmeric is one of the few substantive colours fixing itself on both cotton and wool, without any aid from a mordant or fixing agent. Magenta was also a substantive colour, but Alizarin was certainly not one of this class.

_Adjective Dyestuffs._--Some of these substances are definitely coloured bodies, but in some of them the colour is of no consequence or value, and is quite different and distinct from the colour eventually formed on the fibre, which colour only appears in conjunction with a special mordant; but, again, some of them are not coloured, and would not colour the fibre directly at all, only in conjunction with some mordant. All the polygenetic colours are, of course, comprised in this class, for example Alizarin and logwood (Hæmatein), whilst such monogenetic colours as annatto and turmeric are substantive, for they will fix themselves without a mordant on cotton and wool. The adjective colours can be conveniently subdivided into--(_a_) those existing in nature, as logwood (Hæmatein) and Cochineal; (_b_) those artificially formed from coal-tar products, as Alizarin (madder), Gallein, etc.

_Mineral and Pigment Dyestuffs._--These colours are insoluble in water and alcohol. They are either fixed on the fibre by mechanical means or by precipitation. For example, you use blacklead or plumbago to colour or darken your hats, and you work on this pigment colour by mechanical means. I will show you by experiment how to fix a coloured insoluble pigment in the fibre. I take a solution of acetate of lead (sugar of lead), and to it I add some solution of bichrome (potassium bichromate). Acetate of lead (soluble in water) with bichromate of potash (also soluble in water) yields, on mixing the two, acetate of potash (soluble in water), and chromate of lead, or chrome yellow (insoluble in water), and which is consequently precipitated or deposited. Now suppose I boil some of that chrome-yellow precipitate with lime-water, I convert that chrome yellow into chrome orange. This, you see, takes place without any reference to textile fibres. I will now work a piece of cotton in a lead solution, so that the little tubes of the cotton fibre shall be filled with it just as the larger glass tube or vessel was filled in the first experiment. I next squeeze and wash the piece, so as to remove extraneous solution of lead, just as if I had filled my glass tube by roughly dipping it bodily into the lead solution, and then washed and cleansed the outside of that tube. Then I place the fabric in a warm solution of bichromate of potash (bichrome), when it becomes dyed a chrome yellow, for just as chromate of lead is precipitated in the glass tube, so it is now precipitated in the little tubes of the cotton fibre (see Lecture I.). Let us see if we can now change our chrome yellow to chrome orange, just as we did in the glass vessel by boiling in lime-water. I place the yellow fabric in boiling lime-water, when it is coloured or dyed orange. In each little tubular cotton fibre the same change goes on as went on in the glass vessel, and as the tube or glass vessel looks orange, so does the fabric, because the cotton fibres or tubes are filled with the orange chromium compound. You see this is quite a different process of pigment colouring from that of rubbing or working a colour mechanically on to the fibre.

Let us now turn to the substantive colours (Group I.), and see if we can further sub-divide this large group for the sake of convenience. We can divide the group into two--(_a_) such colours as exist ready formed in nature, and chiefly occur in plants, of which the following are the most important: indigo, archil or orchil, safflower, turmeric, and annatto; (_b_) the very large sub-group of the artificial or coal-tar colours. We will briefly consider now the dyestuffs mentioned in Group (_a_).

_Natural Substantive Colours._--Indigo, one of the most valuable dyes, is the product of a large number of plants, the most important being different species of _indigofera_, which belong to the pea family. None of the plants (of which _indigofera tinctoria_ is the chief) contain the colouring matter in the free state, ready-made, so to say, but only as a peculiar colourless compound called _indican_, first discovered by Edward Schunck. When this body is treated with dilute mineral acids it splits up into Indigo Blue and a kind of sugar. But so easily is this change brought about that if the leaf of the plant be only bruised, the decomposition ensues, and a blue mark is produced through separation of the Indigo Blue. The possibility of dyeing with Indigo so readily and easily is due to the fact that Indigo Blue absorbs hydrogen from bodies that will yield it, and becomes, as we say, reduced to a body without colour, called Indigo White, a body richer in hydrogen than Indigo Blue, and a body that is soluble. If this white body (Indigo White) be exposed to the air, the oxygen of the air undoes what the hydrogen did, and oxidises that Indigo White to insoluble Indigo Blue. Textile fabrics dipped in such reduced indigo solutions, and afterwards exposed to the air, become blue through deposit in the fibres of the insoluble Indigo Blue, and are so dyed. This is called the indigo-vat method. We can reduce this indigo so as to prepare the indigo-vat by simply mixing Indigo Blue, copperas (ferrous sulphate) solution, and milk of lime in a closely-stoppered bottle with water, and letting the mixture stand. The clear liquor only is used. A piece of cotton dipped in it, and exposed to the air, quickly turns blue by absorbing oxygen, and is thus dyed. The best proportions for the indigo-vat are, for cloth dyeing, 4000 parts of water, 40 of indigo, 60 to 80 of copperas crystals, and 50 to 100 of dry slaked lime. The usual plan is to put in the water first, then add the indigo and copperas, which should be dissolved first, and finally to add the milk of lime, stirring all the time. Artificial indigo has been made from coal-tar products. The raw material is a coal-tar naphtha called toluene or toluol, which is also the raw material for saccharin, a sweetening agent made from coal-tar. This artificial indigo is proving a formidable rival to the natural product.