Chapter 4

The first point in discussing this question of fast and fugitive dyes is to define the meaning of these terms "fast" and "fugitive." Unfortunately, as frequently employed, they have no very definite signification. The great variety of textile fabrics to which coloring matters are applied, the different stages of manufacture at which the coloring matter is applied, and the many uses to which the fabrics are ultimately put, all these are elements which cause dyed colors to be exposed to the most varied influences.

The term a "fast color," then, may convey a different meaning to different individuals. To one it implies that the color will not fade when exposed to light and atmospheric conditions; to another that it is not impoverished by washing with soap and water; to a third it may indicate that the color will withstand the action of certain manufacturing operations, such as scouring, milling, stoving, etc.; while a fourth person might be so exacting as to demand that a fast color should resist all the varied influences I have named.

It is well to state at once that no dyed color is absolutely fast, even to a single influence, and it certainly cannot pass unscathed through all the operations to which it may be necessary to submit individual colors applied to this or that material. Many colors are fast to washing or milling, and yet very fugitive to light; others are fast to light, but fugitive toward milling; while others again are fast to both influences. In short, each color has its own special, characteristic properties, so that colors may be classified with respect to each particular influence, and may occupy a very different rank in the different arrangements.

It is, however, by no means necessary to demand absolute fastness from any color. A color may "bleed" in milling, and therefore be very unsuitable for tweeds, and yet be most excellent for curtains and hangings, because of its fastness to light. So, too, a dye capable of yielding rich or delicate tints, but only moderately fast to light, may still be perfectly well adapted for the silks and satins of the ball room, or even the rapidly changing fashion, although it would be quite inadmissible for the pennon at the masthead.

The colors of carpets, curtains, and tapestry should certainly be fast to light, but no one expects them to undergo the fatigue of the weekly washtub; and just as little as we look for the exposure of flannels and hosiery, day by day and week by week, to the glare of sunlight, much as we desire that the colors shall not run in washing.

For all practical purposes, then, it seems reasonable to define a "fast color" as one which will not be materially affected by those influences to which, in the natural course of things, it will be submitted. Hence, in speaking of a fast color, it becomes necessary to refer specially to the particular influences which it resists before the term acquires a definite meaning. To be precise, one should say that a color is "fast to light," or "fast to washing," or "fast to light and washing," and so on. Further, it is necessary, as we shall see afterward, to give always the name of the fiber to which the color is applied.

All that I have said with respect to the term "fast" may be applied with equal propriety to the term "fugitive." This, too, has no very definite meaning until a qualifying statement, such as I have referred to, gives it precision.

The most important question to be considered is

THE ACTION OF LIGHT ON DYED COLORS.

That light can effect radical changes in many substances was known to the ancients. Its destructive action on artists' pigments, e.g., the blackening of vermilion, was recorded 2,000 years ago by Vitruvius. Since that time it has been well established, by numerous observations and experiments, that light possesses, in a high degree, the power of exerting chemical action, i.e., causing the combination or decomposition of a large number of substances. The union of chlorine with hydrogen gas, the blackening of silver salts, the reduction of bichromate of potash and of certain ferric salts in contact with organic substances, are all familiar instances of the action of light. In illustration of this, I show here some calico prints produced by first preparing the calico with a solution of potassium bichromate, then exposing the dried calico under a photographic negative, and, after washing, dyeing with alizarin or some similar coloring matter. During the exposure under the negative, the light has reduced and fixed the chromium salt upon certain parts of the fiber as insoluble chromate of chromium (Cr_{2}O_{3}CrO_{3}) in the more protected portions, the bichromate remains unchanged, and is subsequently removed by washing. During the dyeing process, the coloring matter combines with the chromium fixed on the fiber, and thus develops the colored photograph.

The prints in Prussian blue are produced in a similar manner, the sensitive salt with which the calico is prepared being ammonium ferricitrate, and the developer potassium ferricyanide.

Investigation has shown that the most chemically active rays are those situated at the blue end of the solar spectrum; and although all the rays absorbed by a sensitive colored body affect its change, it is doubtless the blue rays which are the chief cause of the fading of colors. Experiments are on record, indeed, which prove this.

Depierre and Clouet (1878-82) exposed a series of colors, printed and dyed on calico, to light which had passed through glasses stained red, orange, yellow, green, blue, and violet, corresponding to definite parts of the spectrum. They found that the blue light possessed the greatest fading power, red light the least.

More recently (1886-88) Abney and Russell exposed water colors under red, green, and blue glass, and came to the same conclusion.

But the chemical energy of the sun's rays is not the sole cause of the fading of colors. There are certain contributory causes as important as the light itself.

About fifty years ago, Chevreul showed what these accessory causes are, by exposing to light a number of dyed colors under varied conditions, e.g., in a vacuum, in dry and moist hydrogen, dry and moist air, water vapor, and the ordinary atmosphere. He found that such fugitive colors as orchil, safflower, and indigo-carmine fade very rapidly in moist air, less rapidly in dry air, and that they experience little or no change in hydrogen or in a vacuum. The general conclusion arrived at was, that light, when acting alone, i.e., without the aid of air and moisture, exercises a very feeble influence. Further, it was determined that the air and moisture, without aid of light, have also comparatively little effect on dyed colors. Abney and Russell, in their experiments with water colors, obtained similar results.

These conclusions are exactly in accordance with our common knowledge of the old fashioned method of bleaching cotton and linen, in which the wetted fabric is exposed to light on the grass, and frequently sprinkled with water. If the material becomes dry through the absence of dew or rain, or the want of sprinkling, little or no bleaching takes place.

The one color which Chevreul found to behave abnormally was Prussian blue. This faded even in a vacuum; but, strange to say, on keeping the faded color in the dark, and exposed to air, the color was restored. It was shown that, during the exposure to light, the color lost cyanogen, or hydrocyanic acid, while in the dark and exposed to the air, oxygen was absorbed. Chevreul concluded, therefore, that the fading of Prussian blue was due to a process of reduction.

The prevailing opinion, however, is that the fading of colors is a process of oxidation, caused by the ozone, or hydrogen peroxide, which is probably formed in small quantity during the evaporation of the moisture present, and both these substances are powerful bleaching agents.

It would be extremely convenient to have some rapid method of testing colors for fastness to light, and I believe it is the custom with some to apply certain chemical tests with this object in view. The results of my own experiments in this direction lead me to the conclusion that at present we have no sufficient substitute for sunlight for this purpose, since I have not found any oxidizing or reducing substance which affects dyed colors in all respects like the natural color-fading agencies; further, I am inclined to the opinion that the action of light varies somewhat with the different coloring matters, according to their chemical constitution and the fiber upon which they are applied.

With respect to this last point, Chevreul actually found that colors are faster to light on some fibers than on others, and this fact, which is generally known to practical men, is abundantly shown in the diagrams on the wall. As a rule we may say that colors are most fugitive on cotton and most permanent on wool, those on silk holding an intermediate position. Still there are many exceptions to this order, especially as between silk and wool.

Since the time of Chevreul, the action of light on dyed colors has not been seriously and exhaustively studied. From time to time, series of patterns dyed with our modern colors have been exposed to light, e.g., by Depierre and Clouet, Joffre, Muller, Kallab, Schmidt, and others; but the published results must at best be considered as more or less fragmentary. Under the auspices of the British Association, and a committee appointed at its last meeting in Leeds, I hope to have the pleasure during the next few years of studying this interesting subject.

To-night I propose to give you some of the prominent results already obtained in past years, in the dyeing department of the Yorkshire College, where it has been our custom to expose to light and other influences the patterns dyed by our students. Further, I wish to give you an ocular demonstration of the action of light or dyed colors, by means of these silk, wool, and cotton patterns, portions of which have been exposed for 34 days and nights on the sea coast near Bombay, during the month of February of this year.

I may remark that this test has been a very trying one, for I estimate that it is equal to more than a year's exposure in this country. During the whole period there was cloudless sunshine, without any rain, and each evening heavy dew. I have pleasure in acknowledging the services of Mr. W. Reid, a former student, who superintended the exposure of the patterns, and from time to time took notes of the rate at which individual patterns faded.

These diagrams contain, perhaps, the most complete series of both old and new dyes, on the three fibers, which have been simultaneously exposed to sunlight, and they form an instructive object lesson.

Let me first direct your attention to the diagram containing the _natural coloring matters_--those dyestuffs which were in use previous to 1856. Broadly speaking, they are of two kinds; those which dye textile materials "direct," and those which give no useful color without the aid of certain metallic salts, called "mordants."

Now, among the natural coloring matters, these "mordant dyes," as they may be conveniently termed, are much more numerous than the "direct dyes;" but be it observed, we have fast and fugitive colors in both classes.

Referring first to the wool patterns and to the "direct dyes," we find that the only really fast colors are Prussian blue and Vat indigo blue. Turmeric, orchil, catechu, and indigo carmine are all extremely fugitive.

As to the "mordant dyes," some yield fast colors with all the usual mordants, e.g., madder, cochineal, lac dye, kermes, viz., reds with tin and aluminum, claret browns with copper and chromium, and dull violets with iron.

Other dyestuffs, like camwood, brazilwood, and their allies, also young fustic, give always fugitive colors whatever mordant be employed; others again, e.g., weld, old fustic, quercitron bark, flavin, and Persian berries, give fast colors with some mordants and fugitive colors with others; compare, for example, the fast olives of the chromium, copper, and iron mordants with the fugitive yellows given by aluminum and tin. A still more striking case is presented by logwood, which gives a fast greenish-black with copper and very fugitive colors with aluminum and tin. Other experiments have shown that the chromium and iron logwood blacks hold an intermediate position. Abnormal properties are found to be exhibited by camwood and its allies, with aluminum and tin, the colors at first becoming darker, and only afterward fading in the normal manner.

When we examine the silk patterns, we find, generally speaking, a similar degree of fastness among the various natural dyes, as with wool; in some instances the colors appear even faster, notice, for example, the catechu brown and the colors given by brazilwood and its allies, with iron mordant.

On examining the cotton patterns, we are at once struck with the marked fugitive character of nearly all the natural dyes. The exceptions are: the madder colors, especially when fixed on oil-prepared cotton, as in Turkey red; the black produced by logwood, tannin, and iron; and a few mineral colors, e.g., iron buff, manganese brown, chromate of lead orange, etc., and Prussian blue. Cochineal and its allies, which are such excellent dyes for wool and silk, give only fugitive colors on cotton.

The main point which arrests our attention in connection with the natural dyes seems to me to be the comparatively limited number of fast colors. Very remarkable is the total absence of any really fast yellow vegetable dye, and it is probably on this account that gold thread was formerly so much introduced into textile fabrics. Notice further the decided fastness of Prussian blue, especially on wool and silk; while we cannot but remark the comparatively fugitive character of vat indigo blue on cotton, and even on silk, compared with the fastness of the same color when fixed on wool.

Now, let us turn our attention to the _artificial coloring matters_, derived with few exceptions from coal tar products.

Here again we have two classes, "mordant dyes" and "direct dyes." Both classes are somewhat numerous, but whereas the former may be conveniently shown on a single diagram sheet, it requires a considerable number to display the latter.

First let us examine the wool patterns dyed with the "mordant dyes."

We find there a few yellow dyes quite equal in fastness to those of natural origin, or even somewhat surpassing them, e.g., two of the alizarin yellows, viz., those marked R and G G W. Except in point of fastness and mode of application, I may say that these are not true alizarin colors, neither are they analogous to the natural yellow dyestuffs, for they are incapable of giving dark olives with iron mordants. Truer representatives of the natural yellow dyes appear, however, to exist in galloflavin and the alizarin yellows marked A and C, and, as you see, they are of about the same degree of fastness.

Among the red dyes we have alizarin and its numerous allies, and these are certainly fit representatives of the madder root, which indeed they have almost entirely displaced. The most recent additions to this important class are the various alizarin Bordeaux. The only dyes in this group which appear somewhat behind the rest in point of fastness are purpurin and alizarin maroon.

On this same diagram we notice, also, fast blues and dark greens, of which we have no similar representatives among the natural coloring matters. I refer to alizarin blue, alizarin cyanin, alizarin indigo, alizarin green, and coerulin.

Further, an excellent group of coloring matters, giving fast browns and greens with copper and iron mordants respectively, is formed by naphthol green, resorcinol green, gambin, and dioxin.

The only fugitive dyes of the class now under consideration are some of the yellows, gallamin blue and gallocyanin.

If we now turn to examine the colors given by these artificial "mordant dyes" on silk, we notice, also, a good series of fast colors similar to those which they give on wool; and even on cotton we see many fast colors, of which we have no representatives among the dyewoods.

If we were not prepared to find so few really fast natural dyes, surely we cannot but be surprised to find what a considerable number of fast dyes are to be met with among the coal tar coloring matters requiring the aid of mordants.

On these diagrams, the first vertical column shows the stain given by the coloring matter alone; the remaining columns show the colors obtained when the same coloring matters are applied in conjunction with the several mordants--chromium, aluminum, tin, copper, and iron.

It was formerly held that the office of a mordant was merely to fix the coloring matter upon the fiber; we now know, however, and it is plainly illustrated by these diagrams, that this view is erroneous, for the mordant not only fixes but also develops the color; the mordant and coloring matter chemically combine with each other, and the resultant compound represents the really useful pigment or dye. If a coloring matter is combined with different mordants, the dyes thus obtained represent distinct chemical products, and it is quite natural, therefore, to find them differing from each other in color, and their resistance toward light.

Knowing this, it is clearly the duty of the dyer to apply each coloring matter of this class with a variety of mordants, and to select the particular combination which gives him the desired color and fastness. By adopting this method, however, his selection would ultimately comprise a large number of coloring matters paired with a great variety of mordants. In order, therefore, to avoid the intricacy involved in the use of several mordants, and to simplify the process of dyeing, especially when dyeing compound shades, the dyer prefers to limit himself as far as possible to the use of a single mordant, and to employ along with it a mixture of several coloring matters.

Now the woolen dyer has largely adopted an excellent mordant in bichromate of potash; it is cheap, easily applied, and not perceptibly injurious to the fiber. It is his desire, therefore, to have a good range of red, yellow, blue, and other coloring matters, all giving fast dyes with this mordant. This action and desire on the part of the dyer has more and more placed the problem of producing fast colors upon the shoulders of the color manufacturer or chemist, and right well has the demand been met, for in the diagram on the wall we see how, in the alizarin colors and their allies, he has already furnished the dyer with a goodly number of dyestuffs yielding fast dyes with this chosen mordant of the woolen dyer. Since, however, they yield fast colors with other useful mordants, and upon other fibers than wool, these alizarin colors prove of the greatest value to the dyer of textile fabrics generally. Let us not forget the fact, then, that it is among the "mordant dyes," the very class to which belong most of the natural coloring matters, that we find our fastest coal tar dyes.

When we examine the results of actual exposure experiments, such as are here shown on these four diagram sheets, surely we have no hesitation in declaring how utterly false is the popular opinion that all coal tar colors are fugitive to light, while the good old-fashioned natural dyes are all fast. The very opposite indeed is here shown to be the case. For myself, I feel persuaded that at the present time the dyer has at his command a greater number of fast dyes derived from coal tar than from any other source, and I believe it possible to produce with dyes obtained from this source alone, if need be, tapestries, rugs, carpets, and other textile fabrics which shall vie successfully in point of color and duration of color with the best productions of the East, either of this or any other age.

How, then, does it happen that these coal tar colors have been so long and so seriously maligned by the general public? Apart from the fact that public opinion has been based upon an imperfect knowledge of the subject, we shall find a further explanation when we examine the diagrams showing the "direct dyes" obtained from coal tar. According to their mode of application I have here arranged them in three large groups, viz., basic, acid, and Congo colors. A fourth group, comprising comparatively few, is made up of those colors which are directly produced upon the fiber itself.

The "basic colors" have a well known type in magenta. They are usually applied to wool and silk in a neutral or slightly alkaline bath; on cotton they are fixed by means of tannate of antimony or tin. The "acid colors" are only suitable for wool and silk, to which they are applied in an acid bath. A typical representative of this group is furnished by any one of the ordinary azo scarlets which in recent years have come into prominence as competitors of cochineal. The "Congo colors" are comparatively new, and are conveniently so named from the first coloring matter of the group which was discovered, viz., Congo red. They are applicable to wool, silk, and cotton, usually in a neutral or slightly alkaline bath. Of the dyes produced directly upon the fiber itself, one may take aniline black and also primulin as a type, the latter a dye somewhat recently introduced by Mr. A.G. Green, of this city.

Our first impression, in looking at these "direct dyes," is that they are more numerous and more brilliant than the "mordant dyes," and that they are for the most part fugitive. Still, if we examine the different series in detail, we shall find here and there, on the different fibers, colors quite equal in fastness to any of the "mordant dyes."

Among the "basic colors" we search in vain, however, for a really fast dye on any fiber. Still, Magdala red, perhaps, appears faster than the rest on silk, and among the greens and blues we find a few dull blues on cotton, which, for this fiber, have been recommended as substitutes for indigo, viz., Indophenin, paraphenylene, blue, cinerein, Meldola's blue, etc. The azine greens, also, appear tolerably fast on cotton and on silk, but although possessing some body of color, after exposure, the original dark green has changed to a decided drab.

When we examine the "acid colors," however, we meet with a number of scarlets, crimsons, and clarets, possessing considerable fastness both on wool and on silk. Some, indeed, appear almost, if not entirely, as fast as cochineal scarlet, e.g., Biebriech scarlet, brilliant crocein, etc.

Among the "acid oranges and yellows," we also find a goodly number which are of medium fastness. About ten, either on wool or on silk, may even be accounted really fast, and are fit, apparently, to rank with alizarin colors. Note, for example, on wool: Crocein orange, aurantia, orange crystal, tartrazin, milling yellow, palatine orange; on silk, acid yellow D, brilliant yellow, azo acid yellow, metanil yellow, curcumin S, etc. I may remark that these are some of the fastest yellows on wool and silk with which we are acquainted. It is interesting to note the decided fugitive character, on silk, of tartrazin, aurantia, orange crystal, etc., compared with their great fastness on wool. Observe, also, how, on wool, the pale lemon yellow of picric acid has changed to a full reddish brown.

Among the "acid greens and blues," all the colors are fugitive, both on wool and on silk. Patent blue appears slightly better than the rest. Of the "acid blacks and violets," a few colors are of medium fastness, both on wool and silk, e.g., naphthol black, naphthylamine, black, resorcinol brown, fast brown, etc.