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.
When we examine the Congo colors, amid a number of very fugitive
colors, we find a few which are satisfactorily fast. Among the reds,
for example, diamine fast red is quite remarkable for its fastness,
both on wool and silk, and may certainly rank with alizarin; but on
cotton, it is quite as fugitive as the rest. Of medium fastness on
wool are brilliant Congo G and R, Congo G R; and on silk, diamine
scarlet B, deltapurpurin 5 B, and brilliant Congo R.
Among the "Congo oranges and yellows," we find some of the fastest on
cotton of this class of colors. Still they deserve only the rank of
medium fastness. They are Mikado orange 4 R, R, G. Hessian yellow,
curcumin S, chrysophenin. On wool, we have about half a dozen of
medium fastness, viz., benzo-orange, Congo orange R, chrysophenin G,
chrysamin R, brilliant yellow. On silk, however, we find in this group
about a dozen of the fastest oranges and yellows with which we are
acquainted for this fiber, viz., Congo orange R, chrysophenin G,
diamine yellow N, brilliant yellow, curcumin W, benzo orange, Hessian
yellow, chrysamin R and G, cresotin yellow R and G, cotton yellow G,
and carbazol yellow.
Does it not appear somewhat remarkable that we should find among this
generally fugitive group of coloring matters colors which are so
eminently fast on silk, and which we entirely fail to meet with among
those groups which usually furnish our fast colors, e.g., the alizarin
group?
Passing on to the "Congo violets, blues, and purples," we find few
colors worthy of particular notice for fastness. Diamine violet N
appears, perhaps, of medium fastness on wool and silk, while
sulphonazurin, benzo-black blue, and direct gray may claim the same
distinction on silk.
In the small group of colors which are produced directly upon the
fiber, none seems to call for special notice, except aniline black,
which, notwithstanding its direct derivation from aniline, is probably
the fastest color we have upon any fiber.
Now, in classifying the whole range of coal tar coloring matters into
"mordant dyes" and "direct dyes," and the latter into acid, basic,
Congo colors, etc., I have looked at them from the point of view of
the dyer and arranged them according to color and mode of application.
The chemist, however, classifies them quite differently, viz.,
according to their chemical constitution, i.e., the arrangement of the
atoms of which they are composed, and thus we have nitro colors,
phthaleins, azines, and so on.
In studying the action of light on the coal tar colors from this point
of view, we find that whereas the members of some groups are for the
most part fugitive, the members of other groups are nearly all fast,
and it becomes at once apparent that the chemical constitution of a
coloring matter exercises a profound influence upon its behavior
toward light. Members of the rosaniline group are all similarly
fugitive, while those of the alizarin group possess generally the
quality of fastness. Particularly fugitive are the eosins, and yet
some of these, by a slight modification of constitution, e.g., the
introduction of an ethyl group, as in ethyl-eosin, are rendered
distinctly faster.
In the azo group some colors are fugitive, others are moderately fast,
and it is generally recognized that certain classes of the tetrazo
compounds are distinctly faster than the ordinary diazo colors.
By a careful study of the influence of the atomic arrangement upon the
stability of colors, information useful to the color manufacturer may
possibly be gained, but at present my facts are not yet sufficiently
tabulated to enable one to recognize any generally pervading law in
this direction.
It is scarcely necessary to say that the fastness to light of a color
is independent of its commercial value, this being mainly determined
by the price of the raw material from which it is manufactured, the
working expenses, and the profit desired by the manufacturer. Neither
must we suppose that facility of application necessarily interferes
with its fastness to light, for some of our fastest coal tar colors on
wool, e.g., diamine fast red, tartrazin, etc., are applied in the
simplest possible manner. On the other hand, the intensity or depth of
a color has considerable influence on its fastness. Dark full shades
invariably appear faster than pale ones produced from the same
coloring matter, simply because of the larger body of pigment present.
A pale shade of even a very fast color like indigo will fade with
comparative rapidity. The fugitive character of many of the coal tar
colors is, in my opinion, rendered more marked, because, owing to
their intense coloring power, there is often such an infinitesimal
amount of coloring matter on the dyed fiber. Hence it is that in the
Gobelin tapestries pale shades on wool are frequently obtained by the
use of more or less unchangeable metallic oxides and other mineral
colors, to the exclusion of even fast vegetable dyes.
It is interesting to examine what is the action of light upon compound
colors. Is a fugitive color rendered faster by being applied along
with a fast color?
My own opinion, based upon general observation, is that it is not, and
that when light acts upon a compound color the unstable color fades,
while the stable color remains behind. A woaded color, for example, is
only fast in respect of the vat indigo which it contains, and yet how
frequent is the custom to unite with the indigo such dyes as barwood,
orchil, and indigo-carmine, the fugitive character of which I have
pointed out.
Having thus rapidly surveyed these numerous coal tar colors, both in
their dyed and exposed conditions, I again ask why are they so
generally regarded as altogether fugitive?
First, because we have, especially among these "direct dyes," a very
large number which are undoubtedly very fugitive.
Moreover, all the earlier coal tar dyes--mauve, magenta, Nicholson
blue, etc., belonged to a class which, even up to the present time,
has only furnished us with fugitive colors. They were indeed prepared
from aniline, and it appears to me that the defects of these early
aniline colors, as well as their designation, have been handed down to
their successors without due discrimination, so that in the popular
mind the term "aniline color" has become, as a matter of habit,
synonymous with "fugitive color." But science is progressive, fields
of investigation other than aniline have been opened up, so that now,
although a large number of fugitive dyes are still manufactured from
coal tar, there are others, as we have seen, which are as fast and
permanent as we have ever had from natural sources.
Finally, and perhaps this is the most important cause of all, many of
the fugitive coal tar colors are gifted, I will not say with fatal
beauty, but with a facility of application, and such comparative
cheapness in consequence of their intense coloring power, that the
dyer, tempted by competition, applies them not unfrequently to
materials for which, because of their ultimate uses, they are
altogether unsuited; and so it comes about that we find the most
fugitive colors applied indiscriminately and without due discretion.
As we look upon these multitudinous colors, one other thought cannot
fail to cross our minds. Is there not surely an overproduction of
these fugitive coal tar colors? Is not the dyer bewildered with an
_embarras de richesses_, so that he knows not where to choose?
There is indeed much truth in this. With rare skill and ingenuity an
army of chemists is busy elaborating these wonderful dyes; but in such
quick succession are they introduced into the dye house that the busy
dyer has no time sufficiently to prove them, and it is not surprising
therefore that he is liable to commit errors in their application.
But if there is an over-production of fugitive colors, there is also
at work, as in the organic world around us, the counteracting
influence of the law of the survival of the fittest. Sooner or later,
the fugitive colors must give way to those which are more permanent,
and already the number of coal tar colors which have been discarded,
for one reason or another, is considerable.
Not unfrequently one is asked the question, Is there no method whereby
these fugitive colors can be made fast? Knowing the efficacy of
mordants with certain coloring matters, is there no mordant which we
can generally apply with this desirable object in view? The discovery
of such a universal mordant I believe to be somewhat chimerical, and
yet, curiously enough, a number of experiments have been recorded in
recent years, which almost seem to point in the direction of selecting
for such a purpose ordinary sulphate of copper.
Some of these diagrams before you this evening show clearly the
fastness to light generally of the lakes formed with copper mordant.
This peculiarity of the copper compounds has not escaped the notice of
other observers. Dr. Schunck, for example, during the progress of his
research on chlorophyl, noticed the very permanent green dye which
this otherwise fugitive coloring matter gives in combination with
copper.
Then there is the assertion of practical dyers, that the use of copper
sulphate in dyeing catechu brown on cotton assists materially in
rendering this color fast to light.
The use of copper mordant with phenolic coloring matters is perfectly
natural. Some time ago, however, it was successfully applied, for the
purpose of rendering more permanent, to certain of the Congo colors on
cotton, e.g., benzo-azurine, etc., in the application of which,
metallic salts had not hitherto been deemed necessary.
Noelting and Herzberg have also observed that the fastness to light,
even of basic colors, e.g., magenta, methyl violet, malachite green,
etc., is increased by a subsequent treatment of the dyed fabric with
copper sulphate solution, although in many cases the color is much
soiled thereby.
Still more recently, A. Scheurer records that by impregnating or
padding certain dyed fabrics with an ammoniacal solution of copper
sulphate, the colors gain considerably in fastness to light. As the
result of his experiments Scheurer concludes that this protective
influence of copper on dyed colors is a general fact, apparently
applicable to all colors; that it is not necessarily due to its action
as a lake-forming substance, since intimate union between the coloring
matter and the copper salt is not necessary. He seems rather inclined
to ascribe its efficacy to the light being deprived of its active rays
during its passage through the oxide of copper.
Knowing, however, the strong reducing action of light in many cases,
and with the absence of positive knowledge concerning the cause of the
fading of colors, it seems to me that the beneficial influence of the
copper may just as probably be due to its well known oxidizing power,
which counteracts the reducing action of the light.
It is interesting to note, in connection with Scheurer's view, that,
many years ago, Gladstone and Wilson (1860) proposed to impregnate
colored materials with some colorless fluorescent substance, e.g.,
sulphate of quinine, evidently with the idea of filtering off the
active ultra-violet rays. How far some such method as this might prove
successful I cannot say, but since we cannot keep our dyed textile
materials in a vacuum, as Chevreul did, nor is it desirable to
impregnate them with mastic varnish for the purpose of excluding air
and moisture, as Mr. Laurie proposes, in order to preserve the colors
of oil paintings, it is perhaps well to bear in mind the principle
here alluded to as a possible solution of the difficulty.
I have dwelt rather long on this important question of the action of
light on dyed colors, but I have done so because I thought it would
most interest you. With the remaining portions of my subject I must be
more brief.
(_To be continued._)
* * * * *
To introduce free fat acids from an oil, it must be decomposed. This
may be done by the use of lead oxide and water or by analogous
processes. To clarify an oil, expose to the sun in leaden trays. Often
washing with water will answer the purpose.
* * * * *