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.