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.