An identical experiment in an ordinary glazed frame resulted in a reduction of tone from 10° to 1°, while the residual hue was a greenish grey.

The effect, if any, of the several iron reds, such as light red, Venetian red, and Indian red, in accelerating the fading of indigo in sunshine has been tested in many ways. Provided the three red pigments above-named be free from soluble salts, and especially from sulphates, they are equally innocuous; the indigo disappears with or without them at the same rate. In one instance only a wash of Venetian red with indigo lost its indigo completely three weeks before the destruction of the blue in the parallel experiments was complete, which was the case in six months. It is impossible to assert that this result was not due to a smaller amount of indigo having been present in this particular case, but I believe it to have been due to this sample of Venetian red having been an imitative one prepared from colcothar.

In comparing under the same conditions the relative stability, when exposed to light, of different indigoes, it appears that 'Bengal refined' is superior to the best 'raw Bengal,' and even to the indigotins obtained by the green vitriol and glucose processes.

The superiority, when exposed to light, of refined indigo - that is, of indigo which has been purified by the treatment with acid and the other solvents already named - although observed in recent experiments, seems to have been ascertained in the time of De Mayerne. One of the authorities (Elias Feltz, of Constance) quoted by De Mayerne affirms that this pigment may be rendered safe by steeping it in vinegar and exposing it to the sun for several days; the vinegar is then to be poured off, and the paste when dry ground in oil. Another plan recommended (in a note, dated 1642) by Feltz consists in grinding the indigo with a little calcined alum; this plan is, however, objectionable for several reasons. The treatment of the indigo with the solvents previously named, which are adapted to remove its natural impurities, is the only legitimate plan. It is scarcely necessary to add that there is abundant evidence to prove that in De Mayerne's day indigo was generally regarded as an unstable colour.

Under all circumstances no indigo should be employed in painting unless it be completely free from acids.

Although it is not possible to ascertain the exact richness in indigotin of commercial indigoes by any tests save those in use by professional chemists, yet it is easy to learn a good deal about any particular sample by means of a few simple experiments. A good sample should appear homogeneous, and should float on water. Dried at 100° C, it should not lose more than 6 parts per 100 of moisture. When rubbed with a hard smooth substance it should show a coppery lustre. One hundred grains when burnt should not leave more than 10 grains of grey ash. It should dissolve perfectly in four times its weight of fuming sulphuric acid. Starch, gypsum, clay, chalk, steatite, and Prussian blue are amongst the adulterants of indigo.

Indigo dissolved in four times its weight of fuming oil of vitriol (kept cool) forms a liquid from which, after slight dilution and filtration through asbestos cloth, potassium carbonate precipitates a fine blue compound, which has been used in water-colour painting under the name of indigo carmine. It is not a safe pigment.

The artificial or synthetic indigo, although characterized by freedom from impurities, both organic and inorganic, does not offer any advantages over the natural product, if properly purified, when employed as a paint in water-colour or oil. It may be added that those derivatives of indigotin, in which some of the hydrogen atoms have been substituted by bromine or chlorine, are no more permanent than the original dyestuff. This is the case with that form of dibrom-indigotin (the punicin of the late H. E. Schunck), which occurs naturally in, or rather was obtained from, certain marine molluscs (Purpura lapillus and Murex brandaris), and which has been known for many centuries as Tyrian Purple. The very minute yield of the natural pigment would have made its general use impracticable, but now that the identical substance can be freely made from indigo, it is found that its lack of permanence should exclude it from the 'selected' palette of the artist of to-day.

In examining old water-colour drawings, it will often be found that the parts protected by the mount or frame show the indigo used in compounding the greys of clouds and the greens of vegetation perfectly intact, while it has completely left the exposed parts. In many cases where indigo is supposed to have withstood long exposure to light, it will be found that the blue used has been Prussian blue, modified by admixture with other pigments. But in many old pictures and drawings it will be found that the unnatural bluish hue of the foliage represented is due to the complete loss of rapidly evanescent yellows, rather than to the entire stability of the indigo with which the greens have been compounded. Nor must it be forgotten that indigo was not the only blue in such mixed greens, blue carbonate of copper and even lapis-lazuli having been extensively employed at that time. The latter pigment is perfectly permanent except in the presence of acids and alum.