W. N. Hartley's Experiments With Pigments

On September 4, 1886, the late Sir W. N. Hartley read, before the British Association at Birmingham, a paper on 'The Fading of Water Colours.' His trials as to the effect on pigments of a comparatively brief exposure to intermittent sunshine in pure air may be thus summarized. Washes on the best drawing-paper were the subject of the experiments:

Gamboge

Pale washes were completely bleached in three days; in a week strong washes were much lightened in colour, and rendered dull, even three hours' exposure producing a very visible effect.

Crimson Lake

Six hours' exposure to sunlight and air almost bleaches pale washes, while three days or eighteen to twenty-four hours of intermittent sunshine cause dark crimson tones to become very much lighter, the hue of the pigment being altered.

Light red, Indian red, and vermilion were unaffected.

Olive green and brown pink were rendered lighter in colour by six hours' exposure, the former pigment becoming bluish and the latter brownish in hue.

Indigo, cobalt, and artificial ultramarine were unaffected.

Brown madder became rather lighter after eight days' or forty-eight hours' exposure.

Bistre faded with great rapidity, a light wash appearing much paler after six hours.

Sepia

A pale wash became colder in hue, but not very perceptibly paler.

In a second series of experiments, sectors of paper discs, washed with various pigments, were enclosed between glass-plates, the edges of which were fastened with gummed paper. Under these circumstances, crimson lake and bistre were found to have been considerably altered by five hours' exposure - somewhat more so, indeed, than was the case when these pigments were freely exposed to the air.

All the results above noted are in practical accord with those obtained by other observers. The exposure to intermittent sunshine 'for six hours a day during fourteen days,' does not produce a sensible effect upon vermillion and indigo. Had Sir W. N. Hartley extended his observations a few weeks longer, his conclusions as to these pigments must have agreed with those which we have given, and therefore with the unanimous verdict of all other scientific observers. His statement that ' indigo is permanent' (British Association Report, 1886, p. 581) must, therefore, be modified into, 'indigo appears to have suffered no change after fourteen days' exposure to intermittent sunshine.' A similar alteration is demanded with regard to the stability of vermilion.

Sir W. N. Hartley's experiments with water-colour washes on paper enclosed between glasses require a few words of comment. He is clearly and rightly dissatisfied with this method of trial. A supply of atmospheric oxygen, and of hygroscopic moisture, amply sufficient for large chemical alteration and oxidation of the enclosed pigments, was certainly present. And the glasses did accelerate the action, not because of 'the very slight tint of the plate-glass,' but in spite of it. This acceleration of change is mainly caused by the continued presence of moisture in the confined space between the two glasses - it cannot escape as it freely, and to a very great extent does escape, when a piece of tinted paper is exposed to sunshine in free air. I showed, indeed, in my lectures at the Royal Academy, so long ago as 1880, that the fading of many fugitive pigments is greatly lessened, when not altogether prevented, by enclosing the paper washed with them in a glass tube, the air of which is kept dry by means of some strongly hygroscopic substance.

When both moisture and air are excluded (using a sealed vacuum tube), the suspension of fading and alteration of hue is still more marked and general.

It should be added here that Sir W. N. Hartley found that cadmium yellow and Indian yellow are bleached by peroxide of hydrogen, and changed into a muddy yellow by sulphurous acid. This reagent bleaches artificial ultramarine and dulls vermilion. He attributes the partial or complete destruction of the blue component of the hues in certain old drawings, which have been long exposed to air and light, to the presence of acids or acid substances in the air, in the paper, or in the red ferruginous pigments with which the blue colouring substances in question have been associated. These blue pigments could have been nothing other than Prussian blue, indigo, or natural ultramarine. I have ascertained, by direct experiments on old drawings, that the latter was but rarely employed for mixed tints, but it is quite probable that the reds prepared from colcothar, with which it may have been occasionally mingled, would sometimes contain enough acid salts (certain ferric sulphates) to destroy its colour. The products of the burning of gas and of coal would also be rich enough in sulphuric acid to produce the same effect.

I am unable to endorse Sir W. N. Hartley's statement that the best drawing-papers contain free sulphuric acid, at all events when fresh from the mill, but they soon acquire it when kept in an urban atmosphere.