Dr. Russell and Sir William Abney add, indeed, the proviso, 'If painted with any but the more fugitive colours.' But this condition cannot be said to have been fulfilled by the works in question; for in the great majority of them, most of the six fugitive pigments which we have just named were freely employed. And it is these very pigments which have been proved by the reporters themselves to suffer 'marked deterioration' by an exposure of twenty-one months only to strong daylight without direct sunshine. Moreover, it must not be forgotten that the fading of a single important pigment in a water-colour drawing is ruinous to the whole effect, destroying the balance of the chromatic scheme of the artist more effectually than a slight, but equal, degradation of all the hues.

An instructive set of trials of water-colour paints was commenced in May, 1894, and continued for four years by a sub-committee of the Burlington Fine Arts Club. The results of these experiments have been embodied in three reports. Winsor and Newton's moist water-colours were used, flat washes on Whatman's 'not hot pressed' paper of the year 1888 being exposed to light in the windows of the billiard-room of the club. One of the sets so exposed was in an ordinary glazed frame, another set was enclosed in hermetically sealed glass tubes containing ordinary air; while a third set was contained in similar tubes, but the atmosphere present was maintained in a dry state by means of a reservoir of burnt lime which absorbed all or almost all traces of water in the pigments, the paper, and the mount. The final comparisons were made with parallel sets which had been kept in absolute darkness. We give here, in tabular form, the condition of the paints at the end of the fourth year of exposure: reference must be made to the original reports for information as to the results of six months' and of thirteen months' exposure to sunshine.

* In the preceding brief résumé of certain parts of the South Kensington Report no reference has been made to an argument, developed in §§ x. to xv., in which it is contended that 'if a certain tint be exposed to an intensity of radiation which we will call 100, and which bleaches it in, say, 1 hour, then, if a similar tint be exposed to an intensity 1, it will require 100 hours' exposure to it to effect the same bleaching.' The universal applicability of this conclusion cannot be conceded by those who are familiar with numerous instances in which no chemical or physical change occurs when certain substances are exposed continuously for long periods to a particular temperature, yet, when they are heated but a degree or two higher, instantly alter, decompose, or react, as the case may be.

Results Of Exposure For Four Years, From May 25, 1894, To May 25, 1898

(Original depth = 10)

The most striking results of these trials was the complete stability of Prussian Blue and of Indigo when exposed in air kept dry. The further remark may be made that moist air, that is, ordinary air containing moisture and confined in a sealed tube, inflicts more injury upon alterable pigments than ordinary air enclosed in a frame which does admit of some amount of ventilation occurring. There is no difficulty in so constructing a frame as to introduce a water-absorbing substance which may be renewed from time to time. Thus we shall be able to employ with confidence in water-colour painting madder brown, madder purple, Prussian blue, indigo and sepia - five pigments which under ordinary conditions of exposure to sunshine suffer serious changes. But no method has yet been devised by means of which we may safely use gamboge, Indian yellow, vermilion, crimson lake and Vandyke brown, although it must be stated that instances have been recorded in which vermilion as a water-colour has stood very severe exposure-tests, especially when white of egg has been mixed with it.

Amongst the series of trials of oil-paints made by the author of this handbook, one set first arranged in 1880 may be described here. Chance's colourless plate-glass was employed as the painting-ground, so as to avoid all interference with the pigments from the surface on which they were spread; glass presents the further advantage of permitting a complete examination of the back of each specimen, and of changes in its translucency, opacity, or texture. Each glass measured 8 inches by 6; the complete series was prepared in duplicate - one for preservation in darkness, the other for exposure to all the light that could be secured (in Kew) during live years in a window facing nearly south-west. The majority of the paints tried were obtained from four firms (Messrs. Winsor and Newton, Messrs. Roberson and Co., M. Edouard of Paris, and Schoenfeld of Düsseldorf). Specimens of each pigment were reserved for further examination and analysis. Some of the chief results obtained are given in the annexed table; a few remarks on the changes observed in some of the pigments which had been mixed with flake white are added:

A series of trials of seventeen madder colours in oil was carried out in 1893. These were all prepared by Lefranc of Paris. The samples were spread on thin lantern glass 4 in. x 4 in., and when dry each glass was cut in half. One half was exposed to sunshine from March 26 until October 26, the other half of each specimen being kept in darkness. At the end of the seven months the halves of each sample were rejoined and carefully mounted, and thus an instructive set of slides obtained. The results are given in the following table, in which I have grouped together those varieties of 'laque de garance' which resembled one another in their degree of stability:

In the fourth or least stable group, comprising the varieties of madder brown, it was noticeable that the yellow constituent of the colour was most affected by exposure, the brun foncé and the nuance bitume losing all their characteristic beauty of hue and becoming of a dull, poor, rusty tint and not retaining over 20 per cent. of their original depth. Even in Group II. the golden hue of the rose dorée was the only chromatic element of this madder paint which had been lost to an appreciable extent during the six months' exposure.

Experiments as to the degree of stability possessed by many other oil-paints have been made; the results will be found for the most part incorporated with the accounts given of the several pigments in Part III. One remark may perhaps be usefully introduced in this place with reference to the differences observable in the quality and behaviour of pigments bearing the same name but obtained from different artists' colourmen. It is a good plan to place side by side on three trial-plates several 'makes' of the same paint and to keep one set in darkness, and to expose a second set to sunshine and a third set to strong diffused daylight. It will sometimes be found that the fascinating colour-quality which at the first glance recommends one sample is not preserved after exposure, although the reverse experience is not uncommon. It must not be forgotten that although the composition and constituents of the vast majority of pigments, both natural and artificial, are known, yet there are differences in the methods of preparation which, in some cases at least, are kept secret, and which result not in differences of nuance only, but in differences of stability also.