Others have not unreasonably supposed that when pigments are locked up in varnishes and oils, they are safe from all possibility of change. The assumption would be more warranted if we had an impenetrable varnish—and even that would not resist the action of light, however well it might exclude the influence of air and moisture. But, in fact, varnishes and oils themselves yield to changes of temperature, to the action of a humid atmosphere, and to other influences: their protection of colour from change is therefore far from perfect.
Want of attention to the unceasing mutability of all chemical substances, as well as to their reciprocal actions, has occasioned those changes of colour to be ascribed to fugitiveness of the pigment, which belong to the affinities of other substances with which it has been improperly mixed and applied. It is thus that the best pigments have suffered in reputation under the injudicious processes of the painter; although, owing to a desultory practice, the effects and results have not been uniform. If a colour be not extremely permanent, dilution will render it in some measure more weak and fugitive; and this occurs in several ways—by a too free use of the vehicle; by complex mixture in the formation of tints; by distribution, in glazing or lackering, of colours upon the lights downward, or scumbling colours upon the shades upward; or by a mixed mode very common among the Venetian painters, in which opaque pigments are combined, as umber and lake.
The fugitive colours do less injury in the shadows than in the lights of a picture, because they are employed pure, and in greater body in shadows, and are therefore less liable to decay by the action of light, and by mixture. Through partially fading, moreover, they balance any tendency to darken, to which the dead colouring of earthy and metallic pigments is disposed.
The foregoing circumstances, added to the variableness of pigments by nature, preparation, and sophistication, have often rendered their effects equivocal, and their powers questionable. These considerations enforce the expediency of using colours as pure and free from unnecessary mixture as possible; for simplicity of composition and management is equally a maxim of good mechanism, good chemistry, and good colouring. Accordingly, in respect to the latter, Sir Joshua Reynolds remarks, "Two colours mixed together will not preserve the brightness of either of them single, nor will three be as bright as two: of this observation, simple as it is, an artist who wishes to colour bright will know the value."
There prevail, notwithstanding, two principles of practice on the palette, opposed to each other—the one, simple; the other, multiple. The first is that of having as few pigments as possible; and consists, when carried to the extreme, in employing the three primary colours only. The second is that of having a number of pigments; and consists, also when carried to the extreme, of employing as many, if possible, as there are hues and shades of colour.
On the former plan, every tint requires to be compounded; on the latter, one pigment supplies the place of two or more. Now, the more pigments are mixed, the more they are deteriorated in colour, attenuated, and chemically set at variance. Original pigments, that is, such as are not made up of two or more colours, are purer in hue and generally more durable than those compounded. Hence pure intermediate tints in single, permanent, original pigments, are to be preferred to pigments compounded, often to the dilution and injury of their colours. Cadmium Orange, for instance, which is naturally an orange pigment and not composed of red and yellow, is superior to many mixtures of those colours in a chemical sense, and to all such mixtures in an artistic sense. At the same time, it is quite possible for the artist to multiply his pigments unnecessarily. Colours are sometimes brought out under new names which have no claim to be regarded as new colours, being, indeed, mere mixtures. Compound pigments like these may most frequently be dispensed with, in favour of hues and tints composed extemporaneously of original colours upon the palette.
It may be inferred from the foregoing that, between the modes of employing as few pigments as possible, and of having as many as there are hues and shades of colour, a middle course is the best. But, whatever the practice adopted, permanent original pigments should be used as often as the case will admit; it being borne in mind, that a pigment may be compound, although its colour may be primary. As a rule, the less colours are mixed, the purer, brighter, and more lasting they will be found.
To the practice of producing tints and hues by grinding pigments together, instead of blending them on the palette, may be attributed some of the peculiarities of the tints and textures of the Flemish school; they being, perhaps, results of intimate combination from grinding, and consequently of a more powerful chemical action among the ingredients compounded. This method has, in a great measure, fallen into disuse, and undoubtedly it conduced to foulness when the colours of the pigments ground were not pure and true, and did not assimilate well in mixture chemically.
The superiority of Rubens and the Flemings, and of Titian and the Venetian school, in colouring and effect, is due in a considerable degree to their sketching their designs in colours experimentally with a full palette. This practice, as derived from Reynolds, is common with the best masters of our own school, who, in executing their works, resort also to nature, with an improved knowledge of colours and colouring. Such attention to colouring and effect, from the first study and ground of a picture to the finishing, contributes a beauty to the painting no superinduced colouring can accomplish.
The durability of colour in substances is to a great extent dependent upon the condition in which they exist chemically. If pigments, for example, be in a state which chemists have termed protoxide, they are liable to absorb oxygen on exposure to light, air, or moisture, and becoming what is called peroxidized, may, by consequence, change or fade. In like manner, lakes and carmines thrown down upon a base, may owe some of their fugacity to the oxidation of that base, as well as to the natural infirmity of their colouring matter. On the other hand, pigments and bases are subject to deoxidation, or to a loss of oxygen, in which case the colour is apt to deepen. Pigments generally are more affected by oxidation and fading in a water vehicle, and by deoxidation and darkening in one of oil.