It must be observed that each colour may comprehend an indefinite series of shades between the extremes of light and dark, as each compound colour also may comprise a similar series of hues between the extremes of the colours composing it. And as the relations of colours have been deduced regularly, from white or light to black or shade; so the same may be done, inversely, from black to white. On this plan the tertiaries, olive, russet, and citrine, take the place of the primaries, blue, red, and yellow; while the secondaries still retain their intermediate station and relation to both.
Thus, russet and olive compose or unite in dark purple; citrine and olive in dark green; russet and citrine in dark orange. The tertiaries have, therefore, the same order of relation to black that the primaries have to white; and we have black primaries, secondaries, and tertiaries, inversely, as we have white primaries, secondaries, and tertiaries, directly. In other words, we have light and dark colours in all classes.
[CHAPTER IV.]
ON THE DURABILITY AND EVANESCENCE OF PIGMENTS.
Pigments may be defined as colours in a solid or insoluble state, prepared for the artists' use. Hitherto, we have treated of colours in the abstract sense, as appealing to the eye only: we have now to consider them as material bodies.
As colour itself is relative, so is durability of colour relative. For the reason that all material substances are changeable and in perpetual action and reaction, no pigment is so permanent as that nothing will alter its colour. On the other hand, none is so fugitive as not to last under some favouring circumstances. Time, of long or short continuance, has often the effect of fire, more or less intense. Indeed, it is some sort of criterion of the stability and changes of colour in pigments, that time and fire are apt to produce similar effects thereon. Thus, if fire deepen, or cool, or warm a colour, so may time; if it vary its hues, so may time; if it destroy a colour altogether, so may time ultimately. The power of time, however, varies extremely with regard to the period in which it produces those effects, that are instantly accomplished by fire.
That there is no absolute but only relative durability of colour may be proved from the most celebrated pigments. For instance, the colour of native ultramarine, which will endure a hundred centuries under ordinary circumstances, may be at once destroyed by a drop of lemon juice; and the generally fugitive and changeable carmine of cochineal will, when secluded from light and air, continue fifty years or more; while fire or time, which merely deepen the former colour, will completely dissipate the latter. Again, there have been works of art in which the white of lead has retained its freshness for ages in a pure atmosphere, but has been changed to blackness after a few days' or even hours' exposure to foul air. These and other peculiarities of colours will be noticed, when we come to speak of pigments individually; not for the purpose of destroying the artist's confidence, but as a caution, and a guide to the availing himself of their powers properly.
It is, therefore, the lasting under the usual conditions of painting, and the common circumstances to which works of art are exposed, that entitles a colour to the character of permanency; and it is the not-so-enduring which attaches to it rightly the opposite character of evanescence: while a pigment may obtain a false repute for either, by accidental preservation or destruction under unusually favourable or fatal circumstances.
Many have imagined that colours vitrified by intense heat are consequently durable when levigated for painting in oil or water. Had this been the case, the artist need not have looked farther for the furnishing of his palette than to a supply of well-burnt and levigated enamel pigments. But though such colours for the most part stand well when fluxed on glass, or in the glazing of enamel, they are nearly, without exception, subject to the most serious changes when ground to the degree of fineness necessary to their application as pigments, and become liable to all the chemical changes and affinities of the substances which compose them. These remarks even apply in a measure to native products, such as coloured earths and metallic ores.