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. 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.