In oil painting, the shadows or dark portions of the picture are painted thinly, while the lights are laid on or impasted with a full pencil and a stiff colour. In the lights of the foreground, and of parts not intended to be remote, or to retire, the impasting should be bold and free; while, in the more brilliant lights, it cannot well be too solid. There is, however, a reasonable limit to the practice; since actual protuberance or prominence of the paint itself will, in certain lights, produce a false shadow, and therefore a bad and false effect. This will be understood, from observing that the loading of thick masses of colour upon the picture, so as to make them project considerably from the surface, is done with the view of their being strongly illuminated by light actually incident upon the picture, and of thus mechanically aiding in the production of roundness and relief, or in giving a sparkling effect to polished objects or glittering points. But this artifice must be had recourse to sparingly and cautiously, else it defeats its own object, and produces a coarse and vulgar air and effect.
The palette knife has always been a favourite instrument of this impasting, or laying on of colour, capable as it is of producing an agreeable brightness on, and of giving an appropriate flatness to, the pigment. A clear and. appropriate tint, for instance, skilfully swept across a sky by these means, often produces a surprisingly brilliant and charming effect.