The last thing in the order of our analysis is the ground and basis on which colours, pigments, and vehicles, are applied in painting; and as the basis of fresco-painting is plaster, and that of water-colour painting is principally paper, the subject of grounds is chiefly of consideration with respect to painting in oil, in which mode a great variety of grounds have been employed, which have afforded a subject of wide speculation and experiment, - of many hopes and many failures, while the charm of Venetian art has been nearly as fruitful in exciting the invention of grounds as of vehicles.

The subject of grounds belongs to our inquiry only so far as regards their influence upon colours, and needs no very elaborate consideration here. Among the various bases upon which grounds have been laid are the metals, stone, slate, plaster, woods, card, vellum, and cloths, in all their variety. The qualities requisite to a perfect basis are durability, ineligibility, and inflexibility, which neither of these substances comprise in perfection. Metals are durable and infrangible in the highest degree; but they expand and contract by the mere alterations of temperature, and are, on this account, subject to detach or throw off portions of the ground, and to craze the painting and varnish. Linen cloths, parchment, and paper bases, are infrangible and durable in a high degree, but very flexible, which is remedied in a measure by straining and stretching; they become, therefore, variously eligible bases. Wood comprises all the qualities of a good basis in a medial degree, and hence, upon the whole, panel affords the best baas for small works. For works of a moderate size. in which cost may be of no objection, silk, as said to have been employed by Guido, who had remarked its extraordinary durability in an antient tomb, and also cloths of hair and wool might become suitable bases. To treat of the peculiar qualities of all these in their various kinds, would carry us far beyond the bounds of utility, and the limits of our subject, to which the grounding of these is more intimate.

Grounding or priming is not in all cases necessary, as, for example, when stone, slate, glass, porcelain, etc, are employed, as was the case in some paintings of undoubted extreme antiquity; but when grounds are necessary, as upon metal, wood, and canvass, to be eligible, such grounds must partake all the qualities of a good basis, in being neither soft, friable, nor perishable.

The early painters in oil, being also painters in fresco, and accustomed to plaster grounds, appear to have prepared their panels, etc. with plaster or stucco, upon which they employed their colours, in some cases in water, in the manner of fresco or distemper, using size to fix them, and finishing with oil vehicles and varnish; and many such pictures have stood the ordinary effects of time admirably well, as appears among the works of Paul Veronese, Titian, Correggio, and others; but, upon cloth and flexible bases, such grounds are too stiff and friable; such bases require, therefore, a ground more of their own yielding and elastic nature, and better suited to assimilate with the materials of oil-painting, such as is afforded by tempering earths and metallic oxides with the most tenacious drying oils, and laying them evenly upon the cloths, first coated or primed with size.

The preparing of grounds on cloths, etc. is now, however, so well performed by several of our principal colourmen, and with so much improvement, as to require little comparative attention from the artist, beyond such general knowledge of their qualities as may enable him to choose such as are best suited to his purpose. The colour of his ground is also a matter of choice, and some artists attach great importance to peculiar hues; but, as the best must depend upon varying circumstances, there can be no tint suited to every case: and we know but of one general rule on this subject, which is, that the highest light and prevailing tone of the intended work is best adapted for the ground; but c c as this sublition is the first step in every picture, it belongs, with the design, to the artist, and not to the preparer of the ground; and the first work of the brush may, with regard to the colouring.be well employed ill harmonizing the colour of the ground to the key and effect of the intended picture. It may be worthy of remark, however, that nature generally colours upon a white ground, and works entirely with transparent colours, whence the purity and splendour of her colouring: and it is to the white grounds of Paul Veronese we are greatly to attribute his vividness and purity. It was, probably, according to this rule that Titian chose to paint on a red ground, when he intended to introduce much flesh in his design, or to render red principal in his picture. It is related of the same great master, who is a prime authority in all things relating to chromatic art, that, to secure the durability and cohesion of his grounds, he imbued the canvass at the back with bees-wax, dissolved in oil, a substance well calculated to resist damp in such a situation as Venice.

To preserve the elasticity of grounds, some drying oil should be introduced into the glue or size with which they are prepared; for the same purpose bees'-wax, sugar, treacle, albumen, etc. have been added with various degrees of eligibility and success. If the ground give way in any respect, the upper surface of the picture must fail also, and this is one of the principal causes of cracking, although by no means the only one. Those substances, however, which occasion cracking in the ground will occasion cracking in the painting; hence the importance of homogeneity of process in both these subjects. Any discordance in this respect may induce cracking in a variety of ways: if a picture be painted in varnish, or even with some addition of oil, exposure to sunshine will inevitably crack it, by drying and contracting the upper surface, while it softens and swells the under coat upon which it is applied. Heat of any kind will in a less degree produce the same effect. As oils and resins imbibe moisture, damp will have the effect of expanding the upper surface and of cracking, blooming, and chilling soft varnishes. Glue or animal size in the ground unprotected will, by expanding and contracting upon damp or very dry walls, have the same effects. Thick coats of varnish, applied too rapidly, will also dispose the surface to crack by the same mechanism. Indeed, a rapid drying of the upper surface before the under-painttng is fixed, notwithstanding the firmness of the ground, will generally produce cracking: this is the foundation of an artifice, of which the imitators of antiques avail themselves, by applying solutions of gum and glue over varnishes newly laid on, so as to craze the surface all over in the manner often produced by time in old pictures, etc. So powerful, indeed, is simple solution of gum in this respect, that, when applied upon ground-glass, and dried thereon, it will disrupt and tear up the surface of the glass itself by the force of its contraction; and this is a property which belongs in a degree to those varnishes of the hard resins which contract in drying, such as copal, when employed over surfaces of a tenacity inferior to their own.

Other causes of cracking might be enumerated, not peculiarly attributable to the ground, such are over-stretching and mechanical violence, which do most injury to weak and inelastic substances, but against which none are entirely secure. It is apparent, therefore, that this disease of pictures, so desirable to avoid, and so often attributed to the grounds, may belong equally to the vehicles, the varnish, the pigments, or to the entire process of a painting.

It has been supposed that some grounds hare impeded, and that others have promoted, drying, and that, consequently, the first or latter painting have dried more or less speedily; and for this there may be some reason according with the materials of the grounds. Litharge and burnt umber are in this and other respects useful additions in the grounds. The best remedy in every case of ill-drying from the grounds will be to sponge with a weak solution of sugar of lead in water previously to the first painting.

With respect to the improvement of the ground of a picture, it may be worthy of experiment and inquiry, whether caoutchouc, judiciously introduced upon a proper basis, would not afford the best of all grounds for oil-painting?

We have only to remark, with respect to painting in water-colours, that pure paper is essential to the permanence of colouring: if the bleaching acid employed in manufacturing remains ever so little in the paper, both the texture and the colours will suffer in permanence; and if, in the concern of the paper-maker to neutralize such acid, the paper be surcharged with alkali or alkaline earths, they will prove no less injurious in these and other respects: it is highly necessary, therefore, that these circumstances, together with the proper sizing and aluming of paper, should be attended to, and that, if wanting, or if the paper happen to have been long made, the artist should reprepare it himself, by a judicious application of weak isinglass size and roach alum. And as to the practice of miniature-painting, ivory and unglazed porcelain afford excellent and adequate bases entirely free from injurious action on colours.