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-painting 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, &c. 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 have impeded, and that others have promoted, drying, and that, consequently, the first or latter paintings 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.

CHAPTER XXV.

ON PICTURE CLEANING AND RESTORING.

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THE diseases and disorders which injure and destroy pictures are almost as numerous as those of animal nature, and dependent on similar causes and accidents hence picture-cleaning has become a mystery, in which all the quackery of art has been long and profitably employed, and in which every practitioner has his favourite nostrum, for doctoring, which too often denotes destroying, under the pretence of restoring and preserving. The restoration of disfigured and decayed works of art is, nevertheless, next in importance to their production; and, as it chiefly relates to the colouring of pictures, it is a part of our inquiry with which we will close the technical portion of our work.

Of the importance of this minor function of the art of painting, a just estimate may be formed by considering that there is no limit to the time an oil-painting may be preserved with all the merit of its original production, by ordinary care and attention, that a picture so preserved is of even increasing value and interest; and that, but for

the spirit of negligence and destruction, modern art might have been elevated, the heart might still have been warmed, and possessors ennobled, by the famed works of a Zeuxis, a Protogenes, or an Apelles, the loss of which we have so much reason to deplore.

The medication of pictures is then no mean subject of art, but is, when divested of quackery and fraud, as honourable in its bearing as any other form of healing art; and, to be well qualified for its practice, requires a thorough education and knowledge in every thing that relates to the practice of painting, or the production of a picture, but more particularly to its chemical constitution and colouring. As, however, a picture has no natural and little of a regular constitution, it will be difficult to give general rules, and utterly impossible to prescribe universal remedies for cleaning and restoring pictures injured by time and ill-usage; we will, therefore, briefly record such methods and means as have been successfully employed in cleaning and restoring in particular cases, with such cautions as seem necessary to prevent their misapplication, confining our remarks to oil-paintings in particular.

These are subject to deterioration and disfigurement simply by dirt,—by the failure of their grounds, by the obscuration and discolourment of vehicles and varnishes,—by the fading and changing of colours, by the cracking of the body and