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 invariably 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 on 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 some varnishes of the hard resins, 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 painting.

With respect to the improvement of the ground of a picture, it may be worthy of experiment and inquiry, whether caoutchouc judiciously intro

duced 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 porcelain afford excellent and adequate bases entirely free from injurious action on colours.

CHAP. XXV.

ON PICTURE-CLEANING AND RESTORING.

THE diseases and disorders of pictures are almost as numerous as those of animal nature, and dependent on innumerable accidental circumstances; 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.

This 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 surface, by damp, mildew, and foul air,-by mechanical violence, by injudicious cleaning and painting on,-among a variety of other natural and accidental causes of decay.

The first thing necessary to be done in cleaning and restoring is to bring the picture to its original plane and even surface, by stretching, or, if sufficiently injured to require it, by lining, which, with the transferring of pictures to new canvases, is an operation admirably well performed in London by experienced hands. In cases of simple dirt, washing with a sponge or a soft leather and water is sufficient, with subsequent rubbing of a silk handkerchief; which latter, occasionally used, is eminently preservative of a painting.

After restoring the surface to its level, and washing, the next essential in cleaning is to remove the varnish or covering by which the picture is obscured; and this in the case of simple varnishes is usually done by friction or solution, or by chemical and mechanical means united, when the varnish is combined, as commonly happens, with oils and a variety of foulness.

In removing varnish by friction, if it be a soft varnish, such as that of mastic, the simple rubbing of the finger-ends, with or without water, may be found sufficient; a portion of the resin attaches itself to the fingers, and by continued rubbing removes the varnish. If it be a hard varnish, such as that of copal, which is to be removed, friction with sea or river-sand, the particles of which have a rotundity that prevents their scratching, will accomplish the purpose.

More violent means are sometimes resorted to, but never without danger or injury.

The solvents commonly employed for this purpose are the several alkalies, alcohol and essential oils, used simply or combined. Of the alkalies, the volatile in its mildest state, or carbonate of ammonia, is the only one which can be safely used in removing dirt, oil, and varnish from a picture, which it does powerfully; it must, therefore, be much diluted with water, according to the power required, and employed with judgment and caution, stopping its action on the painting at the proper time by the use of pure water and a sponge. These cautions are doubly necessary with the fixed alkalies, potash and soda, which ought to be employed only as extraordinary means of removing spots that will not yield to safer agents. Spirits of wine or alcohol, and ether, act in a similar manner, and their power may be in like manner tempered or destroyed by dilution with water. The uniform disadvantage of all these agents is that they obscure the work, so that the

operator cannot see the good he is doing, or the mischief he may have done, in the progress of his work, except by revarnishing or oiling out.

This inconvenience is, however, avoided by the safer and better mode of cleaning and removing the varnish at once by spirit of wine, tempered more or less with oil of turpentine: the practice in this case is to apply the spirituous mixture to the surface of the picture with a brush, or with carded cotton ; and when, by the motion of either, the liquid has performed its office, its farther or injurious action on the design is to be stopped by another brush or cotton imbued with linseed oil, and held in the other hand; thus alternately proceeding with these tools, till the cleaning and removing the varnish is accomplished. The brushes act rather the better of the two, but the cottons imbibe the dirt and foul liquid, and are then easily exchanged for new ones. The great advantage of this method is, that the design and colouring bear out, and the progress of the cleaning is apparent.

If more action is requisite than the spirituous mixture affords, the more active essential oils may be employed, or the pure alcohol, with the addition of sulphuric ether in extreme cases; and if their action be too strong, the turpentine alone may be employed, or linseed oil added to the mixture.

Many other methods of cleaning have been recommended and employed, and in particular instances, for sufficient chemical reasons, with success; some of which we will recount, because in an art so uncertain, it is good to be rich in resources, although the legitimate doctor may deem them empirical.

In an instance of difficulty, where much care was required, we succeeded upon a picture entirely obscured by various foulness, by varnishing over the whole, and when thoroughly dried, removing the varnish by the above means, bringing off with it the entire foulness and original varnish of the picture, with which, in this instance, the new varnish had combined. Strong solution of gum or glue will sometimes effect the removing a foul surface mechanically, but requires care.

A thick coat of wet fuller's earth may be employed with safety, and, after remaining on the picture a sufficient time to soften the extraneous surface, may be removed by washing, and leave the picture pure,—and an architect of the author's acquaintance has succeeded in a similar way in restoring both paintings and gilding to their original beauty by coating them with wet clay.