Not only is varnishing sometimes badly done, but it is done too soon and at the wrong moment. Since varnish solidifies very quickly, it is quite obvious that it should not be applied until a painting is perfectly dry. This will prevent it from softening the paint even to the slightest extent, and thus causing it to crack by drying differently. Care should also be taken never to varnish a picture while it has dust on it; otherwise impurities will remain between the paint and the varnish and will in time spoil the pic ture. The coat of varnish should be extremely thin and applied firmly and regularly, since it loses its qualities and dries cloudy if it is put on thickly. Varnishing should not be done on a wet day, or moisture will be imprisoned between the varnish and the canvas. If this precaution is neglected, there will be a bluish film over the picture.
One of the most frequent stumbling-blocks for the painter is the result known as "matt" effect. The dulling of the paint, which has already been mentioned as one of the reasons for which varnish has to be used, may be produced while the picture is being painted, and will be especially marked if the first sketch is painted over before it is properly dry. The oil of the second coat may be absorbed so rapidly that the last touches which are put on appear to be of a dull grey nothing like the real colour. The "matt" effect at once disappears if any liquid is passed over the picture ; hut it often causes worse damage in the end, for the repeated imprison ment of moisture between the various layers of paint constituted by retouching will slowly but surely destroy the picture.
Siccatives, heavy oils and mixtures of all kinds are really re sponsible for much of the damage which is attributed to varnish. It is a matter of common knowledge that Delacroix was not always as strong on the technical side as he was lofty in his con ceptions. It is difficult to say which did most to destroy his paintings—the curious mixtures of liquids which he used, or the disastrous qualities of the materials which entered into the com position of the colours with which he habitually painted. We can obtain some idea of the splendour which his works ought to have retained by looking at those canvases which he executed simply, such as the "Algerian Women," "The Artist's Studio" and the "Still Life" in the Moreau-Nelaton collection in Paris. And very often that delicate artist Prud'hon made excessive use of heavy oil and bitumen, which caused immense cracks to form in his pictures, as for example in his "Christ" in the Louvre.
How then, it will be asked, is it possible to correct a passage which is seen in the course of painting a picture to be unsatis factory? And once the picture is painted, how can it be made fit for exhibition if it cannot yet be varnished without danger, and if the colours have a "matt" effect.
There are on the market a number of varnishes known as re touching varnishes, the merits or demerits of which cannot be discussed here. It may, however, be said that the lighter and more transparent the liquid to be applied over the piece which it is desired to repaint, the less danger there is that it will impair its freshness in time to come.
When the painting is completed, the general effect can be clearly seen if it is washed over with a mixture of white of egg, water and sugar. This preparatory varnish does not affect the colours in any way, and it can be sponged off with water when the time comes, about a year later, to apply the final coat of varnish which it has by this means been possible to defer.
Some have thought that varnish can be replaced by glass, and many have followed the example of the British picture galleries by glazing oil paintings. Glass is an effective protection against damp, especially if the back of the picture is felted. This is the reason why it is used in London, where the air is very damp. At the same time glass protects the picture from the fingers of clumsy visitors to public galleries, and from smoke in private houses. But it can never be a substitute for varnish ; indeed it often makes it impossible to see and appreciate a picture properly. Not only is glass no substitute for varnish, but it transforms the picture into a mirror, so that the spectator sees a reflection of himself rather than the painting.
The disadvantages of glass are not so serious in the case of pictures in which the general tone is light. The difficulty still exists, but it is lessened because white does not transform a sheet of glass placed in front of it into a mirror. Many modern painters who paint in light tones have refrained from varnishing their pictures, as it does not help very much to bring out light tones. The objection to varnish dates, naturally enough, from the Im pressionist period. The Impressionists wished their colours to produce as vibrant an effect as possible, and they therefore took care not to cover the paint with a foreign substance which was liable to develop in a way which they did not like.