Varnish is made of resin dissolved in oil, essence of turpentine, or alcohol—to mention only the most usual solvents. Just as the fusing of siliceous sand, itself an opaque substance, produces a transparent substance, glass, so the solution of resin produces a transparent substance, varnish. The principal use of varnish is that it preserves the intensity of colour of pigments. In addition, however, even the extremely thin layer of varnish which is all that should be applied to a picture has an extraordinary isolating effect. Varnish protects a painting from emanations of all kinds which might destroy the brilliance of its colouring as well.as from the dust which progressively accumulates on it owing to the damp ness of the air.
There are some old pictures which have never been varnished; in such cases it is an exceedingly difficult matter to remove the impurities adhering to the painting itself ; the painting is also injured by the fact that its different colours have been differently affected by chemical action. For example, it has occurred that in a picture representing the setting sun, what was originally the brightest part of the sky has been transformed into a dark patch because it contained vermilion, a colour containing sulphide of mercury, which is particularly liable to deteriorate. This would not have occurred to nearly the same extent if the painting had been varnished. The influences which damage paint also damage varnish, but the varnish can be removed, and the painting itself is then found intact, without injury to the specially vulnerable colours.
Varnishes are of course not all of equally good quality, and they may become darkened to a serious extent. This is generally due to the excessive use of varnish often made by collectors or dealers who want to give an appearance of increased freshness to the pictures in their possession. Even the best varnish, however, does not retain its specific qualities if it is applied at more than a certain thickness; it darkens and becomes opaque.
It may happen that a picture requires revarnishing. Before this is done, however, an attempt should always be made to regenerate the old varnish. If the old varnish has ceased to be transparent, it is possible to re-dissolve the molecules of resin which cover the painting by means of alcohol vapour. This is an ingenious method by means of which it is possible to restore the varnish to its original condition, which it will retain for a considerable number of years, without loading the painting with a fresh coat of varnish. Thus was Rembrandt's "Ronde de Nuit" restored in 1889 (fig. 3).
In order to regenerate old varnish, the picture must first be wiped, and then washed by means of wads of cotton dipped in water without soap. This.removes the dust which has been more or less fixed on the picture by the moisture in the air. A receptacle (I) the exact size of the picture and 15-2o centimetres deep is then prepared ; a wooden box will serve.
The bottom of the receptacle is covered with a layer of down thoroughly soaked in 90% spirit. (2) The picture is laid on the box with the painted side downwards, i.e., facing the down soaked in spirit. The time required for the regeneration of the varnish varies in the case of different pictures from 15 minutes to several hours; in some cases the desired effect is not produced at all.
If a picture has been damaged and needs to be restored, or if the colours are altered by too many coats of varnish, it needs de varnishing. One is often painfully impressed by seeing famous masterpieces in the various museums of Europe the colours of which are completely distorted by the darkening of the varnish; and a general de-varnishing of thousands of pictures might be undertaken with advantage. In theory it would be better to see great pictures as the artist painted them rather than as they appear under the influence of old varnish. Unfortunately, however, it must be admitted that the work cannot be carried out because there are not enough expert workers available. It is therefore better to exercise great caution, and not to be in too great a hurry to restore fine pictures, but to wait for the day, which must soon come, when a sound tradition has been created. It is only fair however to recognize the good work which has been done in the past by intuitive artists who have tried processes which, if they had been developed, might have hastened that day. But the experi ments which were tried were isolated, and no tradition was created. Thus only too many restorers have, with a desire to avoid trouble, tried to de-varnish pictures simply with a mixture of essence of turpentine and alcohol. This method is easy and rapid and un fortunately it sometimes gives excellent results. We say "unf or tunately" because the fact that the method is sometimes successful leads to its being applied in all cases, and this is to be regretted because the method sometimes causes damage which cannot be foreseen. Some pictures which are painted strongly, in light tones and without scumbling will stand the process perfectly well; in others the surface is at once spoilt, and in addition, alcohol is dangerous for dark and transparent tones and for all scumbling.