When a picture restorer wishes to remedy a loss of paint due to a hole, a tear, or scaling off, he should confine himself to reconstituting that part of the picture from which the paint is absent and has had to be replaced by mastic. We do, not consider that he is morally entitled to go one-tenth of a millimetre beyond the area of the mastic.
If the paint is rubbed or worn away, a much higher technique is needed for its restoration, for it is a temptation to employ the easiest method, which would be to repaint it. On the other hand taste, power of selection, respect for the artist whose work is being restored, and a thorough comprehension of his art are needed to restore scumbling which one is certain has been worn away. In any case, no attempt should be made to repair the effects of wear unless they really interfere with the unity of the painting.
There was actually a time when great picture galleries re varnished all their paintings whenever an important personage was to make them an official visit. Similar errors have been com mitted in the actual restoration of pictures; there is absolutely reliable documentary evidence to show that in some of Rubens' pictures the red draperies have been entirely repainted, and that when there was an injury to an arm or a leg, the whole of the arm or leg was repainted. This was what happened also to Raphael's "St. George" mentioned above. One of the legs had been entirely repainted and these useless retouchings had to be removed. Many similar restorations may still be seen at the present time. There is another reason which explains why ancient retouchings, even if conscientiously carried out, are so conspicuous at the present day; this is that they were done with oil paints. This brings us to important aspects of the technique of retouch ing in the strict sense of the term.
It would at first sight appear normal to retouch oil paintings with paints prepared in the same way. It must not however be forgotten that all paints are subject to more or less rapid modi fication under the influence of time. In the case of tempera, fresco and gouache painting, the transformation is almost in stantaneous, while oil painting does not complete its development until months and sometimes many years after it was executed.
When that stage is reached, if the painting as a whole is in a good state of preservation, the colour harmony may be con siderably different from the colouring of the painting when fresh. Retouchings executed with oil paints, even very skilfully, on an old picture the colours of which have been modified by time, may be perfect when it is done, but will only remain so for a few days or at the most for a few months ; after that the new colour, which at first exactly matched the old, will in its turn be modified and will inevitably show. A clever restorer will of course take
pains to prepare a very light ground with unmixed colours and will cover this with a very light film of colour in order to arrive at the tone finally required, and in this way will do all that is possible to prevent the paint from changing a great deal. In spite of everything, however, the results thus obtained will only be satisfactory for a certain time, and attempts have therefore been made to mix oil paints with all sorts of other ingredients in order to retard the action of time on them. The next experi ment was to grind colours with turpentine and without oil. and in order to give them consistency, to paint with varnish with a resin and turpentine basis. This represented real progress. The colours were still subject to modification, but they changed more rapidly, and it was easier to foresee the future effect of restora tion. Most restorers still use this process. Although the varnish complicates the work, the colours remain easy to handle, and it must be admitted that restorations executed in this way may last for a long time, particularly if the ground is carefully prepared and the final coat very light.
The object in view is not, however, to carry out restorations which will have to be done over again, even at a comparatively distant date. After a number of experiments with water-colours, gouache colours, etc., it was decided to try restoring pictures with brilliant tempera colours, i.e., colours ground with yolk of egg. It is true that this process renders the task of the restorer in comparably more difficult. Tempera colours dry very rapidly; long practise is necessary to make them produce the fluid effect of certain old paintings. Moreover, the colour becomes much higher in tone after a few hours, and the extent of the change varies enormously according to the mixture used. It is thus necessary to foresee what the colour will be after it has changed in order to bring it into accord with the fixed colour of the original work. It will readily be understood that much experience is needed in order to obtain satisfactory results with tempera colours. On the other hand, the change which takes place in tempera colours after a few hours is complete in a few days. In other words, the transformation which takes many years for oil paints is accomplished in a week. Moreover, tempera painting takes varnish just as well as oil painting.