In the upper church at Assisi there are a series of frescoes hitherto attributed to Cimabue. Tourists as a rule pay little attention to these paintings, which are much dilapidated and extremely ugly; but last year a friend of mine, knowing of the experiments undertaken at the " Sorbonne " in connexion with the Holy Shroud, was struck by the fact that in the frescoes representing the Crucifixion a whole group of figures appeared to be painted negatively. My friend called my attention to this remarkable fact, and I at once procured a photograph of the fresco. An enlarged fragment of this photograph is reproduced on Plate vi.
The upper part of the plate shows the painting in its actual condition. The sky remains light, and the aureole of the figure on the left is still represented in a normal manner; but the heads and draperies are absolutely negative. The transposition is not complete throughout the fresco, and the figures below a sharply defined limit remain positive. The limit corresponds to a change in the plaster on which they are painted. Under the facsimile of the fresco we have placed the reproduction of the photographic plate, giving the portions which were negative in the fresco restored to their positive condition, whilst the sky and the figures on the left are now negative. The plate has been reproduced, so as to give this aspect of the fresco, and also in order that the reader may easily recognize the figures and may get the impression of the work as the painter intended it to be.
This inversion of the picture, the result of chemical change, is particularly striking in the head which occupies the centre of the picture. Before its inversion this head was a really remarkable portrait, but the flesh-tints have now become almost black and the beard completely white. M. Chopin could hardly desire to find a seemingly better example in favour of his thesis. The chemical change produced in so regular a manner hardly alters the artistic value of the painting, and many of the heads are very beautiful.
Granted that it is possible for a painting such as the fresco at Assisi, or a Byzantine icon, to become absolutely negative under certain exceptional conditions, we still contend that it is impossible for the figures on the Holy Shroud to be affected in the same way. For this we give three distinct reasons, the importance of which will be at once recognized.
In the first place, the very fact that the material of the Shroud is very light and supple proves that it could not carry the amount of paint requisite for a picture which could change in the manner described. If such a painting had ever been made on this cloth it would have long since crumbled away and have left hardly any trace.
Let us consider the question of a painting either in oil or water-colour, these being the only methods which need be considered when chemical inversions are in question. Oil-colours with a lead base, whether mixed with sulphide of mercury or not, constitute insoluble substances ; they can only be moistened with oil or water. They then form a semi-fluid paste, and can be spread over the required surface in layers, which may vary in thickness.
When water is used to moisten such paint, a little gum or albumen is added to give the required elasticity and to make it adhere to the canvas. That done, the painting may be preserved as long as the cloth remains flat. Even to roll it up would be unwise, unless it were rolled over a cylinder of fairly large diameter. It would be quite impossible to fold it like a handkerchief, as the painting would crumble off at the folds almost at once. Also, so long as the picture remained on it, the cloth would lose all its flexibility, for the threads would be clogged together by the paint.
Consider any oil-painting. Oil-paint adheres much more firmly than water-colour, and does not flake off so easily. We all know how rigid a canvas becomes when covered with oil-paint. . Even before beginning to paint with colour, a coat of white lead is commonly spread over the canvas, sufficient to make it quite stiff.
These facts are well known. Every one has handled or seen canvases prepared for oil painting. Painting with gum is less well known, being now very little used for large pictures. It is used sometimes for screens made of light gauze, and if the painting is taken out of its frame and rubbed between the fingers the paint quickly scales off in powder and is effaced.