Here is another hypothesis—so ingenious that it arouses our admiration.
Suppose we take a large blackboard and draw a figure on it in brown chalk. The chalk will stand out light against the black background ; therefore we can mass it thickly for the high lights, and let the black show through the chalk for the deepest shadows. The portrait finished, transfer it on to a white cloth by laying the cloth on the blackboard. Behold we have a negative.
The thickest part of the chalk will now be the deepest shadow, and vice versa , just as in the Shroud. But the difficulty will be to give the modelled contours. We cannot outline our figure in chalk, because on our blackboard the chalk represented the high lights. This knowledge of technique is just possible in a frequenter of the studios of to-day ; but such a complicated process as we have described would have been impossible for the painters of the Middle Ages.
Here then is another step gained. We proved that the impressions on the Shroud are negatives ; now we know that such negatives could not have been produced by any pictorial process,
If it is claimed that the stains on the Shroud are the work of some painter, then we must assume that they were painted in the positive or ordinary manner ; further, we must admit that the chemical change wrought by the centuries which have elapsed since their imprint has transformed this positive painting into the negative which we have to-day. If this hypothesis is not tenable we must give up the idea that the Holy Shroud has been painted.
It is M. Chopin again who puts forward this new hypothesis.
He maintains that the picture on the Shroud, though not a real negative—i.e. a negative to our human eye—is nevertheless a photographic negative. He admits that the light colours, without undergoing visible inversion, may none the less have become impervious to photography, and thus even more inert than the shadows. He even suggests that the dark tones may in part have disappeared.
Our answer is that the human eye could perfectly well distinguish a negative picture on the Shroud.
The technical arguments used by M. Chopin to explain this possible change in the quality of the colours as regards photographic effect may be used with equal reason to support the hypothesis of some change which has taken place and is perceptible to the human eye. We will quote M. Chopin's own words:
" The flesh tints may have been painted with a mixture of white paint, which is usually an oxide of lead or of zinc combined with reds (sulphate of mercury), ochres, or naturally tinted earths ; the shadows may have been done with black paint mixed with the same ochres and natural or burnt earths, or even with bitumen. The lighter portions painted thickly and the shadows less so, in order to give depth.
"Painted in this way, which is the ordinary way, the Holy Shroud passed through critical periods, such as the fire at the Sainte Chapelle in 1532, when the constitution of its colours must have been considerably modified. The oxides of lead in the light colours must have become clouded by the sulphur of mercury in the reds, or by external causes, such as one always sees at work in old paintings. The natural earths may have been calcined by the heat—the bitumen burnt, evaporated, vanished !— all this is possible.
" What would now remain after the fire ? A rude image whereon all the light tints which had been mixed with white would be proportionately black, and portions of which would be more or less obliterated."
We can dispose at once of the theory regarding the fire of 1532, since, as we have shown in Chapter IV., the impressions were already negatives at the end of the fourteenth century.
We must also call M. Chopin's attention to the fact that painting executed in zinc medium does not get blackened by sulphur ; this, however, is of little consequence, painting with a lead medium being much more usual.
Again, it is not the case that the early painters painted in high lights very thickly, and it is not a good method to give depth to the shadows by painting thinly. To-day we know many painters do this, but always at the expense of the clairoscuro.
It is unnecessary to make too much of these trifling points. M. Chopin passes in review the causes which may produce alteration in old paintings.
Such alterations are generally produced by the action of sulphur on colours whose main ingredient is lead.
It is more difficult to see how the shadows could get darker in proportion to the light parts getting lighter, but this must be the case if our opponent is to prove his case.
We will ourselves cite an example far more convincing than any cited by M. Chopin in support of his own theory.
