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The physicist and the antiquarian

This is not the title of a fable. It is, on the contrary, a very real thing. However unlikely it may seem to you, physicists sometimes do not disdain to come out of their ivory tower. to mingle with the debates that take place between certain antique dealers and their overly trusting clientele.

The laboratory offers, in fact, a certain number of means of control and observation unsuspected by laymen... and counterfeiters. We can name a few.

Optical means, first. We know that our eye can experience the same colored impression while being illuminated by lights of a different nature. Suppose, for example, that an object sends on our retina blue and red simultaneously, it will have the same effect on us as another which sends us pure violet; but if we place in front of our eye a red glass, like that of a photographic lamp, this glass, which only lets through the red rays, will let through part of the light coming from the first object (the red) whereas it will completely stop all purple light coming from the second one. One will therefore appear clear and the other dark.

This admitted (and for many other colors than purple, what will happen if we restore, for example, an old painting? The artificial colors that painters use are all colors that send us mixtures that are quite complexes of simple rays, rays which necessarily differ greatly depending on the origin of the paint used, while giving us the same colored impression. To the naked eye, the retouching will merge with the original colors; but it will often suffice to interpose in front of the eye a colored glass to detect their difference.

Moreover, this will sometimes appear on a simple photograph of the painting, especially if an orthochromatic plate has not been used, because ordinary plates do not have the same sensitivity as the eye for the various simple radiations; red, for example, is found there very attenuated, and the colors which contain it will be more modified than those which, without containing any, gave our retina the same impression.

Postage stamps which, without being antiques, are, like these, subject to many tricks, are also subject to colored glass for their examination. A stamp viewed through glass of its own color will seem to disappear almost completely, while cancellations or overprints of a different color will appear very black against a light background.

But it is above all X-rays that provide us with precious means of investigation. I will only recall for the sake of their well-known property of passing through objects opaque to light, such as wood or flesh. They can even penetrate (especially the harsh rays given off by cathode-ray tubes) through stones and metals, the less easily the heavier they are. As they can, on the other hand, impress a photographic plate placed beyond the object traversed, they will easily reveal, by its shadow, any heavier object concealed in another. see as easily as a wounded man a bullet in his body.

Paintings, too, often hold surprises for those who “pass” them through X-rays. You should know that old colors are, for the most part, very different in composition from modern ones. The latter are very often aniline colors, therefore derived from organic materials, quite light, therefore transparent to X-rays. The old colors, on the contrary, were of mineral origin, heavy and opaque, copper grounds, like the greens and certain blues, iron salt like Prussian blue, lead salt like "silver" white, lead and chromium salt for yellow, mercury salt for vermilion or cinnabar. If, therefore, one examines by X-rays an old painting, the chipped parts of which have been ingeniously redone, these will appear white on an almost uniformly dark drawing. A Trouillebert sold for a Ruysdael will be almost completely transparent when it should be opaque.

But the most amusing thing is that the X-rays have made it possible to find old paintings on canvases that were later covered with more modern paint to take advantage of a venerable-looking support. Thus, under a painting attributed for a long time to Van Ostade, but which it is certainly only an imitation above all for at most a few glosses, we found under the peasant scene visible in ordinary light a group of barnyard animals whose silhouettes appear alone in an x-ray. And that, of course, without damaging the so-called Van Ostade in any way.