Recently the X-ray has been added to the expert's equipment. And it has become exceptionally useful, when useful at all. It combines in one test opportunities to study the physical consistency of a picture and the psychology of the artist. It reveals the varia tions in density and thickness of the material forming the work of art, thus recording the touches of pigment which differ from one another in kind as well as in handling ; it also may show the strokes of underpainting or preparatory workmanship not visible on the surface. Consequently the X-ray film records an inside view and adds to the technical data at the critic's command.
The efficacy of the X-ray depends on two conditions. First, the pigments in use to-day lack generally the density of the pigments in use prior to about 1800, a date that seems to mark the beginning of extensive research in the chemistry of colours. This means that a forgery in chemically compounded materials differs in density and thus differs in appearance on the X-ray film from a picture painted in mineral and earth colours that have been coarsely ground by hand. Secondly, the degree of skill and sensitiveness of an artist is usually manifested in his brushwork or habits of applying pigment. And this means that a modern forger, even if he uses dense colours ground ever so skilfully in imitation of an old method, nevertheless leaves traces of his personality and peculiarities as an artist in his brushwork.
Unfortunately the X-ray itself has not been perfected to the point where the critic can wholly control it ; the rays, for instance, have their effects in straight lines and must penetrate all of an object in order to be recorded on a sensitized film. For the purpose of art criticism it would be valuable to direct the rays through only one layer of material, that is, to obtain an X-ray of pigment only, without having to penetrate a ground of gesso or lead paint, as well as a panel or canvas, sometimes coated on the reverse with more material, all of which confuses the appearance of the artist's workmanship. This limitation, however, does not affect the X-ray's
value under better conditions. Given a picture forged in modern material, regardless of style or craft, the X-ray shows the lack of density of the paints and the incompleteness or shallow nature of the cracks which, when hammered or scratched on the surface, differ from the cracks which have developed with the slow drying and long condensation of the materials. If the forger has utilized an old picture as the basis for a new one, the X-ray easily reveals the hidden part. Repair work that borders on the fraudulent is consistently visible in the X-ray test, since no amount of skill on the restorer's part can blend fresh paint and old paint so accurately that they match in density, thickness and style. Finally the X-ray aids a just choice between two pictures that are alike in pigment, aesthetic effect and age. By revealing which picture contains changes in design or corrections characteristic of an artist creating a picture, and which does not, it betrays the mere imitator. With the X-ray's help the critic may explore the process and even the progress in the making of a forgery.
In this way the expert keeps one jump ahead of the forger, who is ever forced to new efforts as the scientific aids in the detection of false painting are developed.