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Degrees of Real. The most important reason behind all the scholarly furor is that, with fully accredited Rembrandts costing millions apiece, the 20th century has become far more persnickety in its definition of genuineness than were the 18th and 19th centuries, or for that matter, even Rembrandt's contemporaries. When the artist began plying his trade in Amsterdam in the 1630s, he acquired—as did most successful painters of his day, most notably Rubens—a studio of between 40 and 60 "pupils," who in essence acted as artistic extensions of the master's right arm.
As commissions came pouring in, Rembrandt would direct the composition of a painting, but the pupil, who in many cases was a gifted artist in his own right, would do the work. The master touched up the results and, if the work came up to his standards, even signed the picture. The result, as one National Gallery official in Washington puts it, with a shadow of a smile, is that "there are degrees of real Rembrandts."
Moreover, before the invention of photography, line-for-line copying was not only a common method used by neophyte painters to educate themselves but also a perfectly legitimate means of reproduction. Some 18th and 19th century "copies after Rembrandt" emigrated to the U.S., and were eventually willed to U.S. museums. These, as a rule, were tactfully accepted by most museums, in order not to offend wealthy donors, but rarely hung as Rembrandts, because they were the easiest to weed out. Chemical analyses, known and used for many years, reveal differences in paints and varnishes, since in most cases latter-day copyists did not try to use 17th century materials.
Hidden Handwriting. The knottiest problems lie in distinguishing between oils by Rembrandt and those done by his disciples. Signatures are no guide: not only did Rembrandt sign studio work, but in many cases the original canvas has been cut down—and the signature cut off. Copyists often added the artist's signature for the sake of verisimilitude. Some dealers have even been known to inscribe a spurious "Rembrandt" on top of a genuine "R. H. van Rijn." Though the artist's full name was really Rembrandt Harmensz van Rijn, the simple "Rembrandt" with which he signed his work in later years is better known than the full signature he used as a young man. Not even documents are much help: only seven letters by Rembrandt, referring to a few paintings, survive.
Most scholars rely on their knowledge of Rembrandt's telltale way of handling a brush, as distinctive as an ordinary man's handwriting. They are convinced that they can recognize Rembrandt's, even though it changed and evolved in the course of his 40-year-career. More information can be gleaned by probing beneath the surface of an oil with X rays and infrared photographs to see how the artist progressed from preliminary sketch to finished oil. In Rembrandt's work, the early draft was always much rougher, more bitter and cruel than the finished work, showing a constant spontaneous response of the artist to his model. Another trait, primarily visible in the portraits of Rembrandt's later period: the bright side of the face is prepared with white underpaint, the shadowy side is built up with glazes.