The light microscope, using only photons rather than electrons as an imaging medium, is demonstrably the single most useful tool for the authentication of art and archeological objects. It is the only technique able to identify all paint media, pigments, and supports. With the identity of these components and their known dates of first use, the microscope establishes the earliest possible date for that object. Microscopy reveals the molecular identity rather than the elemental content of each of the components. Elemental analysis, by any other means, would not differentiate between many pigments that have similar compositions, e.g., iron earths (Fe₂O₃(DOT)nH₂O), red lead (Pb₃O₄), and massicot (PbO) or litharge (PbO), chromium oxide (Cr₂O₃) and viridian (Cr₂O₃(DOT)2H₂O), alizarin and madder (both 1,2-dihydroxy anthraquinone), three forms of calcite (CaCO₃), three forms of vermilion (HgS), etc. X-ray diffraction can identify only crystalline compounds and is useless for alizarin, madder, prussian blue, cobalt blue, smalt, carbon black, charcoal, van dyke brown and iron earths other than hematite. Infrared absorption requires highly skilled analysts and can't readily detect zinc white, massicot, or litharge or other diatomic molecules.

Accelerator Mass Spectrometry is used to detect the cosmic- ray-produced radioactive isotope, ¹⁴C, in the organic material associated with artworks. The measured radiocarbon content of an artistic artifact, combined with a tree-ring calibration curve that relates radiocarbon content to calendar age, gives information concerning the age of the artifact. This age information can then be used to assist in establishing the authenticity, or lack thereof, of the artifact. Applications of the technique to the study of several artworks will be described.

The archaeological gold contains usually 10 - 80 ppb U²³⁸ as a trace element. Under radioactive decay U²³⁸ produces approximately 3.7 alpha particles in 1 mg of metal containing 1 ppb of U²³⁸ per one year. The a-particles (as He⁴) are accumulated inside the metal as gold appears to be extremely retentive for He. When the metal is heated to its melting point all accumulated helium escapes thus establishing 'zero-time' for the He-radiogenic clock. This way the clock starts at the point when the object is manufactured. The new ultrasensitive He-recycling mass-spectrometer providing the sensitivity approximately 2.5(DOT)10⁴ He-atoms has been developed and manufactured for the museum usage. Taking a gold sample of 5 - 10 mg with typical 10 ppb U content in gold one can expect to find 185,000 - 370,000 He atoms in metal manufactured 1000 years ago. These quantities can be easily detected by the instrument thus providing rather reliable authentication (and in some cases -- absolute dating) tool for detection of fakes.

Dendrochronology is a discipline of the biological sciences which makes it possible to determine the age of wooden objects. Dendrochronological analyses are used in art history as an important means of dating wooden panels, sculptures and musical instruments. This method of dating allows us to ascertain at least a 'terminus post quem' for an art-object by determining the felling date of the tree from which the object was cut, in other words the data after which the wood for the object could have been sawn. The method involves measuring the width of the annual rings on the panels and comparing the growth ring curve resulting from this measurement with dated master chronologies. Since the characteristics of the growth ring curve over several centuries are unique and specific to wood of differing geographical origins of wood, it is possible to obtain a relatively precise dating of art-objects. Since dendrochronology is year specific it is more accurate than other scientific methods. But like other methods it has limitations. The method is limited only to trees from temperate zones. And even among these, some woods are better than others. A dating is possible for oak, beech, fir, pine and spruce. Linden and poplar are not datable.

Several techniques involving X-rays are routinely applied in the study of works of art. These include radiography, X-ray diffraction, and X-ray fluorescence (often coupled with an electron beam instrument such as a scanning electron microscope or microprobe). Radiography provides information on condition and previous restorations or repairs. In the case of sculptures, the technique also sheds light on the manufacturing process; in the case of paintings, otherwise invisible aspects of the painting technique are often revealed. X-ray diffraction is used primarily to provide specific identifications of crystalline inorganic materials, such as pigments, corrosion products, etc. Stand-alone X-ray fluorescence systems provide qualitative or quantitative information on elements in an object or sample. X-ray fluorescence in electron beam instruments provides general elemental composition information, and in some cases is used for quantitative analysis of a wide variety of inorganic materials, including metals, ceramics, glass, stone and paint samples. This analysis also provides microstructural information at the same time. All three procedure can provide important evidence when the authenticity of an artifact has been questioned.

Painting techniques and materials employed by Caravaggio are analyzed by X-ray fluorescence and radiography of unquestioned genuine works and compared to replicas of his earlier work. This approach is important in the attribution of recently discovered paintings and copies from the XVII century.

ARCOS is a high speed double beam spectrophotometer system which determines contact-free the spectral and topographic data of any of the single cells of e.g. a picture in form of an identification index (IID). The IID is a measure for the agreement/disagreement between two cells as of their material pigment composition (resin, binder, additive, etc.). A period comparison of two paintings (attributed to picasso) is done and the results of IID are shown as 3D graphic.

There is a law in my country, France, which is supposed to deal with forgeries in several fields including the arts. It is called the 'Erreur sur la substance' Error on substance. It means that after an object was sold, if the former owner or buyer can prove that he or she was not aware of the substance of the object, meaning in this particular case its authenticity or lack of authenticity, the sale can be cancelled. We should keep in mind this theme of what makes up the substance of a work of art when we study forgeries. The making of forgeries is probably as old, if not as art itself, at least as old as the art market. Making forgeries is a response to the demand of the market, in close connection with art historical activity. The critics and the art historians inside and outside museums contribute to the fame of artists, which stimulates the art market, the rise inspires the demand for more works, thus the making of copies and of forgeries. Before presenting a few studies made in our laboratory, I would like to make a short survey of the examination and analysis methods we use that particularly help us in the detection of forgeries of old masters paintings. The Research Laboratory of French Museums (LRMF) generally makes a distinction between two main categories of working methods: The first one, Examination, concerns mostly photographic and imaging techniques. The second one, Analysis, is a structural study for which the goal is the identification of the constitutive materials of a work of art and how they were used.

Contrary to popular belief, there is a very great difference between the detection of forgery versus the authentication of works of art. Sciences is generally very good at producing evidence of falsification but often is equally poor at proving authenticity. The primary reason for these gross differences is that connoisseurship and art history are more strongly involved in the process of authentication than are scientific testing and analysis. There is also a pronounced lack of substantive interaction between art conservation professionals, scientists and art historians. The case of a recently discovered painting by John Constable will be used to illustrate the difficulties and opportunities of a balanced and systematic approach to the process of authentication. There is much more than science involved in such endeavors and this would not surprise anyone who has attempted to introduce works of art through non-traditional channels. Great problems arise when the curatorial community is asked to consider works that do not so easily 'fit' into a neat art historical period or stylistic pigeonhole. Connoisseurs often will only accept the best works of an artist and discount the inevitable products of the artist's evolution -- less accomplished works. Scientific principles and technical evidence can and must be used in order to elevate the practice of authentication.

The origin of works of art can often not be attributed to a certain artist. Likewise it is difficult to say whether paintings or drawings are originals or forgeries. In various fields of art new technical methods are used to examine the age, the state of preservation and the origin of the materials used. For the examination of paintings, radiological methods like X-ray and infra-red diagnosis, digital radiography, computer-tomography, etc. and color analyzes are employed to authenticate art. But all these methods do not relate certain characteristics in art work to a specific artist -- the artist's personal style. In order to study this personal style of a painter, experts in art history and image processing try to examine the 'structural signature' based on brush strokes within paintings, in particular in portrait miniatures. A computer-aided classification and recognition system for portrait miniatures is developed, which enables a semi- automatic classification and forgery detection based on content, color, and brush strokes. A hierarchically structured classification scheme is introduced which separates the classification into three different levels of information: color, shape of region, and structure of brush strokes.

The Kodak DCS460IR Digital Camera has been applied in the Hermitage Museum for photographing the museum objects in visual luminescence under UV, in the near IR and for X- radiography in the 10 - 80 kV range using the fluorescent screens. We found the instrument to suit quite well for the qualitative examination, providing the possibility for substitution of the corresponding conventional photographic materials. In case of the near IR the sensor permits to work in the range where the infrachromatic materials are not commercially sensitized. The quantitative estimates for the images were difficult at the same time.