Invasive analyzes - Cross section and stratigraphic analysis
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The analytical and diagnostic investigation techniques applicable to works of art are usually divided into two broad classes:
– invasive, which require the taking of a sample, i.e. the removal of minimal quantities of material from the work, to be subjected to various tests (chromatography, histochemical coloring, etc.);
– non-invasive, which, like X-ray radiography or infrared reflectography, can be performed directly on the work by interacting with the surfaces through various forms of energy.
The invasive methods are in turn distinguished in destructive (the first and for a long time the only ones to be used), which involve the modification or destruction of the examined sample and non-destructive which, such as reflectance spectrophotometry or X-ray fluorescence, allow the study of the nature and structure of the sample without it being modified or altered.
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When the possibility of choosing among the various investigation techniques arises to the solution of a specific problem that must be investigated, the current trend is to give as much space as possible to non-invasive techniques. However, it is important to underline that among the techniques that require sampling, the only ones used are the ones that require very small samples. Sampling, that is the choice of the most significant and representative areas, is arranged in such a way as to limit itself to the truly indispensable samples and to the less important areas in relation to the expressive content of the work. In addition, samples will be taken preferably on the margins of existing lacunae dby experts and with techniques and tools chosen on a case-by-case basis: scalpels, syringes, adhesive tapes, core drills, brushes, etc.
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The need to resort to invasive tests is linked to the solution of specific analytical problems (for example for the identification of organic and inorganic materials, of natural or artificial origin) and to a series of limitations related to non-invasive methods (some imposed by the materials themselves as in the case of X-ray fluorescence which is activated only by inorganic products).
The destruction or modification of the sample is necessary for example with chromatography which allows to separate and dose the components of a mixture or with microanalyses which provide for the identification of the materials through the observation under the microscope of crystal formations or characteristic colors as a result of induced chemical reactions.
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The possibility of keeping the sample intact (invasive non-destructive examinations) makes it possible to continuously update it according to the development of the analysis techniques. The manipulation of the sample therefore allows its incorporation in polyester resin
and a perpendicular cut on the surface to allow the study in section (stratigraphic section). This allows to obtain numerous analytical data on the composition of each single layer, on their succession, on any alteration or modification to them, contributing to the development of methods capable of stopping any degradation processes, the UV fluorescence identification of additions or remakes, as well as to clarify some aspects of the constituent materials making significant contributions in the field of attributionary historiography.
Estratto da: Artis (Art and Restoration Techniques Interactive Studio), Direzione scientifica: Manfredi Faldi, Claudio Paolini. Cd Rom realizzato da un gruppo di istituti di restauro europei, con il determinante contributo della Commissione Europea nell’ambito del programma d’azione INFO2000.
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