Infrared reflectography is the most effective technique for revealing the presence of preparatory drawings made by the artist on the ground preparation layer and covered over by paint layers. This type of examination can provide different types of data that allow the art historian to make comparisons of various hypotheses. Verifications can be made on a single painting as well as on the work of an individual artist. In addition, the data can be compared with other methods that can furnish information on the techniques of a given period in history. This type of examination reveals a series of elements that were destined to be hidden from view, and therefore not only does it increase documentation possibilities, but it allows for a sharper reading of the work itself. In this manner, the work is revealed as a final image of a creative process during which some specific moments can be identified. As has already happened with X-ray radiography, this technique has opened new paths for comparison and research that art historians will use more and more often.
In order to carry out this type of investigation, the painting is illuminated with everyday incandescent lightbulbs that are attached to a tension transformer and positioned before the work. The infrared radiations reflected from the painting are measured by a videocamera modified with a vidicon tube sensitive to I.R. rays up to a wave length of 2,000 nanometers having a sensitively peak at about 1,300nm. This instrument also has a filter that can limit the illumination to a specific I.R. band. The signal is converted into a black and white image that is immediately visible on the screen of a normal high-resolution television monitor. At this point, the capture of the images can be made by photographing the monitor with a still camera or by registering the sequences on a magnetic tape. (It is also possible to digitalize the image directly onto a personal computer equipped with a video card).
Assembly of the images
Diagnostics. Infrared reflectography The digital elaboration of the images obtained with the I.R. videocamera improve the legibility and can be used for re-composing the entire drawing made with the single tapings. In fact, due to the low resolution of the television system only small areas in succession need to be taped. They can be then put together in a type of mosaic. The quality of the images can be improved by using a CCD high-resolution videocamera instead of a traditional infrared videocamera with the vidicon tube. The extension into a lower wave length (1,100 nm), is compensated by the greater sensitivity and by the low signal/noise rapport. In addition, even though the paint film has to be thinner at these wave lengths, it must be remembered that at greater wave lengths there is less reflectance from the gesso in the ground preparation and consequently a loss of contrast between the image and the underdrawing.
Different permeability of the layers
In order to overcome these problems, an infrared scanner has recently been constructed that is capable of analysing large areas up to a square metre in size. This instrument can furnish images that do not have geometrical distortions, and that have excellent contrast and uniform lighting.
The types of images that can be obtained however, are tied to the permeability by the infrared wavelengths into the layers of colour. This tendency is determined not only by the chemo-physical characteristics of the pigments, but also by the thickness of the paint and the characteristics of the underlying components that can be seen only if there is a difference in their reflectance. For example, a drawing made with iron gall ink will be difficult to distinguish because its reflectance is very similar to that of a ground preparation made with a gesso and glue base.
The drawing under the surface
The fact that a drawing is not evident under the painted surface does not mean that the examination has been a failure. This is especially true if we do not limit ourselves to isolated research on one work, but we look toward a systematic application consistent to a particular artist or period. To this end, an archive of data can be formulated that can allow for a comparative judgement. The preparatory drawing was used by some artists as a reference to follow for every brushstroke. Thanks to a cross-hatched or stained chiaroscuro it could be used to create a greater depth in the shadows. At times a very free sketch that the artist more or less followed was. The thickness, the intensity and the fusion of the drawn lines – as well as their absence – are therefore all elements of equal value when a broad comparison is made.
Manfredi Faldi – Claudio Paolini
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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