The electromagnetic radiation that the human eye perceives is contained in an interval of wave lengths that goes from approximately 400 to 750 nanometers. Beyond these limits, the wave lengths are not visible to the naked eye, but they do retain the capacity to interact with the material just as visible light does (by absorption, refection, transmission etc.). The observation of these wavelengths requires special techniques. In the case of near infrared (to about 900 nm) normal photographic methods can be used. Of course, a special film that is sensitive to these wave lengths and a filter that blocks out visible light must be used. In this manner an image is obtained with different dark and light values – similar to a black and white photograph – but comprised only of infrared radiation.
Photographed in this manner, the surface of a painting will seem quite different from the way it normally appears. A layer of colour that is opaque in visible light, can appear partially transparent under infrared, thereby evidencing what is underneath, whether it be a pentimento, a preparatory drawing or a lacuna. Naturally, not the entire painting will become transparent, nor will this always occur. The result depends on the type of pigment used, the thickness of the paint, the way the paint was ground, the type and quantity of the binder and finally, the wave length of the infrared radiation used. For these reasons infrared reflectography is the preferred method for viewing possible underdrawings. Even though in this case the image is seen on a monitor and therefore is less detailed, it can register longer length radiation and therefore the penetration on the pictorial film is greater.
The most interesting results in this sense are obtained examining paintings from the Flemish school because of the specific technique that was used during the execution of the work. Even if it is not always possible to reveal what is underneath the pictorial film, the examination is capable of rendering the layers of finishing varnish transparent, even when they are heavily discoloured. This examination allows for an initial look through the altered varnish layers, and therefore the subsequent cleaning of the painting can be carried out with greater precision and safety. It should be remembered that the relationship between the various parts of the work can be altered under infrared radiation. However, the figures, the drapery and the painted objects will be seen with their chiaroscuro modelling and form, without being visually flattened out because of the altered varnishes.
A final contribution of infrared photography in artistic diagnostics is the identification of non-original parts. Materials with different chemical composition can show similar behaviour under visible wavelengths; for example, verdigris and cobalt green, or, azurite and artificial ultramarine blue can appear to be the same in colour. However, they do not necessarily behave in the same fashion under radiation from different parts of the spectrum. Many different pigments on a painting, both pure or mixed, are the same colour visually, and therefore can not be distinguished from one another. But with infrared photography, these colours can appear lighter or darker. In this case, it is important to remember that different materials even though they are the same colour can be the result of retouchings or other restoration interventions.
Always in an attempt to establish how much of the surface is original, the technique known as colour infrared can show the results of I.R. radiation as well as a part of the visible spectrum. By using a special colour film and a filter that excludes blue radiation, we can obtain on the photographic image of green, red and infrared radiation reflected by the painting. The film will arbitrarily assign colours: I.R. will appear red in the photo, and the red and green with respectively appear green and blue. This technique allows for further evaluations and will show the difference between apparently similar pigments. For example, verdigris and cobalt green with show up magenta and blue respectively. In terms of pictorial retouching, it is possible that this technique will reveal nothing. Nevertheless, it is also possible that the colour infrared analysis can evidence non-original areas that the ultraviolet fluorescence did not pick up because of the presence of old varnishes.
The properties of the I.R. radiation can also be used to improve examination of paintings on canvas in transmitted light. In fact, with respect to I.R. techniques the complete absence of a reflection on the surface of the outermost layers improves the possibility of seeing preparatory drawings or pentimentos. In the I.R. reflected techniques the external layers have to be passed through twice: the first time the radiation has to reach the inner most layer, be partially reflected, and then pass through the layers to reach the outer one. When transillumination is used, the radiation has to pass through the canvas and the preparation, but it will pass through the paint layer only once. The transmitted I.R. can be used to integrate data obtained with the x-ray radiography for paintings with a low radiopacity and with ground preparations with white lead, or even in those cases where a preparatory drawing made with ivory black or carbon black can not be seen under radiographic techniques.
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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