The possibility of obtaining varied and singularly significant images from one single painting is tied to many factors. Among these are: the characteristics of the object under examination, changes in the direction of the illumination, the different kind of wavelengths used, and the relationship between the enlargement and the object. When a non-conventional use of a normal light source is employed, one can imagine the infinite variables when the direction of that light on the object changes. With a painting, it is possible to accentuate the differences in surface texture by finding the specular reflection angle of the area that interests us. This oftentimes occurs when a painting is held in order to look at it up close. However, another possibility exists that is often overlooked: the observation of the object by moving the position of the light source. This is what we do in an examination under transmitted light, or transillumination.
With a band of light that shines through a canvas painting and that is placed between the light source and the viewer, much information can be obtained about the ground preparation and the homogeneity of the pictorial layers. In this examination, the presence of any details under the visible surface can be identified. In a very simple way, recourse to other types of examinations can often be avoided. Examinations do not necessarily give more precise or better information just because they entail more sophisticated methods. This type of examination is also useful in identifying those situations when the support or the ground preparation do not permit the transmission of luminous rays; just as the use of infrared and x-rays may be indispensable for obtaining different and specific types of information.
Transillumination consists in lighting the painting from the back and observing from the front the behavior of the rays that pass through the layers. Logically, the limits imposed by some types of support can not be overcome. Even on some paintings on a canvas support, where there is usually a very good transparency, the light does not always pass through. Nevertheless, this latter case also provides some very important information because it indicates the presence of a thick, colored preparation. Even if the canvas is very heavy or has already been lined, it partially transmits light. It is hard for the pictorial film to completely block out the transmitted light because of the transparency of the binders and some pigments..
In the case of preparations that are completely opaque, the examination under transmitted light can hold unexpected surprises: an accentuated series of ageing cracks show up very clearly, evidencing the breadth and the variations in the size of the network.
A patch glued to the back of a painting usually covers a laceration or rip in the support. In this case, transmitted light can be of use in evaluating the existence, the extent and shape of the damage.
In transillumination, a retouching may clearly be seen as a dark spot, especially when it is thick or the materials used shut out more light than the original pictorial film. However, it is more frequently found that the overpainted area is also transparent. Due to the fact that these overpaintings cover some of the transparency of the original paint layer, these original areas appear darker than the lacunae. A comparison made using other examination techniques – particularly ultraviolet fluorescence – is absolutely indispensable in this case. This should be done because often the areas of paint that are highly abraded, that is, reduced in thickness, can take on a very strong transparency.
Naturally, the thickness of the pictorial “impasto” will block out the passage of the light rays, just as there are pigments themselves that can be more or less transparent. These two aspects will give the contrast to the image, and can supply us with useful information about the working methods of the artist. Whether the “impastos” are dense or not, the sureness of the brushwork, the working over of some areas, all contribute to the critical moment in giving an attribution to a work. In this situation, it is very important to insist that the comparative images be taken by the same photographer, under the same lighting conditions, and to insist on a full diagnostic archive of secure works that can be used for confrontation.
From a technical point of view, this examination does not require special equipment. It is necessary however to work with caution so that the painting is not damaged. A band of light is concentrated on the back of the work and it tends to accumulate heat rather quickly. A fan should be placed near the work, and in any case, the work should be illuminated only for the brief periods necessary to take the photograph.
A valid alternative in this sense is the use of a lamp with optic fibers. It limits the band of light – and therefore eliminates any ‘renegade’ rays on the painting’s surface – and the illuminated area does not heat up.
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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