xeroradiography for paintings on canvas and wood

Maney Publishing

Xeroradiography for Paintings on Canvas and WoodAuthor(s): Patrizia Magliano and Bruno BoesmiSource: Studies in Conservation, Vol. 33, No. 1 (Feb., 1988), pp. 41-47Published by: Maney Publishing on behalf of the International Institute for Conservation ofHistoric and Artistic WorksStable URL: http://www.jstor.org/stable/1506239 .

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XERORADIOGRAPHY FOR PAINTINGS ON CANVAS AND WOOD

Patrizia Magliano and Bruno Boesmi

Abstract-Xeroradiography, a relatively uncommon form of radiographic investigation, was used to ana- lyze four paintings, two on wood and two on canvas. In each case the layers underlying the surface of the paintings were identified by xeroradiography. These results are discussed together with the technique which is analyzed critically and compared to others such as conventional radiography.

1 Introduction

The term xeroradiography comes from the Greek xeros--dry--since this technique pro- duces images without chemical reactions, but rather with an electrostatic process. The first demonstration of this technique was given between 1903 and 1907 by Righi, who produced a latent image on an ebonite plate with an electrostatic system [1]. Thirty years later, in 1938, Carlson reproduced this technique using a zinc plate loaded with a fine layer of sulphur and exposed to light: a latent electrostatic image could be made visible with black powder, which adhered to the plate only in the areas unexposed to the light. The technique was referred to as 'electrography' [2]. The same effect could be reproduced using ionizing radiation instead of light. In 1950 the first xerographic reproducer was commercially produced. The technique was then applied to medicine; xeroradiography has also been employed for the investigation of ceramics [3].

We are now reporting our initial experience with xeroradiography for the study of paintings on canvas and wood.

2 Materials and methods

2.1 Technique The xeroradiographic unit is composed of a 'conditioner' and a 'processor', as in the Xerox 125? system (Figure 1).

Received 25 January 1985 Received in revised form 6 July 1987

CONDITIONER

PROCESSOR I ? I

5 3

Figure 1 Detailed outline of xeroradiographic technique, conditioning of plates, exposure to radiation and processing of the plate. Further details in text.

The conditioner serves to stock the plates when they are not in use, and prepares them to be exposed to radiation. If the plates have already been used, the conditioner will remove the developer and residual surface charges. Thereafter each plate is left for 90 minutes at 600 in a 'relaxing chamber'. This step is important because in a previously exposed plate the layer of selenium is converted from insulator to con- ductor. With proper use a plate of selenium can be used 1500 times. The plate is charged with a uniform electrostatic charge (900-1600 volts) (step 1 of Figure 1). The plates are then exposed to radiation like any other X-ray film, and par- tially decharged according to the thickness and density of the object placed between the radiation source and the plate. A latent image is formed on the plate.

The processor makes this latent image visible. After exposure to radiation the plates are placed

Studies in Conservation 33 (1988) 41-47 41




in the 'processor' (step 2 of Figure 1) and the surface exposed to a cloud of blue powder (developer). A sheet of treated paper is then laid on the plate covered with the blue powder, and the latent image is transferred to the paper with an electric field (steps 3, 4, Figure 1) and heat- fixed (step 5 of Figure 1). It should be noted that the image can be developed as 'positive' (radiotransparent material appears as white) or 'negative' (radiotransparent material appears as blue).

The xeroradiograph at this point is ready and can be seen by the operator (step 6). The plate which has been used needs to be cleaned with a rotating synthetic brush (step 7). It is then stored in the 'processor' (step 8) until five plates have been used, and thereafter transferred manually to the 'conditioner' (step 8). Here the plates remain at 60'C for 90 minutes (step 9): this step is necessary to cancel completely the image of the previous exposure. The plates are then stored (step 10) and electrically recharged (step 11) to be used again.

Figure 2b Xeroradiograph of the painting in Figure 2a: the breast ofthefigure is clearly visible and does not appear to be obscured by the mantle (exposure 50 seconds, 60k V, 1OOmA).

Figure 2a 'Allegory of Summer', oil painting on canvas, Italian, nineteenth century, 70 x 50cm. The whole painting during cleaning.

Figure 2c The painting after restoration.

2.2 Xeroradiographic technique for paintings The first step is to outline within the painting a certain number of areas. This is accomplished with nylon threads pulled from side to side of the painting and from top to bottom: the square

42 Studies in Conservation 33 (1988) 41-47



Xeroradiography for paintings on canvas and wood

areas which are formed should be large enough to fit exactly one xeroradiographic plate.

The painting is then placed horizontally with the pictorial layer side up, facing the radiation source. The area under investigation is centred by means of a beam of light. The xeroradiographic plate, prepared as described, is then placed under the picture exactly, or as close as possible, under the area and exposed to X- radiation. Standard xeroradiography is performed with 60kV, 100mA, and exposed for 60 seconds. Xeroradiography allows for a wider latitude of exposure than conventional radiography: in other words the quality of the image will not differ greatly if exposed for 40 or 60 seconds. The plate is then removed, taking care not to exert pressure with the fingers, to avoid imperfections during the printing process.

2.3 Examining a xeroradiograph The image which will be obtained is composed of white and different shades of blue: in the 'negative' image, the whites represent poorly radiotransparent parts of the painting, whereas an increasing tonality of blue will identify increasing radiotransparency. In principle this is not different from conventional radiography: however, the different shades of blue offer a greater possibility of discrimination between colours when compared to different shades of grey in conventional radiography. In addition the use of 'positive' and 'negative' prints, as mentioned previously, expands these possi- bilities. Indeed it is quite convenient to print both a 'negative' and a 'positive' xeroradiograph and then examine both: the sequential rapid examination of positive/negative prints may be of great help to study relevant details.

3 Results

Four paintings were studied with xeroradiography using a Xerox 125 automatic system.

3.1 Painting 1 An Italian oil painting on canvas of the nineteenth century. It depicted a female figure as the 'Allegory of Summer'. The main problem was a mantle most probably painted to cover her breasts.

At first examination the mantle (Figure 2a) appeared clearly added to the figure at a later

period, as indicated by the different thickness of the paint layers when compared to the rest of the painting. A cleaning test of the mantle disclosed an intact underlying layer of incarnato. Xero- radiography was performed at this stage and showed an intact and uniform pictorial layer underlying the mantle: the xeroradiograph did not show the mantle at all (Figure 2b). Clearly evident also was the direction of the brush stokes and their consistency, as well as the weave of the canvas. The final appearance of the picture is shown in Figure 2c.

Figure 3a 'Madonna and Child', painting on wood, fifteenth century, 74 x 46cm, before cleaning.

3.2 Painting 2 A mixed oil/tempera painting on wood, a 'Madonna and Child' of the fifteenth century





from the Cathedral of Savona (Genoa). The halo of the Holy Child was completely re-gilt with new gold leaf.

In this painting (Figure 3a) the appearance of the paint layers and the fillings indicated that the panel had been largely repainted. However, before removing the re-gilt halo of the Holy Child, it was important to know how much of the original was left underneath. Xero- radiography clearly outlined the shape and dimension of the original halo fragments (Figure 3b). These were found to be relatively small; nevertheless it was decided to remove the repainted halo, mostly because of the poor quality of the gold and of the fillings. In addition large lacunae could be identified within the Virgin's dress and mantle. These can be seen in the painting after final cleaning, shown in Figure 3c.

Figure 3b Xeroradiograph of the head of the Holy Child, showing traces of the original halo (exposure 40 seconds, 60kV, JOOmA).

Figure 3c The painting after final cleaning.

3.3 Painting 3 A 'Sacra Conversazione' by Domenico Fiasella, seventeenth century Genoese school. A bouquet of spring flowers was painted in the hand of the little Saint John, being offered to the Holy Child (Figure 4a).

After initial test cleaning, the problem was the interpretation of a pentimento. At first sight the bouquet of flowers offered to the Holy Child appeared to be composed of tulips and daffodils (Figure 4b). However, in the process of cleaning, some thin rays of light pigment appeared in the background of the bouquet. Xeroradiography was performed at this stage: examination of negative (Figure 4c) and positive (Figure 4d) xeroradiographs revealed some ears of corn painted in a first preparation. Conventional radiography (Figure 4e) also showed the pres- ence of ears of corn, but the image is not as clear as with xeroradiography. The bouquet of tulips and daffodils was left untouched and the lower paint interpreted as a pentimento.





Figure 4a 'Sacra Conversazione', Domenico Fiasella, seventeenth century, Genoese School.

Figure 4b Detail of the bouquet offlowers offered to the Holy Child, after initial cleaning.

3.4 Painting 4 An 'Annunciation' on a lunette of a wood panel owned by the Confraternity of S. Domenico in Savona, sixteenth century, which appeared to be completely repainted, as suggested by discrep-

ancies of the colours on the surface of the painting and in the lacunae.

After initial cleaning, the lunette (Figure 5a) exhibited a gesso-like pictorial layer, rather dis- continuous. In addition a discrepant pictorial style was observed when the lunette was compared to the painting in the architrave below. These observations prompted further investigations with radiography and xeroradiography.

Xeroradiography revealed traces of a decoration of acanthus leaves in the area corresponding to the figures of the Madonna and of the announcing Angel (Figure 5b). The resolution of such traces of original pigment with radiography was much less clear.

The reason for the change became evident when it was known that the painting was origi- nally the property of the Confraternity of San

Figure 4c Xeroradiograph (negative) of the same detail showing traces of ears of corn among the flowers.





Figure 4d Xeroradiograph (positive). Some details can be read more clearly than with the negative photo (exposure 20 seconds, 62k V, 1OOmA).

Domenico, which later joined the Confraternity of the Annunciation. In consideration of the historic reason for the change of decoration, the Madonna and Angel were retained.

4 Discussion

We have shown in the present study that xeroradiography can be successfully applied to the investigation of paintings on canvas and wood. We find this technique has several advantages over conventional radiography. In the first place the painting can be investigated by xeroradiography in one appropriate exposure, whereas conventional radiography requires different exposures to highlight different details. Other advantages are as follows: (A) The grain of the wood support can be

clearly identified. (B) Details of the canvas can be studied and

can give valuable information, especially

Figure 4e X-ray of the same area.

Figure 5a Detail of the 'Annunciation', sixteenth century, Confraternity of S. Domenico, Savona.

Figure 5b Xeroradiograph of the same detail revealing traces of acanthus leaves in the area corresponding to the figures of the Madonna and of the announcing Angel (exposure 30 seconds, 65kV, 1OOmA).





when the painting has been relined; in this situation it may be difficult to inspect the original canvas.

(C) This holds true also for dark areas which would appear dark on conventional radiography. For example in painting 3 the bouquet of flowers appears as a dark area on a conventional radiograph; when the same detail is seen with xeroradiography the craquelure shows more clearly.

(D) The use of a positive print in xeroradiography offers the possibility of comparison with the negative print, and reveals other important details. We find that the sequential rapid examination of positive/negative prints may be of great help in localizing rapidly small but relevant details.

(E) The impasto shows in xeroradiography almost in three dimensions and gives the observer an impression of the texture of the painting.

(F) The traces of original painting can be outlined more precisely: for example in painting 2 the detail of the original halo could be clearly identified thus allowing the original gold leaf to be freed from the thick layer of plaster completely covering it. Other details such as different supports, the pictorial layer, the craquelure, can be clearly seen.

(G) The 'border effect', typical of xeroradiography, outlines the images very clearly, thus contributing to a good resolution of the technique, as compared to conventional radiography [4- 6].

Developing is quick and does not require a dark-room. The print on plastic paper makes the final product easy to handle and apparently durable with time.

At present, limitations of xeroradiography include the fact that the apparatus cannot be readily moved, the plate has a short life of approximately 30 minutes, and its dimensions are 24 x 35cm.

Further investigations on this technique applied to the study of works of art seem to be warranted.

Acknowledgements

The support of Dr G. Terminiello-Rotondi, Sopra- intendente per i Beni Storici e Artistici della Liguria,

is gratefully acknowledged. The authors also wish to thank Dr B. Ciliento for his helpful advice.

References 1 RIGHI, A., Die Bewegung der lonen bei

Elektrischen Entladung, J. A. Barth, Leipzig (1905).

2 LUZZATI, G., and BORASI, G., 'La xeroradiografia: presentazione del metodo e basi fisiche', La Radiologia Medica 60 (1974) 721-724.

3 JOHNSTON, R. H., 'Xeroradiography: a new imag- ing modality. The application of a new form of X-ray to the study of ancient ceramics' in First International Congress on Nondestructive Testing in Conservation of Works of Art, Rome (1983).

4 BORASI, G., and Tosi, G., 'Le caratteristiche dell'immagine xeroradiografica', La Radiologia Medica 60 (1974) 737-740.

5 WOLFE, J. N., 'Xeroradiography: image content and comparison with film roentgenograms', Am. J. Roentgenol. 117 (1973) 690-692.

6 DUTREIX, J., BISMUTH, V., and LAVAL-JEAUTET, M., 'Le detection de l'imagine radiologique' in Traite de Radiodiagnostic, Masson et Cie, Paris (1969).

PATRIZIA MAGLIANO is a graduate restorer from the Restoration School of the Academy of Arts, Genoa. Technical co-ordinator of the Cooperative 'Conser- vazione e Restauro', 1978-82. At present working as an external restorer for the Soprintendenza per i Beni Storici ed Artistici della Liguria. Author's address: Via Maggiocco 14, 16035 Rapallo, Italy.

BRUNO BOESMI is Deputy Head of the Radiology Department, Ospedale Galleria, Genoa, and a radiology consultant to Biotest, Genoa, Italy.

Resum---La x6roradiographie, relativement peu employ&e en radiographie, a ete utilis6e pour examiner quatre peintures, deux sur bois et deux sur toile. Dans chaque cas, les couches sous-jacentes ont ete identifi6es par la x6roradiographie. On discute des r6sultats, ainsi que de la technique qui est &tudi6e et compar6e aux autres m&thodes telles que la radiographie traditionnelle.

Zusammenfassung-Zur Untersuchung von vier Gemilden, zwei auf Holz und zwei auf Leinwand, wurde die Xeroradiographie herangezogen, die eine eher unuibliche Form der Radiographie ist. In jedem Fall wurden die Firnis-, Mal- und Grundierungs- schichten sowie der Bildtrager mit Hilfe der Xero- radiographie identifiziert. Sowohl die Ergebnisse als auch die Methode werden kritisch bewertet und let- ztere anderen Verfahren wie der konventionellen Radiographie gegeniibergestellt.




xeroradiography for paintings on canvas and wood

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