Εξέταση, ανάλυση και τεκμηρίωση της Βάπτισης του Χριστού του Δομήνικου Θεοτοκόπουλου με μη καταστρεπτικές φυσικοχημικές τεχνικές

Περίληψη

The restoration of El Greco’s Baptism of Christ, undertaken by the Conservation Department of the Benaki Museum, Athens, provided a unique opportunity to examine, analyze and document the material and technical aspects of the painting by using non-destructive physicochemical techniques, as presented here, and micro analytical techniques, as presented elsewhere in this volume. The Baptism of Christ, dating from 1567, was painted by El Greco soon after his arrival in Venice, where it is assumed that he joined Titian’s workshop and became acquainted with the works of Tintoretto and of the Venetian School. Recent studies of the material and technical aspects of late 16th - early 17th century Venetian paintings have brought to light specific information relating to individual masters such as Titian, Tintoretto, Bellini, Lotto and others. The non-destructive techniques used prior to conservation were radiography, computer aided tomography (CAT), holographic interferometry, multi-spectral (UV-Visible-IR) imaging, optical microscopy and X-ray fluorescence spectroscopy (XRF). Finally, following completion of the restoration, the painting was analyzed by using near infrared (FT-NIR) and laser induced breakdown spectroscopy (LIBS). In addition, scanning electron microscopy combined with energy dispersive X ray analysis (SEM/EDX) was used on selected micro-samples obtained during the conservation process.
The results allowed the drawing of conclusions on all aspects of the history of the panel, i.e. both the technique and materials used by El Greco, as well as structural information and evaluation of the present condition of the painting before and after conservation. More specifically, the presence of gypsum (FT-NIR spectroscopy) for the ground layer reveals the application of the gesso technique. By taking into account the ratio of intensities for M and L lines of lead (Pb) (XRF results), in combination with the X-radiography, it became clear that lead white was used for both the preparation layer (imprimatura) and the highlights. The very detailed drawing of the figure of Christ was made with a thin brush (IR imaging), most probably with the use of organic black and/or umbrae (XRF, SEM/EDX). St John and the angels are formed with rather rugged outlines. Detailed examination of the painting with the same technique revealed the existence of a date (MDLXVII) in the lower left part of the panel which was hidden under the layers of subsequent overpaintings and retouchings. All FT-NIR spectra are characterized by the presence of an oil substance which points in the direction of an oil medium. Slight differences in the FT-NIR spectra of the orange-yellow himation of the right angel and the observation of micro-craters under the optical microscope may also suggest the selective application of an oil-egg emulsion.
The pigments used for the blue were lapis lazuli (LIBS technique), smalt (cobalt rich glass, XRF) for the light blue and well crystalised azurite (XRF) for the dark blue of the sky (center of panel) and Christ’s loincloth; gold paint has been used for the details of Christ’s loincloth and the date; umbrae, possibly mixed with organic black, for the brown and dark; arsenic sulfides (orpiment and/or realgar) mixed with umbrae and yellow ochre for the orange himation of the right angel; most probably verdigris (basic copper acetate) for the foliage of the trees and the green garment of the right angel which in an oil medium forms a copper glaze; organic red (lake) for St John’s garment, the towel held by the angel and the red tints in the sky; cinnabar for the miniature orange-red figure which stands out in the crowd on the bank of the river Jordan (between St John and Christ); cinnabar, most possibly in thin underlying layers, has also been used for the flesh of Christ while the shadows are worked with umbrae; lead tin yellow type II (mixed with lead-tin type I) has been used selectively for the yellow leaves of the trees and the yellow highlights of the sky around the Holy Spirit. Poorly crystallized azurite, ochre and gypsum have been used for the painting of the wooden substrate (on both sides of the arch) into which the panel was laid after its separation from the original triptych; titanium oxide and a blue that is non-identifiable with the XRF (lapis?) were used for an extended retouching on the sky in an attempt to cover cracks and craquelure, clearly visible under UV imaging; titanium oxide has been also used for the most recent retouching before or on the occasion of the appearance of the panel on the market.
An interesting feature revealed by the XRF analysis in most of the spots tested was a characteristic pattern between Co-Fe-As, expressed as stable ratios between the elements. The results from SEM/EDX technique verified the presence of cobalt glass inclusions in the preparation layer of imprimatura, which are colourless or vary in colour from grey-blue to pale grey. The tint of the glass does not present any correlation with the chemical composition and can probably be attributed to discoloration. The proportion of glass inclusions over lead white in this layer varies greatly. Another interesting feature in the Baptism is the use of almost pure glass mixed with small quantity of lead white, for the underlying transparent layer, for the depiction of the clear waters of the River Jordan and for the sky in the upper part of the panel. Smalt or cobalt glass of the same composition has been used mixed with lapis lazuli for the blue of the sky. The presence of Fe, with simultaneous detection of Ni, as trace element, shows in the use of minerals in the series of arsenopyrite-smaltite, such as cobaltite (Co,Fe)AsS and erythrite [Co,Ni]3[AsO4]2.8H2O for the production of zaffer, parent material of smalt, which are found both in Central Europe (Saxony and Bohemia) and in North Italy. The colour of glass inclusions (i.e. blue-grey) is discussed, especially in view of the significant amount of Fe which usually results in green hues. The data presented here are in agreement with those recently revealed by the re-analysis of samples of paintings by Bellini, Giorgone, Titian and Tintoretto and clearly show that Greco soon, after his arrival in Venice incorporated in his work the “Venetian palette” as well as materials and colorants not specifically designed for use in oil paint (i.e. glassy materials-“smalti”, lead tin type II, blue smalt).
This paper also puts forward a new methodological approach for the examination of paintings based on the combined use of visual, structural and analytical non-destructive techniques. Finally, the application of digital holographic interferometry allowed a thorough structural examination of the panel with the identification of detachments, cracks and other damages underneath the surface and facilitated the conservator’s work. The re-examination of the panel after the completion of restoration has provided it with a strong documentary base for the future.