The pictorial decoration and the process of its creationz
Beneath the paintwork, on all three pieces, there is a white plaster and animal glue preparation, very typical in painting of that time, which was applied in more than one coat.
In all of the samples taken from the crestwork there is a coat of white primer on top of this preparation, based on a lead compound. This primer is observable, however, on only a few of the samples taken from the baldachin panel and the beam. On the panel, strips of cloth were glued over the edges of the planks that made up the support, below the preparatory layer. This was done to prevent cracks appearing in the painted surface as a result of any movements of the wood. This precaution can also be appreciate in the beam, a pine trunk in a single piece.
On top of this preparation, the painter made an incised sketch that served to fit together the various elements of the composition, to distinguish the areas that were to be painted from the areas to be gilded, and to define the outlines and main elements of the figures. These incised lines, which can still be clearly seen on the surface of the three pieces when illuminated with angled light, also guided the artist in the application of more plaster, either a pastiglia or with a brush, in the chosen areas (the mandorla with its precious stones, the central strip of the beam with its lion motifs, and martelé silvered background of the crestwork). In this way, decorated reliefs were produced that imitated stone or the embossed metalwork of the gold and silversmiths. Although the underlying preparatory layer is now clearly visible, these areas were originally completely covered with tin. A few remains of this metal can still be made out, now rusty and blackened, which may have been covered with a yellowish varnish to confer on it a gilded appearance.
In terms of the painting, the technique used throughout the piece is that of egg-based tempera, which bound together the mineral pigments and the rest of the materials. Technical differences can, however, be discerned between the beam and the wooden panel, on one hand, and the crestwork, on the other. Above all, this can be seen in the pigments |
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used to produce the reds, in the preparation of the materials and in the technique of layering the paint. Scientific analyses allow us to appreciate differences in the methods of grinding the pigments and of applying colour in accordance with the importance of the area concerned, as well as differences between the three pieces that make up the whole. The dark blue on the beam, around the medallions, and that used as a background to the mandorla of the painted panel, around the figure of Christ, were obtained by mixing blue and black pigments and applying the colour in a single coat. On the other hand, the blue of the tunic of the angel of St Matthew (on the panel), or the blue of the haloes on the crestwork, were produced by applying two coats of different colours: one an organic blue (a material that was imported and may have come from either the north of Europe or from India) mixed with lead, on top of another coat of carbon black, which in turn was applied on top of two coats of preparation. This procedure required a good knowledge of technique and of the compatible mixtures of materials that were required to obtain the desired results. It was, furthermore, a slower and more sophisticated method that required the complete drying of one coat before the next one could be applied.
The same thing happens with the reds. On the crestwork itself, the red was produced by mixing cinnabar with lime and then covering it with a layer of lacquer. However, to produce the red for the tunic of an Apostle, also on the crestwork, coats of different colours were superimposed (garnet-red organic lacquer on cinnabar, black, cinnabar, white lead, and plaster). Here, the pigment was very finely ground, with the grain being almost imperceptible so as to achieve the effect of interactions and transparencies between the various layers. This system was not applied to the beam, where the reds were obtained using cinnabar as a base with a compound of lead and coarsely ground minium. The recipes were to be found in manuscripts of the period, which prescribed the mixtures to be used and warned of the perils of incompatibility.
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