Sandro Botticelli – The technique
The transition from egg tempera to oil technique was, unlike what is usually thought, particularly slow and gradual and meant much more than a simple and sudden replacement of the binder. Most of the works carried out between the second half of the Fifteenth Century and the Sixteenth Century testify to the research carried out by painters to make the tempera more fluid and smooth, slow down the drying process and, in essence, make the color more manipulable. To this end, as early as the Fourteenth Century, oil and natural resin were added to the mixture, albeit for a limited number of colors or for the final glaze coatings. However, it was in the second half of the Fifteenth Century that this practice began to become no longer casual but programmatic, so much so as to define a set of rules of a specific technique, subsequently called tempera grassa, due to the greater viscosity acquired by the color.
The recent analyzes carried out by the Opificio delle Pietre Dure in Florence on various works by Sandro Botticelli allow us to identify in this artist one of the great users of tempera grassa and to retrace his technical procedures which, after an initial period of experimentation, seem to remain substantially unaltered in the course of its business. In many respects the method followed in the master’s workshop is still the one indicated in the Book of the art of Cennino Cennini: on the poplar panel (the paintings on canvas, such as the “Birth of Venus”, are still an exception) a preparation with two different layers of gypsum and glue is applied, the first thicker and more porous, the second more compact and made of a finer material and richer in binder. However, chemical analyzes have identified the presence of oil already in these layers: the preparation thus acquires a translucent appearance and loses most of its absorbent power, in order to facilitate the application of color and glazes.
Already in this first phase, Botticelli’s technique appears aimed at obtaining the maximum brilliance and luminosity.
On the perfectly smooth preparation, the artist executed a charcoal drawing with which, according to the rules of tradition, he set the composition and identified the forms.
The drawing, once the excess charcoal was swept away, served as a limit for the figures and as a trace for the engravings that identified the areas of the gilding.
After applying the bole and the gold leaves and before tracing over the drawing with ink, the painter applied a colored primer based on pigment, egg and oil, the utility of which was not only to further reduce the absorbent capacity of the preparation, but above all to create a colored base in relation to the desired chromatic effect that would be achieved in later stages.
Keeping in mind the final result to be obtained, the artist thus spread a black primer under the vegetation, light yellow under the green mantles, orange under the red robes, and finally white under the flesh tones and robes.
The white background allowed him to obtain effects of brightness that were not achievable where the dull color of the translucent preparation was maintained, which, moreover, had to be, at the end of the gilding process, partially “dirty” with bole and gold leaf.
On the trace left by the linear outline drawing in charcoal, Botticelli made the final drawing with a brush, with diluted black color (charcoal ink). Three different types of strokes have been identified: very thin and precise strokes that delimit the complexions, less thin lines for the outlines of the garments and large brush strokes to watercolor the shaded parts. The latter are sometimes completed with all necessary chiaroscuro indications, in other cases only hinted at and rather approximate. This differentiation can be attributed to the different chromatic effects that the artist already knew he wanted to achieve when the painting was finished. For the areas to be finished with a thin and transparent mixture, the underlying chiaroscuro effects were exploited more. Where, on the other hand, opaque and full-body colors were provided, the watercolor was approximate. Finally, in the feminine complexions, in order to achieve maximum brightness effects, the background was left without watercolor.
Being an exceptional connoisseur of all the techniques of his time, Botticelli was constantly striving to achieve the maximum effects of brightness and chromatic saturation: he chose the brightest pigments, did not use earth pigments (apart from ocher) and made extensive use of lacquers and other artificially transparent colors (copper resin and natural ultramarine blue with quartz), he used thin and uniform coats, grinding the color finely. The binder used is an egg and oil emulsion but, in the final glazes, the natural resin is substituted for the egg. The colors are applied pure without ever mixing them and the resulting transparencies create infinite tonal variations while maintaining maximum saturation. For the drapery, the covering and transparent properties of the various pigments are best used: the red robe, for example, is obtained with glazes of varnish on a carefully studied chiaroscuro design.
The blue robe of the Virgin is obtained with light coatings of natural transparent ultramarine blue, using the white of the primer for the highlights and obtaining the shadows with the addition of lacquer glazes.
We observe how, while keeping the number of pigments limited, the painter is able to obtain a wide range of tones.
Even if rarely, he painted drapery with full-body color and even very full-bodied mixtures (see impasto, impasto a corpo), which he, in any case, always completes with transparent glazes.
The thickness of the paint layer in the paintings is quite varied. The vegetation shows, for example, a fairly thick body also due to a coarse granulation of the pigment, unlike the flesh tones for which the pigments are very finely ground and the thickness is generally rather thin. On the white primer the paint was spread with thin brushstrokes of pure color in successive layers without waiting for the complete drying of the previous layer, an extremely difficult painting method which allows to obtain the perfect fusion of the layers while maintaining maximum brightness. As for the faces, the drawing was watercolored only for the masculine skin tones, for which the painter sought effects of greater expressiveness. For the complexions of young people and women, once again in search of light, the basic chiaroscuro was not used in order to make the most of the effects of transparency on the white preparation.
On the complexions red lacquer glazes were then spread. The finer and more transparent final glazes were often enriched with a small amount of natural resin. The features of the face were finally retraced with a thin dark line, continuous and uniform.
On the Virgin’s mantle a few touches of pure lead white mark the maximum lights.
The painting phase of the work closes with the veil, painted with lead white only, more or less diluted, depending on the degree of transparency required.
Further enrichments were then given with interventions of oil gilding and of shell gold technique.
Both techniques, already described by Cennino Cennini together with the more traditional water gilding, appear to be anything but abandoned: despite the low consideration shown by some humanists towards gilding, it is actually in this period that this art reaches one of its highest moments, in the search for a variety of effects unknown in previous centuries.
Manfredi Faldi – Claudio Paolini
Painting by Francesca Berni
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.
Quest’opera è distribuita con Licenza Creative Commons Attribuzione – Non commerciale – Condividi allo stesso modo 4.0 Internazionale.
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