Giotto: A collection of 131 works (HD)

Very little is known of Leonardo da Vinci's pictorial technique. That is, because his manuscripts are generally obscure and difficult to interpret, and because of the understandable reluctance to take samples from the artist's works. From the scarce available scientific data and the unfinished paintings, it appears that all of Leonardo's works are executed with oil, on walnut or poplar panel technique, with a gypsum and glue preparation, followed by lead-white priming. Some of the author's paintings, such as the Adoration of the Magi in the Uffizi, or the Saint Jerome in the Vatican Museums, in their unfinished state, allow the inspection of the construction stages of Leonardo's work, especially the layers under the primer, and thus allow us to analyze the artist's method. On the ground preparation layer, Leonardo carries out the preliminary graphic design with dry tip, and engraves the architectural construction guidelines. The drawing is then reinforced with the tip of the brush, using a blackish-brown wash applied directly on the gypsum preparation. In some of his works, Leonardo partially uses cartoons and the pouncing technique for the realization of individual figures, but most of the drawing is done freehand, as if the artist were in front of a large sheet of paper. In fact, there are numerous variations and pentimentos identified in his paintings. In this phase, Leonardo also outlines the draft volumes of the bodies using watercolor, with quick and broad strokes, the artist then marks the shading and builds the volumes with a brush dipped in blue, and, in some areas, red watercolor. Sometimes the two layers mix, or overlap, with a purplish effect. For the red shade, he uses a lay color. The blue one is probably a dye of vegetable origin, perhaps indigo, and not a pigment, given that no chemical elements are found in its composition. The choice of blue color anticipates what Leonardo himself will write in his treatise. Because in the evening, the shadows of bodies cast on a white wall are blue. After the drawing and the first chiaroscuro, he applies a thin and semi-transparent primer, lead white and boiled linseed oil, in order to make the underlying drawing appear gray. The transparency of this layer allows the artist to continue seeing the drawing and use it as a guide for the next steps. In other words, this expedient allows Leonardo to tone down the graphic elements that he does not intend to develop further in the pictorial phase, fix the drawing, and it also acts as an insulator, in order to avoid the penetration of the color binder into the ground preparation. The artist also spreads the primer with his hands, leaving fingerprints and a fresh mixture. The analysis of some samples of this layer have revealed the presence, together with the lead white, of traces of carbon black. This is probably due to a first hint of chiaroscuro on the still fresh primer. Once the primer layer is dry, Leonardo begins the actual pictorial phase, with a general yellow-brownish application, modeled and modulated with the addition of warmer shades depending on the functionality to define the reliefs, areas and figures. This underpainting, which functions as a half-tone for the modeling of the flesh tones and to trace the shadows, is painted with brun-brown colors, often mixed with green pigments. Practically, this is a second layer of primer, based on yellow ochre, on which he spreads marked monochrome brushstrokes with burnt umber and copper-based pigments to strengthen the chiaroscuro. In a large part of the composition, the pigments appear to have been spread with the fingers, in order to soften the clear contours of the figures. However, brushes are also used. While this layer is still fresh, in some areas of utmost lightness, Leonardo also begins to apply white brushstrokes in order to define some reliefs of the modeling. While the underpaints can be considered relatively simple in both layering and pigment constitution, the outermost paint is made with many superimposed layers, particularly fine and translucent. Through X-ray fluorescence spectroscopy, applied to seven masterpieces by Leonardo, scientists have detected the presence of up to 30 ultra-thin layers of paint and varnish. Such glazes were used in Flemish painting to clearly delimit the contours of the forms, while Leonardo, on the other hand, used them to blur the silhouettes of objects and figures from the foreground to the background. Da Vinci himself described the technique as a smoke, where lines blur, soften and blend to create a subtle effect of depth and shading. This refined overlapping of ultra-thin layers is called sfumato in Leonardo's painting. The juxtaposition of the terms sfumato and velatura could make us believe that we are dealing with two totally opposite practices, but the sfumato is exactly the stratification of transparent and semi-transparent layers of color, meaning glazing, which, precisely because of its overlapping nature, progressively loses contact with the drawing underneath. The color is also frequently blended with the fingertip, which distributes the mixture in a circular manner, creating that visual effect of fading. Furthermore, both the practices of rubbing and dabbing the color are attested. The gradual fading of the colors and tones is also obtained by the artist through the smorzatura technique, a glaze not too dark on a white background, but light on a dark background inversely, and with the dry brush technique, carried out by spreading, or rather rubbing, some color pouring binder on the dry underlying paint, using a hard and short brush. These are intermediate processes between the traditional glazing and the use of an opaque film, in a free and complex layering of translucent and opaque glazes. The contours of the figures thus merge imperceptibly, with soft gradations of light. In Leonardo's paintings, the transitions from light to dark are imperceptible. Even under the microscope, it is impossible to detect a trace of a brushstroke. This technique, which needs an oil binder to obtain the transparent layers of color, makes use of the most common pigments, such as lead white, lead or tin yellow, cinnabar, red and yellow ochres, organic lakes, azurite, lapis lazuli, verdigris. Recent investigations attest that the flesh tones show the use, in addition to the colors mentioned in the Treaty, of cinnabar and yellow ochre, to which, in the shadows, earth umber is added. The landscape is painted with yellow ochre and green earth, mixed with lead or tin yellow, while the sky is made with a first layer of lead white and azurite, and then completed with ultramarine applied as a glaze. When applied to landscapes, in particular to the rendering of the distance between objects, the sfumato technique is called atmospheric perspective. Atmospheric perspective is a method of representing space that studies the variations of light intensity and tongue gradations in relation to distance. Leonardo observes that the air is not a completely transparent medium. As the distance, and therefore the thickness, of the interposed air layers increase, the contours become more blurred, the colors less and less sharp, and their hue tends towards light blue. Consequently, in his painting, he makes the shapes crisper in the foreground and increasingly blurry as the distance grows. Transparent glazes blur the color transitions and blend the palette in order to represent the color of the atmosphere. Leonardo's treatise on painting does not provide us with much information on a more or less particular use of binders and varnishes. It recommends the use of walnut oil thickened under sunlight, or mixed with resin, and gives detailed indications for its correct preparation. In light of scientific, historical, and critical data, Leonardo's technique would seem to exclude the use of finishing varnishes.