You probably noticed that the sketch of David slaying Goliath is much rougher than the other sketches for the Jesuit cycle, and is nearly monochromatic rather than richly coloured like the others. Rubens actually made two sketches for each composition: the first a quick, rough, monochrome one which represents the first expression of his idea for the picture, and then he refined, consolidated and clarified this first design in a second, coloured, more polished sketch. This is the working process that he was to follow for all of the major decorative cycles that he was commissioned to paint throughout his career.
Whilst Rubens was not the first artist to make oil sketches, he seems to be the first make them as standard practice. Throughout the sixteenth century, artists usually worked out their compositions by making a series of careful drawings. It says a lot about Rubens’ virtuosity and confidence that he felt able to entrust his first ideas for a work to the relatively unforgiving medium of oil paint rather than graphite on paper.
The swiftness and bravura of the sketches is amazing. He would usually begin by coating the panel with a thin layer of pale ivory-coloured ground. Sometimes he would then make a very summary drawing with black chalk directly on this ground. He would then take up the brush and away he went! The paint is often applied so thinly that the ground shows through, as is the case with this sketch of Bounty triumphing over Avarice. In many of the preliminary, rough monochrome sketches, he adopts a technique which bears some similarity to drawing: with the point of a relatively fine brush, he first draws the outline and main contours of the figures, then goes back and adds more ‘painterly’ touches for shaded and highlighted areas, as can be seen in the David slaying Goliath. It is as though he is drawing, but with paint. It is clear that these sketches really are the first expressions of his ideas, because there are often visible pentimenti, where Rubens made changes (to a figure’s pose, for example) as he worked. His second, more finished sketches (see, for example, the St Gregory) tend to be much more painterly - like smaller, less polished versions of the final work. Further changes were still not ruled out even at this stage, however, as a comparison between many of his oil sketches and the final paintings demonstrate.
In the sketch of The Assumption of the Virgin, the thinness of the paint and the lightness of Rubens’ touch in the upper area where the Virgin is being borne up to heaven by putti produce a spectre-like, semi-transparent, almost ethereal effect - as vaporous and intangible as an idea. In these curiously diaphanous forms tenuously emerging from the inchoate background, we are witnessing Rubens’ ideas in the process of coming into being, of precipitating into solid form.