First of all, what is an oil sketch? It’s a work executed in oil paint in preparation for a larger, finished work. It is usually small, or at least painted on a smaller scale than the finished work is intended to be.
The fact that you are right now looking at oil sketches on a website - and that you can walk into the Courtauld Gallery and see them hanging on the walls - is in a sense paradoxical. Oil sketches were never intended for public consumption. They were private, personal works, made for the artist’s own use - they were the medium through which he explored, tried out, worked through and perfected his ideas for a composition, before putting brush to panel to paint the final work.
In some cases, however, an oil sketch might be shown to the patron. It is possible to divide oil sketches into two rough categories, though a given sketch might serve both functions. Firstly, there is the sketch that an artist would make when he was initially conceiving the idea for a composition. These preliminary sketches tend to be quite small and rough and are characterised by their spontaneity and their free, swift brushwork, such as in this wonderful sketch by Peter Paul Rubens of Hercules Killing the Hydra.
The second type of sketch tends to be more ‘finished’, more polished, less ‘sketchy’ or rough, and is made when the artist has already developed and refined his ideas for the composition, either in a preliminary sketch as described above or through a series of drawings. This type of sketch is often made to show to the patron (or potential patron) for he or she to approve the design before the artist paints the finished work. The patron then has the opportunity to request the artist to make any necessary changes before work on the final picture begins. A splendid example is Rubens’ sketches for the Descent from the Cross triptych, which were presented to the members of the Guild of Harquebusiers for approval - the finished triptych was the altarpiece for the Guild’s chapel in Antwerp Cathedral. In this case, the sketch functions as a sort of contract between the artist and patron, as the basis for the agreement between them about what the final product will look like. Sometimes, an artist might make this type of sketch to submit in competition with another artist to a potential patron, who will decide on the basis of the sketches which artist should receive the commission.
A third type of sketch might be made by an artist specifically to serve as a model for an engraver to make a print after the artist’s design. This Crucifixion by Anthony van Dyck served just such a purpose. The sketch is monochromatic, executed almost entirely in shades of brownish-grey. This makes it easier for the engraver to render the light and dark tonal values when he translates a painted design into one composed of systems of lines. If the sketch were in colour, this would be more difficult for the engraver to do.
A further function of oil sketches - and one that demonstrates particularly well how the sketch is ‘closer’ to the artist than many of his finished paintings - is that they could be passed to members of the artist’s workshop who would then execute the final work based on the artist’s sketch. This became fairly standard practice among successful painters who received a number of large commissions, often all at once, and who would delegate part or most of the execution to their workshop. Rubens is an excellent example of this. This was a recognised and accepted practice at the time and not considered untoward in any way. For example, when Rubens was commissioned to paint 39 large ceiling paintings for the new Jesuit church in Antwerp in 1620, the contract he signed specifically states that Rubens must make the oil sketch for each painting with his own hand, but that members of his workshop would use those oil sketches to execute the final paintings, with Rubens adding the finishing touches. What was important to the Jesuits was that Rubens was responsible for the intellectual aspect of thinking up the design for each picture - they did not object if he delegated the more ‘manual’ work of executing the final painting based on his own design to talented members of his workshop. It was Rubens’ authorship of the designs that guaranteed the authenticity of the works, not whether he executed every single brushstroke of the final paintings with his own hand - in the seventeenth century, a very different notion of authenticity and authorship prevailed from that we hold today.