By Rubens’ time, the creative process of a great artist - the specific route by which he arrived at the final painting - was seen by connoisseurs as being intrinsically interesting. Any artefact from that creative process, such as an oil sketch, was hence seen as being valuable in and of itself. Sketches were seen as being closer to the artist and closer to ‘the real thing’ not only because they were executed by the artist’s own hand whereas the final painting may not be, but because they represented a stage in the creative process that was closer to the original creative act - the near-divine moment when the artist initially conceived the idea for a work of art. It is that idea that was so highly valued, and this explains why the sketches themselves, as much as the final works, were so sought after by patrons and collectors in Rubens’ day as much as in our own.
This growing appreciation of oil sketches is reflected in the fact that in the contract for the ceiling paintings for their new church, the Jesuits gave Rubens the choice between handing over the 39 oil sketches after the final paintings were finished, or else painting another large altarpiece for a side altar. The Jesuits were keen to acquire the oil sketches - even though they of course had the final paintings to enjoy - for the reason outlined above. But Rubens was very protective of his sketches, which he regarded as his intellectual property. They formed an important part of his studio stock as he was constantly reusing motifs from one composition in another. He chose to keep the sketches and to paint the additional altarpiece instead.
You can see that these sketches for the Jesuit Church ceilings of Esther before Ahasuerus, Solomon receiving the Queen of Sheba, The Temptation of Christ and The Coronation of the Virgin appear more finished and are strongly coloured. This is not only because they were to be shown to the Jesuits for approval, but also because they were then used by members of Rubens’ workshop, including the young Anthony van Dyck, to use in executing the final paintings. Rubens had to ensure that his intention was clearly communicated to his assistants, without leaving any ambiguities in the design that could give rise to misunderstandings; hence these sketches are very clear, detailed and refined.
So, on the one hand, oil sketches are about a direct, unmediated expression of the master’s creativity, about spontaneity and freedom. On the other hand, they are also about control - the patron’s control of the imagery and the master artist’s control over his workshop, ensuring quality control and that his assistants carried out his intentions faithfully.