During the 1640s and 1650s Castiglione produced a series of large oil drawings, uniform in size and dazzling in their execution, but whose function is puzzling. None corresponds to a known painting, so it is unlikely that they were direct preparatory studies. Some may have been an attempt to wriggle out of a contract with a dealer, signed in 1644, that required Castiglione to execute one picture a month; others may have been intended for possible sale to collectors, or simply exercises in bravura draughtsmanship.

Whatever their explanation, these oil drawings are unique. No other artist of the period worked in this technique in such a sustained way. We can see Castiglione constantly testing both himself and the limits of the medium – exploring subtle variations of colour, the relationship between contour and modelling, the effects obtainable with a wet or a drier mark, and the tension between clear definition of detail and expressive brushwork.