This artwork is dated at around 1621 and is now a part of the Department of Prints and Drawings at British Museum in London. The Museum itself has a broad range of antiquities spreading across thousands of years and many different civilisations but their collection of art is still worthy of note just by itself.
The lower half of this drawing is very faint, suggesting that this was a study drawing, with the intention of painting his wife at a later date. Perhaps he was trying to get an understanding of her facial features and how he was to deliver them in a more complex art medium. The focus of detail is therefore on her face and this is delivered to a fairly complete level, as his her hair.
Many of Peter Paul Rubens' drawings were constructed using a technique called trois crayons which involves the use of three colours of chalk together, typically red, black and white. The drawing is sized at 15 inches tall by 11.6 inches wide and that would suggest that it came from a standard sketchbook where each artwork would have been a consistent size and the book would have been left around his studio for occassional practice on a whim.
Besides Isabella, husband Peter Paul also drew portraits of his sons Albert and Nicolas and also produced several paintings of all three of them. As a proud husband and family man, these drawings would have been part of his collection of art that was not for sale, whilst many others would be traded for other artist's work.