Bonington's Tiny Watercolor

Richard Parkes Bonington (1802-1828) did this plein-air watercolor at a small scale: 7½ by 6inches or 19.4 by 15.2cm., about the size of a paperback book or a travel guide.  

The subject is the Castello Estense, Ferrara, and it was painted in 1827. That makes the section below about the size of a business card.

There's some rubbing out to lift out that texture on the wall. Near the top of the detail are some diagonal scratches with a fingernail or brush handle. He uses some white or body color. In the top white rectangle, he came in later with some blobs of darker tone. Some of those lines are extremely fine and careful, including the white mullion on the gray window.
----
Link to Sothebys listing

Thanks, Matt McCleer