À la villa Farnèse: Two Poplars
oil on paper, 10"x15"
Artists have been painting oil sketches in the open air since the 17th century, and by the late 18th century the practice was fairly common, especially in Italy, which attracted painters from all over Europe.
One of these was Pierre Henri de Valenciennes. He was best known for historical landscapes and his treatise on perspective (Elemens de Perspective Pratique a l'Usage des Artistes), which he studied with a mathematician in Rome, but he was also probably the foremost advocate of the importance of plein air painting.
He emphasized to his students (two of whom became teachers of Camille Corot) that capturing the particular details of a specific location (a "landscape portrait") was an essential practice for a painter, and he was committed to raising the status of landscape painting.
Stair Sainty Gallery
The National Gallery, London