This type of oil sketch is also done in more "planning" way by Jan the Younger and his works offer us some insights about whole studio process. Some of these are in Ertz--see cat. #333 of his Jan the Younger book. A painting of the Tower of Babel by Jan the Younger and Josse de Momper that actually uses that oil sketch was auctioned at Christie's London (6 July 2006, #14). Important evidence of how collaborative thinking worked.