While we may imagine that Monet's studio was his garden, Daniel Wildenstein, author of his catalogue raisonné, casts doubt on his outdoor practice. Degas despised it so much that he sarcastically mocked the habit, while Renoir returned to working in the studio, trying to forget the term altogether. Although we tend to associate the plein air technique with the Impressionists, it had in fact been practised since the invention of tube paint thirty-five years earlier. Initially, people were wary of the raw colours involved in plein air painting, but it eventually became popular in studios.