Until the time of the painters of the Barbizon school in mid-19th-century France, it was normal practice to execute rough sketches of landscape subjects in the open air and produce finished paintings in the studio. Part of this was a matter of convenience. Before the invention of the collapsible tin paint tube, widely marketed by the colour merchants Winsor & Newton in 1841, painters purchased their colours in the form of ground pigment and mixed them fresh with an appropriate medium such as oil. The new tubes filled with prepared colours, as well as the invention of a lightweight, portable easel a decade later, made it much easier to paint out-of-doors. Despite these advances, many of the Barbizon painters continued to create most of their work in the studio; not until the late 1860s, with the work of Claude Monet (Monet, Claude), Pierre-Auguste Renoir (Renoir, Pierre-Auguste), and Camille Pissarro (Pissarro, Camille), the leaders of Impressionism, did painting en plein air become more popular. This change came about from 1881, when Monet, in his efforts to capture the true effects of light on the colour of landscape at any given moment, began to carry several canvases at once into the out-of-doors. On each he began a painting of the same subject at a different time of day; on subsequent days, he continued to work on each canvas in succession as the appropriate light appeared.