According to the Artlex Art Dictionary, the term "plein air" is taken from the French phrase "en plein air." It is used to describe works of art painted outdoors rather than in the studio.
Even when painters moved from the direct application of paint on a wall to painting on canvas, it was still difficult to paint outdoors. A breakthrough came when the paint tube was invented in 1841 by American portrait painter John G. Rand. Soon all kinds of other portable equipment began to appear. No longer required to work from sketches and memory alone, artists began to look at landscape painting in a different way.
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