Working in layers
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Working in layers is used extensively in oil painting for paintings that require more than one session. For a painting that develops over several days, allowing for the oil paint to dry for a given layer, it is helpful to work with explicit painting layers. The first layer may be a ground, usually applied all over the surface. Then an underdrawing in outline may follow. Then comes underpainting, overpainting, and finally semi-transparent glazes and varnish. All of these layers will affect the appearance of the final painting. To understand the role of underpainting, one can use metaphor and think of the underpainting as a base-rhythm in music, and the overpainting as a solo played over this. Areas not underpainted, outlining the space for a figure for example, are said to be reserved.
Working in layers has been utilized by many schools of art over many centuries, although the overall trend in Western art since the Middle Ages has been towards a simplified and quicker technique. For example, in the early 15th century Cennino D'Andrea Cennini describes how to paint in layers in the egg tempera medium. In contrast, his directions for painting in fresco, done in one session on damp plaster, offer a different system although even here, there is some layering employed. The important distinction is that in fresco, a second layer of paint will physically blend with the first, whereas in egg tempera, which dries rapidly, a second layer will cover and optically blend with the first layers. When a new layer is added to a still-wet earlier layer, this is called wet-in-wet painting. A significant change in the history of western painting occurred in the course of the Renaissance when the white grounds of earlier painting were replaced by dark ones, and darker underpainting.
Modern techniques such as X-rays and infra-red reflectograms often enable lower layers of paint and underdrawing to be seen, and reveal pentimenti, or changes of mind by the artist in the course of work.