The process of oil painting [varies from artist] to artist, but often includes certain steps. First, the artist prepares the surface. Although surfaces like linoleum, wooden panel, paper, slate, pressed wood, and cardboard have been used, the most [popular surface] since the 16th century has been canvas, although many artists used panel through the 17th century and beyond. Before that it was panel, which is more expensive, heavier, less easy to transport, and prone to warp or split in poor conditions. For fine detail, however, the absolute solidity of a wooden panel gives an advantage.
The artist might sketch an outline of their subject prior to applying pigment to the surface. "Pigment" may be any number of natural substances with color, such as sulphur for yellow or cobalt for blue. The pigment is mixed with oil, usually linseed oil but other oils may be used as well. The various oils dry differently creating assorted effects.
Traditionally, an artist mixed his or her own paints for each project. Handling and mixing the raw pigments and mediums was prohibitive to transportation. This changed in the late 1800s, when oil paint in tubes became widely available. Artists could mix colors quickly and easily without having to grind their own pigments. Also, the portability of tube paints allowed for plein air, or outdoor painting (common to French Impressionism).
The artist most often uses a brush to apply the paint. Brushes are made from a variety of fibers to create different effects. For example, brushes made with hog's bristle might be used for bolder strokes. Brushes made from miniver, which is squirrel fur, might be used for finer details. Sizes of brushes also create different effects. For example, a "round" is a pointed brush used for detail work. "Bright" brushes are used to apply broad swaths of color. The artist might also apply paint with a palette knife, which is a flat, metal blade. A palette knife may also be used to remove paint from the canvas when necessary. A variety of unconventional tools, such as rags, sponges, and cotton swabs, may be used. Some artists even paint with their fingers.
Most artists paint in layers, a method first perfected in the Egg tempera painting technique, and adapted in Northern Europe for use with linseed oil paints. The first coat or "underpainting" is laid down first, painted normally with turpentine thinned paint. This layer helps to "tone" the canvas, and cover the white of the gesso. Many artists use this layer to sketch out the composition. This layer can be adjusted before moving forward, which is an advantage over the 'cartooning' method used in Fresco technique. After this layer dries, one way the artist might then proceed is by painting a "mosaic" of color swatches, working from darkest to lightest. The borders of the colors are blended together when the "mosaic" is completed. This layer is then left to dry before applying details. The artist may apply several layers of details, using a technique called 'fat over lean.' This means that each additional layer of paint is a bit oilier than the layer below, to allow proper drying. As a painting gets additional layers, the paint must get oilier (leaner to fatter) or the final painting will crack and peel. After it is dry, the artist will apply "glaze" to the painting, which is a thin, transparent layer to seal the surface. A classical work might take weeks or even months to layer the paint, but the most skilled early artists, such as Jan van Eyck, also used Wet-on-wet painting for some details. Artists in later periods such as the impressionist era often used this more widely, blending the wet paint on the canvas without following the Renaissance layering and glazing method. This method is also called "Alla Prima." When the image is finished and dried for up to a year, an artist would often seal the work with a layer of varnish typically made from damar gum crystals dissolved in turpentine. Contemporary artists increasingly resist the varnishing of their work, preferring that the surfaces remain varnish-free indefinitely.
