Oil painting is the process of painting with pigments that are bound with a medium of drying oil — especially in early modern Europe, linseed oil. Often an oil such as linseed was boiled with a resin such as pine resin or even frankincense; these were called 'varnishes' and were prized for their body and gloss. Other oils occasionally used include poppyseed oil, walnut oil, and safflower oil. These oils confer various properties to the oil paint, such as less yellowing or different drying times. Certain differences are also visible in the sheen of the paints depending on the oil. Painters often use different oils in the same painting depending on specific pigments and effects desired. The paints themselves also develop a particular feel depending on the medium.
Traditional oil painting techniques often begin with the artist sketching the figure onto the canvas with charcoal or a "clean", which is thinned paint. Oil paint can be mixed with turpentine or artist grade mineral spirits or other solvents to create a thinner, faster drying paint. Then the artist builds the figure in layers. A basic rule of oil paint application is 'fat over lean.' This means that each additional layer of paint should be 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. There are many other painting media that can be used in oil painting, including cold wax, resins, and varnishes. These additional media can aid the painter in adjusting the translucency of the paint, the sheen of the paint, the density or 'body' of the paint, and the ability of the paint to hold or conceal the brushstroke. These variables are closely related to the expressive capacity of oil paint. When looking at original oil paintings, the various traits of oil paint allow one to sense the choices the artist made as they applied the paint. For the viewer, the paint is still, but for the artist, the oil paint is a liquid or semi-liquid and must be moved 'onto' the painting surface.
Traditionally, moving paint was accomplished with paint brushes, but there are other methods, including the palette knife, the rag, and even directly from the paint tube. Oil paint remains wet longer than many other types of artists' materials, enabling the artist to change the color, texture or form of the figure. At times, the painter might even remove an entire layer of paint and begin anew. This can be done with a rag and some turpentine for a certain time while the paint is wet, but after a while, the hardened layer must be scraped. Scraping may also be used to smooth a portrait before scumbling and glazing. Many oil paintings reveal evidence of scraping on close inspection, particularly when the surface itself is examined. Oil paint dries by oxidation, not evaporation, and is usually dry to the touch in a day to two weeks. It is generally dry enough to be varnished in six months to a year. Art conservators do not consider an oil painting completely dry until it is 60 to 80 years old.
Most Renaissance sources, in particular Vasari, credited northern European painters of the 15th century, and Jan van Eyck in particular, with the "invention" of painting with oil media on wood panel, however Theophilus (Roger of Helmarshausen?) clearly gives instructions for oil-based painting in his treatise, On Divers Arts, written in 1125. Early Netherlandish painting in the 15th century was however the first to make oil the usual painting medium, followed by the rest of Northern Europe, and only then Italy. The popularity of oil spread through Italy from the North, starting in Venice in the late 15th century. By 1540 the previous method for painting on panel, tempera had become all but extinct, although Italians continued to use fresco for wall paintings, which was more difficult in Northern climates.
A still-newer type of paint, heat-set oils, remain liquid until heated to 265–280 °F (130–138 °C) for about 15 minutes. Since the paint never dries otherwise, cleanup is not needed (except when one wants to use a different color and the same brush). Although not technically true oils (the medium is an unidentified "non-drying synthetic oily liquid, imbedded with a heat sensitive curing agent"), the paintings resemble oil paintings and are usually shown as oil paintings.
Traditional artists' canvas is made from linen, but the less expensive cotton fabric has gained popularity. The artist first prepares a wooden frame called a "stretcher" or "strainer". The difference between the first and second is that stretchers are slightly adjustable, while strainers are rigid and lack adjustable corner notches. The canvas is then pulled across the wooden frame and tacked or stapled tightly to the back edge. The next step is for the artist to apply a "size" to isolate the canvas from the acidic qualities of the paint. Traditionally, the canvas was coated with a layer of animal glue (size), (modern painters will use rabbit skin glue) and primed with lead white paint, sometimes with added chalk. Panels were prepared with a gesso, a mixture of glue and chalk.
Modern acrylic "gesso" is made of titanium dioxide with an acrylic binder. It is frequently used on canvas, whereas real gesso is not suitable for that application. The artist might apply several layers of gesso, sanding each smooth after it has dried. Acrylic gesso is very difficult to sand. One manufacturer makes a sandable acrylic gesso, but it is intended for panels only, not canvas. It is possible to tone the gesso to a particular color, but most store-bought gesso is white. The gesso layer will tend to draw the oil paint into the porous surface, depending on the thickness of the gesso layer. Excessive or uneven gesso layers are sometimes visible in the surface of finished paintings as a change in the layer that's not from the paint.
Standard sizes for oil paintings were set in France in the 19th century. The standards were used by most artists, not only the French, as it was - and evidently still is - supported by the main suppliers of artist materials. The main separation from size 0 (toile de 0) to size 120 (toile de 120) is divided in separate runs for figures (figure), landscapes (paysage) and marines (marine) which more or less keep the diagonal. Thus a 0 figure corresponds in height with a paysage 1 and a marine 2 .
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.