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Oil painting is done on surfaces with pigments that are ground and mixed into a medium of oil especially in early modern Europe, linseed oil. Other oils occasionally used include poppyseed oil, walnut oil, and safflower oil. These oils give 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 mediums. 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. Traditional oil painting techniques often begin with paint mixed with turpentine. 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 mediums that can by used in oil painting, including cold wax, resins, and varnishes. These additional mediums 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 variable 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, so a reality in many painter's studios is the removal of oil paint from the painting. 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. Many oil paintings reveal evidence of such scraping if you look closely, and pay attention to the surfaces themselves. 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.
Oil paint was probably developed for decorative or functional purposes in the High Middle Ages. Surfaces like shields both those used in tournaments and those hung as decorations were more durable when painted in oil-based media than when painted in the traditional tempera paints.
Many Renaissance sources credit northern European painters of the 15th century with the "invention" of painting with oil media on wood panel Jan van Eyck is often mentioned as the "inventor". The popularity of oil grew in 16th century Venice, where a water-durable medium was essential. Oil painting was ideal for the northern European painters, because the preferred fresco painting media did not work as well in their cooler climate. The linseed oil itself comes from the flax seeds, and this flax was a common fiber crop. Recent advances in chemistry have produced modern water miscible oil paints that can be used with and cleaned up with water. Small alterations in the molecular structure of the oil creates this water miscible property.
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.
Process of oil painting
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, pressed wood, and cardboard have been used, the most versatile surface is canvas. While many famous paintings were painted on panel (Da Vinci's Mona Lisa), the size of the work was limited by weight and the thickness of the wood. Stretched canvas, on the other hand, only depended on the width of the roll fabric and could be unstretched for easier transportation.
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 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 ground (or size) to isolate the canvas from the acidic qualities of the paint. Traditionally, the canvas was coated with a layer of rabbit skin glue and primed with subsequent layers of finely ground chalk (or marble dust) and rabbit skin glue. Later the process was changed to a sizing of rabbit skin glue with subsequent layers of white priming (gypsum, chalk, barium oxide, titianium(IV) dioxide mixed with linseed oil). Modern gessos are made of titianium dioxide with an acrylic binder and are not "real" gessos in the true sense of the word. The artist might apply several layers of gesso, sanding each smooth after it has dried. Sanding the primed surface is important to roughen the generally slick surface so the subsequent layers of oils will properly adhere. 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.
Next 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 sulfur 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 1800’s, 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. After it is dry, the artist will apply "glazes" to the painting, using a process of "Fat over Lean" which means more oil/paint ratio than the previous layer. A classical work might take weeks or even months to layer the paint. Artists in later periods such as the impressionist era often blended the wet paint on the canvas without following the Renaissance layering and glazing method. This method is 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.
From Wikipedia, the free encyclopedia
Mona Lisa, Oil on wood panel painting by Leonardo da Vinci
La Donna Velata, painted in 1516, Oil on wood panel painting by Raphael
The above explanation comes from 出典: フリー百科事典"ウィキペディア"