Oil paint is any paint made by mixing pigment, oil, and a binder (to keep the pigment and oil from separating). Linseed oil is often used for the oil part of the paint, however, other types of oil are also commonly used. Safflower oil and poppyseed oil are frequently used in making oil paints, either alone or in combination with linseed oil. Almost any type of vegetable oil can, theoretically, be used for oil painting, though most painters use linseed, safflower, and poppyseed oil because their properties are well known - there is little question as to how they will age or change with time.
Oil paints have been used since the Middle Ages. In the early Renaissance, they became the chief media for painting, as painting changed from a stationary work done for the interiors of churches to a personal thing that would change hands over time. Oil paints were the chief media for artists until the middle of the 20th c., when it became acceptable to work in a greater variety of media, like collage, assemblage, photography, watercolor, printing, and acrylic paints.
Oil paintings are done on stretched linen or canvas. The canvas is primed, either with a traditional gesso, made from rabbit skin glue and marble dust (calcium carbonate) or with acrylic gesso. The canvas must be primed with some sort of gesso, because otherwise, the oils in the paint will cause the canvas to rot. After a canvas is painted, and the paints are completely dry (usually 6-12 months), the artist may choose to varnish the painting to increase the permanence and enhance the colors of the work. Paint thinner, turpentine, and other acids will remove oil paints, even those that have been dry for a long time, as seen by the attack on Rembrandt van Rijn's Danae in the Hermitage, St. Petersburg.
Oil paints are valued for their permanance, vibrant colors, and long drying time. Oil, as a plant product, is relatively stable, providing that it is kept under reasonable climate controls and out of excessive sunlight. Oil paints produce more vibrant colors than acrylics because they use oil. The oil reflects light hitting the painting far better than acrylics can. This is why a Vermeer or Rembrandt still appears to glow, centuries after it was painted. The long drying time of oils is desirable because it allows for subtle mixing of colors on the canvas. Oil paints also look better than acrylics because more pigment can be suspended in oil than in acrylic medium. The darkening in older oil paintings is not due to the painting itself, but to the varnish on top of the paint, which has darkened. Layers of varnish, applied over centuries, can cause soot to be trapped in the varnish, and make the painting even darker.
Acrylic paints, because they are made from plastic, are more prone to fading and changes in color with time. They dry more quickly, which makes it more difficult to mix colors. They are easier to work with when beginning, are much cheaper and less messy than oils. They produce decent results, but for some applications, only the beauty of oil paints will do.