Oil paintings are among the most beautiful and realistic, with their texture and depth bringing a photograph-like quality to painting. Oil paint is a simple mixture of pigment and oil, usually linseed, walnut, or poppy oil. It's slow to dry, which allows an artist to alter his or her work before the paint sets.
Before oil paint was invented, artists used tempera paint, made from egg yolks. Tempera paint is a much faster drying paint and lends itself to be a very precise style of painting. When using tempera paint, you’ll use small brush strokes, in layers, because it dries so fast. It's also associated with classical art and, oddly enough, poster paint.
Jan van Eyck is credited with inventing oil paint in the 1400's. Oil paints gave artists another medium for catching and reflecting light that would allow them to realistically imitate life. You'll notice that many oil paintings look a lot like a photograph. This realism would popularize oil paint until the 50's, when acrylic paint replaced it as the paint of choice for many artists.
While painting with oil paint gives an experienced artist options, it can challenge many budding artists. All the things that are loved about oil paint also create challenges for those not used to working with the long drying times. The term "fat over lean" is a concept, in painting, for artists to increase the amount of oil with each layer. When top layers dry before bottom layers, the paint can crack. By adding more oil to the top layers you can slow down the drying time of the top layers.
Linseed oil dries the most quickly and is good for bottom layers or underpainting. It's important to note that linseed oil tends to yellow, which very noticeable with light colors. Avoid it for upper layers in soft hues. Poppyseed oil is a good replacement, but it is slower to dry. When drying your art work, never let it dry in the dark. Oil can rise to the surface, creating a thin film, and cause yellowing. If this happens, put your art in a sunny window for the day to reverse any damage.