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 poppy seed 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 Oiler than the layer below, to allow proper drying. Traditional oil painting techniques often begin with paint mixed with turpentine. As a painting gets additional layers, the paint must get Oiler (leaner to fatter) or the final painting will crack and peel.