Through Leonardo’s sfumato technique, the transition between light and shadows in portraits has been realized with the extremely delicate tonal gradations. Emulation of sfumato begins in the underpainting. A thick paint was spread with a dry brush (or a sponge) in places where the shadows in the flesh tones were crossing in to the lighter areas. This method eliminates every brush stroke and it creates smooth modeling of the surface. The resulting gradation of tones was used as a base layer for the upcoming layers of paint. The following glazes of pure tones create homogeneity of these transitions, while highlights of pure white tint helps to emphasize a three dimensional display.

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 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 poppy-seed oil, walnut oil, and safflower oil. These oils confer various properties to the oil paint, such as less yellowing or different drying times.

http://en.wikipedia.org/wiki/Oil_painting

Leonardo di ser Piero da Vinci (April 15, 1452 – May 2, 1519), was an Italian polymath: painter, sculptor, architect, musician, scientist, mathematician, engineer, inventor, anatomist, geologist, cartographer, botanist and writer. Leonardo has often been described as the archetype of the Renaissance man, a man whose unquenchable curiosity was equaled only by his powers of invention. He is widely considered to be one of the greatest painters of all time and perhaps the most diversely talented person ever to have lived.

http://en.wikipedia.org/wiki/Leonardo_da_Vinci