In the 15th and 16th centuries, there was a gradual movement from tempera paints to oil paints as the primary medium. Egg tempera paints dried quickly, had less color saturation, and had low transparency, whereas oil paints dried more slowly which made them easier to work with, had greater color saturation, and increased transparency. This shift in painting medium corresponded with the themes of humanism and realism in the Renaissance by allowing artists to represent their subjects more realistically than was possible with tempera paints. Source: http://itech.dickinson.edu/chemistry/?p=610 Image source: http://www.the-athenaeum.org/art/list.php?m=a&s=tu&aid=3550 (public domain)