Tempera is typically applied onto a prepared surface. Wood panel paintings, for instance, were prepared with layers of gesso (a mixture of size and chalk) to form a smooth surface. The tempera was then applied (over a prepared drawing or sketch) and built up slowly in a series of thin, transparent layers. Unlike oil paint, tempera cannot be applied too thickly, and thus lacks the deep colouration of oils. But tempera paintings are very long lasting and colours do not deteriorate over time, unlike oil paints which tend to darken or lose colour with age. It dries rapidly, and when dry it produces a smooth matte finish. The main disadvantage - apart from the time-consuming need to apply it in thin layers - is that tempera paintings cannot usually attain the deep colour saturation that oil paintings are able to manage.