When the painting is finished, it is not very brightly coloured, until it has been varnished. This makes the colours bright and shiny. The good thing about tempera painting is that the colours do not change over the years, whereas oil paintings change, get darker and go more yellow. Tempera paintings can last well for at least two thousand years. Because tempera paintings take a long time to do, many of them are small. Sometimes artists painted large altarpieces in tempera. One of the biggest tempera paintings is Duccio's picture of the Madonna and Child in the Uffizi Gallery. It is about 15 feet high. Another very famous large tempera painting is Botticelli's "Birth of Venus" which is also in the Uffizi. 