The principle of painting 'fat over lean' is one of the fundamental
concepts of oil painting and one to follow to reduce the risk an oil
painting cracking. 'Fat over lean' has got to do with the varying drying
times of oil pigments (which can vary from a couple of days to a
fortnight) and ensuring that upper layers of paint don't dry faster than
'Fat' oil paint is oil paint straight from the tube. Mixing it with an oil
makes it even 'fatter' and increases the length of time it takes to dry
completely (even though it may feel dry to the touch, it will still be
drying under the surface). 'Lean' oil paint is oil paint mixed with more
turpentine (white spirit) than oil, or oil paint mixed with a fast-drying oil. 'Lean' oil paint dries faster than 'fat' oil paint.
'lean' is painted over 'fat', it will dry first, making the 'lean'
layer of paint vulnerable to contraction (shrinking) and cracking when
the 'fat' layer dries underneath it. Lower layers also tend to absord
oil from the layers above them.Therefore every layer in an oil painting
should be a little 'fatter' than the previous one, or have a greater
proportion of oil in it.
The drying times of artist's quality
oil paints will vary because they are usually made only from pigment and
oil; cheaper paints may have drying agents added to make the drying
times more consistent.
Paints which tend to have a low oil
content, and thus dry quickly, include Prussian blue, ultramarine, flake
white, and titanium white. Oil paintings with a medium oil content, and
which dry within about five days, include cadmium reds and cadmium
'Fat on Lean' Oil Painting Tips: