Naked Science Forum
Non Life Sciences => Chemistry => Topic started by: thedoc on 23/05/2013 18:30:01
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Maggie Siner asked the Naked Scientists:
On your recent episode (May 2, 2013 Art and Antiquities) a painting conservationist discussing oil painting stated that they need to be constructed on principle of 'Fat Over Lean', which is correct, but her reason stated was not completely correct. She said it was so the underneath layers could dry first. That is sort of true but really the "Fat Over Lean" rule has to do with the binding of layers and the movement of paint during the drying (i.e. oxidation/polymerization) process.
Bonding of layers: The leaner paint film forms a rougher surface, (the proportion of pigment granules is greater than the amount of oil binder) so the subsequent layer of paint can grab and stick to it very well. A fatter (more oily) paint film forms a smoother surface (fewer pigment granules in proportionately more oil) so the next layer does not have as much rough surface to bond to.
Movement of paint during drying: A lean paint, has a low oil content. Fat paint has a high oil content. Oily layers tend to be thicker and of course they dry more slowly. Paint 'dries' from the top down; the outside skin polymerizing first, and then gradually hardening down into the depths of the paint layer. That means there will be a more liquid paint underneath, especially in an oily paint. Liquid expands, contracts and moves. A lean paint film dries all the way down more
quickly and does not remain liquid as long. If a layer of drier leaner paint were on top of oilier paint, then that dry layer would crack due to the disturbance of the more flexible layer underneath. In addition the leaner paint would not bond as well to the smoother
oily paint underneath. In addition, every pigment requires a different proportion of oil to
make it into a functional paint (has a different oil absorption index) and this means a painter must know which colors are fat and which are lean if they are to maintain a secure construction of paint layers that bind well to each other.
I love your program (http://www.thenakedscientists.com/HTML/podcasts/) and learn so much from it.
What do you think?