Fat over Lean

This could be just called “your painting is going to crack eventually”, because that’s likely true no matter what you do to try and avoid it. It’s just the nature of oil paint. To understand why, we need to look at the chemistry of how oil paint “dries”.

Although this process varies slightly depending on the specific oil used as the suspension medium for the paint, all oil paints go through two different phases of drying. The first is “autoxidation” where oxygen molecules bond with the lipid structures through the same free radical process you and I take vitamins to prevent from happening to us. During this phase, the oil medium dries to a film and becomes “touch dry”. This is also known as the weight gaining phase where the paint can actually increase mass by up to 14%. The oxygen uptake process creates a three dimensional network of bonds. It also results in the creation of volatile compounds that diffuse out of the paint film after autoxidation. This is the second phase of “drying” where the paint actually loses mass — in some cases more than the mass that was gained in the first phase.

There are a lot of factors that go into how this happens. Linseed oil tends to lose less mass than any other drying oil used in paint. Other oils tend to break down more during the second phase. Other issues influence mass loss over time too, such as metal ionization (where metal ions break down the three dimensional bond structures over time causing more volatile compounds to diffuse out of the paint film). Metal ions are in the paint only if the pigments contain them, or if you use certain additives that contain them (such as “dryers”). Many pigments do contain metals, such as flake white or cremnitz white (contains lead), earth tones usually contain iron-oxide, some contain manganese. Cobalt and copper are also found in pigments (cobalt blue, any pthalo pigments).

There’s also something called “hydrolization” which can continue to damage the paint film over time if the oil in the paint was subjected to water. I know many people think exposing oil paint to water is safe because “oil and water don’t mix” but I’m afraid they do — or at least the interact chemically.

So, this is what happens in each layer of paint. But what delineates the layers? The autoxidation process is what really separates layers. So, if you are working with wet paint and painting wet paint on top of other wet paint, it’s all one layer as far as the chemistry is concerned. If you wait until paint is touch dry then continue working with glazes, etc., then you are adding new layers (that will need to undergo a similar process, depending on the glazing medium you use). So, if you are worried about fat over lean because you tone your canvas and then paint on top of it in one sitting, don’t worry. It’s all going to be part of the same layer once the oxygen molecules do their thing.

I often hear and read that the old masters really understood this “fat over lean” rule and their processes and their materials reflect the great wisdom of their mastery. Unfortunately, this is not true. It turns out that their mastery was in their painting and not in their understanding of chemistry. I don’t know of a single oil painting more than 150 years old (and some even younger) that haven’t cracked. Basically, they all crack — and so will yours and mine one day.

So, worry less about fat over lean in general. Stop worrying at all of you paint wet into wet. If you use any modern “plastic” mediums (such as Liquin), you might be better off than the old masters just because the chemistry is better, but we’ll have to wait a hundred years or more to find out.

If you want to read more about the research on this topic, I recommend this site, and also this paper.

In any case, keep painting.