What causes cracking in oil painting? Fat over lean explained

Cracking in oil painting happens when a lean layer of paint, one with little oil in it, dries over a fat layer with more oil. The fat layer underneath stays flexible and keeps moving for longer, while the lean layer on top becomes brittle, so the top layer splits. The rule painters follow to avoid it is fat over lean: each layer should have a little more oil than the one beneath. Easel by Zaweba simulates this, using solvent and linseed oil among its mediums, so lean paint dried over fat cracks just as it would on canvas.

It is one of the first technical rules an oil painter learns, and one of the easiest to break by accident. Seeing it happen on screen is a far cheaper lesson than seeing it happen on a painting you spent a month on.

What fat and lean mean

Oil paint is pigment ground in a drying oil, usually linseed. Fat paint has a lot of oil in it, either straight from the tube or with extra oil added as a medium. Lean paint has had its oil diluted, usually with a solvent, which thins the paint and makes it dry faster and more matt.

Oil paint doesn't dry by evaporating. It cures by reacting with air, and as it does, it moves and shrinks slightly over a long time. A fat layer stays flexible for longer than a lean one. Put a flexible layer on top of a stiff one and all is well. Put a stiff, lean layer on top of a flexible, fat one, and as the fat layer keeps moving, the brittle top layer cracks.

How painters follow the rule

A traditional way to work is to start lean and get fatter. The first layer, the underpainting or block-in, is thinned with solvent so it dries quickly. Each following layer has a little less solvent and a little more oil. The final touches, including thick impasto highlights, are the fattest of all.

The same idea explains why you should not lay a thin, solvent-heavy glaze over a thick, oily passage. If you want to glaze over fat paint, the glaze needs at least as much oil as what's below.

Seeing it in Easel

Easel's mediums and additives include linseed oil, glazing medium and solvent, along with retarder, flow improver, gloss and matt medium, and others for watercolour. In Easel, oil stays wet for impasto, and lean paint dried over fat cracks.

To see the rule at work, try a small test. On a canvas or panel, paint one patch fat, using linseed oil, and let it dry. Easel's drying speed slider runs from 0.25 to 4 times, and Ctrl+D dries the paint now. Then paint a lean layer, thinned with solvent, over half of it and dry that too. Next to it, paint the same in the right order, lean first and fat on top. The difference is the whole lesson.

Cracks on purpose

Sometimes you want cracks. A painting meant to look centuries old usually has a fine web of craquelure. Easel has a separate Age the picture tool that yellows oil and varnish, fades fugitive colours and cracks old oil. Breaking fat over lean in a chosen area is another way to place cracks where you want them, so they follow the structure of the painting.

Habits that keep a painting sound

Thin the first layers and keep later layers richer. Avoid adding solvent to paint you lay over a thick, oily passage. Let underlayers dry before working on top. And save your thickest, oiliest strokes for the end.

These habits come naturally once you've watched a lean layer crack over a fat one. Easel by Zaweba makes that safe to try as often as you need, with a saved .zeasel file keeping wet paint wet so you can come back and carry on.