THE "FAT-OVER-LEAN" RULE

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When painting in oils and dealing with painting mediums, it is necessary to pay close attention to a very ancient but very important rule: Fat-Over-Lean. This rule causes quite a deal of confusion among students, beginners and even more accomplished painters, chiefly because of lack of understanding of what happens when an oil dries. A drying oil does not evaporate, it simply changes chemically in contact with oxygen in the atmosphere, it is said to polymerise and when dry, it forms an elastic material. This elasticity characteristic of a drying oil such as walnut or linseed ensures that the paint surface remains in theory flexible. Well... Not always!

The more oil is added to a paint layer, the more flexible that layer will be. The less oil, the less flexible the layer. Turpentine or mineral spirits (white spirit) make a layer thinner, less fat and therefore less flexible.

Temperature, humidity and other factors all influence an oil painting. They make the paint layers to move microscopically, their molecules adapting or changing slightly according to the environment conditions. If there is enough oil (fat), the layer is elastic enough to move without major problems. If however, the  layer is not flexible enough, its surface risks cracking.

As a matter of principle and as a Rule-of-Thumb, the more fat layers always go on top of less fat (or lean) layers. This is to be observed without question, in order to prevent an oil picture from cracking. A lean layer on top of a fat layer will crack, because it doesn't have enough elasticity to cope with the moving layer underneath. So, always FAT-OVER-LEAN or FLEXIBLE-OVER-LESS FLEXIBLE.