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“FAT OVER LEAN”

What exactly does this mean?

The easiest way to understand what this age old rule means is to first consider how a painting is made up. Essentially it is a laminated or multi-layered structure, starting with the support (canvas, board etc.) the ground and then the paint layers. From this basic starting point comes the principle that each layer should be at least as flexible and susceptible to expansion and contraction as the preceding layer. If not one runs the risk of cracking, flaking and peeling paint.

This brings us back to “Fat over Lean”. Oil paint is essentially dry pigment ground in a drying oil, be it linseed, walnut or poppy oil. To this in painting can be added various oils and varnishes in glazes and mediums.

The “Fat” part of this rule refers to these ‘fatty’ oils. “Lean” is the opposite, in that the paint layer contains less of these oils, as when paint is thinned with Turpentine. To put it simply, to paint “Fat over Lean” means painting in layers of paint that contain successively greater amounts of the ‘fatty’ drying oils.

Why is this important?

Oil paint does not “dry up” by evaporation as for example do water based paints. Rather, oil paint dries by oxidisation or absorption of oxygen from the air. Once dry this oil film is a new substance differing in both physical and chemical properties, which cannot be returned to its original state by any means.

As it dries the oil paint layer moves; those layers with less ‘fatty’ oil, the “lean”, dry quicker and before any subsequent “fat” layer. That this order is adhered to is important because if a “lean” layer were to be painted over a “fat” layer, the former would dry quicker than the latter, oxidise and become inflexible. The “fat” layer underneath, still moving as it oxidises more slowly, can then cause cracking in the already dry layer above it.

In a nutshell that is what painting “fat over lean” means.

The role of paint itself.

In addition to considering the drying oils already mentioned, linseed, walnut and poppy, each pigment colour has its own drying and ‘fatty’ characteristics, which effect the whole equation. Each colour in its dry, pigment state has different properties which determine how much oil is needed to turn it into a paste. In the days before commercially prepared oil paint, each artist would have had to grind his own, and by so doing would have known which colours required more or less oil. The artist would then proceed to paint his picture with this knowledge, using those colours with less oil content in his under-painting. Today this knowledge is largely lost, but not all colours we squeeze out of the tube have the same properties. Below, is a basic, not exhaustive list of colours which have high oil content and are slow drying, and are thus not suitable for use in under-painting. Alongside these are colours which have a low oil content but are equally slow drying and again unsuitable for under-painting. Lastly are those colours which have low oil absorption and are fast drying and therefore particularly suited to under-painting.

High oil content, slow drying

Low oil content but slow drying

Low oil content, fast drying

Alizarin Crimson

Rose Madder

Raw Umber

Cadmium Orange

Zinc White

Burnt Umber

Cobalt Blue

Titanium White

Raw Sienna

Ivory Black

Lemon Yellow

Burnt Sienna

Lamp Black

Cadmium Yellow

Cerulean Blue

Aureolin/ Cobalt Yellow

Yellow Ochre

Cobalt Green

Phthalocyanine colours

Flake White

Quinacridone colours

Cremnitz White

Viridian

Lead White

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