Dear Mitra,
It seems that everyone describes fat over lean as simply placing more oily layers over less oily layers. But are certain oils in and of themselves more fatty than other ones if equal amounts are set out? For a example if I set out an equal amount of a polymerized linseed oil and refined linseed oil is one fattier than the other? Since one was polymerized does this change its fat/lean state?
Thank you,
Hector Hernandez
The idea behind the ‘fat-over-lean’ rule is pigment volume concentration (PVC), which is the percentage of all pigments in relation to all solids (pigment and binder) in the paint film.In my Painting Best Workshop I introduce artists to the concept of pigment volume concentration (PVC) to help them understand the relationship between pigment and binder in cured paint films. Understanding and using this concept is easier than working with the so-called fat-over-lean rule, which is archaic, cumbersome and often misunderstood.
Pigment volume concentration (PVC) is the volume of pigment compared to the volume of all solids. If paint has a PVC of 30, for example, than 30% of the total binder/pigment is pigment and 70% is binder solids. ‘Lean’ oil paints have high PVC and are typically underbound (not sufficient binder), whereas ‘fat’ oil paints have low PVC and have an excess of oil (binder). The type of oil does not alter this relationship, but rather it is the amount of oil in the paint film.
The point at which there is just enough binder to wet pigment particles is called the critical pigment volume concentration (CPVC).
Films with lower concentrations of pigment have more gloss and less tensile strength, but are less permeable to moisture. Films with high percentages of pigment are more permeable to moisture and susceptible to solvents. This is because with more pigment, there is less binder to fill the voids between pigment particles. This porosity leaves the film open to the environment.
Image of PVC
Thanks George. Another issue to point out about relative “fatness” of different oil binders relate to their viscosity when they are undiluted. Polymerized oil is generally more viscous than raw or alkali refined drying oils. In its undiluted state, its high viscosity would make it difficult to grind as much pigment into it and would result in a paint with a very fat glossy quality. If the polymerized oil were diluted to the same consistency as the raw oil, it would take up a more similar amount of pigment. This will probably not be exactly the same, but closer.
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