Confusing Concepts in Oil Painting: Fat Over Lean

Fat over Lean… Thick Over Thin… Thick Over Lean… Whatever… It’s Time to Get Rid of It

Let’s toss out the concepts of “fat over lean” and for that matter “thick over thin” (or the confused “thick over lean”) while we are at it, and let’s consider the physical structure of the paint.

To help you to understand the properties of oil paint, it is helpful to understand the relation between the pigment and oil. One way to think about the relationship between pigment and binder is a brick wall. Every mason knows there is an ideal ratio of mortar to brick. Too much mortar and the wall is weak. Not enough mortar and the bricks fall apart.

The same relationship exists between the pigments and binder in dried paint. We call this relationship or ratio the pigment volume concentration or PVC. Pigment volume concentration (PVC) is the volume of pigment compared to the volume of all solids. If paint has a PVC of 30, than 30% of the total binder/pigment is pigment and 70% is binder solids.

The point at which there is just enough binder to wet pigment particles is called the critical pigment volume concentration (cPVC). For almost all colors this is between 45% and 55% PVC. Films with lower concentrations of pigment have more gloss but as the PVC increases they become increasingly matte. 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. Films with higher pigmentation have increasingly lower tensile strength. (See the graph.)

As the PVC increases past the cPVC, the dried paint film will contain voids. The porosity will increase, moisture vapor transmission will increase, wet hiding will decrease, but dry hiding will increase. As the paint dries, a condition will exist where the paint has too much pigment and too little oil such that the internal voids created will entrap air or solvent in the vapor phase.

Most paint in tubes contain enough binder to wet and envelope pigment particles and so are at about the critical pigment volume concentration or cPVC for that pigment or mixture of pigments. The variation in the pigment volume of particular colors between brands are due to the type of oil used, the amount of oil (some paint makers may choose to make their colors softer or thicker) and the amount of additives used in their formulation.