Fat over Lean... Thick Over Thin... Thick Over Lean... Whatever...
It’s Time to Get Rid of Them
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 the paint has a PVC of 30, then 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). This is between 45% and 55% PVC for almost all colors. Films with lower pigment concentrations 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 increasing 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, and 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 paints 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 is 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.
So the best way to discuss paint is by understanding this relation between pigment and oil, expressed as the pigment volume concentration (PVC). Oil paint straight out of the tube is in the form of a paste and is usually near its cPVC. Adding oil to paste paint lowers the PVC (low PVC) while adding solvent potentially increases the PVC (high PVC). Hence the ideal paint is a paint of the consistency of paste, and this is why the admonition to “apply thick paint thinly” or work with paste paint in thin layers is the best practice for oil painting.
From this, it is clear that paint consists of both liquid and solid components. In the case of oil paint, the liquid component is a drying vegetable oil. The oil undergoes chemical and physical processes that change it from a liquid to a solid, called “oxidative polymerization.” Oil is the binder or glue that holds the pigment particles together and onto a substrate.
You may think of paint as pigmented or colored adhesive. When it comes to adhesion in oil paint, oil is the glue. While many artists believe that mechanical adhesion is most important when it comes to paint, what is more important is “dispersive adhesion,” which does not rely on the absorption of the binder or oil into a porous substrate or on surface texture (although they can improve dispersive adhesion) but instead on surface energy and polarity. Hence, paste paint adheres better than paint that has been heavily diluted with a solvent (high PVC) even if it is partially absorbed into the substrate because the latter has less oil or glue to adhere pigment particles to the surface of the substrate.