MITRA Forum Question Details

​Some pigments are not as absorbent as others, and this affects how much vehicle is needed to make paint. So-called "lean" pigments form a workable paste with less vehicle- the original Dutch Boy white lead paste was 89% white lead, 9% linseed oil, 2% turpentine (reformulated later to 88% white lead, 10% linseed oil, 2% mineral spirits). Lean pigments usually tend to release or "shed" oil in storage- some brands of lead-based white, for instance, seem very oily if stabilizers are not used, even though the pigment load is relatively high. "Fat" colors, on the other hand, can look lackluster and dry if the pigment has taken up a lot of vehicle, even though they might contain relatively more oil than leaner colors. As to why there might be some difference with tempera vehicle, I imagine it is the water content of egg medium which helps wet the pigment with less binder compared to oil.

Sarah Sands covers the problematic nature of the traditional way in which we measured oil absorption (or even binder requirements). Since it used weight to volume and not volume to volume, some colors appeared far “leaner” in comparison with other colors than they actually were. See her article on the subject here:

https://www.justpaint.org/volume-weight-and-pigment-to-oil-ratios/

It does not cover what aspects of the pigment influence the proportion of binder required to make an adequate paint. I did find this online article that appears to cover the subject well. In essence, it is the size, shape, and density of the pigment.

https://www.european-coatings.com/Editorial-archive/Oil-absorption

I did worry about particulars on this because, as I understand it, the issues are a bit more complicated than can be succinctly stated…and a bit beyond my total understanding. I believe that particle size and morphology have the biggest impact and that it is about surface area. Density or weight will affect the manner in which a pigment stays in suspension. This is less of an issue with oil paints containing a lot of gelling stabilizer.

However, there is also the role of chemical affinity and/or reactions between the oil and pigment. Lead white, in particular, will require less oil if that oil has a higher acid number. Some portion is converted to metal soaps. Sorry that I cannot give you a perfect answer on this. I will reach out to Dr. Jaap Boon and perhaps another conservation scientist to see if they can provide a more accurate explanation.

To continue to discuss this topic we need to understand the meaning of the terms in play here, such as CPVC or critical pigment volume concentration. First, the absorption rate is actually based on the absorption of oil not any other media, so it is properly called “oil absorption” (OA). The reason for this is that historically this was measured for oil paints and secondly, the entire amount of the liquid (oil) is the binder, which is not the case for acrylic latex paint, egg yolk tempera or all other waterborne paints. The solvent or diluent (which in the case of egg tempera is water) is not included in determining the oil absorption of a pigment, which in the case of egg tempera can be as much as 70% or more of the fluid mixed with the pigment.

Let's examine the definition of oil absorption (OA) more closely: OA is the mass of linseed oil required to form a coherent putty-like mass with a specified mass of pigment under specified conditions. The OA or amount of oil to make a paste is therefore expressed as x grams of linseed oil per 100 grams of pigment. The method to determine the oil absorption of any given pigment is described in the following ASTM methods: Oil absorption using the Gardner-Coleman method, D 1483; and oil absorption using the rubout method, D 281. These tests give a rough idea of how much oil is needed to make a stiff paste.

What happens during an OA test is approximately the following: The surface of each particle of pigment is wetted with oil, that is, each particle becomes enveloped in a thin layer of oil. The amount of oil required depends on the specific area of the pigment, that is, particle size, roughness of the surface of the particle, and presence of cracks and pores in the particles. Other things being equal, it depends on the vigor and duration of rubbing and on the wettability of the pigment. With the addition of more oil, the interstices between the particles (covered by a thin layer of oil) now become filled with oil. The amount of oil required for this stage depends on the type of packing assumed by the pigment particles. It is also influenced by the presence of aggregates (clusters of pigment not broken up by the procedure) and agglomerates (clusters formed after particles have been wetted). Agglomeration is influenced by the nature of the oil.

Pigment Volume Concentration (PVC) and Critical Pigment Volume Concentration (CPVC) while calculated from mass, are expressed as the volume of pigment compared to the volume of all solids in a paint film. The PVC of paint is determined by separating the pigment from a weighed paint sample by, for example, extracting the binder and analyzing the residue.

When paint is at the CPVC of the pigment or mixture of pigments, the pigment particles are at a maximum packing density, and the interstices are completely filled with binder. With smaller amounts of binder (high PVC), the interstices are incompletely filled and there are voids. With larger amounts of binder (low PVC) the interstices are filled and there is additional space between pigment particles occupied by the excess binder. The CPVC thus represents a pigment concentration boundary at which abrupt changes in the properties of the paint film occur.

Now that we have established the definitions it becomes clear as to why there are differences in the amount of binder required for oil paint and for egg tempera to make the same consistency of paint even for the same pigment and same conditions.

In answer to your questions, CPVC is determined to a large extent by the pigment surface area when calculated based solely on the density and volume of the pigment and binder, but will in practice be different because it is influenced by the wettability of the pigment.

The density of a specific pigment is based on how compact the pigment is and is commonly expressed as kilograms per cubic meter (kg/m3) or pounds per cubic foot (lb/ft3), although kilograms per liter (kg/l) and pounds per gallon (lb/gallon) are also sometimes used. The pigment density is used as part of the calculation for CPVC:

CPVC Formula

The density of basic lead carboante (lead white) is about 6.5 that of water (specific gravity) or expressed as 6.582 g/cm3. The OA of lead white is 8–12 grams of oil per 100 grams of pigment. on the other hand, the density of alizarin crimson is 1.540 g/cm³, and its OA is 80–100 grams of oil per 100 grams of pigment. The large differences in OA between these two pigments are due to the density of the pigments.

Read more about PVC and CPVC

​Thanks George.

It now strikes me that some pigments actually are sponge-like. Some earth colors contain clays that do absorb oil and as George mentions pits, fissures, and cracking in the pigment particle will hold oil. In each case they have greater surface area than would a smooth, nonporous pigment.

What is not often mentioned in literature is the wettability of the pigment surface with various media. A dramatic and often overlooked example is ultramarine blue. I have measured the OA of a specific ultramarine blue pigment in walnut oil and linseed oil and found a remarkable difference: 48 grams of walnut oil, and 56 grams of of linseed oil for 100 grams of pigment. Walnut oil wets the surface of ultramarine pigment bettter than linseed oil, because it is a polar pigment and walnut oil is more polar than linseed oil.

Koo, the answers are a lot easier than you imagine.

1. If more pigment (we have to be clear as to which solids we are discussing, because the binder is considered a solid in a dried paint film) is added to paint, the PVC is increased. If more binder is added the PVC is lowered because the percentage of pigment in relation to the binder decreases. As I wrote in my previous answer, solvents and diluents are not part of the calculation of PVC. Egg tempera paint is never at the CPVC of the pigment or pigments in the paint, because egg yolk does not envelop pigment particles in the dried paint film. If you examine egg tempera paint films at high magnification the egg yolk would appear as tiny glue bridges connecting pigment particles. As a consequence egg tempera paint is a naturally high PVC system.

2. Paint at its CPVC theoretically does not contain voids, because pigment particles are enveloped by the binder and because of pigment packing interstices between pigments are the smallest possible, so that particles are nearly in contact within one another except for a thin film of binder surrounding each particle.

3. Your understanding is correct, but when painters prepare egg tempera paint they often add additional water so I included this water content which may comprise 70% more or less of the liquid vehicle (binder and solvent) added to pigment. The percentage is not, of course, an absolute number because it depends on how the paint is prepared.

4. As water in egg tempera paint evaporates the non-volatile portions of egg yolk adhere to pigment particles, forming air voids throughout the paint film.

5. The absorption of other binders, mostly alkyds, have been studied to better understand how these impact formulations and the calculations of PVC and CPVC. I do not believe there is any practical benefit to knowing the absorption of egg yolk, since egg tempera is a high PVC paint.

6. Adding more pigment to paint (increasing PVC) does not change the CPVC (that's a fixed value based on the amount of binder that completely envelops pigment particles with the densest pigment packing), Changing the PVC by increasing it above the CPVC changes paint film properties, such as increasing permeability (making it more susceptible to solvents and water vapor), decreasing tensile strength, and reducing gloss. Adhesion may also be affected because there is less binder to wet the substrate, decreasing the total surface area of the paint in contact with the support. Paint that is less flexible is typically more susceptible to cracking. Air voids increases the potential for diffusion of oxygen and water vapor through the paint which is why it is more susceptible to fluctuations in relative humidity. In addition, high PVC films (>CPVC), such as egg tempera, are not good candidates for varnishing, because varnish is absorbed into the paint film, thereby becoming part of the painting and is very dfficult if not impossible to remove. To some degree paint films with high PVC are also susceptible to abrasion, but this is also dependent upon the type of pigments in the paint film.

Due to the high PVC nature of egg tempera it is more brittle compared to oil paint and more susceptible to changes in the environment. For this reason it performs best when applied to rigid supports. Varnishing egg tempera paintings is more complex compared to varnishing oil paintings; applying an isolating coating first to tempera paint film should be  followed by a final protective coating that can later be removed.

7. Paints can be ranked in terms of their properties, such as tensile strength, but that is only one aspect of the complete picture, and is perhaps not entirely useful, since a painting is a composite structure where the support, preparatory, pictorial and protective layers interact with each another as one object (that is a zen-like statement but it fits well the physical nature of paintings).

​Geroge We are so happy to have you contributing here. Thanks