MITRA Forum Question Details

I contacted Pieter Keune about casein-acrylic. Here is a selection of a few answer that may be relevant. But please add or comment, since I'm only an artist with limited knowledge about Physics and Chemistry. I understand what he says, but lack the knowledge of a broader perspective.

The text is translated from Dutch.

Fine if you post my answers on the university forum. I have fond memories of my visit to the conservation and restoration department at the invitation of Prof. Hilton Brown in 1988. He had previously visited my department in Maastricht. I heard from my daughter Katrien, head of research at the Rijksmuseum Amsterdam, that she has been invited for next year.

Pure casein becomes brittle, but added in not too large quantities to an acrylic will not cause many problems.

Addition of a casein solution to an acrylate produces two effects.

First, it increases the surface tension of the dried film. An acrylate primer (often incorrectly called acrylgesso) is increasingly being applied as a primer suitable for all techniques. On the market as a universally prepared canvas. This primer has a low surface tension, so that ordinary oil paint can easily flow out on it. This also applies to most acrylic paints where additives give the paint a low surface tension. But paints based on water, egg tempera and water-based oil colors, for example, will bead. The paint does not flow sufficiently and that is at the expense of color strength. A casein-acrylic substrate is therefore suitable for water-based paints. Namely, water has a high surface tension and flows well on surfaces with a high surface tension and poorly on those with a low surface tension, compare a drop of water on a greasy surface.

Note; oil paint ages and also acquires a higher surface tension over time. Oil on an oil-primed linen is therefore always preferred.

A second effect is that casein acts as a crosslinker for acrylic. That is, acrylic molecules are interconnected by the casein. In practice this means that the plastic substance acrylic turns into an elastic substance. An impact on a universally primed linen causes permanent deformation, while a linen prepared with casein acrylic springs back. The effect as a crosslinker is also the reason that a prepared mixture thickens. It is then recommended to prepare the mixture yourself shortly before use.

and further (after I asked some questions):

When acrylic paint came onto the market here in 1964, it was perceived as a miracle paint. Dry quickly and then the real work with oil paint could quickly begin. A few decades later, the paint of many oil paintings in our national collection started to fall off. Now that we have a better understanding of the way in which oil paint ages, it also becomes clear that oil paint becomes much more polar over a period of 7 to 10 years, thus achieving a higher surface tension. The manufacturers have tried to overcome the mismatch that develops over time between oil paint and the acrylic primer by adding more chalk to the acrylic. Slightly more than the CPVC so that some oil is absorbed and the chalk chemically combines with the fatty acids from the oil. Time will tell whether this will be enough.

The other approach is to introduce a component in a ground layer that ensures a higher surface tension. When applying the oil paint, the adhesion is mainly based on van der Waals forces, later on polar attraction.

It is unknown to me that acrylic ground has a favorable synergistic effect on the layer of oil paint. On the contrary. I dealt with the case of an artist whose oil paint on the acrylic ground was severely cracked. In short: the linseed oil expands by 17% in volume when dried. The acrylic is deformed by the drying oil paint. The acrylic deforms plastically. The oil paint then shrinks, but the deformed acrylic no longer returns elastically. Due to the tensions now created, the dried oil paint cracks.

In principle, the oil from the paint does not penetrate through a preparation layer of casein acrylic in the canvas. The fact that some oil is absorbed into the surface layer is only beneficial.

A casein paint dries under high tension. Such a paint can only be applied on a very solid surface. But the protein molecules in the casein-acrylic ground also function as connections between the acrylic molecules. They crosslink. This means we no longer have two separate materials next to each other. Compare the function of sulfur in the vulcanization of natural rubber.

A casein-acrylic ground certainly does not make the linen stiff. The canvas now gives a certain amount of counter pressure to the brush. It springs back when pressed. That does have some influence on the brush stroke.

After the corona-time, I will go to the factory to take a closer look at this ARA product that was not created with my cooperation. It seems to me that it has a limited shelf life.

About the increase in volume/weight. The Dutch text in the e-mail is:

"de lijnolie zet bij droging 17% in volume uit". This can be translated as: "The linseed oil expands by 17% in volume when drying”.

Basically Mr. Keune’s point is (if I understand him correctly): the initial surface energy of oil paint is low, the surface energy of acrylics is made low ("This primer has a low surface tension, so that ordinary oil paint can easily flow out on it. This also applies to most acrylic paints where additives give the paint a low surface tension".) However, the surface energy of oil paint increases as the paint ages. This creates problems with adhesion in the long term.

I asked him indeed, by referring Yonah Maor’s article about delamination (2008), if this might be due to the presence of zinc in the paint film. He says: “Until now I have disregarded zinc white. Zinc white reacts quickly with fatty acids from the oil and forms zinc soaps that can settle between soil and paint layers. This can cause paint coat delamination. This problem can be seen in some of Van Gogh's paintings. But the danger of this should not be exaggerated. The fear of this has meant that the Belgian manufacturer Claessens of oil-primed linen could no longer export its product to the USA.”

Further: he says that by adding casein in small amounts you can increase the surface energy of acrylics, and thus evade adhesion-problems in the future. Adhesion in the beginning is based on van der Waals-forces (I guess that’s why casein is chosen?). After a while on polar attraction. Sounds logical to me.

But although I’m really interested in the more theoretical side of it all, this is also leading me down the rabbithole (I haven’t painted in a month, considering and reading all kinds of articles, but also learning a lot from George O’Hanlon’s Best Practices-webinar). For example I think: “but maybe failure in adhesion is also influenced by the size of the canvas, large canvasses experiencing more adhesion-problems than smaller one (like the ones I paint on), which is maybe the reason why Gottsegen recommends no oil on acrylics when the surface >2,4 square metres. And maybe there is a difference between acrylic as a layer, and acrylic as a size, which is brushed into the weave, not really creating a film (I only use it as a size, not as a ground, which is what Mr. Keune seems to talk about).” And so on. And I realize I’m only guessing.

To be clear, I haven’t experienced any adhesion-problems with my Golden acrylic-size. I just wanted to know the (very) best practice. And indeed, mounting a canvas might be a much more effective solution, certainly for a small format. So, if there’s no clear answer to it, then I’ll just continue painting, and see what the future brings :) (Nevertheless I thought it an interesting product, and I will try make my own, as Mr. Keune recommended)

Thanks,

Brecht Koelman