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I'm looking for the best way to size my linen, when using a lead-oil ground.
A combination of Golden GAC 400 en GAC 100, or 2 coats of GAC 200 gives good results. However, I have also tried casein-acrylic binder by Ara Colours (https://www.aracolours.com/about-ara/auxiliary-products/). They are a part of Old Holland.
The descrition says: "Casein acrylic binder V350 is a modern alternative to rabbit glue. It gives a strong flexible film for the preparation of the canvas. Casein acrylic binder is insensitive to moisture after drying."
It becomes indeed very stiff (much more than the GAC 400 or 200) even after a year or so, which is probably due to the casein. This stiffness is very desirable of course, but I also read that casein becomes brittle. Is it possible that this brittleness is counterbalanced by the acrylic? And is casein-acrylic able to block oil absorption? So, can casein-acrylic binder be a good size if you want you painting to last?
I have mailed Ara Colours several times with this question about how it works, but never got an answer, so I hope someone can help me here.
Move this whole section up, swapping places with the section above it.
I was unaware of this product and can't really comment without more information. I will ask around.
This product is unknown to me, too. However, I am familiar with a 1991 European and Canadian patents for an acrylic casein glue. I wonder if this is related?
Casein has long been investigated as a source of “plastic” material. There is a 1938 patent on thermoplastic proteins, for example, featuring casein. In fact, there was an entire industry based on casein plastics in Great Britain and the U.S. at the beginning of the 20th century. Casein objects were fabricated from stock material such as sheet, rod or tube. Button blanks were stamped from sheet or cut from rod but in the early years most were trepanned from sheet material.
Same here, have only read about casein acrylics as potentially promising, biodegradable alternatives for architectural coatings. From the sparse literature I have found, it looks like the internal structure of the film forms like regular acrylic rather than through simple evaporation, so I would not expect the physical properties of this product would be the same as old-fashioned Casco Glue-type casein.
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.
We haven't come across this product either and if you are uncertain about its long term performance, you could resort to PVA sizes or three layers of acrylic gesso before to block oil strike-through, before applying an oil ground. Ideally you would adhere the linen canvas (if you like painting over linen texture) to a rigid board to provide maximum support for the oil paint layers. We have also seen some pre-primed canvases show adhesion failure (https://www.goldenpaints.com/adhesioncanvas), even with acrylics applied on top. The formulation for acrylic dispersion paints, mediums and grounds can be complex and there are great differences in quality and performance among acrylic grounds, which is why I think drawing a general conclusion from one or a couple case studies, as Peter Keune did, is problematic. (I have great respect Peter Keune's work and have learned a lot from his articles and also met him in person during a workshop he gave at the University of Amsterdam.) We know that linseed oil gains about 17% in weight during the initial curing process (see https://justpaint.org/weighing-in-on-the-drying-of-oils/- are you sure you translated this part correctly), but I have not come across studies that show increase in paint volume. It would be very interesting to have related references. It's not unlikely that the oil layers in the case studies that Peter Keune mentioned contained zinc, which we know to create severe embrittlement. At Golden, we have naturally aged samples of oil paints over acrylic layers from the 1990s that are still in great condition with oil paint showing excellent adhesion.
This is such a small world. I teach the classes at UD which were formerly
instructed by Professor Hilton Brown.
Mirjam, the article you cite by Sarah Sands is really excellent. BTW one needs
to copy and paste it into one’s browser as the link goes to an “Oops that Can’t
be Found” page.
I am guessing that the Pieter Keune statement, “the linseed oil expands by
17% in volume when dried” is a slight mistranslation. An initial increase of
17% of the weight of an oil film would be far more in line with the Mechlenburg
and Golden studies I have seen.
The one paper I know that looked at volume changes, as well as weight, of different linseed oils is 'Determination of changes in mass and volume of linseed oil during drying' by P. Svane
All of the oils gained considerably in mass, with a maximum of 12% within seven days. The volume, however, decreased with large individual differences from one oil to another. A cold-pressed raw linseed oil decreased in volume by almost 15% whereas the volume of an alkali-refined linseed oil only decreased by a few percent.
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)